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Providing Outdoor Combustion Air for a Wood Stove

We brought ducted outdoor combustion air to our wood stove by punching a hole through the back of the unused fireplace

Posted on Oct 14 2013 by Chris West

In November 2012, I started on a deep energy retrofit of my 1976 raised ranch in northwestern Vermont, in the shadow of Mount Mansfield. As a Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. consultant, I wanted to make my leaky (8.25 ach50) house with fiberglass-filled 2x4 walls and a tuck-under garage much more energy-efficient.

My hopes to make the renovation hit the EnerPHit standard (a standard for Passivhaus retrofits) were dashed because of some of the basic realities of my house. The orientation was all wrong, and I had already replaced my home's old single-pane windows with what I was told by my contractor were the best windows on the U.S. market — namely Andersen super-low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. double-hung sash windows, not the Passivhaus windows I would have chosen had I known better at the time. Ugh!

The inability of the house to hit the EnerPHit numbers didn't deter me from superinsulating the house and getting as close to Passivhaus goals as I could. In the end I decided to remove the existing vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding and wrap the whole house in 6 inches of repurposed (used) rigid polyisocyanurate insulation above grade. Below grade, I installed 6 inches of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. on the outside and 2 inches of EPS on the inside.

There were lots of little improvements I made in addition to these changes; I've written about how I managed these improvements in I have been keeping on the retrofit. Today I'd like to share details about the decisions I made concerning my wood stove.

A chimney is a thermal bridge

Wood is a very common heating fuel in Vermont. More than 50% of Vermonters use wood as part of their heating fuel mix. Wood is plentiful and cheap. It’s even free if you have land to get it from, and most of us in my neck of the woods do. So we will be keeping the chimney and the wood stove.

When it comes to thermal efficiency, chimneys have lots of problems. They are huge thermal bridges (especially if they are on the outside of the house like mine is); they are hard to air seal; and most wood stoves are atmospherically vented. This means that the air that the wood stove uses for combustion comes from the room it is sitting in.

To burn or not to burn wood?

Wood stoves are great. OK, maybe they aren't great. The efficiency sucks (usually less than 75% for older models), but the fuel is cheap, readily available, and renewable. I'm not saying that burning wood doesn't produce CO2 and CO and NOx and SO2, but it is a way to heat your house that is very popular here in Vermont.

The cheap part is also part of fuel independence. I can go out into the woods around my house and cut down enough wood to keep my house warm for a whole season. Just saw, split, stack, and use.

For a Passivhaus, a wood stove is in most instances a deal-breaker. For starters, a wood stove usually has a rated output that is way more than any Passivhaus can use. A 55,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h stove in a house with a 7,000 Btu/h load is just overkill.

Secondly, most wood stove installations introduce air-sealing problems that make it hard to get a home's airtightness down to 0.6 ach50.

My old Pendelton Avalon wood stove dates back to the 1990s. I bought it from the fellow I bought the house from. It is a good little stove with lots of years of use still in it. My wife and kids love the fire and it gives off 55,000 Btu/h when it is running hot. It is also an atmospheric combustion stove.

An atmospherically vented stove (one which gets its combustion air from the room where the stove is located) is fine if your house is leaky, but becomes a problem when your house is tight. If your house is tight, you need to bring in outdoor air for the combustion process.

I thought that my wood stove had the ability to be outfitted with a duct to bring outside air into the house, but upon further investigation I found this wasn't so. I ended up buying a new Hearthstone wood stove with a larger firebox (and with a ten-hour burn time instead of six hours) and the ability to add the sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. fresh air duct I needed.

If you are going to do this type of retrofit on your wood stove and chimney, make sure that your wood stove can be outfitted with the outdoor air supply first!

A tight home’s wood stove needs to breathe from somewhere

We then need to figure out where to get this fresh air from. A friend of mine suggested that there are plenty of people who run the fresh air duct up the chimney and pull the air supply down from the top of the chimney. I could use this approach, because my chimney has two flues, one of which I’m not using for heating. I could just break through to the other flue and pull air from there.

The only problem with this scenario is the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. Hot air rises. The warming of the chimney shaft next to the fresh air channel could cause an updraft that could make the “draw” of the fresh air difficult, which could decrease the efficiency of the wood fire.

My next idea was to just cut a hole in the back of the chimney and run a pipe through the chimney directly out to the outside. (I planned to install the wood stove as a fireplace insert.) This approach would give us a short horizontal duct with access to fresh air. A short run decreases the resistance caused by the inside surface of the pipe and the horizontal run would eliminate possible problems from the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season..

As I said before, one problem with outside chimneys are that they are horrible thermal bridges. To help cut down on these losses, I had 6 inches of rigid rock wool board put on the outside of the chimney. To learn exactly how I did this, you’ll need to check out . The rock wool gives me about an R-24 and means that I had an additional 6 inches plus to get through after I was through the bricks of the chimney.

To ensure that all of the work would be covered by my homeowner's insurance, I hired a professional chimney company named Brickliners to do the installation.

They came and made a hole in the back of the chimney, and drilled a small hole through the Roxul. I widened this hole to allow for the 3-inch stainless-steel duct to come through. Getting the duct to come through the hole was no problem once the hole was big enough.

Air sealing details

Chimneys are difficult to air seal. If you are installing the type of wood stove that is designed to fit in an existing fireplace, you have a big hole just above the insert area up into the flue. At my house there was now also a hole at the back of the chimney to bring in fresh air. Both of these holes needed to be air sealed.

To air seal the bottom of the fireplace flue I made a baffle, which is just a piece of sheet metal cut to the shape of the chimney flue just above where the wood stove sits. This needed to be screwed into place and caulked. Again, I left this job to the professionals. I could have done the work, but I would rather that their liability insurance cover the work, not mine.

The fresh-air duct going out the back of the chimney was a bit different. I first needed to fill the space between the duct and the walls of the hole. I filled this with leftover Roxul insulation that I had lying around.

The black membrane you see in many of the accompanying photos is Mento 1000. This is a special European membrane that has a variable vapor-permeance rating. When the relative humidity is low, it has a perm rating of 4. When the relative humidity is high, which might happen if moisture gets into the wall, it has a perm rating of 34.

This variable permeance allows any moisture in the wall to migrate out of the wall while still preventing vapor and bulk water from entering the wall. Compared to Tyvek or Typar, Mento 1000 is expensive. But unlike Tyvek or Typar, it is a "smart" vapor retarder.

Since I had worked so hard to get the airtightness of the house down from 8.25 ach50 (that was the air leakage rate shown by the blower door test that was performed before the renovation began) to 2.0 ach50, I needed to make sure that any penetration through the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. was properly sealed.

Fortunately, there are some great products available for this purpose. I used Tescon tape, which has an acrylic-based adhesive. This acrylic actually penetrates the top molecules of the substrate it is being applied to and cures there to become one with the substrate material. This means that the adhesive doesn’t dry out like other tapes. The bond actually becomes stronger over time. These tapes have a 60-year guarantee!

I used this tape in conjunction with a silicone gasket which I got from 475 High Performance Building Supply in Brooklyn, New York. They also supplied the membrane and the tapes I used for this project. The photos below show how I sealed the gaps around the outdoor air duct.

Once the duct was properly sealed, I needed to add a piece of 3/4-inch plywood to make the plane of the PVC plate flush with the plane of the siding. The plywood has the same thickness as the strapping.

We hit our airtightness target

After the siding was installed and the new stove was in place, we performed a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. and found that we had hit the 2.0 ach50 target. We also found that the chimney workers had failed to seal two important seams in the chimney baffle plate. We invited them to come back and they fixed the problem. I still have to do another blower-door test to see what kind of change that made.

I’ve used the stove a few times — more to try it out than because I needed to for heating the house. It worked fine. It will be putting out way too many Btu/h for the house. I'll be using less energy over all.

For more information about my retrofit please visit .

Chris West has a background in mechanical engineering and lived in the Netherlands for ten years during the 2000s. There he learned about advances in building science; he brought this knowledge back with him when he moved back to the U.S. in 2010. He is a Certified Passive House Consultant and is currently president of the Passive House Alliance of Vermont, husband and father of two wonderful children who love living in the wilds of Vermont.

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Image Credits:

  1. Chris West

Oct 14, 2013 7:58 AM ET

Thermal Bypass
by shane claflin

A chimney is a Thermal Bypass

Oct 14, 2013 8:04 AM ET

Response to Shane Claflin
by Martin Holladay

It looks like your forgot to read the article. Chris West wrote, "When it comes to thermal efficiency, chimneys have lots of problems. They are huge thermal bridges."

Or are you referring to an air leakage path that Chris failed to discuss? I think that Chris is aware of the disadvantages of brick chimneys, especially exterior brick chimneys.

Oct 14, 2013 11:03 AM ET

Best blog
by aj builder, Upstate NY Zone 6a

This blog is why I am open GBA.

Thank you Chris for sharing. Any residential contractor contemplating similar undertakings will be aided greatly by this blog and your own.

Oct 14, 2013 2:19 PM ET

Edited Oct 14, 2013 5:40 PM ET.

Code-schmode, where's the commode?
by Dana Dorsett

" A friend of mine suggested that there are plenty of people who run the fresh air duct up the chimney and pull the air supply down from the top of the chimney."

Having looked at this movie several times in the last 18 months (including for my own home) that would constitute a code violation! The combustion air intake that is higher than the fire box of the combustion unit is EXPRESSLY DISALLOWED under the IRC:

" R1006.2 Exterior air intake.

The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling or from spaces within the dwelling ventilated with outside air such as nonmechanically ventilated crawl or attic spaces. The exterior air intake shall not be located within the garage or basement of the dwelling nor shall the air intake be located at an elevation higher than the firebox. The exterior air intake shall be covered with a corrosion-resistant screen of 1/4-inch (6 mm) mesh."

Not to put too fine a point on it, your air intake as implemented without a 1" clearance between the air intake duct and the Mento 1000 / Tescon / gasket/ plywood /PVC at the air intake is also a code violation (unless it's more than 5 running feet of duct away from where it hooks up to the stove or insert) :

"R1006.3 Clearance.

Unlisted combustion air ducts shall be installed with a minimum 1-inch (25 mm) clearance to combustibles for all parts of the duct within 5 feet (1524 mm) of the duct outlet."

Though the risk is low, under adverse conditions that back-draft the stove embers can theoretically pile up at that point and light it off. One code-compliant solution is to use a sheet-metal air-barrier sealed sealed with a fire-rated duct mastic at that location.

I'd be very curious to see more details of how the chimney was insulated too- the link points to a thermal bridge analysis. The fact that the IRC specifies clearance limits between the exterior of the masonry that are not allowed to be filled with insulation (R1003.18,) nor does it allow the chimney to mechanically support either the insulation or the siding have been a constant PITA for retrofitting insulation & siding-surrounds on chimneys (R1003.8 ) If a P.E. signed off on the analysis for the "extra" loading it would meet code, but it would be a required step for MY local inspectors to sign off on it. (YMMV). It looks in the pictures like the rock wool & furring is being supported by the masonry. Even though R24 rock wool between the masonry and Mento 1000 & furring etc is a DAMNED good thermal barrier against ignition, without the 2" gap is doesn't meet the letter of R1003.18 without the 2" gap.

The letter of the IRC code around some of this stuff seems nearly self-contracdictory adding undue complexity to retrofits, but enforcement officials don't have the liberty to simply ignore it, even where common sense says it's not a real problem. Anyone diving into it is strongly advised to read the code carefully, make up detailed drawings, and get pre-approval from the building department before going ahead with insulating an exterior chimney.

[edited to add]

I found the blog discussion of the chimney insulation details here:

"The insulation will be brought on in two three inch layers. My contractor, Jim Bradley, was unable to find masonry screws that were 7" or 8" long so he came up with a solution that should work. We are going to use 4" masonry screws and use this to put the first layer of 3" of insulation on. This will then be covered with a layer of 1/2" plywood. The following layer will then be fastened to the plywood using regular screws. "

The intermediate layer of plywood and the lack of 2" of un-filled clearance to the masonry puts the assembly at odds with the letter of the IRC and it's exceptions. While I don't forsee a functional issue with that, it IS an assembly that would require a variance. (I'm more leery of the lack of clearance at the combustion air intake port than the non-compliant 3" of rock wool between the plywood and masonry, or the weight of the insulation & siding being carried by the masonry.)

[edited a second time to add]

So the outer layer of insulation on the chimney is 3" iso? It wasn't clear from the blog text that the chimney too had a foam layer, as seen in this pic:

I hope a 1" clearance around the air duct was maintained at least where it passes through the foam & plywood(?).

Oct 16, 2013 7:52 AM ET

by shane claflin

Isn't there a difference between a bypass and bridge? I thought a bridge loses heat through conduction. A much slower process than convection, as in a bypass. I could be wrong, I was wrong once.

Oct 16, 2013 8:52 AM ET

Edited Oct 16, 2013 8:57 AM ET.

Response to Shane Claflin
by Martin Holladay

You are correct that the term "bypass" usually refers to an air leakage path. Unfortunately, the lexicographical waters were forever muddied by Sam Rashkin of the EPA, who chose to call a list of thermal envelope defect locations -- some of which were air leakage paths, and some of which were thermal bridges -- the "Energy Star Homes Thermal Bypass Checklist." Ever since Rashkin's unfortunate misnomer became enshrined in the literature, the word "bypass" has been misconstrued.

I suspected that you might be referring to an air leakage path, which is why I asked, "Are you referring to an air leakage path that Chris failed to discuss?" I'm sorry that my earlier answer included a misunderstanding, and I regret my disparaging remark. Sorry.

To answer your point: yes, the flues of traditional brick chimneys represent a big hole in the air barrier of a home. So you are correct. But, as I wrote earlier, I think that Chris is aware of the disadvantages of brick chimneys.

Oct 16, 2013 9:23 AM ET

what am I missing?
by Greg Smith

" R1006.2 Exterior air intake.

The exterior air intake shall be capable of supplying all combustion air from the exterior of the dwelling or from spaces within the dwelling ventilated with outside air such as nonmechanically ventilated crawl or attic spaces. The exterior air intake shall not be located within the garage or basement of the dwelling nor shall the air intake be located at an elevation higher than the firebox.

Maybe I am not seeing something really obvious, but if a stove with external air intake is installed in a basement how is the external air intake supposed to be below the level of the firebox?

Oct 16, 2013 9:33 AM ET

Response to Greg Smith
by Martin Holladay

The way I read that code provision -- assuming that there are no exceptions provided in other provisions of the code -- it sounds like a wood stove with an exterior combustion air duct has to be located above grade, unless you are willing to dig a level trench that leads from the bottom section of your basement wall all the way to daylight. (This approach will work for homes in Vermont that are built on a hillside, but won't work in Kansas.)

Oct 16, 2013 11:08 AM ET

That's my take too...
by Dana Dorsett

Installing the wood stove below grade with ducted combustion air (or even without ducted air) will always be a problem. The issue is that there is always some potential for back-drafting the firebox into the air intake, but when the air intake is above the firebox it essentially becomes another flue- the natural stack pressures would have both the main flue an the air intake drawing FROM the firebox. The higher the air intake is above the firebox, the bigger the stack-effect draw pulling from the firebox.

On further analysis I'm less & less comfortable with the insulation stackup on this retrofit. With ~R15-R20 of rock wool between a 500F-800F stack and the plywood, and R18-R20 of polyiso & siding to the exterior, there is potential for sections of the plywood to run at temps north of 200F for sustained periods. Over time & repeated heat soaking at those temps it results in slow-oxidation, even charring, and I'd expect most plywood would delaminate and lose structural integrity after months/years, but the risk of charring is real. or an idea of what heat-soaking wood over 150F for extended periods does, see:

Polyiso's service temp limit is also 250-300F, above which the polymer begins to degrade (it won't ignite at those temps though- unless the wood does) and it's highly likely that it'll see those temps. When I first read it quickly I had the impression that it was an all rock-wool show out to the outer layers, which would be fine, but that's clearly not the case- it's a brick | rock-wool | plywood | polyiso | WRB sandwich with more R in the iso than the rock wool.

Oct 16, 2013 5:57 PM ET

No poly in chimney insulation sandwich
by Chris West

Hi All,

It took me a few days to get to this blog. Life intervened, which it likes to do on occasion. I'd like to thank Martin for giving me the opportunity to tell you about my retrofit in Jericho, Vt.

Dana has raised some important questions which I don't have all of the answers to, but I'll try to answer some of his concerns.

The sandwich which the chimney is surrounded in is three inches of rockwool with 3/4" plywood and then another three inches of rockwool NOT polyiso. I would never want to bring the polyiso that close to the chimney. Therefore the issues Dana suggested about the temperature problems with the polyiso are moot. The closest polyiso is a good seven inches away from the chimney in any direction. It was only installed on the walls of the house stopping before the chimney to allow for the installation of the Roxul.

The question about the combustible material near the air intake. The gasket I used was six inches wide. I recently reopened the construction around the air intake and increased the distance from the paper to the pipe. I will revisit this on his suggestion to ensure that there is at least 3" from the fresh air pipe to the combustible paper(or other materials) either by using metal flashing or some other method. This should, I hope satisfy his second concern.

The last concern is interesting. None of the people I was working with on this project mentioned any of these concerns when I explained the layers. Chimney techs were on site when we were installing the pipe. I will get in touch with them and others involved to ask them about your concerns with the plywood overheating and charring.

Dana asked me in a separate email if I would be willing to monitor the temperatures of the plywood at different places to see what temperatures it is experiencing. I think this is a great idea and I will incorporate this into my monitoring plan.

I will be starting, however by monitoring the flue temperatures and the temperature of the chimney where it comes out from the insulation (just below the roof line). These measurements should give some indication as to the danger for the plywood. Just until the monitoring sensors can be put into place. If my monitoring shows that there is a danger of charring or of ignition I will change the construction to be safer. I suggested to Dana moving the wood to the outside of the construction.I am waiting to hear his response to this suggestion. On the outside of the construction the temperature of the plywood should stay way below the 170F he gave as the maximum to avoid low temperature charring and ignition.

Thanks Dana for your careful, thoughtful and expert advice on these points.

Thanks to all of you for applying your critical minds to my project. Through the type of exposure the GBA can offer the issues around super-insulating various types of buildings can be discussed in forums like this. Hopefully we are all working towards the healthiest, safe, comfortable and energy efficient homes we can build.

Oct 16, 2013 7:28 PM ET

Is the need for make-up air made up?
by Derek Roff

In a related Dec 2011 GBA article, "How to Provide Makeup Air for a Wood Stove", Peter Yost tells us that neither the EPA nor any other authority that he could find offers solid guidance on whether makeup air is needed for a sealed wood stove, nor how to supply it. In the comments, Martin says, "there should be enough air indoors to supply the fire with oxygen -- just as there should be enough air indoors to supply you with oxygen." Various people indicate that the amount of air consumed by a sealed wood stove is small, with one calculation offering "1.36 cfm" as a credible air consumption rate.

If new scientific information has been published since that article, I'd love to see a reference. I'm thinking that most people are still going by seat of the pants guesswork. And their pants are supplying a lot more makeup air, than is justified by the calculations.

Not mentioned in these comments so far, is the concern about spraying super-cold air on a fire. The direct air intake for the Vermont home described in this article could easily be taking in outside air at -40 degrees (C or F) some of the time, and not much warmer, for a lot of the time that the stove is in use. Spraying air of that temperature on a fire that you want to burn cleanly is counterproductive. Wood stoves start to get into the clean burning range at about 1,200 degrees F (650 C), and burn cleaner still at 2,000 degrees F (1,100 C). Feeding the fire -30 degree outside air instead of 70 degree inside air can increase pollution by two to ten times, according to some references.

If a given wood stove in a given house burns well without makeup air, it seems foolish to connect it to the outside air with a direct air duct. If it burns poorly until you open a window, then makeup air would seem to be needed. I'm pondering whether a little home-built, stove-connected heat exchanger could be designed to supply the stove with the proper amount of warmed air, at whatever level of makeup air is needed.

Oct 16, 2013 9:31 PM ET

power vent
by shane claflin

If you power vent the air intake, I don't think worries of backdrafting exist. You might have to have a fan on the flue as well, I don't know.

Oct 17, 2013 5:50 AM ET

Edited Oct 17, 2013 5:51 AM ET.

Response to Derek Roff
by Martin Holladay

The amount of combustion air required for a wood stove varies; while the volume of air per minute is small for most of the burn, it is larger when the fire is first lit.

There is no doubt that all virtually all homes have enough air leakage to supply the combustion air for a wood stove. Problems only arise when the wood stove has to compete with exhaust fans (bathroom fans, range hood fans, and clothes dryers, for example). It can therefore be argued that it's not the wood stove that needs a makeup air system -- it's the range hood.

Many of the disadvantages of combustion air ducts that connect directly to the firebox of a wood stove (especially worries about backdrafting that blows hot embers into the fresh air duct) can be avoided by providing a fresh air duct that terminates near the wood stove, but is not connected directly to the firebox. Of course, this approach introduces its own set of disadvantages.

Oct 17, 2013 5:59 AM ET

A question for Chris West
by Martin Holladay

A caption to one of the photos reads, "Hole from outside showing brick, 3" polyiso, 3/4" plywood, 3" poly iso and wrap."

You recently posted a comment on this page claiming that "The sandwich which the chimney is surrounded in is three inches of rockwool with 3/4" plywood and then another three inches of rockwool NOT polyiso."

Please clarify this apparent contradiction so that I can provide accurate captions to the photos published here. Thanks.

Oct 17, 2013 6:03 AM ET

Response to Shane Claflin
by Martin Holladay

You suggested using a fan to provide combustion air to a wood stove. That approach would provide too much combustion air for most stoves, and would cause such a fast rate of burn that the house would probably overheat. (The technique is used for some wood-fired boilers with large storage tanks, but that's another story altogether.)

Installing a fan in the flue is a bad idea for the same reason, as well as an additional reason: the fan would quickly clog with creosote.

Oct 17, 2013 7:46 AM ET

Edited Oct 17, 2013 7:52 AM ET.

As Chris says, external
by David Coote

As Chris says, external chimneys are terrible thermal bridges. Here's some IR images from our house last year.

We have a wood combustion insert in the old fireplace chimney. I had the wood heater running earlier that night to ensure we had some nice IR images.

On a related note, some of the Middle European wood combustion vendors are selling log systems they claim reach over 90% efficiency on Lower Heating Value. And there are residential scale wood burning systems for sale in Europe that meet regulations in areas with smoke controls.

IR_2041.jpg IR_2043.jpg IR_2045.jpg

Oct 17, 2013 10:39 AM ET

Regarding re-building iwith the plywood on the exterior...
by Dana Dorsett

As noted in my second private email to Chris, putting the plywood on the exterior would mitigate the fire risk, but in a VT climate plywood is too vapor retardent to fly without ANY type of interior vapor retarder (how vapor retardent is the brick, eh?), and that the likely hood of mid-winter frost build-up at the rock-wool/plywood interface would be high.

One solution would be to install sheet metal vapor & air barrier either between the masonry and inner layer rock wool, or between the rock wool layers. But it's probably cheaper and easier to instead of plywood use a vapor open, fire-proof structural sheathing that isn't susceptible to moisture, such as GP DensGlass. The DensGlass itself doesn't need an exterior WRB, but putting a WRB on the exterior to meet air-tightness goals would be OK.

Oct 18, 2013 12:44 AM ET

Make-up air likely not needed, and...
by Steve Paisley

I agree with Derek and Martin that make-up air is not likely to be needed for the wood stove in this house, unless there's an unusually large range hood or other oversized vent fan running. And: if a rangehood, dryer, exhaust fans, etc... are depressurizing the house with respect to the outdoors more strongly than the negative pressure provided by the chimney draft, then the stove will leak smoke (and CO) into the living space, EVEN IF direct outdoor combustion air is supplied to the stove. Thus Martin's suggestion to supply make up air to the living space, not to the stove, is a good one--at least this way the incoming air will not pick up CO and smoke before it enters the room, as it would if it were connected to the stove. Also, there's no reason (other than the not-thought-out code requirement) for an air intake to terminate near the stove, which has a farily small air needs in any case. You'd be better off putting it near the range hood, say, or at some place where an incoming cold draft is least likely to cause discomfort.

Another point to consider is that winds generally create negative outdoor pressure on three sides of a house, and if your air intake happens to be located on one of those three sides, a strong wind can easily overpower a chimney draft. This, again, is a reason A) NOT to connect an air intake directly to a wood stove or masonry heater, and B) to preferably have not a single air intake at all, but rather have small ventilation openings balanced on all sides of a house. This happens naturally, of course, in all but the most meticulously and heroically airsealed new construction.

If it were my house, I'd skip the air intake altogether, and see if it's needed in actual use. I'd be really surprised if it were.

Oct 21, 2013 3:19 PM ET

The amount of leakage MATTERS! (response to Steve Paisley)
by Dana Dorsett

Sure air tight wood stoves aren't exactly air tight or sealed combustion, but the cross sectional area leakage between the firebox and conditioned space of a unit with ducted combustion air is but a tiny fraction of the 2-8 square inches represented by the combustion air intake port on the stove.

I'm not buying the argument that proximity-air results in lower net pollutants introduced just because one of the infiltration paths in a depressurization situation then doesn't pass through the firebox, given that the remaining path that DOES pass through the firebox is at least an order of magnitude or two larger than the combined seam & gasket leakage on the unit.

A house that tests in the neighborhood of 2ACH/50 has enough combined air leakage that getting the stove to draw under normal conditions won't be a problem, but the exterior combustion air isn't really about that, it's about the backdrafting issue. A house that tests under 2ACH/50 has a real potential for a backdrafting issues with a clothes dryer running simultaneously with all kitchen & exhaust vents.

Without ducted combustion air intake the wind-generated depressurization causing it to run backwards is still an issue, except that now the back draft path runs through the house, not the air intake. In tighter homes where the heat recovery ventilation is the largest air leak, HRV side will be the low-pressure side of the house just as often as the dedicated combustion air intake would be, but the volume backdrafting that enters the conditioned space air would be an order of magnitude or so larger.

Bottom line, even imperfectly sealed combustion is generally safer that one that uses room air on atmospheric drafted appliances.

Oct 31, 2013 3:04 PM ET

prewarming air intake
by Guillermo Metz

A couple of questions and comments (and thank you for these blogs!):
1) why a 3" air duct? That seems excessive to me considering what the stove is calling for at any given time.
2) my understanding of why to provide dedicated air to combustion equipment such as a wood stove is that even in a home that is not tight -- one that can provide plenty of air to the indoor space and the fire -- is that you're drawing your air from the outside, right through every crack in the walls, etc, which makes for a drafty house. Bringing in dedicated air does not depressurize it (as much) and (at least) lessens this.
3) can the issue of bringing very cold air into the wood stove be dealt with by using something like a 3/4" flexible copper pipe that is wrapped in several loops along the back of the wood stove before it enters the stove (or ends at a point near the stove's primary air intake)? You wouldn't want to make it too long, but it would also prevent back-drafts from being hot enough to still be a concern when they get to the wall layer. (Is the concern with using something like 3/4" pipe and adding something like a loop or two that it would not pull well? That is, not provide enough pressure to pull outside air in.)
4) I'd like to hear more about the issues of bringing outside air to near the wood stove rather than directly into it (from the end of "Response to Derek Roff by Martin Holladay", above). To me this seems like a sufficient set-up and one that solves some problems.

Oct 31, 2013 3:20 PM ET

Response to Guillermo Metz
by Martin Holladay

Roughly speaking, you can use the diameter of the flue connected to a combustion appliance to get an idea of the diameter of the required air intake duct. If the wood stove needs a 6-inch-diamter flue, that's how much air is leaving the building through the flue, so it stands to reason that it's a good idea to give it a 6-inch-diameter duct to supply fresh air.

That said, the idea bothers many people, and they downsize the air intake duct. After all, you don't really need the 6-inch flue unless the fire is roaring, and most stoves can supplement their combustion air from envelope air leaks (assuming we are talking about a duct that terminates in the proximity of the wood stove rather than connecting to the firebox).

Looping copper tubing around a hot part of the stove won't ensure that the air in the tubing won't backdraft. If the tubing gets hot, the hot air just might travel outdoors rather than meandering indoors. I suppose if you terminated the fresh air duct near the ceiling, and pulled outdoor air from the rim joist, you might prevent backdrafting. But then the air wouldn't be supplied near the stove's air intake.

Nov 5, 2013 1:17 PM ET

'My' Outside (Heated) Air
by John F Cross

In the mid '80's (pre-certified) I bought a maker-modified, Sedore, furnace model, wood stove, with a 3" inlet, near the bottom of the ??air surround??, around the combustion chamber ('my' location point). With a slide-gate shutoff, and ABS pipe going through an unheated crawlspace, to the outside, about 10' away. The outside, air is heated by the stove, warms the room - combustion air, from the room, burns and goes up a SupraFlu R5/inch insulated, inside, masonry chimney.

'My' theory is " cold o/s air is heated, slightly pressurizing the inside, so that warm air flows to the extremities, rather than pulling (cold) air from the extremities, with room-stale air being combusted. I realize this violates my current building code!

The Sedore Stove is a bottom burner, loading from the top on one side of a baffle that separates the chimney side. It can burn at very low (fire) temperature levels, as long as there are coals-flame under the baffle to burn any creosote gasses. Very effective, as I can heat at -30'F. At +50'F, I allow the coals to (almost) burn out - add wood chips-wood pellets and a new log and re-create a renewed, fire-coals, under the baffle, warming the room. There is a users YouTube video, showing the loading door 'wide open' when the stove is in use! NOT the way 'my' stove operates, with good draft, from the o/s air.
I realize, the importance of the building code - not allowing 'my' method, even though it ensures fresh air in the room. It took time to train my kids to open the slide gate to bring in the o/s air. Otherwise, the draft is poor, with the risk of a backdraft. Until you learn (any) a stoves operation, do not go away, leaving it alone!
'My' Sedore, rarely operates with a chimney temperature above 200'F - except at (fresh) start-up and (about) minus -30'F. I burn almost anything - softwood, pine needle (chipped) boughs, etc. I get very little creosote in my annual chimney sweep. I 'like' the heat holding, masonry chimney vs a metal, insulated chimney; and the burny baffle, feature. With about 30 years experience with the Sedore stove - both year round weekend use, and now full time, year round, use, now also using a Mini-split (my kids are 'lite' on the (small) amount of firewood needed - I have learned and mastered the quirks of the Sedore, in my situation.
I don't know how, I would (legally) configure a similar, current configuration, without a direct-heated air, connection. Even with a furnace, a combustion air, outside connection, extremely, varies the density and temperature of the air!

Dec 4, 2013 4:17 PM ET

Data is the key
by Chris West

Hi All,

I hope this isn't to late for you to get and that you all get a chance to see this information.

Dana Dorsett was concerned about the chimney construction. He, rightly so, was worried that the construction, which is 3" of rigid Roxul then 3/4" plywood and then 3" of ridid Roxul on top of that would cause a fire hazard.

Specifically Dana shared a scientific paper which showed that wood in contact with relatively low temperatures can char (become charcoal) over extended periods of days or months of exposure. The temperature that is the low level limit is 170 F. If wood is exposed directly to temperatures of 170 F or higher it becomes charcoal which has a lower ignition temperature than solid wood. It can even burn/smolder anaerobically (without oxygen)!

This was news to me. It was obviously important to test this to see if my chimney was in danger from this low temperature charrification. I contacted Dana to ask him what test conditions would give an acceptable reflection of worst case conditions. Dana suggested that if I were to keep the wood stove at it's highest temperature for 48 hours and monitor the temperature at the inside surface of the wood that would tell us.

Dana's concern was based on the assumption that the temperature gradient inside the construction was linear. This means that if the chimney is 500F and outside is 0F that the inside of the plywood would be 250F (halfway in the construction would be one half of the temperature difference).

The data from the testing will tell us this.

So I bought a thermocouple data logger from Extech (SD-200) which can take up to three thermocouples. I used thermocouples because they can measure temperatures up to 900F, well above the temps we are expecting.

I wanted to test at two heights just to be sure that I was getting an accurate measurement of the temps. I measured from just above where the stove pipe enters the chimney flue (expected to be the hottest point) and five feet above this.

The test was started on November 21st at 2:45 pm and finished on November 22nd at 3:00pm a good 48 hours.

The test wast to take readings at two heights at three positions at each height, on the left side of the chimney, spot in the middle where the actual flue runs and on the right side of the chimney.

The heights were to be 1) one foot above where the stove pipe enters the chimney flue and 2) about five feet above that (2 feet from the top of the insulation).

I just finished the 48 hours of testing a few weeks ago.

First I'd like to note that the testing was done during relatively warm weather, which means that the outside temperature is high relative to what it might be during days I would normally use the chimney.

This means that it is a worst case scenario for the test. Warm outside temps (upper 20's to low 40's) should mean that the temperatures at the inside of the plywood would be about as bad as they can be. Lower ambient outside temperatures should mean that the temperature at the plywood would be lower since the temperature gradient is influenced by the heat from the chimney side and the ambient temp from outside.

The good news is that with flue temperatures averaging 420 F (as hot as I could get them), the top temperature the sensors read was in the mid 80's F at the first location and in the low 90's at the higher location.

This means that after two days of constant heating the chimney was 80F lower than the allowed 170F where charring can occur. This was at the sensor closest to the flue. The two side sensors didn't even get past 55F.

I may have a reason why my chimney is acting this way and not the way that Dana expected. My chimney has a liner. The liner is inside of the brick chimney flue. There must be some sort of air space between the brick and the liner. I am not certain of this, but the chimney flue opening in the unused flue in the basement does have a clay liner.

Another possible factor is that the chimney has two flues. One flue is not being used, nor will it be used now or in the future. This means there is an air space between the active flue and the right side sensor, though when you look at the data it shows that the two side sensors are tracking very closely.

It means, as far as I can tell, that there is no danger of the plywood in the construction turning into charcoal or starting an anaerobic fire inside the rockwool sandwich. The danger is just not there.

It is very important to me to get this right of course. No one wants to cause an unsafe situation and lots of this super insulating stuff is new. Fortunately we have data monitoring available for a reasonable price. We can test to see if we are causing a dangerous situation by accident.

The main sensor temperature had not leveled out completely at the end of the test and was approaching 90F, which is well within the acceptable range (below that 170F by 80F or so). I am considering monitoring this further but am confident that we would not be reaching the 170F as the temperatures for most of the day were between 80F and 88 F with a dip in the early afternoon today.

The second height I took data at was for a shorter amount of time but was tracking the same types of temps but two to three degrees higher (90.8 at the highest).

There you have it. Dana was concerned, I got the data, now I am certain that it is fine.

Dana, thanks for your help understanding the possible hazards of wrapping my chimney in this way. I appreciate it. We are all working together to make super energy efficient homes, both new and retrofit. It is important that we don't create hazards while trying to cut down on energy use.

Fortunately we live in a time where low cost monitoring gets even the average person access to on-site data.

Thanks again Martin for letting me share my project on GBA.

Chris West

Dec 4, 2013 4:20 PM ET

Chimney wrap construction clarification
by Chris West

The chimney insulation is 3" rigid Roxul - 3/4" plywood - 3" rigid Roxul - 3/4" strapping/rain screen space.

The polyiso is only on the framed part of the house and not near (6" + away from) the chimney. The temps we got on the sides of the chimney during our test (see post above) was in the 60F range. Well within the acceptable range for polyiso.

The main part of the house has two layers of 3" polyiso screwed into the framing of the house.

Hope that clarifies it for you.


Dec 5, 2013 6:45 AM ET

Response to Chris West
by Martin Holladay

Your chimney test and monitoring procedure are good examples of an unusually conscientious approach. I'm impressed that you pursued this issue so thoroughly. Thanks for sharing your findings.

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