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Air Sealing an Attic

If you haven’t done it yet, it’s time to climb into your attic and plug as many holes as you can find

Posted on Sep 20 2013 by Martin Holladay

If you want to improve the energy performance of an older house, one of the first steps is to plug your attic air leaks. Although many GBAGreenBuildingAdvisor.com articles address aspects of attic air sealing, no single article provides an overview of the topic. This article is an attempt to provide that missing overview.

I’ll try to explain how you can seal air leaks in a conventional vented, unconditioned attic. If your house has cathedral ceilings — that is, insulated sloped roof assemblies — the air sealing tips in this article don’t apply to your house.

There are four basic steps to sealing attic air leaks:

  • Inspecting your attic;
  • Patching the big holes;
  • Sealing the cracks and small holes; and
  • Weatherstripping the access hatch.

Once this air sealing work is done, you may want to add more insulation to your attic floor. If you want to add insulation, remember that air leaks have to be sealed first.

Inspecting your attic

The easiest way to find air leaks is with a blower door. In some cases, a theatrical fogTo fog a room or building is to use a fog machine during a blower door test, revealing locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely non-toxic. machine is also very useful. (For more information on these two pieces of equipment, see Blower Door Basics and Pinpointing Leaks With a Fog Machine.)

If you don’t have a blower door, you’ll have to find your attic air leaks using your eyes and your powers of deduction. You’ll also need a powerful flashlight. If you don’t like balancing on joists, bring a couple of 2 ft. by 3 ft. pieces of plywood to step on while inspecting your attic. Needless to say, you don’t want to step between the floor joists and punch a hole in the ceiling.

Warning: if your attic floor is insulated with vermiculite, a type of insulation that may contain asbestosMineral fiber once commonly used in many building materials, including insulation, fireproof siding, and resilient flooring. Inhalation of invisible asbestos fibers can lead to chest and abdominal cancers as well as scarring of the lungs. The use of asbestos in some products has been banned by the EPA and the U.S. Consumer Products Safety Commission; manufacturers also have adopted voluntary limitations on its use. When found in older buildings (most commonly in floor tiles, pipe and furnace insulation, or asbestos shingles), the product's friability is a major determinant in how it must be handled during renovations. More information: http://www.epa.gov/iaq/asbestos.html, don't touch the insulation. Don't attempt any air sealing work in an attic insulated with vermiculite. Removal of vermiculite insulation should only be performed by a certified asbestos abatement contractor.

If your attic has no vermiculite, the first step is to get your bearings and look around the entire attic. You may be surprised at what you discover. For example, it isn’t too unusual to find gross insulation or ventilation system defects in the attic, including joist bays with no insulation and bathroom fans that direct exhaust air into the attic. Needless to say, these defects will need to be remedied. The first order of business, however, is to seal the air leaks.

If the attic floor is insulated with fiberglass batts, look for stained or dirty insulation. The most common cause of dirty fiberglass batts is air leakage; if the batts are very dirty, it means that dusty indoor air has been rushing through the batts for years. The batts strain out the dust from the flowing air, just like a furnace filter. If you lift the dirty batts, you’ll probably find a crack or a gaping hole.

Look for sunken batts. These are often a clue that there is a soffit or suspended (dropped) ceiling under the insulation. In most cases, these soffits and dropped ceilings lack any ceiling drywall — and therefore lack an air barrier.

Look for plumbing vent pipes, ducts, and exhaust fans. Areas near these items are frequent leak locations.

Think about the layout of the floor directly under the attic. If you know of any dropped ceilings or soffits, try to locate those areas in your attic. You’ll probably want to lift the existing batts (or redistribute the blown-in insulation) to find out what’s underneath.

If you take your time and you’re thorough, you should be able to find all of the big holes in your attic using these techniques.

Patching the big holes

Big holes in your attic floor — holes above soffits, dropped ceilings, and utility chases — can be patched with gypsum drywall, plywood, or OSB. Perhaps the easiest material to use, however, is foil-faced polyisocyanurate, since it’s easy to cut and easy to tape.

Whatever type of sheet material you use to seal your large holes, cut a piece of material so that it covers the hole, and secure it in place with nails or screws. The perimeter of each piece of material can be sealed with caulk, non-hardening acoustical sealant, or canned spray foam. Here’s a link to a GBA video that demonstrates this type of air sealing: How to air seal soffits and chases in an attic.

Gaps around brick chimneys are dealt with differently than holes above soffits. Because chimneys can be hot, these gaps should be covered with sheet metal, not rigid foam. After nailing four pieces of sheet metal in place — one on each side of the chimney — the seams where the metal pieces overlap and the gaps between the metal and the chimney can be sealed with high-temperature silicone caulk.

Gaps around metal chimneys are sealed with techniques that are similar to those used for brick chimneys. The easiest way to seal around a metal chimney is with two overlapping pieces of sheet metal; of course, you'll need to cut each piece with a half-moon hole that corresponds to the chimney diameter.

Manufacturers of metal chimneys and most building codes require a 2-inch air space between the chimney and any framing lumber. Respect this air space; avoid the temptation to fill the air space with insulation.

Finally, it's worth checking whether chimneys are in use before you begin your air sealing work. Unused chimneys represent a thermal bridge as well as an air-leakage path, so all unused chimneys — both brick chimneys and metal chimneys — should be removed. At the very least, the top section of an unused chimney should be demolished down to a level that is lower than the ceiling air barrier, so that the penetration through the attic floor can be patched.

Sealing cracks and small holes

Here are some of the cracks and small holes that need to be sealed in a typical attic:

  • Cracks near recessed can lights;
  • Cracks around ceiling-mounted duct boots;
  • Cracks around bath exhaust fans;
  • Cracks around plumbing vent pipes;
  • Cracks between partition top plates and partition drywall;
  • Cracks at ceiling electrical boxes;
  • Holes drilled in top plates for electrical cable.

Cracks near recessed can lights. Recessed can lights are bad news; most are responsible for very significant air leaks. The best solution to the can light problem is to permanently remove the can lights and replace them with surface-mounted fixtures. If you aren’t willing to do that, you may be able to install airtight covers on the attic side of the recessed cans to reduce air leakage. For more information on this topic, see .

Cracks around ceiling-mounted duct boots. If your house has ceiling-mounted HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. diffusers or grilles, then you’ll need to seal the crack between the galvanized duct boots and the ceiling drywall. It’s usually easier to seal these cracks from below than from above.

Cracks around bath exhaust fans. Most ceiling-mounted bath fans have a removable plastic grille. Remove the grille from below and caulk the crack between the fan housing and the drywall.

Cracks around plumbing vent pipes. These cracks need to be sealed with caulk or acoustical sealant; some builders prefer to use European air-sealing tapes.

Leaks at ceiling electrical boxes. These leaks are fairly straightforward to seal. Using caulk, seal the crack between the electrical box and the ceiling drywall. Then seal around the knockouts at the sides and back of the box, as well as any location where electrical cables enter the box. Don’t make the mistake of filling the box with canned spray foam — that would be a code violation and would create a fire hazard.

Holes drilled in top plates for electrical cable. Again, these leaks are fairly straightforward. Seal them with caulk or canned spray foam.

Cracks between partition top plates and partition drywall. A surprising volume of indoor air can escape through these cracks. Usually, the conditioned air enters the partition stud bays through electrical outlets and cracks at the bottom of the wall. Unless the drywall contractor installed a bead of adhesive along the top plate — most don’t — air will continually escape along this crack.

To seal these cracks, peel back the insulation above each partition top plate. The cracks can be sealed with caulk. If the top plates have lots of wiring penetrations, it’s often easiest to seal the entire top plate with spray foam from a two-component spray foam kit. Here’s a link to a GBA video that demonstrates how to seal these cracks: How to Air-Seal Ceiling-to-Wall Connections in Attics.

Access hatches and pull-down stairs

There are two problems with attic access hatches and pull-down attic stairs: they usually aren’t properly insulated, and they are rarely weatherstripped.

It’s easier to deal with an attic hatch than pull-down stairs. As long as the existing hatch is sturdy, the usual solution is to fasten or glue multiple layers of rigid foam on the attic side of the hatch. (Make sure that any adhesive you use is compatible with rigid foam). Then install weatherstripping on the lip where the hatch rests, as well as at least two latches that pull the hatch tightly against the weatherstripping. For more information, see How to Insulate and Air Seal an Attic Hatch.

To stop air leaks at pull-down attic stairs, you’ll need to install an elaborate cap. For more information on this topic, see these two articles:

Kneewalls

So far, we’ve mostly been talking about ceiling leaks. But air leakage routes get more complicated if your attic includes kneewalls.

A typical Cape-style home has several different attics. There are usually two cramped attics behind the second-floor kneewalls; there is also a low attic above the second-floor ceiling. Kneewalls are also common in some new homes built in the 1980s and 1990s, especially large homes with chopped-up roofs, dormers, and multiple ceiling planes.

There are at least two problems with most kneewalls:

  • Kneewalls often lack an attic-side air barrier, even though they are insulated with air-permeable insulation (usually fiberglass batts) that require such a barrier.
  • When builders install kneewalls, they often forget to install solid blocking between the floor joists under the kneewall bottom plate, as well as solid blocking between the rafters above the kneewall top plate. If the kneewall is insulated, such blocking is essential; moreover, each piece of blocking needs to be carefully air sealed. Without the blocking, cold outdoor air can migrate horizontally between the floor joists, robbing heat from the house.

If you discover that your kneewalls are uninsulated, don’t despair. It’s possible that your builder decided to insulate the sloped roof assembly above the attic instead of the kneewall. If that’s the case, you’re in luck. As long as the work was done conscientiously, the best way to insulate attics behind kneewalls is at the sloping roof.

For more information on air sealing at kneewalls, see . To watch a video showing how to seal leaks at kneewalls, see How to Seal Attic Kneewalls.

Ventilation baffles, insulation dams, and duct leaks

Once you have completed your air sealing work, there are a few more details that you may need to address before you add more insulation to your attic: ensuring that the ventilation channels that connect your soffit vents to the attic aren’t blocked by insulation; making sure that all of your insulation dams are high enough; and checking any attic ducts for problems.

If your attic has soffit vents and ridge vents, you'll need a ventilation gap under the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. at the perimeter of your attic — a clear channel that allows air to flow from your soffit vents to your attic. In some attics, this area is blocked by insulation. The solution is to install ventilation baffles — either commercially available baffles or site-built baffles. (For more information on this topic, see Site-Built Ventilation Baffles for Roofs.)

If you need to install these baffles, you’ll also need to install insulation dams from the top plates of your exterior walls to the underside of the ventilation baffles. Here’s a link to a GBA video that shows how to do this work: How to Ventilate Rafter Bays When Adding Insulation.

If you plan to add more insulation to your attic floor, you’ll probably need to extend the insulation dam around your attic hatch. This dam needs to be at least as high as your anticipated insulation depth. You can use either 2-by lumber or plywood to build this insulation dam.

Remember that installing insulation on top of live knob-and-tube wiring is a code violation and a possible fire hazard. If you have knob-and-tube wiring in your attic, you can only insulate on top of the knob-and-tube if you are sure that it has been permanently disconnected. When in doubt, call an electrician to have your wiring evaluated before proceeding.

Finally, if there are any ducts in your attic, you’ll want to inspect them. Make sure that there are no disconnected ducts, that all duct seams are sealed with mastic or high-quality HVAC tape, and that the ducts are adequately insulated.

It’s usually a bad idea to locate ducts in an unconditioned attic — but just because it’s a bad idea, doesn’t mean that attic ducts don’t exist. If you’ve got them, you’ll have to do your best to deal with them. For more information on attic ducts, see these three articles: Sealing Ducts; How to Install Flex Duct Properly; and Keeping Ducts Indoors.

Martin Holladay’s previous blog: “All About Wood Stoves.”


Tags: , , , , ,

Image Credits:

  1. Fine Homebuilding
  2. EPA

1.
Sep 20, 2013 7:14 AM ET

Nice Job Martin. By the way,
by Chris Brown

Nice Job Martin. By the way, its not just brick chimneys that present challenges. My home has a gas fireplace installed in a "kick-out" in the family room. This is in the center of the house and the correct metal piping was used for the chimney. However, at the termination of the kick-out in the attic, I was faced with an open chase of 3' x 5' with the chimney pipe squarely in the center of the open chase. Closing this one single huge opening made an unbelievable difference in the conditions in both the Family room and kitchen behind it. I used cement board to cover the majority of the chase along with sheet metal around the chimney pipe sealed with intumescent caulk. All metal flues should be dealt with in the manner you describe for brick chimneys.


2.
Sep 20, 2013 7:45 AM ET

Response to Chris Brown
by Martin Holladay

Chris,
Thanks for the feedback. In response to your comments, I have edited the article to include information on sealing around metal chimneys.


3.
Sep 20, 2013 9:02 AM ET

Another way to air seal can lights
by Devan Anthony

Most recently we've been air sealing can lights using replacement LED trim kits. The Cree CR6 has a warm color temperature (2700k) and is fully dimmable (no buzz) and we sell it to the client as a dual energy upgrade - more efficient lighting and air sealing in one. We order the trim kits online in bulk packs.

The install is easy, I think the trim kits are faster to install than boxing out the light in the attic. We use caulking to seal the edge of the trim to the drywall. And then, so long as your fixture is rated for Insulation Contact (IC) we blow insulation right over the lights.


4.
Sep 20, 2013 9:19 AM ET

Response to Devan Anthony
by Martin Holladay

Devan,
I agree with your recommendation, and it's one of the recommendations I made in my article, . (I'm not sure whether you noticed the link, but I recommended that article in my discussion of can lights.)

In the Fine Homebuilding article I linked to, I wrote, "Another way to cut down on air leakage through a can—especially one installed in a cathedral ceiling—is with an LED conversion kit. These kits include airtight compartments that can be inserted to replace an existing fixture’s can and lamp trim. Examples of LED conversion kits include the Cree LR6 LED conversion kit (about $130) and the Cooper All Pro LED conversion kit (about $80)."


5.
Sep 21, 2013 10:02 PM ET

Response to Martin
by Devan Anthony

Martin,

Of course you already made this recommendation! I admit that I did not click through to your Fine Homebuilding article.

Here's a review of the CR6 after they've been installed in a new home for 2 years:

The homeowner loves them.

The Cree CR6 can be found online for $35-40. Difference between the CR6 and the LR6 that you priced in your article seems to be the size of the heat sink and the integrated trim ring. The LR6 has longer life due to the bigger heat sink (50000 hrs), but the CR6 still is rated for 35000 hrs, which at 6 hrs a day is nearly 16 years!


6.
Sep 21, 2013 10:15 PM ET

Wish I'd gotten this article sooner!
by Robert Guico

We're having an insulation company come out in 3 days to insulate (mostly with batts) the kneewall in our sidecrawls (as we call them.) I'm not sure if we've discussed blocking at all.

The only discussion regarding penetrations we've had was regarding a water supply pipe that takes the long way through the attic to our bathroom (and yes, it gets cold in the winters).

Prior to today I had assumed that insulating the sloped ceiling was inherently bad because we just had mold remediation done on the (formerly) insulated north sloped ceiling. Humid air got trapped behind the insulation.

However in your diagram of a sloped ceiling, I'm guessing that's what the rigid insulation is meant to protect?


7.
Sep 22, 2013 4:10 AM ET

Edited Sep 22, 2013 4:11 AM ET.

Response to Robert Guico
by Martin Holladay

Robert,
There is nothing "inherently bad" about insulating sloped ceilings. What's bad is getting the details wrong.

The usual mistakes include: (a) failing to include an air barrier on the interior as well as the exterior side of air-permeable insulation like fiberglass batts, (b) failing to install enough insulation to meet the minimum R-value requirements of the building code, and (c) failing to include a vent channel between the top of air-permeable insulation and the underside of the roof sheathing.

Here is a link to an article that tells you how to do it right: How to Build an Insulated Cathedral Ceiling.

If you had mold in an insulated sloped ceiling, I'm guessing that your ceiling had air leaks, or insufficient R-value, or an ineffective ventilation channel, or all three problems.


8.
Sep 23, 2013 9:49 AM ET

Electrical Boxes
by But Why?

I've never really understood climbing into an attic to seal an electrical box. Why not just drop the fixtrure and caulk it from below? Granted you can't use spray foam but a good caulk and your finger can effectively seal the boxes from below which is WAY more comfortable, can be done while the football game is on and is much more accessible to the refrigerator. If you are opposed to getting a tingle every now and again, shut the power off first and a piece of extra tubing can help direct the caulk easier than you might be able to with the caulk gun. You can pull outlets and switches out of the box as well and seal them too with little effort....power off is almost a must because twisting the metal caulk gun to hit all corners will almost certainly result in touhing terminals (the resulting shock shouldn't be fatal but flinging a messy caulk gun on your wifes carpet just might be!)

Also, depending on your local HVAC practice, don't forget to check the return air ducts. Here locally they depending on the wall board to create their chases and I have seen cases where the return air is sucking outside air from the attic (which you might catch in attic sealing or it can even be linking to the outside horizontally through electrical holes and plumbing pipes. I had a brand new home with frozen water pipes in the middle of the house before (should never happen btw) the pipes ran through the return air which was drawing direct outside air two floors above in the attic. Because it was very cold the furnace ran more meaning more cold air came in meaning more run time etc etc. Eventually, the run time was long enough that the pipes froze in the basement which is always 60 degrees. My prefered method for dealing with the return airs is to cut a 2x4 the width of the stud bay and drive it into place above the return air grill and then seal with caulk. WARNING, #1 make sure this return air chase doesn't also serve another on the floor above. #2 you may have to have the HVAC guys come back and re calibrate the system. His original sloppiness may have given him plenty of return air while your careful sealing may resulting in a lack of return air. I always try to further open the hole to its maximum.


9.
Sep 23, 2013 10:34 AM ET

Response to B.W.
by Martin Holladay

B.W.,
You're right that it's possible to seal leaks at a ceiling electrical box from below. However, if you're in the attic anyway, pulling up batts to look for problems, and you are in full air-sealing mode -- equipped with rubber gloves, a caulk gun, and some canned spray foam -- why not address the electrical boxes when you come across them? You're up there anyway. It's not as if it makes sense to climb down the hatch at that point, look for a screwdriver, and remove the ceiling fixture.

The bottom line: either way works, and you should use whatever approach seems simpler to you.

As far as return air ducts are concerned: your points are well taken. Personally, however, I wouldn't try to retrofit stud bays to make them "better" paths for return air. The right way to fix this problem is to install new galvanized ductwork, properly sized, and to box it in where necessary with a new chase. After all, a stud bay only measures 3 1/2 inches by 14 1/2 inches -- nowhere near big enough to provide many cfm.


10.
Sep 25, 2013 9:44 PM ET

air sealing with vermiculite
by Stephen Martinson

Any thoughts on air sealing when you attic has vermiculite insulation?


11.
Sep 26, 2013 2:00 AM ET

Old furnace / water heater chimneys
by Bryce Nesbitt

For every five condensing water heaters installed, there are probably 3 open chimneys left penetrating through everything. These are chimneys that are no longer even used, but probably lead straight outside.


12.
Sep 26, 2013 6:56 AM ET

Response to Stephen Martinson
by Martin Holladay

Stephen,
If you have vermiculite insulation in your attic, that's bad news. The safest approach to attic vermiculite is to assume that the vermiculite contains asbestos. That means that you shouldn't disturb it.

Properly air sealing an attic with vermiculite insulation requires that the vermiculite be removed. That's an expensive proposition, unfortunately, because the work must usually be done by an asbestos abatement contractor.

For more information on vermiculite and asbestos, see:


13.
Sep 26, 2013 7:01 AM ET

Edited Sep 26, 2013 4:06 PM ET.

Response to Bryce Nesbitt
by Martin Holladay

Bryce,
You make a good point about unused chimneys. Unused chimneys represent a thermal bridge as well as an air-leakage path.

Unused chimneys -- both brick chimneys and metal chimneys -- should be removed. At the very least, the top section of the chimney should be demolished down to a level that is lower than the ceiling air barrier, so that the penetration through the attic floor can be patched.

I have edited the article to reflect your suggestion. Thanks.


14.
Sep 30, 2013 8:59 AM ET

Eliminate unecessary access / vermiculite
by Andy Kosick

You wouldn't believe how often I see a house with an attic access in the house and in the attached garage, or where one could easily be put in the garage. In this case seal the one in the house shut (or drywall in smooth if you no longer wish to look at it) and cover it with a full depth of insulation, then use or make one in the garage. I've performed this conversion many times just make sure you can access the entire attic from the garage as this is not always the case.

Also, vermiculite is a big problem in my area (middle of Michigan) and my worst nightmare as a home performance contractor. I believe the most important aspect of public safety is missing in the case vermiculite (and lead paint as well ), awareness. Practically nobody seems to know about this stuff and "don't touch it" is a terrible police. As you read this a plumber, electrician, and mechanical contractor are pushing it out of their way to do something (which brings up the issue of panned framing for return air in these attics), the stuff pours in the room every time some changes a light fixture and getting home owners not to store stuff up there is actually laughable. Of course, the cost of remediation has stopped every project I've looked at in it tracks, which probably means the home owner hired another insulator to blow over top of it. So, I'm writing this to ask Mr. Holladay, or anyone else reading with more influence than I, to help think of more realistic solutions to vermiculite, because its not going away on its own. There's probably 10 million homes with vermiculite in them, that's a home performance issue, not to mention an IAQ problem. Suggestions anyone?


15.
Sep 30, 2013 9:28 AM ET

Response to Andy Kosick
by Martin Holladay

Andy,
Permanently sealing the attic hatch in a house with an attached garage is an excellent suggestion.

I don't have any suggestions or innovative ideas when it comes to vermiculite. I used to be a certified lead abatement contractor (I got the certification, although I never worked in the field), and I know enough about hazardous materials to say that it's best to leave this topic to the experts.

I'm not an asbestos expert. I don't recommend that builders innovate in this field. Abatement is expensive, but it's the best way to proceed if you want to air seal your attic.


16.
Sep 30, 2013 11:20 AM ET

Response to Martin Holladay
by Andy Kosick

Thanks for the response.

I didn't mean to imply avoiding proper abatement in the case of vermiculite just how do we get it to actually happen. For instance, my only idea with an merit is to require inspection for and acknowledgement of vermiculite the time of sale (as if a home sale needs more paperwork), and while it wouldn't force abatement to take place, it may force the issue at a time when it's likely to be taken care of properly. It would also open up the possibility of rolling the cost into a mortgage making it an easier pill to swallow and for that matter more work for abatement contractors might increase production and help bring the cost down. All I know is I've done audits for a few new home owners and had to tell them they just purchased a HASMAT problem.


17.
Nov 1, 2013 3:44 PM ET

Gun foam pricing
by Nick Welch

I've looked at getting one of those foam guns and using Great Stuff Pro or equivalent foam, but the "pro" foam always seems to be WAY more expensive. I can get Great Stuff for about $0.30 per ounce, but the local places don't seem to stock the pro foam, and the cheapest I can find it online is about $0.50 per ounce plus significant shipping cost. And then there's the cost of the gun.

Where does everyone get this foam at a reasonable price?


18.
Nov 15, 2013 11:36 AM ET

Foam pricing
by Nick T - 6A (MN)

i see it on amazon for $0.38 per ounce. for the normal red greatstuff.

the blue window/door is more - at $.48

havn't seen it much cheaper than that...


19.
Jan 22, 2014 8:49 PM ET

Great Stuff Pro
by Erich Riesenberg

I have bought Great Stuff Pro from Home Depot, Lowes and Menards. I have tried 3 versions. The cheapest is around $0.50 per ounce but I try to buy on sale. I also use a non Dow gun, Foam N Seal, for instance, has worked as well as the Dow brand gun, at half the price.

Hardest part for me is figuring out when the can should be tossed. There seems to be a dispersal agent which runs out before the foam, I can hear liquid in the can when shaking the can, but it only trickles out.

I find the foam is useful when the gap is more than 1/4 inch, smaller gaps are easier with caulk.


20.
Sep 25, 2014 7:21 PM ET

Bath fan dampers and stack effect
by Venkat Y

Are bathroom fan dampers typically spring-loaded to stay closed unless the fan is running so as to prevent heated air from escaping thru the fan exhaust? If not, don't these fans represent significant leakage points? Should one consider adding a spring-closed-motor-opened damper inline with the exhaust duct from the fan? Thanks in advance.


21.
Sep 26, 2014 4:43 AM ET

Response to Venkat Y
by Martin Holladay

Venkat Y,
Bathroom exhaust fans include backdraft dampers that limit (but do not absolutely prevent) reverse air flow. For technical reasons, it's hard to make these backdraft dampers absolutely airtight.

It's theoretically possible to add a motorized damper downstream of the fan, electrically tied to the fan motor. But nobody does that, for two reasons:
1. Adding a motorized damper adds complexity to the system, and adds a component that may fail in the future and cause problems.
2. The potential energy savings are too low to justify the cost of the equipment.


22.
Sep 26, 2014 9:19 AM ET

Conditioned air leaving building
by Venkat Y

Hi Martin,

My understanding of backdrafting is unconditioned air "entering" the building envelope via the exhaust vent and I appreciate your comments in regard to that issue above.

But I was referring to the opposite problem: conditioned air "leaving" the building via the exhaust vents due to pressure differentials (either due to wind or because I maintain a slight positive pressure in the otherwise tight building envelope), stack effect, etc., or simply because the damper doesn't close all the way when the fan isn't running. Could the losses here be as bad as a recessed light that's not air sealed or worse?

Thanks in advance.


23.
Sep 26, 2014 11:23 AM ET

Response to Venkat Y
by Martin Holladay

Venkat,
As far as I know, most exhaust fan dampers use springs to hold them in a closed position, to limit air flow in either direction when the fans are not operating. Examine the fan you are thinking of installing to determine how the damper operates.

Again, I don't think that the energy savings associated with a motorized damper would be great enough to justify the investment in the added equipment.


24.
Jan 2, 2013 5:19 PM ET

Around chimney air leaks: followup question
by Ed Osborne

Great overview. Much appreciated.

And now my followup question: Once I seal the code-required gap between the wooden ceiling frame and the chimney with sheet metal and then seal any small gaps with heat resistant silicone, should I then put some fiberglass insulation on top of the sheet metal or is that (1) unnecessary and (2) dangerous? In other words, is the sheet metal sufficient to provide insulation equivalent to the rest of the attic which has two layers of R-30 insulation?


25.
Jan 2, 2013 5:25 PM ET

Response to Ed Osborne
by Martin Holladay

Ed,
If you want to insulate right up to a chimney, it's best to wrap the chimney with mineral wool batts rather than fiberglass (because mineral wool can sustain higher temperatures without damage than fiberglass). Secure the mineral wool batts with loops of steel wire.


26.
Jan 5, 2013 11:35 AM ET

Response to Martin Holladay
by Ed Osborne

Thanks, Martin, for your response. Is there any downside/danger to sealing the chimney-to-ceiling airflow gap with sheet metal and then wrapping chimney with batts?

Also, are mineral wool batts commonly available?


27.
Jan 5, 2013 11:45 AM ET

Edited Jan 5, 2013 11:48 AM ET.

Response to Ed Osborne
by Martin Holladay

Ed,
I'm not sure what you mean by the "chimney-to-ceiling airflow gap."

Codes require a gap between chimneys and wood framing. This gap should not be filled with insulation. However, codes do not require airflow through this gap. Nor do codes prohibit installing insulation above the sheet metal that is installed above this gap.

If you are talking about a metal chimney, you should always follow the installation instructions provided by the metal chimney manufacturer.

Mineral wool batts should be widely available. I have purchased mineral wool batts at Home Depot.


28.
Jan 5, 2013 10:43 PM ET

Response to Martin Holladay
by Ed Osborne

By "chimney-to-ceiling" I meant the gap between my brick chimney and the wood frame. (Sorry for the confusion: I meant to say floor as in ceiling floor.) based on your comments, I'm going to install sheet metal to bridge the gap and then wrap the chimney with mineral wool batts above it. I've already experienced a noticeable improvement in the upstairs office and bedroom after installing attic insulation, and this should be the icing on the cake.


29.
Sep 14, 2013 10:37 PM ET

sealing attic with wooden ceiling?
by David Martin

If the ceiling below the attic is tongue and groove pine, would plastic sheeting between the ceiling and the insulation help make it air tight? If the ceiling is not yet installed should the sheeting be stapled to the bottom of the joists? Do you have other tips for sealing this kind of ceiling? I hope I do not have to put a layer of plywood between the pine and the joists.

Dave


30.
Sep 15, 2013 6:17 AM ET

Response to David Martin
by Martin Holladay

David,
You absolutely must have a tight air barrier under your tongue-and-groove ceiling, since the boards do nothing to slow down air flow. The standard solution -- and the one I recommend -- is to install a layer of 1/2-inch drywall with taped seams first, followed by the tongue-and-groove boards.


31.
Sep 26, 2013 8:06 PM ET

Use air gap around chimney as intake vent?
by Andy Close

Great article, thanks for taking the time. This may seem like one of those dumb questions, however, here goes; I am purchasing a 1662 sq ft home which needs a new roof. It is located in Orange County, CA, very moderate / warm climate, and mild winters. I have a chance to start with a clean slate and fully intend to do so with attic sealing and ventilation. Based on the 1/300 ratio, I am currently good with exhaust vents (gables) at 364 sq in NFVA but lacking intakes at only 216 in sq. I am aiming for 400 in sq of each. I intend to have the roofer add (QTY 3) O' Hagins at 72 sq in each low down on the roof. However, there is a space around the chimney that connects directly to the crawl space below the home which obviously in turn connects to many outside mesh vents at ground level. Can I use that space around the chimney as intake venting and avoid the O" Hagins on the roof? The area around the chimney is sealed off completely from any part of the conditioned living space within the house (or so I believe!). All that would remain would be to cover said chimney gap with wire mesh as a secondary barrier to prevent any rodent infiltration! Thanks.


32.
Oct 19, 2013 9:09 PM ET

tongue and groove ceiling continued
by David Martin

When I asked how to make a tongue and groove ceiling air tight, Martin advised me to put sheetrock above the wood ceiling. Could I use a 1/2" rigid foam panel instead if I tape the joints? Besides adding a bit more insulation, these panels are much lighter and easier to fasten to a ceiling. And they don't seem to have much structural use since the tongue and groove boards will be nailed to the ceiling joists.


33.
Oct 20, 2013 6:32 AM ET

Response to Andy Close (Comment #31)
by Martin Holladay

Andy,
I was unfamiliar with "O'Hagins," but evidently O'Hagin is the name of a manufacturer of roof venting products. (For interested readers, here is a link: .)

I have two comments:

1. Don't worry too much about achieving the 1/300 ratio (or even about attic ventilation in general). The 1/300 ratio is an arbitrary number that has been in the code for decades; there has never been any scientific attempt to explain where this arbitrary number came from. For more information on why you don't have to worry too much about attic ventilation, see All About Attic Venting.

2. It's a bad idea, for several reasons, to use a leaky attic chase that connects your crawl space with your attic as a ventilation channel. The main reason this is a bad idea is that this attic chase connects with your interior air (since it is highly unlikely that the original builders did a good job of air sealing the partitions that surround the chimney).

If you encourage air flow through this chase, you will be pulling conditioned air out of your house and increasing your heating bills and air conditioning bills. So seal up that chase! You should use the air sealing techniques described in this article -- both in the attic and in the crawl space -- to prevent any air from entering the chimney chase or exiting the chimney chase.


34.
Oct 20, 2013 6:37 AM ET

Response to David Martin (Comment #32)
by Martin Holladay

David,
Yes, carefully taped rigid foam can be used as an air barrier. If you are not going to install any gypsum drywall, it's important to check with your local building department to make sure that omitting the drywall is permissible in your jurisdiction. (Some building inspectors insist that rigid foam needs to be protected by drywall for fire safety reasons; other inspectors allow the use of pine or spruce boards as you propose.)

If you install rigid foam, you might also want to install 1x3 or 1x4 strapping, 16 inches on center, on the interior side of the rigid foam. The strapping will make it easier to install the tongue-and-groove boards.


35.
Jun 2, 2016 6:51 PM ET

Question to Martin (Comment 4/5)
by Brian Gray

Martin, question on can light insulation and your recommendation of LED conversion kit as an effective way to air seal can lights. Several years ago I installed rockwool can light covers in each of our attic-exposed can lights, spray foamed in place. Last week I purchased a new toy - a FLIR infrared camera for my iPhone (I know, dangerous for an amateur energy nerd with OCD). I immediately noticed every can light (despite my rockwool covers and ~R50 insulation) was blood red when viewed in IR. This is with the lights off and outdoor temps in the low 80s. My question is - is this just the unavoidable, unintended consequence of can lights or can something be done to reduce the heat loss in winter and gain in summer? I assume the conversion kits you referenced would be redundant to the rockwool and LED lights I already have.


36.
Jun 2, 2016 7:14 PM ET

Response to Brian Gray
by Charlie Sullivan

Your lights might be warm because they hadn't been off long enough to cool down completely, because of air leakage, or because of the insulation being weaker there. If it's air leakage, it might be worth fixing. If you have a house that's otherwise pretty tight, you might be able to do a blower door like test just by turning on all your exhaust fans. If you can feel air coming in and/or the thermal image gets worse, you could try the conversion kit and could probably get a better seal.


37.
Jun 3, 2016 5:55 AM ET

Response to Brian Gray
by Martin Holladay

Brian,
In my article, I wrote, "Recessed can lights are bad news; most are responsible for very significant air leaks. The best solution to the can light problem is to permanently remove the can lights and replace them with surface-mounted fixtures." I stand by those sentences.

In Comment #4, I wrote, "Another way to cut down on air leakage through a can—especially one installed in a cathedral ceiling—is with an LED conversion kit." Note that I wrote that this is a way to "cut down on air leakage," not "eliminate air leakage."

I'm not sure why your insulated can lights are hot right now. (I'm assuming that your house is air conditioned, and that your ceiling drywall is cooler than your recessed can lights -- but you didn't give many details.) Certainly Charlie Sullivan's guesses are a good place to start. Let us know whether the LED lamps were totally cool (off for an hour) when this infrared scan was made.


38.
Jul 19, 2016 1:21 PM ET

Chimney chase ends in conditioned space
by A. Bradford

Hi Martin,

You mentioned that in the case of cathedral ceilings, this work is not necessary. However, I recently converted my unconditioned attic to a conditioned space. My chimney is exposed on this floor. Therefore, my chimney chase ends at this level and is exposed up to the roof framing. I was thinking of sealing the chase at the floor level, but you say this is unnecessary? By leaving it open, it is essentially a heater blowing air up from the basement. Do you generally recommend doing nothing, or should I at least use flashing and caulk at the roof level below my insulation?


39.
Jul 19, 2016 1:43 PM ET

Edited Jul 21, 2016 3:33 PM ET.

Response to A. Bradford
by Martin Holladay

A. Bradford,
The most important place to air seal around a chimney is where the chimney penetrates the thermal envelope. In your house, it sounds like that penetration occurs at the roof line. So you should certainly seal the air leaks around the chimney at this location, by installing metal flashing with sealed seams around the chimney, on the underside of the rafters.

As you note, the chase on the attic floor is not as much of a problem, because your basement and your attic are both inside of your thermal envelope. (If your basement is outside of your thermal envelope, that's another issue.)

Even though this chase won't be responsible for infiltration or exfiltration problems, it might be a good idea to seal it -- if only to reduce opportunities for odor transfer or rodent highways.


40.
Jul 21, 2016 3:14 PM ET

Thank you Martin!
by A. Bradford

Thank you Martin!


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