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Questions and Answers About Air Barriers

Every home needs an air barrier that limits infiltration and exfiltration

Posted on Jan 14 2011 by Martin Holladay

UPDATED on December 12, 2014

Builders of a certain age — say, those older than about 55 or 60 — started their careers at a time when no one talked about air leakage or air barriers. Back in the early 1970s, even engineers were ignorant about air leakage in buildings, because the basic research hadn’t been done yet.

Times have changed, and most residential building codes now require builders to include details designed to reduce air leakage. Today’s young carpenters are working on job sites where air barriers matter.

Q. What materials make good air barriers?

A. A wide variety of materials make good air barriers, including poured concrete, glass, drywall, rigid foam insulation, plywood, and peel-and-stick rubber membrane. (Note that evidence is increasing that OSB is not an air barrier; for more information on this issue, see Is OSB Airtight?)

Although air can’t leak through these materials, it can definitely leak at the edges or seams of these materials. When these materials are used to form an air barrier for your home, additional materials such as tape, gaskets, or caulk may be required to be sure seams and edges don’t leak.

To make a good air barrier, a material not only needs to stop air flow; it also needs to be relatively rigid and durable. If you want to determine whether a material is an air barrier, hold a piece of the material up to your mouth and blow. If you can blow air through it, it’s not an air barrier.

Engineers distinguish between air barrier materials (drywall, for example), air barrier assemblies (for example, plywood with taped seams attached to wall framing), and air barrier systems (all of the materials and assemblies that make up a building's air barrier).

Q. My builder installed Tyvek under my siding, so I already have an air barrier — right?

A. Not necessarily. Although Tyvek and other brands of plastic housewrap are sometimes marketed as air barrier materials, the primary function of housewrap is to act as a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB). In other words, the Tyvek is there to protect the wall sheathing from any wind-driven rain that gets past the siding.

Some builders have experimented with using Tyvek as part of an air barrier system. If the seams of the Tyvek are taped, and if the gaps between the Tyvek and window openings are carefully sealed, and if the transitions between the Tyvek and other materials at the bottom of the wall and the top of the wall are detailed in an airtight manner, then Tyvek can work as part of an air-barrier assembly. But siding contractors often rip holes in the housewrap with their ladders; they also penetrate the housewrap with hundreds of nails and staples. Builders interested in achieving a tight air barrier have found that other air sealing methods are more effective than an approach that depends on housewrap to be a wall’s primary air barrier material.

Q. My home has polyethylene under the drywall. Is the polyethylene an air barrier?

A. Probably not. During the 1980s, interior polyethylene was widely promoted as an interior vapor barrier. Its use in new homes is now relatively rare, except in very cold locations (for example, Minnesota, Canada, and Alaska).

Most polyethylene installations leak a lot of air — especially at the seams between adjacent sheets of poly, at penetrations, and around electrical boxes. That’s not usually a problem, since polyethylene is an effective vapor barrier even when it is not installed in an airtight manner.

Some cold-climate builders have successfully used polyethylene as part of an air barrier system. To act as an effective air barrier, however, polyethylene needs to be installed with careful attention to a long list of fussy details, including the use of acoustical sealant (non-hardening caulk) at all seams and the use of airtight electrical boxes. This type of polyethylene installation is relatively rare.

Q. Where are the most common air barrier defects located?

A. Most air leaks happen at the seams or cracks between different materials: for example, where the mudsill framing meets the foundation, where floors meet walls, and where walls meet ceilings.

Although gaps around windows and doors — the first areas of concern for many homeowners — occasionally contribute to air leakage problems, the most significant air leaks are usually in hidden areas. Because such hidden leaks — called “thermal bypasses” by weatherization contractors — usually don’t cause obvious drafts, homeowners are often unaware of their existence.

Here’s a list of some of areas that are often poorly sealed, and therefore responsible for significant air leakage:

  • Basement rim joist areas;
  • Holes cut for plumbing traps under tubs and showers;
  • Cracks between finish flooring and baseboards;
  • Utility chases that hide pipes or ducts;
  • Plumbing vent pipe penetrations;
  • Kitchen soffits above wall cabinets;
  • Fireplace surrounds;
  • Recessed can light penetrations;
  • Cracks between ceiling-mounted duct boots and ceiling drywall;
  • Poorly weatherstripped attic access hatches; and
  • Cracks between partition top plates and drywall.

Q. What’s the best way to test whether my house has a good air barrier?

A. Tracking down air leaks can be tricky, especially for builders or homeowners who are unfamiliar with the devious paths that air can take. For example, builders are often surprised to learn that significant air leakage paths can occur through interior partitions located far from exterior walls.

The best way to test the integrity of a home’s air barrier is to perform 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.. To learn more about blower-door testing, see “Blower Door Basics.”

Q. During the winter, water sometimes drips from the can lights in my cathedral ceiling. A contractor told me that the problem is that my ceiling doesn’t have an air barrier. Can you explain what he means?

A. If you have punctuated your insulated cathedral ceiling with recessed can lights, it’s very difficult to keep the ceiling airtight. (Unfortunately, even so-called “airtight” can lights are actually fairly leaky.)

In a ceiling like yours, warm humid air enters the insulated rafter bays through cracks around the can-light trim. The air is drawn into the rafter bays by 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.; the air usually exits the rafter bays through cracks near the ridge.

Since fiberglass batts are air-permeable, they do little to slow air movement. Regardless of whether the rafter bays are vented or unvented, there are usually plenty of cracks that allow humid indoor air to find its way to the underside of cold roof sheathing, where the moisture condenses. On cold nights, a layer of frost can build up on the underside of the roof sheathing. When the weather warms up, the frost melts, leading to dripping can lights.

The solution is to create a tight air barrier at the ceiling plane. The best way to achieve this goal is to remove the can lights, patch the drywall, and substitute surface-mounted light fixtures like track lighting.

Q. How do I create an air barrier for my basement?

A. An air barrier system is a three-dimensional balloon surrounding the conditioned area of a home. Assuming that your basement is part of your conditioned area — and I think it makes sense to include it — then there are several areas of concern.

First of all, it’s important to note that 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. depressurizes basements during the winter. In fact, stack-effect depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. is strong enough to pull air through soil under the basement slab. (Believe it or not, most soils are porous enough to allow a connection between the air in the soil — even 7 feet below grade — and exterior air above grade.)

A basement slab is an effective air barrier. However, the perimeter crack where the basement slab meets the footing or foundation wall is a source of air leakage and should therefore be caulked.

Basement sumps can allow significant volumes of air to enter a house. The solution is to install a sump with an airtight lid; these are available from in Mishawaka, Indiana (574-256-5635).

Most builders know the importance of installing sill-seal between the top of the foundation wall and the mudsill. If the sill-seal is ineffective, this joint may need to be caulked or sealed from the inside with spray foam.

If the rim joist in your basement isn’t insulated, it should be. Closed-cell spray polyurethane foam is the best insulation for this location; fortunately, spray foam also helps seal air leaks in the rim-joist area.

To limit the stack effect, you may want to consider weatherstripping the door at the top of your basement stairs.

Q. What’s the “neutral pressure plane”?

A. Several different driving forces affect air leakage rates. Some of these driving forces — including wind, exhaust appliances like bathroom fans, and unbalanced or leaky ductwork used for forced-air heating or cooling systems — are intermittent. However, one driving force — the stack effect — acts continuously on a house, as long as the exterior air temperature is significantly below the interior air temperature.

Because of the stack effect, it’s fairly easy to predict infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. patterns during the winter. Warm air usually exits the house through cracks near the top-floor ceiling. The stream of departing air pulls air into the house through cracks in its lowest level — for example, through the crack between the basement wall and the mudsill.

In other words, the air at the top of the house is pressurized with respect to the outdoors, while the air in the basement is depressurized.

In the center of your house, maybe somewhere in the vicinity of your living-room windows, the indoor air is neither pressurized nor depressurized. It’s at about the same pressure as the air outdoors. This is called your home’s “neutral pressure plane.” Even if your living-room window has all kinds of cracks, not much air will leak through — as long as the wind isn’t blowing. That’s because there is no driving force. The outdoor air and the indoor air are at the same pressure.

What happens to your home’s neutral pressure plane when you turn on an exhaust fan in an upstairs bathroom? It moves upward a few feet, that’s what. More air is now leaving your house, so more air needs to enter the house to replace it. As more air enters — perhaps through those cracks around your living-room window — the neutral pressure plane moves a few feet higher, up near the ceiling.

Understanding these pressure dynamics helps guide the efforts of weatherization contractors when they perform air-sealing work. The most important cracks to seal are those under negative pressure — that is, cracks in a basement or crawl space — and those under positive pressure — that is, cracks in the attic floor. Air leaks in the center of the house — in the vicinity of the neutral pressure plane — are less important.

Q. I don’t know whether my air barrier is on the outside or the inside of my wall. Can I tell where my home’s air barrier is located by looking at the house plans?

A. Probably not. Unfortunately, most house plans don’t indicate the location of the air barrier. As a result, a home's air barrier details — if such details even exist — were probably made up by the builders on the job site.

A wall’s air barrier can be located at the exterior sheathing (by taping the sheathing seams), at the interior drywall (by following the ), or in the middle of the wall (by using spray polyurethane foam). If the details are done correctly, any of these three methods can result in a very tight air barrier.

Q. My designer brags that his house plans include air-barrier information, and that the air barrier can be traced “without lifting your pen from the paper.” What’s that mean?

A. It means you have chosen a good designer. Although it’s rarely done, a good section drawing of a house design should indicate the location of the air barrier. Because this air barrier must be continuous, without any interruptions where the walls meet the ceiling or at other transitions, it should be possible to trace the air barrier on the section drawing — from the basement slab, up the walls, over the ceiling, and down the other side — without lifting your pen from the paper.

Q. I live in a hot climate where heating bills are low and air conditioning bills are high. Do homes in a hot climate need a decent air barrier?

A. Yes. If your home lacks a decent air barrier, lots of cool indoor air can escape through cracks in your walls and ceiling. That departing cool air is replaced by hot outdoor air that sneaks in other cracks, forcing your air conditioner to work overtime.

If your summers are humid, these air leaks carry a double penalty. When outdoor air enters your home, your air conditioner struggles not only to cool the air, but also to wring the moisture out of the air. In states with humid summers, a significant portion of your air-conditioner run time is actually devoted to dehumidification. The tighter your home, the easier it is is to keep your indoor air cool and dry.

Q. I want to make my house as airtight as possible. Is there any way to do that without spray foam?

A. Absolutely. Many builders have achieved very low levels of air leakage without any spray foam at all. Moreover, many homes that have been insulated with spray foam still have high levels of air leakage.

How is this possible? The reason is simple: most walls and ceilings don’t leak in the middle of the wall or ceiling. They leak at edges, penetrations, and transitions. So even when a wall is insulated with spray foam, you still need to worry about the details.

The lowest levels of air leakage are achieved by builders who have studied airtight construction techniques, who think through potential air leakage paths during each phase of construction, who are conscientious and methodical, and who have already built a few homes that were tested by a blower door.

Most air sealing details don’t require fancy materials. In many cases, a few rolls of gasketing material and some tubes of high-quality caulk are all that’s necessary. Attention to detail usually matters more than high-tech tools or equipment.

When it comes to air sealing, the proof is in the pudding — in other words, the blower door results.

Last week’s blog: “Net-Zero-Energy Versus Passivhaus.”

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Image Credits:

  1. Fomo Products
  2. Fine Homebuilding

Jan 19, 2011 2:03 PM ET

Edited Jan 19, 2011 2:04 PM ET.

About sealing ventilation ports
by Martin Holladay

Thanks for your post; it's fine to contradict me. It seems clear that homes are routinely tested both ways.

My source was David Keefe, who wrote in his useful JLC article ("Blower Door Testing"), "Whether or not intentional openings like ventilation ports are temporarily sealed depends on the test being performed. For a description of how an existing house normally behaves, such openings are usually left uncovered. On the other hand, if a new house is being tested for sufficiently tight construction, it may make sense to seal intentional openings, removing them from the measurement."

Here's a link to the article:

Jan 19, 2011 9:58 PM ET

Comparing air barriers.
by Lucas Durand - 7A's actually a pretty common protocol (including part of RESNET's standard test procedure) to seal off continuously operating ventilation systems when performing a blower door test.

Not being a blower-door tester I'm not sure of proper protocol... I will be having one done in the not-so-distant future...
If the goal is to test the integrity of the air barrier, shouldn't it be required to have those types of envelope penetrations sealed off?
For the purposes of determining the "tightness" of an air barrier or to compare one air barrier to another, shouldn't all penetrations be sealed as best as possible?

Jan 19, 2011 10:07 PM ET

Response to Lucas
by Martin Holladay

Both methods have merit. If you want to know the air leakage rate of your home under normal operating conditions, then sealing off vents that are normally open is "cheating."

However, "cheating" is sometimes useful if it helps you track down leaks.

Jan 19, 2011 10:10 PM ET

response to Lucas
by j chesnut

Lucas, Martin already addresses that there can be different results sought:

"Whether or not intentional openings like ventilation ports are temporarily sealed depends on the test being performed. For a description of how an existing house normally behaves, such openings are usually left uncovered. On the other hand, if a new house is being tested for sufficiently tight construction, it may make sense to seal intentional openings, removing them from the measurement."

Do both. A blower door test does not take that long.

Jan 19, 2011 10:18 PM ET

by j chesnut

Isolating factors (such as the building enclosure) and performing tests to quantify their performance results in targeted information that can be used to exploit the isolating factor to desired ends. This is science qua science Martin ; )

Jan 19, 2011 10:37 PM ET

by Lucas Durand - 7A

J, I see that now thank you.

Sorry Martin, I didn't see your comment when I posted my question.

Jan 20, 2011 5:38 AM ET

On "cheating"
by Martin Holladay

That's why I put the word in quotation marks. It's "cheating," not cheating.

Yes, "cheating" is a useful way to gather data.

Jan 20, 2011 8:35 AM ET

blower door set-up
by Michael Blasnik


I think I see some misunderstanding here. My comment about sealing off ventilation systems was specifically about continuously operated ventilation systems -- not intermittent systems. Continuous operation is a key distinction because that means that the "leak" the blower door might measure from that ventilation opening is not going to be leaking like the rest of the building and so should not be included in any infiltration modeling.

I'm quite familiar with this detail having spent the past several months on a RESNET subcommittee drafting new blower door testing standards and this was an area with broad agreement. One can certainly debate some other aspects of proper house testing set-up, but this one seems fairly clear.

Jan 20, 2011 8:47 AM ET

On continuous ventilation
by Martin Holladay

Thanks very much for the information.

So how does one define "continuous"? I know that a lot of HRVs are programmed to run for about 50% of the time. Moreover, some homeowners program their ventilation timers to turn off the ventilation systems while they are at work.

It's also common for exhaust-only ventilation systems (for example, a system using Panasonic bath fans) to operate for only 50% of the hours in a day.

It seems to me that these systems aren't continuously operating. Do you agree?

Jan 20, 2011 10:56 AM ET

continuous ventilation
by Michael Blasnik

I agree -- continuous would mean continuous. Systems that cycle on and off would not be sealed during a blower door test because their leakage will act like envelope leaks whenever the system is off. I realize that many systems cycle but there are also those that run all the time and vary between lower and higher speeds.

Jan 20, 2011 2:57 PM ET

Edited Jan 20, 2011 10:26 PM ET.

Alternatives to poly?
by J. Edison

First off let me say, I have chemical sensitivities; so, everything I plan to do to my house in the next year has to be addressed with this in mind. Since I'm very sensitive to the off-gassing of so many products, I can't even walk into places such as Target anymore without gagging, and often find I have anaphylactic responses from many other products... people's body products, air fresheners, candles, the laundry and body products aisles at all stores. I've known for 30 years now that I've got this problem. I've done the best I could without turning myself into a bubble woman. I have to hold my breath a lot and open windows and doors sometimes to air out my house or car from things that cause me reactions. It's gotten worse in the past 10 years.

I live in Boulder, CO where the weather varies, sometimes erratically, year round. In January, we can get highs as much as 70F one day and 10 the following. The wind can gust here up to 100mph as it screams over the Continental Divide then slams down into the Front Range communities. This happened just 2 nights ago. Funny thing was, it was the first time I ever noticed how drafty my bedroom is, outside of the annoying whistling sound as the air enters through the wood double hung windows, of which there are too many in this house. It was late at night, around midnight when I noticed it. I was reading in bed (about 10 ft from the nearest window) at the time and could feel a definite slight cooling breeze around me. My house thermostat is set to bedtime as 11, so I'm not surprised when at midnight it's cooler, but this was wafting cooler air. Good thing I'm about to do more work on the house.

I'd replace all the windows, but can't afford to do this now; the payback is outrageously long when there are 5 large windows in my bedroom alone. There are 18 windows on the second floor total, 16 windows and 4 doors on the 1st floor and 4 large slider windows in the basement. We're talking a huge layout I don't have to replace them all. I'm not planning on living here long enough anyway to make it worth taking a loan out for this. So, moving on...

My reason for posting today is to ask about air and vapor sealing my crawl space under the basement floor, that doesn't involve spray foams or encapsulating it with some sort of poly sheeting.

I've been reading a lot here, and various other places, on the different methods, but have yet to find any alternative to air and vapor sealing without using some sort of foam and/or plastic sheeting. Even if I wasn't chemically sensitive, I'd never use the foams, now that I know how nasty they all are from cradle to grave.

Seems every insulation company, and my contractor, have said if I don't use foam, then there is only plastic sheeting of some sort. I like the Basement Systems product, but it's made of seven layers of high-density polyethylene, lot-density polyethylene, and polyester cord, and it has some trademark antimicrobial additive on the underside layer, which is either applied on or infused in it. When I asked their rep about this, they said because it's on the bottom side, it won't off-gass into my home. So, how safe is the whole product? I told him I'd want them to air out the poly for at least a couple of days before they bring it here, but they said due to the amount I need, it might not be possible as warehouse space is limited. Guess I'll have to insist on this though. I don't want to be sick every time I go down in my basement to do something fun, mess around in my store room, or for whatever other reason I have to down there.

Any suggestions for alternatives?

Jan 20, 2011 3:10 PM ET

Response to J. Edison
by Martin Holladay

J. Edison,
You never described your crawl space floor, so it's hard to provide advice. I'm going to guess that the crawl space has a dirt floor.

If my guess is correct, you could consider installing a polyethylene vapor barrier under a concrete slab. Because concrete is an air barrier, the concrete will prevent any off-gassing from occurring.

If that makes you nervous -- if the idea of any polyethylene at all makes you nervous, even when it is under a layer of concrete -- you could follow the advice given in books from the 1950s. Building specialist Hubbard Cobb, writing in 1955, recommended the following materials under a concrete slab: 6 inches of crushed stone as a capillary break, topped with a 1/2-inch thick layer of cement grout, topped with a layer of asphalt felt laid in hot-mopped asphalt. Then you pour your slab on top of the asphalt.

If you don't want asphalt, you can always use deep crushed stone as a capillary break -- the deeper, the better -- topped with 2 or 3 inches of concrete. This isn't a perfect moisture barrier, but it contains neither polyethylene nor asphalt.

Jan 20, 2011 10:13 PM ET

Edited Jan 20, 2011 10:52 PM ET.

Response to Martin's response
by J. Edison

Martin, thanks for the alternate suggestion, but I don’t think your way is doable. I’m pretty sure that method would cost way more money than using the polyethylene products, in both labor and material, but I'll discuss it with my contractor.

When I build or remodel my homes (I'm on my 7th) I try to do everything in a healthy, environmentally sound way, not just for energy efficiency and beauty (which is subjective anyway).

This 1994 house is about 3800 sq. ft. finished. It was built in a neighborhood that was once wetlands. Some people have concrete slab basements while others, including me, have crawl spaces with dirt floors and floating plywood subfloors.

I moved here in 1997 and did little in the way of decorating, outside of painting some bedrooms and twice repainting the exterior, which will be done again this summer. I just needed something ready to move into, in a neighborhood with kids, and school bus service. I had young children then, school had begun already, and I had a very busy life then too, and honestly, it hasn't slowed down since.

My basement was finished off in 1998, using fiberglass batts in the walls, recycled PET carpet (which I never liked after it was done), recycled carpet pad, and low VOC paints. It was totally unfinished before that. I didn't know about air and vapor sealing back then. I’ve sure learned a lot since then on how it SHOULD be done, in terms of energy efficiency AND environmental aspects. Some good new stuff on the market, but some not so good (ex: foam insulation). Now that I’ve got some money saved up, I can do it right this time!

However, there is way more to do around the house in terms of air sealing and redecorating in healthy ways, so I can't blow my whole budget in the basement. My money should be able to cover it all, but one never knows. I do what I can as I can afford it. But, I don't want to go overboard. I don't plan to live here more than another year, but then again, I've been saying this for 5 years now :)). Life is funny that way. My kids are grown and gone, for the most part, and I'm ready for a smaller house in a different location. I've never lived anywhere as long as I've lived here, and I really want to build my own home from the ground up. Likely it's going to be a one story, large eat-in kitchen (my kitchen has always been the heart of my homes), 3 bd/3bath, straw bale house, with a separate guest house, and separate art studio, all surrounding an outdoor partially covered patio and garden, as off-the-grid as reasonably as I can be, and NOT in suburbia. I'm an artist, I love company, but I also like my personal space and nature is a must for my happiness. View of the mountains and maybe the ocean somewhere -- Hawaii or Belize? Oh yeah, but I'm allergic to mold too, which is why I live in dryish Colorado; guess I'll have to think more about where to go.

Leaving that tangent... the entire basement floor footprint is ~1315 sq. ft.. (see attached drawing below). The basement rooms are from subfloor to ceiling: 7' 9" in the bedroom and closet, 6' 11" in the bathroom, and 7' 8" in the hallway and large rec. room. The storage and utility rooms (the little room between the bathroom and storage room) are unfinished, so floor to ceiling height is closer to 8’. There are a few soffits in some rooms that lower the ceiling height to 6' 9" or less; so, I think raising the floor up higher is out of the question as I have friends and family members who are over 6' 6" tall.

The crawl space is pretty shallow, averaging 3 ft deep at the most, depending on where you are in it, and the grading of the dirt floor isn't level. There are currently 3 accesses to it. One in the basement bedroom floor very close to the window, a much smaller one in the bedroom closet floor, and the last is in the storage room floor, not far from the sump pump.

I haven't been under the plywood floors myself. I have seen only what can easily be seen when the access hatches up and I'm laying on the floor looking underneath. So, this is what my contractor and the insulation guys have told me. There is a very thin layer of dirt and rock on top of poorly laid 3 or maybe 6 mil plastic sheeting on the dirt floor of the crawl space. It is torn in many places and disintegrating in others. I have no idea what is under the plastic sheeting other than more rocks and dirt. The exterior foundation concrete walls sit on top of pylons. The floating subfloor is plywood, on top of wooden joists, on top of some steel beams. You can't crawl under the steel beams unless you are a rodent, a cat (mine have found their way in there once or twice when a hatch was left open) or a small child, maybe. According to the 5 adults who have been down there, none of them have been able to shimmy under the steel beams, where it's closer to under a foot between the dirt and the bottom of the beams.

To do this air and vapor sealing, several areas of the plywood will have to come up, if not all of it. Some of these need replacing anyway, thanks to my cats soiling them.

I guess, I’ll probably go with the thick polyethylene that can be wrapped up the walls. If only it wasn’t winter! I may have to air out the basement for a while after this is done, if there is an off-gassing odor issue. I’ll also have the basement door that comes up to the 1st floor sealed up tight too. This should also help reduce the stack effect. There are air vents in the crawl space to that need sealing up, I think.

After the proper air and vapor sealing happens in the crawl space, I plan to put R-30 recycled cotton batts under the floating floor. Air and vapor sealing will continue in the walls too. Will have recycled carpet tiles or maybe cork tile laid on top of the plywood. R-19 bats or R-21 will go in the walls. I’m not sure what insulation is in the ceiling, but likely it's more fiberglass batts. Will put cotton batts in there too as the ceiling will have to come down due to all the holes from pool cue sticks and air sealing that will be done around the recessed light cans and the heating ducts. I also find them to be great sound proofing. I have cork tile flooring on the 2nd floor and love it, tho the cats hate it. Nothing to shred and they slide across it when they go tearing around the house... what a hoot! But, they aren’t allowed in the basement anymore anyway, so more likely it'll be the carpet tiles, which are so much fun to mix and match, as well as easy to clean or dry out, if need be. Also the pool table and other heavy furniture won't ruin them like cork or lino tiles. Dents from heavy objects can spring back easier or if need be I'll switch out carpet tiles, also easier to do than anything else.

Basement drawing.pdf 1.7 MB

Jan 25, 2011 3:16 PM ET

Dripping Recessed Lighting
by Mark A. Miller Architect/builder

Great article Martin. Just wanted to add from my experience. We had a new house under construction and were seeing drops of water on the plywood deck below a few recessed lights in a two story space. What we discovered was that warm interior air was entering the conduit feeding the can lights. This conduit went up into the attic space, above our insulation (poor choice by the contractor, that wasn't initially seen). The warm air would condense in the cold part of the conduit, causing the water dripping. Stopping air from entering the conduit was the solution at the time, but since then, I won't be installing any recessed lighting in ceiling/roof planes again. Not worth the worry. Sconces that illuminate the ceiling are a better solution on many levels. Food for thought.

Jan 25, 2011 3:33 PM ET

Response to Mark Miller
by Martin Holladay

Good detective work!

And I couldn't agree with you more: sconces that illuminate the ceiling are much better than recessed cans that light up the floor and cast strange shadows on people's faces.

Sep 28, 2011 5:37 PM ET

Edited Sep 28, 2011 5:55 PM ET.

concrete as air barrier
by Lyle Axelarris

I was glad to see the mention of the stack effect's effect on concrete's performance as an air barrier in the basement section (I think the grammar on that sentence is actually correct, believe it or not :) ). This is critically important in radon-mitigation strategies. A slab IS NOT ENOUGH to keep Radon out of your home. Poly and perf-pipe under the slab (connected to a fan and exhausted outside of the home) is critical for keeping Radon out. I'm surprised at how many green builders aren't aware of this fact (I've even seen NZE home builders ignore Radon mitigation - to save energy, I guess).

I take some exception to the unapologetic promotion of spray foam on this (and other) green building sites. There is an incredibly high amount of embodied energy in petroleum-based spray foam, the off-gassing is a legitimate concern, and perhaps most relevant to this column, spray foam can get leaky through time, because it separates from wood framing members (which expand and contract with temperature and humidity fluctuations). That may seem impossible, but in the interior of Alaska, where indoor RH's drop below 30% and temperature gradients (between inside and outside) are over 100 degrees, this absolutely happens and studies at the Cold Climate Housing Research Center confirm remodelers' experience. Outsulation techniques such as the REMOTE wall or Thorsten Chlupp's hybrid walls (on two NZE and PassivHause homes in Alaska) accomplish ACH50 below 0.4 without poly or spray foam in the wall cavity.

Thanks, Martin, for another great read.

Jan 16, 2012 3:09 PM ET

Building airtight w/o (spray)-foam
by floris keverling buisman

An example how to get away from depending on spray foam is to take the rim-board out of the air-sealing equation by making the plywood sub-floor airtight - this can be done by taping the boards together. Of course services entering the airtight floor of the house (electric and plumbing) need to be sealed as well, with a durable connections. As mentioned most spray-foam and caulks will shrink/dry-out or lose adhesion to the substrate over time, which is a reason age tested tapes and rubber gaskets are a better way make long lasting seals. Disclosure, I sell imported air-sealing tapes, intelligent membranes and accessories that are available at

Regarding the walls and ceilings - a good rule of thumb is to have >66% of the insulation outboard of the most vapor retarding layer- to prevent condensation on cold surfaces in the winter. But if in addition to that, you want the wall to be more vapor open on the exterior than on the interior so any moisture that 'leaks' into the insulation can dry outwards in winter - a reason why Joe loves vented roofs.

However a OSB sheathed buildings with exterior foam could have condensation problems in summer in an airconditioned house, since the vapor drive now is reversed and the hot humid summer air could condense on this (interior) sheathing. The same goes for a interior PE-vapor retarder.

Solution to these condensation issues, mold and potential for structural decay is an intelligent vapor retarding membrane (INTELLO or the paper based DB+). These membranes are vapor closed in winter, vapor open in summer. This allows you to use insulation with low environmental footprints (cellulose or cotton) - and if combined with a service cavity () puts the airtight and vapor control layer in the right locaiton and protects it from the homeowner (which ADA doesn't). It can be used in new construction or renovations without having to resort to floor-outlets, sealing each and every outlets (ADA).

Jan 2, 2013 9:36 PM ET

Stack effect and heated basements
by Katherine Slufik

Martin, I have a finished walk out basement. Should I keep heat turned up a bit higher than the main floor to try to minimize the stack effect? Will this approach save any energy?

Jan 3, 2013 6:04 AM ET

Response to Katherine Slufik
by Martin Holladay

You should adjust your heat to provide comfort. If you feel cold, and you can afford the cost of your fuel, then turn up the heat.

Turning up the heat will not reduce the stack effect; if anything, it will increase it. Nor will it save energy. If you turn up the heat, you will burn more fuel.

The only way to reduce the stack effect is to perform air-sealing work. If you know that your house has air leaks, start sealing them. Begin at the attic and the basement. If you aren't sure where the leaks are, you can hire a blower-door contractor to help you find them.

Mar 21, 2013 11:52 AM ET

Air barriers in vented roofs
by Mark Fredericks

In a standard vented roof assembly, with an air barrier at the ceiling, and the attic insulated with fiberglass, cellulose or other air-permeable stuff, why is there no additional air barrier on top of this insulation? It would seem the air in the vented attic could bypass the insulation. I've read that any fluffy insulation needs a 6 sided air barrier like enclosed in a stud bay to be effective, but I've never seen an air barrier above attic insulation. I can appreciate why more effort is given to ensuring the ceiling air barrier is tight so conditioned air doesn't escape. But I would also want to avoid unconditioned air bypassing the insulation. Vented roofs are common and they supposedly work well, so what am I missing here?

Mar 21, 2013 12:10 PM ET

Edited Mar 21, 2013 12:13 PM ET.

Response to Mark Fredericks
by Martin Holladay

Q. "In a standard vented roof assembly, with an air barrier at the ceiling, and the attic insulated with fiberglass, cellulose or other air-permeable stuff, why is there no additional air barrier on top of this insulation?"

A. There are two kinds of people who ask this question: those who care about thermal performance, and those who don't. The answer for those who don't (old-fashioned builders) is simple: "Because that's the way we've always done it, and because we'll fight tooth and nail if anyone tries to change the code and tell us to do it differently."

The answer for the second group -- those who do care about thermal performance -- is more subtle. Yes, there is a thermal penalty to not having a topside air barrier -- epecially for fiberglass batts or blown-in fiberglass -- but it's not a huge penalty. It turns out that it is cheaper to simply install deeper insulation (enough to make up for the thermal penalty) than it is to try to correct the problem with a layer of housewrap.

So, here's the moral of the story: if you care about this minor thermal penalty:
- Choose cellulose, not fiberglass, and
- Just make the cellulose a little bit deeper if you want to make up for the (slight) thermal penalty associated with the lack of a topside air barrier.

Mar 21, 2013 1:27 PM ET

by Mark Fredericks

Thanks Martin for your quick and clear response. Your answer is not unexpected, I can imagine the headache of trying to seal an air barrier around all the roof framing for only a minor performance gain. However it's comforting to know that just because it's not common, doesn't mean it's not valuable. Thanks again.

Nov 16, 2014 6:12 AM ET

non-drywall ceiling?
by A Lange

Thanks for all the fantastic information. Can you tell me if an air barrier is used at the ceiling with insulation between the joists above, is it still possible to use wood for the ceiling or does it have to be drywall in order to keep the integrity of the airseal? Would using an adhesive or silicone where screw/nail penetrations would occur have any use?

Nov 16, 2014 6:50 AM ET

Response to Enga Lokey
by Martin Holladay

You say that you plan to install "wood for the ceiling." By "wood," I'm assuming that you mean boards -- either square-edged boards or tongue-and-groove boards.

A board ceiling leaks air like a sieve. This is the most common type of ceiling involved with cathedral ceiling disasters, because this type of ceiling allows lots of warm, humid interior air to enter the cold rafter bays and condense.

The nail penetrations aren't really the problem. The problem is the cracks between the boards. There is no way that caulk or adhesive can be used to stop air leaks through these cracks, because normal changes in temperature and humidity cause the boards to swell and shrink, stressing any caulk to the breaking point.

The best (and easiest) solution is to install a drywall ceiling first (with taped joints) before installing the boards. Another option is to install foil-faced polyisocyanurate with taped seams first; in these case, the polyiso is the air barrier. (If you use polyiso, make sure to seal any leaks at the perimeter of the ceiling with caulk or tape.) After installing the polyiso, you can install 1x4 strapping and boards.

Nov 16, 2014 8:25 AM ET

non-drywall ceiling?
by A Lange

OK, so one of the fancy Intello air barriers between the boards and underside of rafters will not make wood cracking and gaps a non-issue? I wasn't thinking the wood was the barrier, but the air-barrier membrane was, although this would have holes in it where nails penetrated to attach the wood. Thanks!

Mar 29, 2013 2:19 AM ET

air barrier wrap
by A Lange

Still wondering about the extent to which the Intello and similar materials are sufficient as an air barrier on the ceiling, regardless of what ceiling lining is used. It would seem that they are an expensive waste of time if airtight drywall or foam sheeting is also needed.

Mar 30, 2013 10:29 AM ET

Resonse to Enga Lokey
by Martin Holladay

I agree. Either gypsum drywall or carefully taped and sealed foil-faced polyisocyanurate makes a good ceiling air barrier. If you are using drywall or polyiso as your air barrier, you don't need an additional sheet membrane.

May 30, 2018 4:05 PM ET

Rim joist insulation: open cell?
by Adam W

Would you discourage the use of open cell foam to insulate the rim joist? (I see the note about a preference for CC in the rim joist)

May 30, 2018 5:08 PM ET

Found the answer... I believe
by Adam W

On I read:

"One advantage of using spray foam to insulate rim joists—an approach sometimes called the critical-seal method—is that a single product performs two tasks: sealing air leaks and insulating. In mild climate zones, either open-cell spray foam or closed-cell spray foam will work; however, in climate zone 6 and colder zones, it’s safer to use closed-cell spray foam."

I'm in Climate zone 4.

May 31, 2018 7:29 AM ET

Response to Adam W (Comments #78 and #79)
by Martin Holladay

Yes, you found the answer. Open-cell spray foam is fine in your climate zone. Good luck.

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