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One Air Barrier or Two?

If your home has an air barrier at the exterior sheathing, do you also need an interior air barrier?

Posted on Jul 16 2010 by Martin Holladay

Although building scientists have understood the advantages of airtight construction details for years, few residential plans include 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. details. That’s nuts.

Do the blueprints show where the air barrier goes?

Ideally, construction documents should show the location of a building’s air barrier, and should explain how the builder is expected to maintain air-barrier continuity at penetrations and important intersections. In a typical house, these intersections might include:

  • where the basement slab meets the basement wall;
  • where the basement wall meets the mudsill;
  • where the mudsill meets the rim joist;
  • where the rim joist meets the subfloor;
  • where the subfloor meets the bottom plate;
  • where the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. meets the vertical drywall; and
  • where the top plate meets the ceiling drywall.

A designer who doesn’t know how to make these areas airtight can hardly fault a builder who fails to intuit details that aren’t even mentioned on the plan. (For more information on air barriers, check out these three resources: Questions and Answers About Air Barriers, the GBA Encyclopedia, and a useful Web page from Oikos, "Advanced Air Sealing.")


Questions and Answers About Air Barriers

Airtight Wall and Roof Sheathing

Getting the Biggest Bang for Your Air-Sealing Buck

Blower Door Basics

Pinpointing Leaks With a Fog Machine

Air-Sealing Tapes and Gaskets

Air Sealing With Sprayable Caulk

New Air Sealing Requirements in the IRC

Navigating Energy Star’s Thermal Bypass Checklist

Prevent Ice Dams With Air Sealing and Insulation

Video Series: Attic Air Sealing

Video Series: How to Hang Airtight Drywall

Fine Homebuilding: How to Hang Airtight Drywall

Energy Smart Details: Airtight Drywall

CAD Detail from GBA Detail Library: Airtight drywall approach

Podcast: Air Barrier or Vapor Barrier?

GBA Encyclopedia: Air Barriers

GBA Encyclopedia: Addressing Air Leaks

Product Guide: Air Barrier Components

Q&A: Is dense-packed cellulose an air barrier?


Air Sealing: A Guide for Contractors to Share with Homeowners

Making Air Barriers That Work

The road ahead is steep

If the day ever comes when most new homes include an air barrier that addresses typical penetrations and the intersections listed above, energy efficiency experts will cheer. That day is a long way off, however.

In the meantime, some progressive builders insist that every home needs not one but two air barriers: an exterior air barrier and an interior air barrier. Although this belt-and-suspenders approach is controversial, it has many strong advocates.

To understand the controversies surrounding double air barriers, it’s important to explain the two positions.

One good air barrier is enough

The basic rule of infiltration is “air out always equals air in.” If you have a good air barrier on one side of the insulation, you have stopped both infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. — so you’re done.

According to this philosophy, it doesn’t matter where the air barrier is located — as long as the insulation is in contact with the air barrier:

  • It can be on the exterior side of the insulation — for example, using taped housewrap.
  • It can be in the middle of a wall — for example, using spray foam insulation.
  • It can be on the interior side of the insulation — for example, using the airtight drywall approach.

If you set a particular airtightness goal — for example, 1.5 ac/h @ 50 Pascals — you can achieve it with any of these methods, as long as you pay attention to penetrations and intersections. Once 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. verifies that you have achieved your airtightness goal, you have a tight house.

You need two air barriers

In the U.S., the most important proponent of the two-air-barriers approach is the Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. program. Since July 1, 2006, Energy Star has required builders to comply with a Thermal Bypass Checklist. The Thermal Bypass Checklist Guide notes, “Generally, the Thermal Bypass Inspection Checklist requires a sealed air-barrier on all six sides of insulation.” The checklist notes that “insulation shall be installed in full contact with [a] sealed interior and exterior air barrier.”

The Energy Star program allows three main exceptions to this two-air-barrier mandate:

  • The exterior air barrier can be omitted above insulation installed on an attic floor.
  • The interior air barrier can be omitted at rim joists.
  • The interior air barrier can be omitted in warmer climate zones (zones 1 through 3).

For the most part, these exceptions are due to political compromises rather than important building science principles, so I won’t be discussing the exceptions here.

The classic problems that this checklist attempts to address include:

  • Fiberglass-insulated kneewalls with no air barrier facing the attic.
  • Fiberglass-insulated skylight shafts with no air barrier facing the attic.
  • Fiberglass-insulated exterior walls behind tub/shower units or zero-clearance fireplaces with no interior air barrier.

The rationale behind the two-air-barrier principle is that many of these areas — especially ventilated unconditioned attics — are subject to wind-washing or convection. If the builder installs air-permeable insulation like fiberglass in these areas, the performance of the insulation will be seriously degraded unless the insulation is protected by air barriers on all sides.

Of course, there is an inherent flaw in the Energy Star Homes mandate for both interior and exterior air barriers — the fact that no technology exists to test the air tightness of each individual barrier. A blower-door test can reveal a home's air leakage rate, but it can't reveal whether a home has one or two air barriers. If an Energy Star builder provides a good exterior air barrier, the interior air barrier can be as leaky as a colander — and building inspectors and blower-door technicians will never notice the flaws.

Double air barriers in Europe

In Europe, meanwhile, there is a significant effort underway to train builders to install two air barriers — an exterior air barrier and an interior air barrier — on all new buildings. One strong proponent of this construction approach is SIGA, a Swiss manufacturer of membranes, tapes, and gaskets.

In its English-language literature, SIGA explains that the interior barrier provides “airtightness” while the exterior barrier provides “windtightness.” (Presumably, these terms are translated from the German; the distinction between “airtightness” and “windtightness” escapes me.)

According to Patrick Haacke, SIGA’s head of product management, most new Swiss buildings include two air barriers installed with great attention to seam sealing. SIGA brochures (and SIGA training sessions for builders) recommend a meticulous series of steps to seal exterior housewrap and roof underlayment seams and penetrations.

SIGA sells a variety of air-barrier products, including housewrap, roof underlayments, underlayment seam tape, tapes for sealing underlayment to skylights and plumbing vents, and even gasketing material (“nail sealing tape”) for use under furring strips installed on top of roofing underlayment. (By the way, the quality of these tapes and gaskets appears to be excellent. I've played around with samples of the products; I'm impressed.)

Once the roof underlayment has been made airtight, SIGA recommends similar air-sealing measures at housewrap seams and wall penetrations. With the exterior air-sealing work (including details to seal the intersection between the roof and walls) complete, one might assume that the Swiss builder would happily call up his blower-door contractor and enjoy his excellent test results. But no — now the builder moves indoors, and repeats all of these steps on the interior.

SIGA brochures show a happy Swiss worker (dressed, of course, in a white collared shirt and a black vest) installing an interior membrane (similar to polyethylene, but more permeable). The builder uses special tape (sticky on both sides) along every stud so that the membrane can be attached to the framing without any staples.

Then the membrane seams are sealed with a special tape. A different tape is used to seal between the ceiling membrane and plumbing vents; still another product is used to seal the seam where a ceiling membrane meets a wall membrane. SIGA also sells tape to seal around floor joists or exposed rafters that penetrate the wall membrane. Of course, tape is used to seal the membrane to windows and to exposed ridge beams.

Are you tired yet?

Not all European builders comply with the SIGA approach, however. Although Haacke reports that it’s now standard practice for Swiss builders to install two air barriers, some PassivhausA 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. builders disagree with the technique. With more than one air barrier, it's always possible that builders will fail to be meticulous; in other words, redundancy can lead to imperfect execution of details. According to Hans Porschitz, a building systems associate at Bensonwood Homes in New Hampshire, German Passivhaus consultants usually advise that “one effective air barrier is better than redundant ineffective layers.”

So, are two air barriers necessary?

There are two possible reasons for installing two air barriers:

  • The two barriers are a belt-and-suspenders solution to the problem of builder sloppiness.
  • The two barriers are an attempt to prevent degradation in the performance of air-permeable insulation due to wind-washing and convection.

The first reason is certainly defensible. For example, some manufacturers of structural insulated panels (SIPs) now insist that panel seams be sealed twice: once with spray foam, and then later with interior tape. This belt-and-suspenders approach is a direct result of a cluster of SIP failures in Juneau, Alaska, where poorly sealed SIP seams allowed exfiltration, condensation, and rot. The air leakage problem in Juneau was caused by job-site difficulties encountered during freezing weather or rain; difficulties in accessing certain SIP seams; and worker sloppiness. The manufacturers’ response — “we need two air barriers” — makes sense.

However, some building techniques don’t need such redundancy. Marc Rosenbaum’s use of the Huber Zip System for roofs and walls, along with early blower-door testing, results in high levels of thermal performance without the need for two air barriers.

If you have a good air barrier, how much air moves through your insulation?

The second reason — addressing wind-washing and convection currents — is subject to debate. Obviously, wind-washing is a concern when fiberglass batts are exposed at soffits or the back of attic kneewalls. Elsewhere, however — for example, in walls with Huber Zip System sheathing and no interior air barrier — it’s hard to imagine that wind-washing or convection currents are significant.

To a large extent, double air barriers are an attempt to respond to the inherent performance problems of fiberglass batts. As more builders choose other insulation materials, there will be fewer justifications for the expense and hassle of double air barriers.

An academic discussion

For many energy consultants in the field — at least on this side of the Atlantic — the discussion about interior and exterior air barriers resembles the debate over how many angels can dance on the head of a pin. Here, most energy experts lament that “American homes don’t yet have a single air barrier; two air barriers are a far-off dream.”

Last week’s blog: “Energy and Construction Photos from Greece.”

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Image Credits:

  1. SIGA

Jul 16, 2010 7:35 AM ET

Drying potential with two air barriers
by Ben Wilson

Is the interior air barrier used by SIGA actually a "smart" retarder, a la MemBrain? If it is not, I would be concerned about the lack of drying potential inside the wall cavity. A perfectly-detailed exterior should keep bulk water out for a long time, but I would tend to prefer air-sealing details that can handle unexpected situations long after installation.

Jul 16, 2010 7:52 AM ET

Response to Ben
by Martin Holladay

The interior SIGA membrane has an s(d) value of 5 m. According to European standards, that makes it a "diffusion-blocking" membrane -- but not "diffusion-proof." In other words, it's a vapor retarder, not a vapor barrier. It is more vapor-open than polyethylene. It is not a smart retarder; in other words, its permeance does not vary with moisture content.

Here are the three categories of permeance according to the s(d) system:

S(d) ≤ 0.5 m => diffusion-open
0.5 m < S(d) => diffusion-blocking
S(d) ≥ 1500 m ≥ 1500 m => diffusion-proof

For more on European permeance categories, see

Jul 16, 2010 9:57 AM ET

Attention to detail equals building performance
by Doug McEvers

Great article,

The best point made is low energy buildings do not happen by accident, effort is needed and the details are critical. I am in favor of the 2 air barrier system, one primary, one secondary because it puts the focus on quality for the entire building envelope. Use the right system for a given climate, get the details right, exterior drainage plane in particular and the building will perform well.

Jul 16, 2010 11:11 AM ET

letting a house breathe?
by Building a House and Home

Your whole argument is based upon the premise that airtight is a good thing because of energy efficiency. My momma always said that a house needs to breathe. We lived in an old farmhouse that was anything but efficient, but it was heated from wood, and never needed air-conditioning because it was always 10 degrees cooler than the outside. So I guess it was more energy efficient than most modern homes. She said that asthma, allergies and other sicknesses could be tied to the airtightness of new houses, not to mention that moisture has no where to go and mold becomes a problem. Studies are proving her right. As I set out to build my own house, I am really pondering these two points of view and trying to find a good outcome. I have heard of people making their house airtight and then installing an air filter that needs to be run constantly and replaced every few months. That seems insane to me. (Doesn't it take electric to run the air filter? and isn't more machinery and more maintenance more work? I don't want to have to change a filter in my house to be able to breathe healthy air!) I would love to hear what you have to say.

Jul 16, 2010 11:25 AM ET

Your momma's wisdom
by Martin Holladay

Dear Building,
1. Mammals breathe to oxygenate their bloodstreams. Buildings don't have any blood to oxygenate, so buildings don't need to breathe.

2. Occupants of buildings definitely need fresh air. If you don't have a mechanical ventilation system, where are you getting your fresh air from? It sounds like you and your momma are depending on random cracks in your building envelope. Random cracks are not a dependable source of fresh air.

In your momma's home, air infiltration ranged widely. During the coldest days of winter, the stack effect brought in way too much fresh air. On still, windless days during the spring and fall, very little fresh air entered your momma's house. Sometimes, the air entering your momma's house came from a moldy crawl space -- because that's where the cracks were.

Jul 16, 2010 11:44 AM ET

Testing two layers of airtightness
by Interested Onlooker

Would this work?

Build framing, add sheathing, membrane and siding/roof. Blower door test. Fix air-tightness of exterior barrier while you can still get to it from the inside. Add insulation, add air-tight drywall. Repeat blower door test. Compare results and tweak inner barrier as necessary.

Jul 16, 2010 11:49 AM ET

Permeance and gas exchange through membranes
by aj builder, Upstate NY Zone 6a

Take a completely sealed home that is absolutely airtight. Get the gas exchange through this permeable airtight membrane correct and indeed a home is breathing like our lungs do to deliver oxygen to the enclosed humans and to take away CO2 and other gases including H2O vapor. They will balance across the proper membrane and no electric/mechanical fan ERV/HRV is needed.

Am I wrong?

Would like someone to chime in and tell me the permeance a home would need for this to work.

Who can site the rate of all gasses that make up air through all building materials available today or being developed.

My feeling is densepacked cellulose held by something better than membrain would do the trick perfectly.

Jul 16, 2010 12:15 PM ET

breathable walls
by j chesnut

I have lived my life in turn of the century homes in a cold climate and it took awhile for me to accept that residential construction should be built air tight and have mechanical ventilation. Part of the reason for my difficulty with the concept was due to the poor performance of the air systems in commercial and institutional buildings; those with all non-operable windows, bad indoor air quality and erratic temperature levels. Residential buildings with operable windows are quite a different scenario as I have come to realize.

We use so much energy to heat and cool a conditioned environment for our sedimentary lifestyles. When temperature conditions outside are uncomfortable or harmful to us we need to provide an airtight enclosure to reduce the amount of energy expended to condition the air. As a consequence of this we need to introduce (passively to the extent possible) pre-conditioned fresh air into the interior of the building mechanically for health reasons (mammals need of oxygen as Martin illustrates). The amount of energy expended by transporting fresh air at low velocities is much less than that expended for heating or cooling a conditioned space.
There is the concept of 'breathable walls' that many use. This idea includes the important issue of how building assemblies deal with water vapor. 'Breathable walls' is probably a poor choice of words because most the issues surround vapor transmission through a wall assembly not air transmission. However some claim with certain wall assemblies made of natural materials adequate fresh air is able to transfer through the solid wall, but I'm fairly certain this is false thinking and they are merely experiencing leaky construction.
'Negative and positive ions' are often part of the discussion also, one of the two being important to our health. While I have been open to this argument I've only seen vague claims to back it up.

Great article. You have an uncanny ability to address some of the more subtle aspects of building science that I was just getting around to wondering about.

Jul 16, 2010 1:01 PM ET

Response to Adkjac
by Martin Holladay

When we measure the vapor permeance of a building material, we are measuring the ability of the material to allow or restrict the transmission of water vapor, not gasses. Measuring permeance provides important information, but it won't tell you anything about whether a material is good or bad at admitting gasses like oxygen.

If you build a structure out of Tyvek, it will be extremely permeable. If you sweat inside your Tyvek tent, the moisture will pass through the tent wall. But you will still suffocate and die unless you include a mechanical ventilation system, because Tyvek restricts air.

It seems to me that your hypothetical building -- one that allow the passage of gasses through the building assemblies -- has already been invented. It's called a leaky building.

Jul 16, 2010 1:05 PM ET

Response to Interested
by Martin Holladay

Your proposed method of air testing -- first testing the exterior air barrier with a blower door, then installing ADA, and then testing the building again -- certainly provides information, and tells you whether your interior air barrier is contributing to an improvement in airtightness.

But I'm not quite sure how you would set your airtightness goal. "As tight as we can get it?" Or do you tighten the exterior air barrier until you reach 0.6 ach @ 50 pascals? Or do you leave the exterior air barrier a little bit sloppy and leaky, because "we can always hit our goal by tweaking the ADA"?

It just seems complicated to me. It's better to choose your air barrier location and aim to do a good job in that one location.

Jul 16, 2010 2:50 PM ET

Water vapor is a gas
by aj builder, Upstate NY Zone 6a

Water vapor is a gas. It does pass through "airtight materials." It will seek to go from high concentration to low concentration through a properly permeable material.

So will all the other gasses that make up the air that we breath and live in.

What I do not know and was hoping someone with the scientific knowledge could do is discuss some permeance ratings for building materials that would allow sufficient exchange of critical gasses to living, mainly O2, CO2, and CO. Nitrogen is the largest component of air but doesn't do much other than give us divers the bends if we don't follow our diving tables suggestions.

Also... I second the motion that Martin is invaluable at explaining building systems.

As to double air barriers... The Swiss... might be overcompensating for their cheese situation?

Jul 16, 2010 3:28 PM ET

Double Air Barrier
by Sam Rashkin

Thanks Martin... good article...
Here's the deal about inside and outside air barriers at exterior walls from our perspective at ENERGY STAR. One air barrier would be adequate if driving forces were predominantly in one direction. Thus, the exemption for interior side air barriers at exterior walls in Climate Zones 1 thru 3. However, if significant driving forces can be exerted both from outside and inside, as experienced in moderate and northern climates that have both hot summers and cold winters, you need an air barrier on both the inside and outside. This will help minimize likelihood relatively humid air can reach a cold surface and condense inside the wall assembly year-round. Hope this helps the discussion.

Jul 16, 2010 3:46 PM ET

Response to Sam
by Martin Holladay

You write that you are worried that humid air can reach a cold surface and condense inside the wall. You write that in northern climates, there are "significant driving forces" exerted from the inside and the outside that can introduce humid air to cold surfaces in the wall.

So, let's talk about a house with an air barrier at the wall sheathing and roof sheathing. It's been blower-tested to 0.6 ach50. What driving force are you talking about that moves humid air into the wall cavity? There's diffusion, of course -- but diffusion has nothing to do with moving humid air. How does the humid air get into the wall? Not the stack effect; we've established an air barrier, remember. Not the wind; we've established an air barrier.

Jul 16, 2010 3:52 PM ET

Response to AJ
by Martin Holladay

You're right; water vapor is a gas. But the reason we build houses out of materials that stop the wind is that we really don't want to have a windy living room. Cheesecloth allows the wind to enter, so if you build your house out of cheesecloth you'll get plenty of oxygen.

I prefer to live in a house that stops the wind. I can open the windows when I need fresh air, or operate a mechanical ventilation system if I prefer.

Jul 16, 2010 3:55 PM ET

ENERGY STAR perspective
by John Brooks

What does Energy Star think about a Rigid Exterior Air Barrier Strategy such as Marc Rosenbaum has employed?
It seems to be "doing the job" of resisting the driving forces from either direction.

Jul 16, 2010 5:01 PM ET

How does the humid air get into the wall?
by Michael Blasnik

How about convective loops?. An airtight exterior won't stop convective loops from forming if there is a path from the warm interior air to the cold side of the insulation. Of course, if the bulk of the R value is exterior to this "exterior" air barrier, then convective loops shouldn't be a problem either.

Jul 16, 2010 6:48 PM ET

Cheese cloth and swiss cheese oh my
by aj builder, Upstate NY Zone 6a

ha haa.. yaa have me smiling Martin. OK... what you are saying is you don't know any more than I do about what perm a material might have that would make a home a sealed but livable space. I know, I am pushing to a knew area of thought with this. But... others are also working on this.

I am not against HRV/ERV use... but I bet there are alternatives other than cheese cloth.

And my main point it this; Membranes can pass enough gas to allow humans to exist within. That's what our lungs are doing everyday.

And they are airtight. When not airtight one needs surgery!

Jul 17, 2010 12:44 AM ET

AJ, your lung anology is faulty
by David Argilla

What you are describing is is a passive gas permeable exterior membrane. Gas exchange for humans is a powered and highly regulated internal membrane system. The heart, diaphragm and other muscles provide the power, the lungs provide a huge surface area (over 100 square meters) for exchange. In reality , an HRV plus duct work is much more similar to biological respiration and circulation than a passive system. I would think that the HRV has better energy recovery though. It seems the passive system you are looking for requires no special materials. Crack a window in each room, you lose heat, but so would an air permeable wall...

Jul 17, 2010 6:17 AM ET

Response to Michael Blasnik
by Martin Holladay

If the wall is poorly constructed, a convective loop is theoretically possible. If you're building a high-performance home, however, you are unlikely to be using materials that allow convective loops. So it appears that these expensive double air barriers are just a response to the fiberglass batt problem.

Imagine a wall with cold sheathing (because the builder didn't use exterior foam) and fiberglass batts between the studs (because the builder didn't want to use cellulose). If you have a hole in the drywall near the top of the wall, some interior air can move into the wall; then it can filter through the fiberglass, find the cold sheathing, drop to the bottom of the stud bay, and look for another hole near an electrical outlet. It's all possible, in theory -- but if you have an impeccable exterior air barrier that eliminates exfiltration, not much air will be moving in such a loop.

If you change just one or two things about the wall -- if you include foam sheathing or switch to cellulose insulation -- the likelihood of such a very small convective loop drops to near zero.

Jul 17, 2010 7:22 AM ET

"ONE undisturbed airtight layer"
by John Brooks

from this Passivhaus page:

An envelope can be airtight only if its consists of ONE undisturbed airtight layer enwrapping the whole volume.

They are not saying that you can not have two... but rather you must have ONE.
I think the key word is "undisturbed".

One sort of good layer outside plus One sort of good layer inside is not the same as
"ONE undisturbed airtight layer"

Jul 17, 2010 7:42 AM ET

response to Martin
by Michael Blasnik

I agree, in part.

You basically say that, if you have a good interior air barrier or fairly airtight insulation like dense-pack cellulose then convective loops won't be a problem. But isn't this an argument in favor of a good interior air barrier?

Your other claim -- that an exterior air barrier will somehow prevent or reduce convective loops -- just doesn't make any sense. Convective looping is based on air paths that connect the warm inside air with a cold surface and a way for the cooled air to get back into the living space. There is no need for any exterior leakage connection.

Jul 17, 2010 9:33 AM ET

Martin, So if I put the air
by Brett Moyer

So if I put the air barrier on the exterior, there's no need for any sort of air barrier on the interior? I could, in theory, leave the wall exposed without drywall? Though I can't debate this scientifically, I am having a problem with the one exterior air barrier approach. Especially when we are talking about homes in cold climates.
I look at the exterior air barrier as something to stop wind and condioned air from infiltrating/exfiltrating the home, and the interior air barrier as something to protect warm moisture-laden air from reaching a cold condensing surface.

Jul 17, 2010 1:38 PM ET

"old house" smell
by Adrian

To me a double air barrier would also help prevent the diffusion of smells from the wood framing to the interior of the house when renovating old homes.
The question is what kind of "plastic" should we use.? I have seen a lot of opposition versus using PE sheeting. How about Tyvek? I understand that it allows moisture to travel so wouldn't it help the house dry through the inside while stopping the smell?
SIGA products don't seem to be available in the US. I looked into the data sheet for the Majpell and I am not quite clear what it is. Any suggestions?
Another way would be to treat the wood but this is another can of worms...

Jul 17, 2010 2:46 PM ET

Response to Michael
by Martin Holladay

There are really three questions here:
1. How much of a problem is convective looping with well-installed fiberglass batts (there has been a lot of debate on this issue over the years)?
2. How much of what people refer to as "convective looping" is really something else (usually involving infiltration and exfiltration)?
3. Is it really necessary to go to SIGA-literature level air sealing to limit convective looping problems?

Jul 17, 2010 2:48 PM ET

Response to Adrian
by Martin Holladay

The answer to your question is MemBrain, not Tyvek.

Jul 18, 2010 10:10 AM ET

response to Martin
by Michael Blasnik


1) not much, if they are well installed. The key question is more about the "well installed" part and I think that's part of the justification for the interior air barrier. I don't really know what the typical energy savings might be from the interior air barrier -- I would guess not a lot in most cases -- but I think reducing the likelihood of moisture problems in colder climates .may be sufficient justification for paying attention to this,.

2) not much, but maybe I over-estimate people's understanding of the term

3) probably not

Jul 19, 2010 3:52 AM ET

One Air Barrier or Two - One
by Interested Onlooker

You're quite right; it is possible to build two complete air barriers and, as I described, test them but it's a complicated exercise. So one air barrier it is. But where? From a purely practical point of view it has to be external - wrapping the outside of a box is easier than wrapping the inside of a box. Do it early and protect it with rainscreen siding and a roof. Test it and fix it. Hide it with internal insulation and drywall and forget about it - happy in the knowledge that it is away from rain and UV and most of the wind. The people living in the house can then live in the house - put up pictures, run extra wiring, move the bathroom and generally move stuff around in a major or minor way. None of this activity is messing up an ADA air barrier because there isn't one. The external air barrier is outside the insulation so it's going to get cold - so it had better be vapor-permeable (at least in parts of the world which are HDD-dominated).

Jul 19, 2010 11:14 AM ET

longevity of tape adhesives
by Don


All of these systems rely on adhesive tapes to seal the joints between membrane layers or between the membrane and the structure. Every tape that I am familiar with has a usable life period after which the adhesive has degraded too much to adequately stick. Eventually the adhesives lose all their strength adn taped joints fail. Have you seen any claims as to how long the adhesives on these construction tapes (Tyvek, SIGA, etc.) will last?

Jul 19, 2010 11:37 AM ET

Adhesive longevity
by Martin Holladay

Your question is a good one.

When I met with a representative of SIGA, he informed me that SIGA tapes have no VOCs. Since there are no volatile organic compounds to evaporate, SIGA adhesives (he claimed) do not dry out like some competitors' adhesives.

I know from personal experience that butyl adhesives are extremely long-lived.

I know of one study of air barrier longevity. A 2004 Canadian study by Gary Proskiw and Anil Parekh found little evidence of air-barrier deterioration after 14 years.

I reported on the results of the study in the March 2005 issue of Energy Design Update:

"A recent study by two Winnipeg engineers, Gary Proskiw and Anil Parekh, provides reason for optimism. In December 2004, Proskiw presented the study,“Airtightness Performance of Wood-Framed Houses Over a 14-Year Period,” at the Performance of Exterior Envelopes of Whole Buildings IX conference in Clearwater Beach, Florida.

"Proskiw and Parekh compared blower-door results for 22 Winnipeg houses that have been extensively studied since they were built in the late 1980s. Blower-door tests were performed at all of the houses soon after completion. Follow-up testing was performed periodically on the houses, with the most recent blower-door tests performed in 2000.

"The 22 wood-framed single-family stucco-clad houses were all built with careful attention to air sealing. The study divided the houses into two groups:
• Nine of the houses had polyethylene air barriers, with poly seams sealed with [Tremco] acoustical sealant.
• Thirteen of the houses were sealed with gaskets using the advanced drywall approach (ADA).

"The houses, all completed during the late 1980s, were very tight, with original blower-door results averaging 1.14 ac/h @ 50 Pa. Measured again in 2000, the average airtightness of the 22 houses had deteriorated only slightly, to 1.45 ac/h @ 50 Pa — still below the stringent R-2000 standard of 1.50 ac/h @ 50 Pa.

"Comparing the houses with polyethylene air barriers to those with ADA air barriers, Proskiw and Parekh noted that the houses with polyethylene air barriers showed slightly less deterioration in airtightness than the houses with ADA air barriers.

"During the 2000 site visits, Proskiw and Parekh looked for leaks. They determined that most of the observable air leakage was occurring at accessible locations not directly associated with either the polyethylene or the ADA portions of the air barrier systems. Some of the leaks were at floor drains, around doors and windows, and at mechanical and electrical penetrations through the envelope. During his presentation in Florida, Proskiw described these leaks as “basically, just old weatherstripping,” noting that most of them could be easily remedied.

"In their paper, Proskiw and Parekh conclude that there is no evidence to indicate that either polyethylene or ADA gaskets are unsuited for use as an air barrier material in residential wood-framed construction. As Proskiw pointed out in Florida, “No catastrophic failures were observed, so the durability of the polyethylene and ADA air barrier systems has been reasonably maintained over their 14-year monitoring period.”

"These results were especially encouraging in light of the fact that 18 of the houses enrolled in the study suffered significant vibrations when an environmental cleanup project required the excavation and removal of contaminated soil from the homes’ back yards."

Jul 19, 2010 11:04 PM ET

air barriers and our existing housing stock
by Gavin

It seems like much of this discussion about air barriers revolves around new buildings, or buildings that are undergoing a gut remodel, or chainsaw remodel for that matter. I think there is a case for working toward establishing an effective air barrier on both sides as opportunites present themselves in existing homes. We need some great problem solvers in the field to tackle these interesting issues on the millions of existing homes. In the end we can judge how effective our efforts are by change in annual utility bills, along with the homeowners preception on the changed enviornment that they live in. Great article Martin.

Jul 20, 2010 2:05 AM ET

Spray Insulation
by Dale Le Grand

A few days ago I had my 1989 Trailer, insulated with blown fiberglass. With the weather at 50-90 degrees I can see the different in the trailer. In about 12 to 24 months I plan in building our home. Due to the extreme weather here in North Dakota, plus 'M a draftsmen and designer and Ineed all the information I can get for my trade. I would like to know more about SIGA

Jul 20, 2010 4:37 AM ET

Change in Utility Bills
by Interested Onlooker


Cost of supplying utility - not likely to fall since it is virtually independent of the amount of utility supplied but dominated by providing and maintaining the infrastructure to deliver it. Especially for renewables...

Number of consumers - pretty steady, rising slowly

Size of Utility Bill - not changing much

The only sure way of reducing your utility bill is to reduce your own consumption significantly and don't tell anyone else how you did it. Their continued high consumption will subsidize your bills. This is not likely to work out well environmentally.

Jul 20, 2010 4:38 AM ET

by Interested Onlooker

Sorry I meant Gavin.

Jul 20, 2010 5:49 AM ET

More about SIGA
by Martin Holladay

SIGA tapes and gaskets are sold in Europe but not available in the U.S.

Gaskets similar to those sold by SIGA are available from Conservation Technology in Baltimore or Shelter Supply.

U.S. builders can use a variety of tapes, including ZIP System tapes, housewrap tapes, and peel-and-stick butyl tapes, to achieve airtightness using the SIGA approach. Whether or not these tapes are the same quality or will have the same longevity as tapes manufactured by SIGA remains to be seen.

I will be writing a future blog on air sealing tapes and gaskets.

Jul 20, 2010 7:41 PM ET

The Best Defense
by Robert Steinberg

I worked in construction for over ten years and nothing leads to failure faster than poor workmanship and cutting corners. Every SIPS manufactures says the same thing about their product. The panels have to stay dry to be effective which means you better be good at what you do and have a plan in place to get the job done correctly using that particular system. Additionally, a commitment to thoroughness is a requirement. Bad process. Bad results. There was time in our History when building it right the first time was a fact of everyday life and then along the way we got all these experts telling us how to build better and truth is you need hands on experience to figure that out as opposed to a degree in "THE THEORY OF'.

Jul 21, 2010 7:35 AM ET

air barrier
by jessimen

Good point to rise whether you need interior air barrier or not. Good recommendations for that. Expanding foam insulation is applied by spraying it on the desired surface. . Insulation must be installed where air cannot flow through it. At least one side of the insulation must be installed against an air barrier such as drywall. Because insulation works by using trapped, non-moving air, any air movement renders the insulation practically useless. Insulation must be installed according to manufacturer's instructions; otherwise it will not be effective.

Jul 21, 2010 8:53 AM ET

Quality ICF Wall Construction and Vapor Barrier
by John

Give me air tight construction (less than .1 AC/Hour) and I can give you clean and healthy air (displacement ventilation) at the least cost (energy efficient). Make it real easy, use Quality ICF's with great thermal break details and Viper Vapor Barrier. You guys make it sound like it is difficult or Rocket Science to build a great wall. It is easy (and economical) to build almost any building 3-10 times more energy efficient than 95% of them currently built.

Jul 21, 2010 9:02 AM ET

Response to John
by Martin Holladay

You claim you can build a house with an air leakage rate of "less than .1 AC/Hour."

Do you mean less than 0.1 ac/h at 50 Pascals? Is this verified by a blower-door test? That would mean that your houses leak at a rate that is less than 1/6 of the rate of a house that meets the Passivhaus standard. That's tough to achieve.

It may not be rocket science, but it takes more than ICF walls to achieve that.

Perhaps your decimal point is misplaced?

Jul 21, 2010 9:04 AM ET

Air barrier
by Matt Belcher

Great discussion! This is just one more reason why GBA is the best!
We build in climate zone 4 (the worst!) We have hot, high humidity in the summers (Right now!) and cold, dry winters. In the not too distant past, we had to work hard to overcome shortfalls in building products by incorporating air barriers and different builders did it different ways (most had no idea even why). The positive side is programs such as Energy Star and Builders Challenge, etc. are leading to better components to address major issues such as these. Improvements in insulating materials and thier application and even advances in components such as SIPS are making it easier for "us practitioners" to address these issues. Keep the discussion going as it keeps getting better!

Jul 21, 2010 9:30 AM ET

If there are 2 barriers...
by Jack

Ok, rookie question here... Assume there are 2 barriers, like polyethylenye (as an air and moisture barrier), and Tyvek (air but not moisture barrier), which should I put inside, and which outside? In colder climates, from what I read sofar, tyvek on the inside, and polyethylene on the outside, and in warmer climates reverse that. (warmer side gets the tyvek). Is this correct? Now as second question, where is the 'changeover' (by climate zone, or other measure)?

Jul 21, 2010 9:38 AM ET

PHI and SIP claims
by M. Steven Dickerson

I wonder, with some PHI folks in discussion saying that SIP construction using OSB sided foam, is sufficiently impermeable to be considered an air barrier, would anyone recommend extra (interior taped plastic sheeting, and exterior house wrap taped) on top of each sides of a SIP constructed structure?

thanks in advance for responses.

Jul 21, 2010 9:45 AM ET

Response to Jack
by Martin Holladay

I think it would help you to get a better basic understanding of vapor barriers and vapor retarders. I suggest you read these two articles:
Vapor Retarders and Vapor Barriers
Forget Vapor Diffusion — Stop the Air Leaks!

As far as Tyvek and poly are concerned, you have it exactly backwards. The traditional recommendation for builders in cold climates was to install poly on the interior and Tyvek on the exterior.

However, that recommendation is obsolete. Unless you are building in colder parts of Canada or Alaska, you shouldn't be using interior polyethylene.

To understand the circumstances that call for summertime control of vapor diffusioni, read
When Sunshine Drives Moisture Into Walls.

Jul 21, 2010 9:47 AM ET

Two barriers to prevent condensation?
by Curious but ignorant

Would the two-barrier approach help with condensation issues though? 1) Internal barrier to prevent condensation when warm, humid inside air meets cooler air in the wall cavity in winter; 2) External barrier to prevent condensation when warm, humid outside air meets air-conditioned space in summer? Maybe I'm confused about air vs. vapor barriers?

I read the entire article, but not all the comments. Sorry if this has been addressed elsewhere. Thanks!

Jul 21, 2010 9:53 AM ET

by John

Yes, More than ICF walls, special attention to all the envelope details (Really Good Contractors) is important. Vapor barrier is not needed on the ICF but everywhere else it is (floor slabs and any wood frame). I like ICF's for the roof also, but for those that like wood trusses I spec taped 10 Mil Viper Vapor Barrier, not violated in any way other than maybe a few sealed wire holes (all ceilings are stepped) for mechanicals, and of a minimum of 2.25" High Density EPS foam sheet under trusses with a layer of HD spray foam above with anything good (no fiberglass) above that. Use great doors and windows like Serious fiberglass 925's and all penetrations are detailed air tight. Ventilation (including bath exhaust) is displacement through a Lifebreath HRV (airtight PVC ducting with dampers and night time bypass cooling) with built in dehumidistat and also use Thermastor Digital controlled Ultra Aire 65 for Dehumidification (UA 65 uses central return with HRV fresh air supply for delivery).

Jul 21, 2010 9:56 AM ET

Response to M. Steven Dickerson
by Martin Holladay

M. Steven Dickerson,
1. OSB is, indeed, an air barrier. Of course, the seams between adjacent panels of OSB (or adjacent SIPs) must be sealed in order to have an air barrier.

2. Interior polyethylene is not required with SIPs that have properly sealed seams. If a builder is worried that the seams may not have been well sealed, he or she might decide to install poly as a redundant interior air barrier. I don't recommend this approach, however. Taping the interior seams makes more sense.

3. Exterior taped housewrap over SIPs is not an air barrier; it is a WRB (water-resistant barrier). The purpose of a WRB is to protect the exterior OSB from wind-driven rain that gets past the siding.

Jul 21, 2010 10:06 AM ET

Response to Curious But Ignorant
by Martin Holladay

Dear Curious,
You are right that condensation within walls is usually a sign of a defective air barrier. The moisture piggybacks on exfiltrating air (during the winter) or infiltrating air (during the summer) and condenses on cold surfaces in the wall. It's also possible for vapor diffusion to lead to condensation problems, especially in summer.

The point of this article is to raise a question concerning the need for two air barriers. In theory, if you have one good air barrier, you have stopped all air movement. In most cases the second air barrier is unnecessary. The exception would be a wall without exterior foam insulation that is insulated between the studs with fiberglass batts.

Jul 21, 2010 10:29 AM ET

by mike


word on the street is SIGA will soon be available in the u.s.

Jul 21, 2010 10:44 AM ET

Response to Mike
by Martin Holladay

Thanks for the interesting information. A little Web sleuthing resulted in the following contact information for the new U.S. distributor of SIGA products:

Albert Rooks
Small Planet Workshop
4646 Oyster Bay Road NW
Olympia, WA 98502
albert.rooks [at] mac [dot] com

Jul 21, 2010 10:47 AM ET

Air Movement
by John

In conventional construction I think the key is limiting air movement in loose fill insulation along with controlling air movement through the envelope. If the interior or exterior details make this sealing process to difficult then 2 barriers are necessary. Use vapor barrier (if desired for one layer) to keep the humidity where you want it and can control it. Does any one agree with that?

Jul 21, 2010 10:59 AM ET

Response to John
by Martin Holladay

I'm not sure what you mean by a vapor barrier. If you are using the term in opposition to vapor retarder, then I presume you mean interior poly. I usually recommend against interior poly in all but the coldest climates, because interior poly limits a wall's ability to dry to the interior.

To control vapor diffusion, all you need is vapor retarder paint. Remember, though, that a vapor retarder is not an air barrier. To create an air barrier on the interior of a wall assembly usually requires gaskets or caulk under the perimeter of the drywall.

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