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Should Insulated Concrete Forms be Air-Sealed?

Where airtight construction is a central goal, are the joints between ICFs a weak point?

Posted on Oct 4 2010 by ScottG

Roger Lin is planning to use insulated concrete forms in a house he hopes will meet the 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. standard of 0.6 air changes per hour at 50 Pascals (ACH50). ICFs are rigid foam building blocks stacked like Legos and then filled with concrete.

Lin has been told by ICF manufacturers they won't need air-sealing, but he's not so sure.

“I understand that ICFs with the concrete will be pretty airtight,” he writes in a Q&A post, “but I still see the joints where the ICFs meet as potential weak spots. Will I need to air seal them and how would you do it?”

The conversation that follows is as much about the Passivhaus standards themselves as it is about leaky ICF walls. Are the standards arbitrary and unreasonable rules promulgated by zealots? Or sensible construction standards that can work anywhere?

First, the air leak question
John Semmelhack is first out of the box with an assurance that a Passivhaus home in Wisconsin built with ICFs tested a very low 0.36 ACH50 with a preliminary blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. And that was without any additional air sealing of the forms. editor Martin Holladay agreed with Semmelhack, noting that “ICFs have a continuous monolithic concrete core that is an excellent 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..” He added a caveat: “Of course, there are many locations where careful air sealing is still required, including: around windows and doors, at the intersection of the walls and ceilings, and at ceiling penetrations.”

J Chesnut adds that a “very diligent” site foreman had a lot to do with the good airtightness results in Wisconsin, but it's not so much the walls that may be leaky but other parts of the structure.

“An important aspect to the air tightness achieved at the PassivHaus project in Wisconsin were the protrusion details, window-to-wall details and wall-to-roof detail,” Chesnut says.

The Optiwin windows were installed with double-sided butyl tape to maintain the air seal with the wall. “The butyl tape with some subsequent taping reinforcement during a blower door test for leakage proved effective,” Chestnut says. “What was found that didn't work as well as an air barrier was the spray foam between the window casings and the ICF wall.”

Well, how do you air seal effectively?
Jesse Thompson has had the same experience as Chesnut. Not only does a bead of spray foam between window and framing make an ineffective air seal, it's probably a lousy water seal as well. He suspects many leaks found in window installations on the Maine coast might rightfully be attributed to that.

“On a recent project, only the blower door test revealed that the spray foam wasn't tightly adhering to the ZIP tape wrapped around the OSB buck forming the window opening,” Thompson says. “The contractor added a bead of caulk to that interface, but the interior tape seal typical in Europe feels like a much safer method of insuring that we can achieve a pressure managed window installation that will hold up over time."

Garth Sproule wonders whether the airtight drywall approach (ADA) might be more dependable. At least, he says, it can be inspected and repaired, and it only requires a small bead of caulk where the drywall meets the window frame.

“ADA has always been more difficult to inspire confidence for us because you can't test your building enclosure for leaks until very late in the construction process,” Thompson replies. “We're much happier these days testing our building enclosures when there is still ample opportunity to repair mistakes.”

Thompson, in fact, thinks testing buildings early in the construction process is the “most drastic change in the residential construction process I've seen in my career.

“Looking back, it seems absolutely absurd to build a complex custom object like a house and have no verification process in place to ensure it's not leaky or defective,” he says. “What's the current typical practice, a plentiful supply of hope and faith?”

Is there such a thing as “too tight”?
David S. has seen the same kind of leaks during construction of an ICF house last year. Tested with a blower door right after the building shell was completed, the house had a leakage rate of 1.0 ACH50. When the crew looked into the problem a little deeper, they found most of the leaks were where window boxes met the ICF forms.

“The owner choose not to go back and seal after the blower door test to reduce the level from the 1.0 ACH50 test,” David S. writes. “When is tight, tight enough?” he adds.

That turns out to be the sixty-four thousand dollar question.

It is Robert Riversong who gets the ball rolling. “If the purpose of tightening up a new home is for efficiency and durability for the life of the house, then an inspectable and repairable air barrier, like ADA, makes sense,” he writes. “If, on the other hand, the purpose of getting a house tight is to meet some arbitrary standard, such as Passivhaus, without regard for long-term function, then exterior taped sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. methods are fine.”

Arbitrary, Thompson asks? The 0.60 ACH50 standard was developed by the Passivhaus Institut as the point where the risk of moisture damage via air movement in the walls was “reduced to a safe level.”

Furthermore, he tells Riversong, “You have a consistent habit of misstating what the Passivhaus standard involves, characterizing it as 'hermetic' and other pejorative language.”

Nonsense, Riversong tells him. “An honest critique appears 'pejorative' only to true believers who cannot tolerate criticism... I continue to assert that [Passivhaus] standards are not only arbitrary but unreasonable and excessive and is a natural progression of the movement toward isolating human occupants from the natural environment with which they evolved for millions of years and which keeps them whole and hearty and healthy.”

The exchange gets testy, but it's also a ringside seat for a debate over a key question in building: Just how tight is tight enough?

An expert's opinion
We asked GBA technical director Peter Yost for his take on these questions. Here's his reply:

1. Air tightening ICFs: Air tightness is always about penetrations, transitions, complex shapes, with window openings being right at the top of the list. To achieve Passivhaus levels of air tightness, dedicated continuous exterior and interior air barriers are strongly indicated.

2. How tight is tight enough or too tight? Homes should be easily coupled and decoupled from the exterior environment because in many (if not most) climates there are times when you want the outside in, and times when you don't. I am talking air here, not views or daylightingUse of sunlight for daytime lighting needs. Daylighting strategies include solar orientation of windows as well as the use of skylights, clerestory windows, solar tubes, reflective surfaces, and interior glazing to allow light to move through a structure.. Who wouldn't want to shut off the HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. /ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. and open windows as often as they can if it's pleasant outside? Here in southern Vermont, we pretty much shut off our whole-house ventilation system from about May through September because we want the house coupled to the outside just about every night and many days.

3. The Passivhaus air tightness standard was developed for a particular climate and it does not necessarily transcend climate as a variable. Keeping walls safe from moisture issues varies by climate and by interior conditions, as managed by the occupants. Too much variation built in for a single air tightness level to be the “safe” level. Having said that, you can't build a house too tight, period. But the more we ask of our walls in terms of managing and restricting the flows of energy and moisture, the better our designs and installations need to be. Materials and components can't rise to the challenge on their own.

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Image Credits:

  1. Phil-Insul Corp.

Oct 4, 2010 11:08 AM ET
by lance williams

Seems to me the Insulated Concrete Form Association at might have details on air sealing typical ICF to framing connection points and penetrations. I took a quick look at this section and did not find anything that directly calls out air sealing details. Perhaps there is a resource for air sealing when using ICF's?

Oct 6, 2010 4:14 PM ET

The interior of an ICF wall
by Mark Miller

The interior of an ICF wall is required by code to be covered with a fire rated barrier, i.e. sheetrock, in most cases. Doesn't sheetrock create the necessary air barrier?

Oct 6, 2010 6:16 PM ET

I will never understand why a
by Lloyd

I will never understand why a polystyrene and concrete sandwich would ever be considered green building. ICFs do not belong on a site called "Green Building Advisor."

Oct 6, 2010 9:15 PM ET

ICF window and door sealing
by Jake

Amvic ICF has a very effective window and door buck which greatly improves the air sealing of ICF window and door openings. The Amvic Buck also makes the windows and doors much more resistant to water infiltration and simplifies bracing the windows and doors during construcition.

To allow a better understanding of why ICFs are green relates to ICFs energy savings which have proven to be exceptional and their structural sustainability, concrete lasts for 1000's of years.

Oct 7, 2010 6:19 AM ET

air tight construction
by Fred

I have been reading all the discussions on a super air tight house and frankly see it as a waste of time and money. After all the expense the building code in our part of Canada requires installation of an air exchanger. Yes it is controlled air changes in the house but it is still blowing heat outdoors and bringing in fresh air which I might add then requires humidity controll quipment. You do not want to have to wear wool socks around the house to keep your feet warm from all the drafty walls and windows but you also do not need a house so tight you can smell a fish for a week.

Oct 7, 2010 6:45 AM ET

Smelling fish for a week
by user-756436

If you can smell fish for a week in your house, it's either time to clean out the refrigerator, or time to increase the ventilation rate on that HRV you disabled in the basement.

Oct 7, 2010 7:43 AM ET

Response to Mark Miller
by user-756436

Mark Miller,
In an ICF home, the drywall doesn't add much in terms of airtightness, since most air leaks in an ICF home are around windows and doors and at penetrations -- problems that drywall doesn't solve.

Most drywall installations are not airtight. To detail drywall for airtightness requires following specific steps that most drywallers don't take. The method is called the "Airtight Drywall Approach." My article on the Airtight Drywall Approach appears in the current issue of Fine Homebuilding magazine (#214). You can read it here:

Oct 11, 2010 12:35 PM ET

"Arbitrary" Air-Tightness
by Graham Irwin

My understanding of the Passive House air-tightness level is that it was developed as being the minimum that was "feasibly" achievable. In most of the recent North American projects with which I'm familiar, the builders have not only found it relatively easy to achieve this standard with diligent focus on the construction process, but in most cases have vastly succeeded it by at least 50%! Futhermore, every one of these builders I've talked to is convinced that this process is not only easily achievable but that going back to previous techniques makes no sense

The PassivHaus Institut is purportedly considering TIGHTENING the standard and it is currently recommended that in challenging circumstances, increased air-tightness be adopted as an energy conservation strategy. As Peter Yost states, there is no benefit to leaky building assemblies, so 0 ACH50 is the theoretical ideal (with theoretical emphasized.)

It IS important to distinguish between preventing air infiltration (which actually causes moisture problems more than anything else) and allowing drying - as a colleague of mine, Kris Knudson, cleverly remarked: "walls don't need to breathe, but they do need to sweat!"

Oct 11, 2010 12:48 PM ET

Response to Graham Irwin
by user-756436

You may be right about the origin of 0.6 ACH50. But every time that Dr. Wolfgang Feist is asked about the origin of the Passivhaus airtightness standard, his account differs from yours.

This is what he said in a British interview (available at this site: ):

Q. “The Passivhaus standard has a maximum air leakage of .6 air changes at 50 pascals. … In a warmer climate such as the UK, slightly warmer than Germany, would it be possible to raise this?”

Fesit: “No. Not at all. The airtightness is one of the things that we really have to stick on in almost all climates. There are only a few climates where this might not be a [requirement], but very few -- like in San Francisco. In San Francisco you might not need to have it airtight, but in almost all other climates you need that. A major part of the airtightness requirement is to avoid structural damage. You have bad indoor air with humidity, and if there is an exfiltration through the construction you get really big problems of condensation in the structure. This is the major reason to make it completely airtight, and even in subtropical climates and of course in tropical climates, it has to be airtight because you get structural damage without airtightness.”

I think that the airtightness standard is a good one, but I think that Dr. Feist is wrong about structural damage in houses with more leakage -- for example, 1.2 ACH50. It's perfectly possible to build a house in many climates with a leakage rate of 1.2 ACH50, without the house being structurally endangered.

Moreover, some types of walls -- like ICF walls -- are virtually immune from structural damage, even if the walls include leakage points. His point really only refers to buildings with wood framing, vulnerable sheathing like OSB, and air-permeable insulation like fiberglass.

Feist's insistence on this point undermines the logic of the Passivhaus standard and encourages critics to conclude that it is arbitrary.

Oct 11, 2010 1:30 PM ET

Every time?
by Graham Irwin


I have seen also this video, and as one who has been interviewed myself, I would say it is challenging to subject someone's ad-libbed responses to interview questions to the same scrutiny as a written piece. To my interpretation, the first part of this response is in respect to energy efficiency, specifically meeting the Passive House Standard. As someone who has done extensive Passive House design in the San Francisco Bay Area, I can state that it is definitely tremendously and dramatically helpful here as well! Dr. Feist was speculating on our climate and perhaps doing some "California dreaming on a winter's day" when he said it "might not" be necessary. ;-)

His comment, IM(H)O, then goes on to discuss the structural impact of air infiltration. As you are well aware, air infiltration is by far the greatest source of moisture introduction into building assemblies. Moisture = rot if the materials can rot, but it also equals mold in any material that can support it, so I wouldn't want ICF walls that were saturated either, even if they would be "extremely durable mold farms." Further, as the article suggests, the main leakage points in the ICF building are around windows and doors, which are more sensitive to rot and moisture damage than concrete.

Additionally, there is the fact that effective ventilation cannot be achieved with a leaky building. Even if one installs an efficient ventilation system, it is rendered ineffective by a leaky building.

Lastly, the business of "optimizing" air-tightness is a tricky one. Most of the practitioners I know have a good overall strategy and a set of best practices, and they apply and then test them - they don't calculate how leaky the end result will be, they strive for a good result. As I alluded to previously, we're seeing a lot of buildings in the range of 0.4 ACH50 without undue heroics.

Oct 11, 2010 1:45 PM ET

Re: Every time?
by Anonymous

I'm just curious about the statement that "effective ventilation cannot be achieved with a leaky building." How's that? Do you mean "efficient mechanical ventilation," "adequate ventilation to prevent mold growth," or what? I'm asking because it would seem that the less air-tight a building were, the less would be required from its mechanical ventilation system. I'm not suggesting that that's necessarily a good thing, only that the original statement sounds contradictory.

Oct 11, 2010 4:14 PM ET

Effective = Predictable & Consistent
by Graham Irwin

By "effective," I mean ventilation that is tied to the actual needs of the building occupants, not controlled by prevailing winds, exterior temperature, size and nature of the holes in the shell, height of the building, climate zone, etc. None of the factors that create "natural" aka "random" ventilation are positively correlated to the fresh air requirements. A correct amount of fresh air may be delivered on occasion under certain circumstances, but it is nearly impossible to predict. For further information, read this article:

On top of that, the energy losses are generally enormous and air leaking into buildings is a primary cause of moisture-related building failure. I call this "ineffective" by any measure!

Oct 11, 2010 5:50 PM ET

ICFs are designed exactly wrong
by Rolf Pechukas

this discussion misses the central point:
for *passive solar* construction, which is presumably what 'PassiveHaus' standards are all about, we want MASS that SUN can actually HIT -
all the mass in the world will do you no good if there is insulation on the INSIDE of it, which is how ICFs are designed -
a better design would have thicker foam on the outside, and a thin, structurally-sufficient-for-the-pour but thermally-conductive material on the inside - like cement backer board -
actually, the ideal material would be some kind of magnesium oxide panel - eg. Dragonboard - because MgO tends to mitigate the ill-effects (on people) of concrete as an internal finish material (ion depletion, radon exposure, etc) -
but with a large quantity of mass INSIDE the insulation envelope, internal temperatures are very effectively moderated, and any form of heat the hits the walls - sun, cooking, bodies - gets absorbed, distributed by conduction, and gently re-radiated -
and when there is a large body of mass, the issue of outside (winter) air becomes MUCH less important -
gasses are comparatively MUCH less dense than solids - if you have many tons of rock and sand and cement (or clay) in your walls, floors (and even ceilings), then you can have doors and windows wide open in February and still feel toasty warm
radiant heat with a lot of mass is a big fat elephant; a few cracks around the windows is a tiny straw hat --- no contest

Oct 11, 2010 7:11 PM ET

Passive House is not passive solar
by Graham Irwin

Passive House construction certainly utilizes passive solar principles and gains, but it is much more aligned with the super-insulation approach. That is, a very efficient shell which benefits from solar heat gains, but also from low losses and "recycling" of internal heat gains. Thermal mass is of benefit, but mainly in summer, where night cooling can be used to provide daytime comfort. An analogy between a highly glazed, high mass passive solar building and a Passive House building is that of a sieve vs a thermos bottle - the passive solar building has wide daily swings of heat gain and heat loss, with mass intended to mitigate. The Passive House is more subtle, favoring smaller external losses and gains, hence the decreased need for thermal mass. Again, not a bad thing at all, but not required. Nor, for that matter, is optimal orientation or "passive solar" architecture.

Oct 11, 2010 8:15 PM ET

Another example ...
by user-756436

Once again, a confusion arises between "Passive House" and "passive solar house" ...

One more piece of evidence supporting my contention that we should all be referring to the European standard as the "Passivhaus" standard -- in hopes of minimizing confusion....

Oct 11, 2010 9:56 PM ET

Detailing ICF Construction
by Vera Novak

Scott, thanks for "airing' this topic, and thanks to Martin for clarifying the question actually being that of airtightening the ICF construction vs the installed ICF, since the concrete is airtight. As previous Technical Director of the ICFA and now PhD Student in Construction, I couldn't resist posting a full description of the points of detailing the construction on my current blogsite:

A few responses to other comments: re: "green". I would rather use oil to make EPS foam which maintains its insulating properties indefinitely and can avoid the burning of oil (or coal, etc) in the heating of the building for many years to come.

Passive Solar - the point of a super-insulated house is to flatten the demand for heating and cooling, so the remaining delta can easily be handled with a few areas of exposed mass in full sun exposure. This approach avoids the great temperature swings and overheating of the 70's passive solar experiments.

Re: How much to airtighten? This isn't really a question of ventilation standards, but the economic/ embodied energy balance of investing material/time/money in the envelope vs into renewable energy standards. Stinking fish are bad no matter what....

Re: Working on window detailing as an Industry. We did start, with the ICFA, to work with the ASTM standards committee to develop some of these details. But associations need membership to be operate - so ICF contractors out there - join the Association!!

Cheers - always thoroughly enjoy the discussions at this site

Oct 20, 2010 11:50 AM ET

a polystyrene and concrete sandwich
by Jeff Rempel

Thanks Scott for an excellent article.

I'd like to address Lloyd's early snippy comment regarding 'a polystyrene and concrete sandwich being sustainable.' While his point has merit, he is simply commenting on the immediate environmental cost of the product, not the long term benefit. While one ton of concrete takes approximately four times the energy to create than a ton of spruce lumber, (Gordon, Ashby and Jones CUED 1984) that isn't the end of the discussion.

ICF's gain their merit by virtue of a 'safer' post construction envelope. This means not only reduced structural and mold issues, but more importantly, longer building life. With the cost of construction doubling every 7-10 years, investments in products that increase the longevity of our homes are potentially far more valuable than those that simply mitigate immediate carbon.

For another take on this concept, check out They have what I think is a brilliant solution to the real issue at hand.

Nov 26, 2010 7:17 PM ET


I agree with Graham. In stick-framed buildings it is feasible to get to 0.4 with minimal incremental cost vs. 1.5ACH@50 (Canadian R-2000).
Tighter buildings = less energy heating infiltration air. And with HRV's, there's no such thing as too tight.

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