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Community and Q&A

Do you need to air seal ICFs?

Roger Lin | Posted in Green Products and Materials on

We are planning a house using ICFs. We are building to the Passive House standard of 0.6 ACH at 50 Pascal. ICF manufacturers have told me that we don’t need to air seal. I understand that ICFs with the concrete will be pretty air-tight 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?


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  1. User avatar
    John Semmelhack | | #1

    Hey Roger,

    I think Tim Eian used ICFs without any additional air sealing on the forms on his Passive House project in Wisconsin.

    They tested at 0.36ACH50 on their preliminary blower door test.

  2. Roger Lin | | #2

    Thanks John! Also Thanks for all the great advice on Sunday.

  3. Riversong | | #3

    In any block wall, whether CMU or ICF, the joints are the weak point in terms of air tightness. A monolithic wall has no joints. I would advise looking into the ThermoMass wall system, which places up to 4" of XPS in between two 4" whythes of concrete. This not only creates an air-tight wall but also places a capillary and thermal break in the centerline so that you still retain the dynamic thermal mass advantage of interior insulated concrete, and keeps the XPS out of reach of insects, UV and physical damage.

    I used ThermoMass for a walkout basement on my latest super-insulated project in VT. So far, I'm quite impressed. No additional cost over an exterior insulated concrete foundation, but several advantages.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Robert and Roger,
    Although you compare an ICF wall to a CMU wall, Robert, they differ. While a CMU wall has many mortar joints, most ICFs have a continuous monolithic concrete core that is an excellent air barrier.

    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.

  5. Roger Lin | | #5

    Thanks. I will also look into thermomass wall systems. It's always good to explore options prior to committing any $. So far, I love the prospect of using ICFs due to that continuous monolithic concrete core. I think the ICFs forms, the concrete core and a 3 " EIFS system and finally a interior gypsum should provide an air-tightness required by PH. I just don't want to leave anything to chance.

  6. J Chesnut | | #6

    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. The project had a very diligent site foremen which was important. The project used Optiwin windows whose installation includes a 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. 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.

  7. Jesse Thompson | | #7


    We've been seeing this as well, that the typical spray foam bead in between a window unit and the framing doesn't actually air seal very well. I also suspect it doesn't water seal very well either, and that many of the leakage problems we see on coastal window installations here in Maine might be related.

    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. 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.

    We have been noticing many pin-hole air leaks where the spray foam expands around voids, even with careful installation.

    Don't get me started on US window flanges and their inherent problems...

    Jesse Thompson
    Kaplan Thompson Architects

  8. Garth Sproule | | #8

    Have you ever considered using the ADA approach with drywall returns to the window frame? Seems like it might be a more dependable, inspectable, and a more repairable solution. It will require only one small bead of caulk where the drywall meets the window frame.

  9. Jesse Thompson | | #9

    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. We're much happier these days testing our building enclosures when there is still ample opportunity to repair mistakes.

    Honestly, early stage building enclosure testing been 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. What's the current typical practice, a plentiful supply of hope & faith?

    Jesse Thompson
    Kaplan Thompson Architects

  10. Garth Sproule | | #10

    I see your point about testing for air tightness early. However, down the road a few years, if a piece of tape loses its grip, or a bead of caulk or foam loosens, you have no way to detect where a problem is, and secondly, no way to fix it.

  11. Jesse Thompson | | #11


    That is indeed a concern, but a bigger concern for us is if the tape or caulk is even there in the first place. In my opinion we need to get that part of construction correct first more than we need to worry about the latter.

  12. Riversong | | #12

    Tape and caulk are, at best, temporary solutions to air-tightness or water-tightness. Tests, for instance, on several varieties of tape on the housewrap diagonal slits required over windows by AAMA installation method B failed under moderate water pressure.

    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. If, on the other hand, the purpose of getting a house tight is to meet some arbitrary standard, such as PH, without regard for long-term function, then exterior taped sheathing methods are fine.

  13. Jesse Thompson | | #13


    The 0.6 ACH50 that is part of the Passivehouse standard was not arbitrarily chosen. It's the level of shell airtightness at which the Passivhaus Institute determined that the risk of moisture damage from air movement through walls of varying construction methods was reduced to a safe level for the life of the building.

    It's a construction quality standard as much as anything else.

    You have a consistent habit of misstating what the Passivhaus standard involves, characterizing it as "hermetic" and other pejorative language. For example, if you look at this page you'll notice the PHI recommends an interior air barrier for cold climate buildings, just like you do. If you can achieve 0.6 ACH50 with air-tight drywall, go right ahead, they'll happily certify your buildings with that technique:

    Jesse Thompson
    Kaplan Thompson Architects

  14. Riversong | | #14


    An honest critique appears "pejorative" only to true believers who cannot tolerate criticism.

    Of course the PH ACH is arbitrary, since no single standard can apply to all climate zones or all construction types.

    We all (should) know that, with increasing insulation levels (and decreasing energy flux through the envelope), it becomes more important to manage moisture. This requires source elimination, whole house ventilation, much more air-tight envelopes than has been customary, and - the element most commonly overlooked - materials that can store and release excess moisture and can tolerate routine fluctuations in relative humidity. Depending on climate zone, an important additional consideration is the relative vapor permeance of the inside and outside skins.

    Another variable to put into the mix is the type of whole-house ventilation. With a "balanced" system, it is more important to get the envelope as air-tight as possible because any natural leakage will unbalance the system and create a positive pressure zone in the upper section, causing winter time exfiltration, which is the air leakage that can result in condensation.

    With an exhaust-only ventilation system, there is no positive pressure zone and the only air leakage can be infiltration, which cannot - in winter - cause condensation, so extreme levels of air-tightness are not required.

    If one wants a home to be truly passive, then it should also be able to exchange some minimal amount of air when the electrical power is down (a PH is not a passive house), such as is possible with the passive make-up air inlets of an exhaust-only system. Yes, there is some energy efficiency compromise without energy recovery ventilation, but keeping a house truly passive and healthy for its occupants is a higher priority for some.

    I continue to assert that PH 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.

    Humans are not designed to live in terrestrial space capsules with life support systems. That's a fact. By creating hermetically-sealed shelters (that's an accurate descriptor), we are removing ourselves from our real life-support system. That, too, is a fact.

  15. Jesse Thompson | | #15

    First, thank you for charmingly accusing me of zealotry and not tolerating criticism, it's a wonderful conversation technique. To continue the conversation:

    So why can't a single air tightness standard apply to all climate types or construction standards? Nothing in the following paragraphs (most of which I agree with, by the way...) explains why you have come to this conclusion.

    What we're down to at this point is that Robert Riversong has decided that with two houses sitting side by side, one built to an 0.6 ACH50 standard, one to 2.0 ACH50, but otherwise identical (vapor open construction methods, mechanically ventilated, etc): one is hermetic, isolated from the natural environment and a dangerous terrestrial space capsule, but the other is humane and will keep people whole, and hearty and healthy.

    That's, frankly, absurd.

    Again, all of the buildings we have designed and built that meet this air-tightness standard have windows that seem to open widely and let breezes and air and the world outside flow happily through them, but close up in the winter if the owners desire, just like all the houses that surround them. And in winter, just like your houses, they have a mechanical fan in a metal box with some kind of controller, that is connected to the outdoors with ductwork.

    But you seem to have decided fairly arbitrarily that one form of a fan in a box connect by ductwork to the outside is acceptable (exhaust fan), but a different type of fan in a box connected by ductwork to the outside is not (HRV, ERV). To me, the differences seem minor in the grand scheme of things.

    Again, nothing in the Passivehouse standard precludes using natural building materials, vapor open construction methods or all the other things you recommend. In fact, if you got out more, you would discover that all those thechniques are recommended by the US Passivehouse organization. Except the buildings they construct use less energy than the buildings you build...

    Jesse Thompson
    Kaplan Thompson Architects

  16. Riversong | | #16


    I didn't "accuse" you of anything, and your continued defensiveness speaks poorly of your ability to consider these issues objectively. I simply stated an obvious truth: that those who insist on perceiving rational analytical criticism as "pejorative" are those with some kind of vested interest or limited perspective or bias or ignorance. For instance, there is nothing whatsoever "pejorative" in a descriptor like "hermetic" as it applies to a highly-sealed and isolated environment.

    If you don't understand why different air-tightness standards are appropriate for different climates and construction systems, even after my explanation, then there's probably little more I can add that will convince you.

    But, for those who are open to a broader understanding: since winter exfiltration presents the condensation and mold/rot problem that we're all trying to avoid, and since the greater delta-T and delta-vapor pressure the greater the problem, air-tightness becomes far more crucial the colder the climate. Different climates, different standards - just as there are for R-values.

    In hot/humid climates, it's infiltration which presents a condensation issue. But the indoor air is not more buoyant than outdoor air, so stack effect leakage is not the problem. Negative indoor pressure and wind pressure is the problem. So exterior skins need to be able to resist normal wind pressures, and the interior environment cannot experience negative pressure, but should always be slightly pressurized. A well-designed HVAC system takes care of this, not a super-tight envelope.

    Additionally, if the reason for air-tightness is primarily the durability of the structural envelope, then the use of hygroscopic (moisture buffering) and moisture tolerant materials can ensure this goal far better than uber air-tightness, which can not be guaranteed for the life of the house.

    If you insist on fabricating a straw man in order to cut it down, then feel free to continue misrepresenting my arguments. I never stated that my houses and PH's fall on two ends of a spectrum - one completely open to the environment and the other not. I would place a teepee or wigwam or igloo or perhaps a log cabin at the opposite end as the PH. But I will insist that my houses have gone about as far as is reasonable in the direction of air-tightness and environmental isolation (likely too far), and that the PH movement has taken a trend to its inevitable extreme, based on the logically false notion that a good thing is made even better by taking it farther away from the norm. There is a vital difference between optimal (which is always a moving target, difficult to pin down) and maximal.

    What is truly absurd is your argument that PHs have windows and doors that can be opened to undermine the extreme source energy consumption and air-tightness requirements of the PH system. If the windows are opened in winter, then it's arguably no longer a PH.

    I agree that there is not a vast difference between a bathroom exhaust fan in a box and an HRV/ERV in a box, at least until you add centralized ductwork in addition to the simple one-directional exhaust duct. There IS, however, a significant difference between a system that fails when the power is down and one that operates passively to a minimal failsafe extent without power. And there is an even more significant difference between a house with a central grid-dependent ventilation system and one (such as I've built) that uses the wood stove as the passive exhaust system, creating the negative pressure which not only protects the structure from condensation but also draws fresh air in through the passive inlets.

    Even were there is not a major difference between two technologies, it's long been an element of wisdom that the simplest solution is the best one. Occam's razor.

    And taking extreme measures to reduce home energy consumption to the absolute lowest value is a perfect example of using the same mindset that created the problem in order to attempt to resolve it. If we were to live wholesome and responsible lives, living in very small and simple shelters that have minimal impact on the earth (considering all impacts and not simply operating energy), and were not addicted to high-tech concentrated fossil energy (and high-tech everything) that destroys the environment at every phase of its development and use, then building PH's would be considered the height of absurdity.

    We don't need to conserve fossil energy. We need to stop using it and return to using only the daily solar energy that the Universe has provided for our use and pleasure - primarily photosynthetic energy converted by our cousins, the green people.

  17. Riversong | | #17

    What we're down to at this point is that Robert Riversong has decided that ...

    I should add that, to use indirect or subtle ad hominem argument by personalizing my statements rather than arguing the factuality and logic of them, simply emphasizes the paucity of your disputation.

    My arguments stand on their own merits, regardless of who makes them. They would be equally valid even if you had stated them ;-)

  18. Jesse Thompson | | #18

    Someone asked me if I'd hit my Riverwall :) I thought that was fairly humorous...

    A few notes:

    Again pointing out agreement in the building physics, the PHI also recommends increasing building air tightness as well as climates grow colder, except they hold 0.6 ACH50 as an upper limit and recommend dropping below that number in what they consider very cold climates (>7,500 HDD or so). Similar understanding of building physics, different recommendations.

    To the point of stopping the use of fossil fuels, the energy consumption levels mandated by the standard were developed as a direct response to the Factor Ten and 2,000 Watt Society research, with the goal of determining how large an energy budget each one of us should receive if our goal is planetary equity. This let to the target of a 90% reduction in heating energy use, which led to the 15 kWH/m2/yr baseline. The goal all along was to develop a construction method that would permit affordable fossil-fuel free living for all. That's a very difficult challenge, and we will certainly develop many different methods for achieving it.

    Single family homes in rural areas with plentiful firewood will necessarily be built with different construction techniques than multi-unit dwellings in urban areas. There are many situations where building to the Passivhaus standard does not make sense, but in my opinion their analysis of building physics and energy equity has been a significant addition to the conversation.

    Back to sealing ICFs...

  19. David S | | #19

    Getting back to the question.. I finished an ICF house last fall and did a blower door test of the shell right after it was completed. We tested at 1.0 ACH50 and went around the structure and found the majority of the leaks to be at the interface of the window boxes and the ICF forms. The window boxes were constructed of 3/4" PT ply ripped to 11.25" (width of the ICF Wall) then framed on the inside, with 2x4's DF, flush to the inside and flush to the outside, this gave us a none PT surface to nail our window flanges and interior trim to. The space between the 2x4's was filled with 1.5" foam board. Scraps of 3/4 PT ply were ripped at 15 degs to form a dove tail, and screwed to the outside of the boxes to form keys that tied the boxes to the concrete.
    The air leakage was at the interface of the 2x4's and plywood and the plywood and the concrete. Next project we will run our keys the full length of the boxes and apply sealant at the 2x4, plywood interface. The owner choose not to go back and seal after the blower door test to reduce the level from the 1.0 ACH50 test, for many of the reasons above, when is tight, tight enough?

  20. Riversong | | #20

    when is tight, tight enough?

    One of the most important questions!

    It is often found to be true that more of a good thing is not necessarily better.

    In other words, maximum and optimum are quite different standards.

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