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Is rigid EPS insulation an effective air barrier?

Steve_Vollstedt | Posted in Green Building Techniques on

The reason for my question is that ENERGY STAR states that rigid EPS insulation cannot be used as an air barrier to address thermal bypass unless it is at least 5-1/2″ thick.

I posed the following question to ENERGY STAR:

Footnote 6 is referenced from Item 3 – Fully-Aligned Air Barriers section of the TES Checklist. My understanding is that rigid EPS foam board is an open cell product. I have builders that routinely use 1″ and 2″ EPS foam board as an air barrier in places such as the attic side of ceiling walls where the ceiling heights change from one section of the house to another. This foam board is foamed in around the perimeters with a canned spray foam product (Great Stuff). My reading of footnote 6 is that the EPS foam board will not qualify as an air barrier in this kind of thermal bypass application unless it is at least 5-1/2″ thick. Is my understanding correct?

ENERGY STAR’s response (by email) is below:

Thank you for your question. You are correct in your interpretation of Footnote 6 of the Thermal Enclosure System Rater Checklist, which states:

“Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 in. or 1.5 in., respectively, to qualify as an air barrier unless the manufacturer indicates otherwise.”

Unless the EPS foam board you use is qualified by the manufacturer as being an air barrier, you must install an air barrier in addition to the EPS foam currently being used.

Thank you for your continued support of the ENERGY STAR for New Homes program.

Best regards,

The ENERGY STAR for New Homes Team

To me the test on an effective air barrier is not being able to blow air through it if you hold it up to your mouth and blow hard; and that it be durable and hold its shape.

Does this make sense???

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  1. GBA Editor
    Martin Holladay | | #1

    Here's why the Energy Star program is wrong: EPS board is a closed-cell product, not an open-cell product. That's why blocks of EPS are used as flotation devices for docks. It's waterproof. Did you ever notice that it is used to make coffee cups? The coffee doesn't leak out.

    Of course EPS is an air barrier. The problem is the seams. EPS can be hard to tape, although that problem can be solved by using foil-fraced EPS. Or you can use canned spray foam to seal the seams.

    1. Turgz | | #7

      Eps is a open cell matrix into which every cell is a closed cell. There is very smalls cavities between each cells, too small for water but not enough for air and moisture (vapor)

      XPS are closed cell form into a closed cell matrix.

  2. d9mweRDYS6 | | #2

    I am not a professional builder, but I have had experience where I used open cell Expanded Polystyrene for several different applications, including shipping dry ice. EPS is manufactured by basically blowing up little balls of the stuff in a mold, it starts out dense and ends up "fluffy". Since it is not a continuous sheet of material (like XPS foam) but rather a matrix of material there are always gaps, very small ones, that work well for a short period of time (ie coffee cup) but not a long time. We are able to ship dry ice for this reason without having to worry that we will blow up the package en route, the gas escapes through these gaps.

  3. Gregory La Vardera | | #3

    Maybe I'm reading this quote differently than you:

    "Open-cell or closed-cell foam shall have a finished thickness ≥ 5.5 in. or 1.5 in"

    They are naming two kinds of foam in series - open-cell or closed-cell, and then giving the respective thickness in series - 5.5 or 1.5.

    I read that as Open-Cell foam you need 5.5"; closed-cell foam you need 1.5".

    EPS is closed-cell. 1.5"

    Does that clear it up?

    1. Turgz | | #8

      EPS is expanded aka open cell. So 5.5"

  4. ErgoDesk | | #4

    EPS Foam after all the years it has been around is still not understood. Perhaps the most beneficial building material on earth we should start getting to know it better.

    Some moisture can be absorbed into any open cells that surround the surface that have been disturbed buy handling or cutting but mostly the cells act as a barrier.

    More information can be found on using EPS Foam in Construction at:

  5. Steve_Vollstedt | | #5

    It seems that I incorrectly believed EPS foam board was an open cell product rather than closed cell. If that is the case, it IS appropriate for my clients to be using EPS foam board an an air barrier.

    Thank you to all who have enlightened me.

    Steve V.

  6. albertrooks | | #6

    As far as EPS barrier goes, SIGA Wigluv tapes to EPS to create an air barrier just fine. we are consulting on a development in the PNW that will use Wigluv to seal EPS sheathing and hit the Passivhaus 0.6 ACH 50. 

    Since this is the first development that is trying to hit the PH infiltration level, and... use only EPS as sheathing with only limited OSB shear panels, it will be extremely interesting to see the performance.
    I'm confident that it will work just fine. The tape to EPS connection is not an issue. It's just the rest of the joints that need to be thought out in a production environment. Great challenge!

    I have personal issues with EPS since I'm such a "wood head", but an affordable super-insulated price point development with a developer that currently has a really strong sales record is interesting...

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