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Add exterior foam — but (woops) poly sheeting?

KyleBH | Posted in Green Building Techniques on

This is a follow up to my question from yesterday – “Add exterior foam — but paper-faced batts?”

Well, at the advice of my builder I cut some test holes and darn.. poly sheeting over the batts, not kraft paper as I thought.

So the same question applies – what are my options, if any, for adding exterior insulation during this window replacement project?

Thanks again.


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  1. iLikeDirt | | #1

    In that case, it's important that you preserve the wall's exterior drying potential. That means unfaced EPS or preferably rigid mineral wool boards. With EPS, you're thickness-limited to preserve adequate permeability, but with mineral wool, you can go as thick as you want.

  2. GBA Editor
    Martin Holladay | | #2

    Your question is asked frequently. I will cut and paste my standard answer:

    Many energy experts worry that it may not be a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly weren't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

    That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

    If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

    To summarize, here are four caveats:

    1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

    3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

    4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

  3. Expert Member
    Dana Dorsett | | #3

    If you're the type of person who would lose sleep over it, rigid rock wool would be a better choice.

    If going with foam, 1-lb density "Type-I" EPS has a vapor permeance of about 4.5-5 @ 1", so even 3" (R11.5 ish) it would run about 1.5 perms and would not be a major impediment to drying. While it's more fragile to handle than 1.5lb "Type-II" EPS, the latter is about 2.8-3 perms @ 1", but even there you'd be OK up to about 2" (R8.4). At 3" it would be less than 1 perm but it would also be R12.6, and if that crosses the dew-point control threshold for you it's probably better to go with 3" rather than 2".

  4. KyleBH | | #4

    One last follow-up. Why don't I just peel the facing off of the 3" polyiso that I currently own? It appears (although I can't find a definitive source) that unfaced polyiso is quite permeable. Since I'm only doing one wall of my house, and the facing peels off quite easily, this seems tedious but doable.

    Why not?

  5. GBA Editor
    Martin Holladay | | #5

    You can do that if you want. However, the facing on polyiso helps retain the blowing agent bubbles, thereby improving the insulation's thermal performance.

  6. Dana1 | | #6

    The total change in R value from polyiso's labeled long-term (15 year average) value and the fully-depleted of blowing agent value is about 10%, which isn't really much in the grand scheme of things. The initial R value is quite a bit higher than it's labeled value, but even with foil facers it's really only the first 2-3 years where it makes an appreciable difference. For 1-lb or 1.5lb density polyiso you're looking at a fully-depleted R between R5.5-R5.8 per inch at a mean foam temp of 75F, with a 30F delta-T (ASTM C518 test conditions.)

    For dew point control design purposes with exterior foam the wall stackup and climate are both important. When the mean temp through the foam is 55F or so it's performance is higher than labeled, but when it's in the 30s and lower it falls off considerably, bottoming out around R2.5-3/inch at true arctic conditions. So while it's average performance is pretty close to labeled performance in US climate zone 4 or lower, in zone 7 you have to take the derating factors seriously. With 3" of foam on the exterior of a 2x4 wall with R13-R15 cavity fill you'd be in pretty good shape up through zone 6 from a dew point control perspective without interior vapor retarders, but maybe not in zone 7. But with the poly vapor barrier on the interior the performance hit matters less.

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