Foil-faced batt insulation effect on wall construction?
MKalustov
| Posted in GBA Pro Help on
I’m in the process of planning energy retrofit for my house located in Philadelphia (zone 4A)
Existing wall construction consists of t1-11 over 2×4 framing with foil-faced batt insulation. The plan is to add 2 layers of 2” polyiso to the exterior taping joints making polyiso my WRB. Is there any concern that moisture trapped in the wall cavity won’t be able to dry to the interior due to foil faced batt insulation?
thanks!
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Michael, a warm wall has warm moisture which is vapor not liquid. Winter will be ok. Summer A.C. Season who knows.... Seal your exterior shearing well.... Before adding rigid foam.
Michael,
Your question is similar to a frequently asked question about adding exterior foam to a wall with interior polyethylene. I will cut and paste my stock answer below; just mentally substitute "foil facing" for "polyethylene," and the answer will work.
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.
If instead of 4" of polyiso you installed 3-4" of UNfaced Type-II EPS (1.5lbs per cubic foot nominal density) you'd have more than 0.5 perms of drying capacity toward the exterior which is WAY better than the 0.05 perms (best case) you'd have with foil-faced foam.
If instead of 4" of EPS you installed 3-4" of rigid rock wool you would have more than 25 perms of drying capacity toward the exterior. You would be counting on the vapor retardency of the T111 + paint to limit the condensation on the foil batt facer during the most humid of summer days, so be sure to air-seal it well.
If the windows are going to remain co-planar with the T111, and not the new siding, use a crinkle type housewrap between the T111 and foam. (Flat housewrap is fine, if rock wool.)
A 2 x 4 T111 wall with fiberglass batts comes in at a whole-wall R of about R9 after thermal bridging. Adding 4" of rock wool or EPS and a layer of rainscreened siding would bring that up to over R25, which is about the limit of what's financially rational in a zone 4 climate. Read chapter 1 of this document, and refer to Table 2, p.10.
http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
The additional R-value you'd get with polyiso instead of rock wool or EPS isn't really "worth it", especially when you look at the additional (however slight) risk that comes with a foil facer.
Thank you all for your responses.
Martin - now that you mention it, the similarity to interior poly example seems so obvious!
Dana – to tell you the truth I was starting to lean towards going with mineral wool previously and your post only reinforces that.
In my case if I go with the mineral wool would there be any particular benefit to using high end air barrier over T1-11 vs. your typical house wrap?
Thanks again.
Dana, and all, is not the problem summer humidity??? Winter weather should be no problem as there is no cold wall. Summer there will be cold foil if A.C. Is used.
Using permeable exterior insulation exacerbates the summer problem.
Note that foil-faced fiberglass on the interior has a little more drying ability than you'd have with a large sheet of polyethylene. It's hard to estimate, because it's a 2-D assembly.
Charlie, you too, is it not a summer AC concern and not a winter concern if any concern?
All of you seem to ignore this fact.
A.J.,
Wall assemblies that get wet in the summer (during the months when the air conditioner is running) suffer from inward solar vapor drive. In most homes, the plywood or OSB wall sheathing is enough of a vapor retarder to prevent this problem. If the wall assembly has a rainscreen gap between the siding and the sheathing, this gap also reduces inward solar vapor drive.
Walls with fiberboard sheathing or board sheathing are at a higher risk of inward solar vapor drive that walls with OSB or plywood sheathing.
Walls with EPS or polyiso on the exterior are well protected against inward solar vapor drive.
The mid-summer outdoor dew point average in PA is about 65F, with daily high dew point averages about 70F. While there are excursions where the dew point hit's the mid-70s, unless you are air conditioning to subtantially cooler than 70F indoors, there will not be significant condensation or moisture accumulation inside the wall cavities. This it true even if the sheathing were fairly vapor open or air-leaky. But it's not:
The vapor retardency of the T111 itself is about 1-perm when dry, but it's also painted. This is Class-II vapor retardency. While big air leaks need to be sealed and bulk-water management details such as lapping the window flashing properly to the WRB, the vapor retardency of the T111 is low enough that if summertime condensation were going to be a problem, it already WOULD Be a problem.
As long as you don't lower the drying capacity of the T111 to something substantially less than the exterior house paint with Class-I vapor retarders (foil facers) or tight Class-II vapor retarders (3" of XPS) , you aren't going to be increasing the moisture susceptibility.