Air sealing and rim joist insulation
First off, thank you everyone for the all the helpful answers I’ve received here.
This may be a pretty basic question, but how is air sealing normally accomplished at the sheathing when using OSB with no exterior foam? Can it be taped? Glued?
This may be a tough one to answer because I’m sure there isn’t a one size fits all answer, but how much air sealing needs to be done to get down to say, less than 3 ACH @ 50 Pa? If my air barrier is located at the sheathing and other framing members, is there much of a benefit to trying to seal some potential leak sources in the drywall (ie, a continuous air barrier outside, but a non continuous one at the drywall)?
I also have a question about insulating and air sealing at the rim joist when using open web floor trusses. According to http://ductsinside.org/
“One consequence of this approach is that the space between floors is now extremely vulnerable to air leakage and vapor condensation. It’s essential to protect the entire rim joist. This can be done by covering the entire rim with high density spray foam, which combines the properties of insulation, air barrier and moisture barrier. Another common approach is to press fiberglass insulation into the joist bay, and then cut a square of rigid insulation (usually extruded polystyrene or foil-faced urethane) and fit it between joists so that it covers the fiberglass. Finally caulk around the rigid insulation to prevent moisture-laden air from reaching the cold rim joist. Both methods will seal and protect the building. “
I’m a bit confused as to how the second method (fiberglass insulation + rigid foam) would work. The open web floor trusses are, as the name suggests, open. If there is fiberglass between the rigid foam and the rim joist, wouldn’t the rigid foam be set back to where the sides would be open?
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Replies
Michael,
Q. "How is air sealing normally accomplished at the sheathing when using OSB with no exterior foam? Can it be taped? Glued?"
A. The usual method is to use tape (for example, Zip System tape or Siga Wigluv). For more information, see:
Airtight Wall and Roof Sheathing
Return to the Backyard Tape Test
Q. "How much air sealing needs to be done to get down to say, less than 3 ACH @ 50 Pa?"
A. Once you have your blower door set up and running, you'll know the answer to that question.
Q. "If my air barrier is located at the sheathing and other framing members, is there much of a benefit to trying to seal some potential leak sources in the drywall (ie, a continuous air barrier outside, but a non continuous one at the drywall)?"
A. The answer depends on how conscientious you are with your exterior air barrier. The exterior air barrier might be sufficient, but only if you are meticulous. For more information, see One Air Barrier or Two?
Q. "I’m a bit confused as to how the second method (fiberglass insulation + rigid foam) would work. The open web floor trusses are, as the name suggests, open. If there is fiberglass between the rigid foam and the rim joist, wouldn’t the rigid foam be set back to where the sides would be open?"
A. Like you, I am skeptical of the rim joist insulation method that calls for fiberglass to be installed adjacent to the rim joist. For more information, see Insulating Rim Joists.
Thanks! I'm surprised I hadn't come across those articles yet. One follow up question. The article states that "Rosenbaum noted that at any sheathing edge that doesn’t abut another piece of sheathing — for example, sheathing adjoining a rough opening or sill — needs to be sealed to the framing."
What is the preferred method? Is construction adhesive sufficient? Caulk (what kind?) between the sheathing and framing as it's being sheathed, or caulk afterwords along the seam?
And another one I just thought of. Right now I plan on having the air barrier at the exterior sheathing and the drywall between the living space and the attic. Obviously it's easier to find and fix leaks before the drywall goes up, but if you have a vented attic and the drywall serves as your air barrier, that makes it impossible. Is there a good solution for this, other than bringing the attic inside the thermal envelope?
Michael,
To seal potential air leaks between sheathing and framing, caulk (for example, polyurethane or silicone caulk) probably works better than construction adhesive. If you are thinking ahead, install the caulk before you secure the sheathing.
After the sheathing is up, a high-quality European tape probably works better than caulk.
Here is a link to a recent Q&A thread about blower-door testing in homes with vented, unconditioned attics: What stage to do a blower door test?