Spray Foam to Seal Air Leaks for Airtight Passive House
The insulation company assured us that using this kind of low-expansion foam would produce an air-tight seal. However, we knew this not to be the case.
Absolutely do not rely on any type of spray foam to seal air leaks alone to achieve air-tightness. It won't. Even if it does in certain instances, it won't likely last. Expansion, contraction, and reductions in moisture levels in the wood causing shrinkage will eventually cause those "airtight seals" to fail. Trust me on this.
In fact, during our preliminary blower door test with Four Seven Five, post their membrane install but before our air-sealing efforts began, it was obvious that foam (and caulk) alone would not do the trick. To be certain of achieving long-term air-tightness, we used some high-end tape to do the heavy lifting.
We decided to use small amounts of fire-rated spray foam to add a layer of fire protection at the base of our double wall system. Ultimately we filled the stud cavities with fire-rated sprayed wet cellulose would also go along way to deter air from feeding any potential fire as well as provide additional levels of "air-tightness" and sound deadening (more on this later).
What we ended up doing was using a combination of all three materials/methods--caulk, tape, and then foam (as the last element). Even when we used a professional spray foam company for "skinning," the first pass, didn't always ensure that we achieved the air tightness in certain spots that we were seeking. We spread the spray foam company's work over a four day period. After each pass, we went around checking for remaining air-leaks (which we found) and then marked them for subsequent remediation. So when I say airtight, I mean airtight, not "nearly airtight."
I would not recommend relying upon spray foam alone, even when applied professionally, to do the job of air-sealing. When an area fails to seal or is not completely covered/filled (and this is especially true in tough to reach areas) it can be very tough to actually find the air leak and simply spraying more foam is no guarantee that you will seal it.
I believe it is best to address any air leaks at the earliest (and most exterior) points of the building envelope. We addressed the actual pinpoint sources of the failure, before any additional layers of air-sealing took place and certainly well before the installation of drywall. We surpassed our Passivhaus air-tightness requirement even before the drywall completely went up. Do not rely on the drywall to air-seal, either, even drywall listed as "airtight." Even if it did work long-term, which it won't, then the penetrations of the drywall itself would become an issue--such electrical outlets, switches, baseboard, etc. You don't want to go there.
Furthermore, you would still have infiltration of colder (or warmer) air into the wall space cavity behind it, compromising your insulation with moisture from the condensation that could result. And that could spell trouble, long-term, from eventual mold formation behind the drywall. Something that would be hard to identify and correct after the fact.
Only after we repeatedly sealed air leaks with caulk, tape, and canned spray foam, did we then rely on skinning over those areas with professionally sprayed high R-value closed-cell foam as a fourth level of air-sealing and additional insulating value prior to spraying with blown cellulose insulation.
Spray foaming for air tightness, sealing air leaks, insulation, thermally breaking, and sound-deadening
As you get more and more into the interior of the building space, it gets harder and harder to eliminate air-leakage. Could there have been shorter paths taken that could have been as effective? Perhaps. But we didn't want to leave anything to chance. Once the drywall is up, the game is over.
Note: More recently, some, including PHIUS, have raised concerns about the use of certain types of spray foam because of their potential for contribution to global warming(GWP) and subsequent off gassing. Others have suggested that improper application may lead to problems after the fact. In light of these developments, I would suggest if you chose to use spray foam, carefully select a company that knows what is it doing and, as we ultimately did, use it sparingly. It will also save you a lot of money and there are other ways to achieve a vapor barrier where you need it, as we did with a special membrane.