Designers, framers, insulators and drywallers think of party walls with two main functions: fire ratings and sound transmission class. These are clearly outlined in both Part 9 and Part 3 of the OBC. What isn’t always very clear is how to deal with SB-12, which suggests that the total air-leakage of a townhouse unit shouldn’t exceed 3.0 Air Changes per Hour (ACH). OBC discusses air-barriers in 9.25 only as a barrier between conditioned and unconditioned space where SB-12 implies that there should be an air-barrier between units. Having tested many “code built” townhouse units, we can say with some certainty that many are somewhere between 4.5 and 6.5 ACH.
The question is, “Why should we care if the homeowners share a bit of air with their neighbours?”
That “bit of air” will not come alone; it will bring a few friends:
and just to top off the list: homeowner complaints and service calls.
The bad news: if it’s not built right the first time around, it’s near impossible to fix (short of tearing the wall apart)
The good news: where block party walls are notoriously difficult to seal, framed party walls make air-sealing a breeze.
Think of the party wall exactly the same as you would an outside wall
How do we get 2.0 – 2.5 ACH on middle units?
Wrap the rims with header wrap. We know from experience that leakage around the rim joists will make or break an air test. It can account for more than 30% of total air leakage.
Caulk the header wrap onto the foundation wall with acoustical sealant. This completes the air-barrier in the basement.
Compartmentalize the party walls. Frame the inside walls separately from the party walls of adjoining garages or bump-outs.
Seal the ends with house wrap. Stopping cold air from entering between the party walls will prevent condensation, mold growth, and heat loss.
Air-seal one wall with poly and the adjacent one with house wrap. We know that using drywall as an air barrier just doesn’t work that well. You just need a clump of dirt or a stray nail caught between the drywall and the bottom plate, and there goes the air-seal. This is an added cost but pays dividends!
Prep the party walls with poly or house wrap and drywall BEFORE installing stairs. Think of stairs and landings as back-framing.
If you plan for success, you can’t avoid being successful!