The importance of air tightness in building construction cannot be overstated. Very few factors affect energy performance in a cold climate more than air moving in and out of a building.

The Mill Creek Net Zero Home (MCNZ) has achieved an air tightness test result of 0.36 air changes per hour (ACH) at a pressure of 50 Pascal. In other words, when it’s really cold out, which creates a big pressure difference between the inside and the outside of a house, the 0.36 of the air in the MCNZH would leak out and be replaced with cold air over the course of an hour. It has the equivalent of a 13.8 square inch hole in it leaking air all of the time.

To put the number (0.36 ACH @ 50 Pa) into perspective, here are some average numbers:

Source: Biggs et. al. 1987

The 0.36 number is extremely small. In fact, the MCNZH is probably one of the most air tight wooden buildings in the world.

Deep Wall System

In the 1980s, Peter Amerongen figured out that, contrary to conventional thinking, it’s okay to have a small portion of the vapour barrier on the cold side of a house’s wall, as long as virtually no air is moving in and out of the house.

From this cut out, you can see that the vapour/air barrier runs along the exterior of the wall at the floor. This allows for a continuous, easy-to-install barrier.

For more information on the deep wall system that allows for super insulation and super air tightness, see this document: Riverdale Deep Wall System.

Costs

The insulation value of R56 and air tightness of 0.36 of the MCNZH were achieved at a rough cost of $4.00 per square foot of wall area ($43/m2).

The additional costs for this deep wall system include (in the case of the Riverdale NetZero Project)

• Increased framing labour costs of about 40 percent over a standard wall, or 10% of the overall framing budget;
• $260 worth of OSB to separate the 2 – 38 mm x 89 mm (2”x4”) outside walls;
• Bigger footprint for same floor area; and
• Other minor expenses include lining the window wells with drywall and finishing the deep sills.

Adoption

The deep wall system described in this post should become standard practice. I’ll leave the last word to Peter Amerongen:

“The credit for this great result belongs to the framer [Adam Larsen of Green Door Builders], the insulator and the Habitat Studio personnel who did the preliminary sealing work.  We recognise that these guys were diligent and experienced, but I think that 0.7 AC/h shouldn’t be that hard to achieve with a reasonable level of care and attention even by workers using this system for the first time.

I think it is important for builders trying to get maximum air tightness to realize that expensive foams are not the only way to get there.  This is not to suggest that spray foams and SIPS are not excellent products.  We occasionally use spray foams (with some remedial sealing of wood to wood junctions) for difficult to seal situations or when space is at a premium.”

Note: Peter tells that there is a minor error or two in this post. I have decided to post it now and correct whatever errors it may contain later. Let me know in the comments if you see anything!

(cross-posted at greenedmonton.ca)