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.


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.


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!

Riverdale Deep Wall System.doc709 KB

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Congrats Conrad!
0.36ACH is a great result. I'd add to what you said above that the majority of the cost increase in your wall design is the extra insulation, not the airtight building techniques.
I think if you do the numbers you'll find the tight building will save you more energy vs R2000 than the extra insulation (i.e. 1.5ACH & R25 walls for R2000)
What did you use for caulking for wall plates? Acoustical sealant?

Thanks Ralph. It was acoustical sealant.

I also noticed your wall spec says 3/8" OSB. Is it readily available in Edmonton or did you have to go with 7/16"?
In NS the only thickness less than 7/16" is 1/4".


With that level of air tightness, have you installed an air exchanger? Is one needed? Are there concerns about air quality?

We absolutely need an air exchanger. I've heard of a house built from basement to roof out of Insulated Concrete Forms (ICFs) that had no air exchanger. These are extremely airtight houses (I've heard of one that was measured at 0.21 ACH), and this particular one had to have its drywall completely gutted and replaced after six months because without an air exchanger the moisture had nowhere to go but into the walls.

An airtight house must have a heat recovery ventilator(HRV) that ventilates the house during the seasons when windows aren't open. The HRV extracts most of the heat (~75% I think) from the stale warm air and passes it to the fresh cold air as it enters the house.

MFR HRV/ERV ratings are available at the HVI site.
There's a nice page with a summary of several models here:

Conrad, at 0.36ACH and no central air you probably need a HRV. I have a large house (54,000 cubic feet of volume) and ended up not running my HRV last winter (my 2nd winter) as the air was already too dry by December (<40%RH). My most recent blower door test in March of 2009 was 1.09ACH@50Pa.


Ralph: 3/8" OSB is pretty standard around here.

We will be installing an HRV soon.

Hi Conrad
I think that it is important to note the differences between an air barrier, a vapor barrier, and a combined air/vapor barrier.
I am attaching a link to a rather dated, yet informative NRC article.

Are you measuring humidity? Did you get your HRV installed?

I suspect you'll really need it this winter since you have all the moisture in the wood and drywall of a normal new house plus your slabs. Do you know the water:cement ratio for your slab concrete? I suspect it would be around .4:1, assuming a superplasticizer was used to give it the high slump. Fully hydrated cement uses 1/4 of its weight in water, and the rest evaporates.


The most recent post is my attempt at answering you:

We did have to run the HRV more than we normally would when we first moved in to get the extra humidity out. It stabilized after a while though.


It would be quite interesting to see how you handled the details at exterior doors, in this deep wall system.

Also, with regards to the vapour barrier being pushed to the exterior plane on the rim joists, was there any issues with the code / inspectors on this aspect.

I've seen a variation of the double stud system where the vapour barrier was placed on the exterior plane of the inner wall, thereby leaving the inner 2x4 chase as utility plane (plus insulation of course), but reducing the work of detailing the penetrations, etc... Was that option considered while building?

Great build!


I'm curious about the variation you are talking about. With the vapor barrier on the inside of the interior wall you have to deal with the detailing of penetrations mostly from power outlets. If you put the vapor varior on the exterior plane of the inner wall, don't you have to deal with the detailing of the penetrations caused from structural support between the two walls? I'm not sure I understand how this would reduce the detailing work. (I'm not a builder, just an interested observer).


I'm having trouble finding the link now.. It was an older reference... Which structural supports would you be thinking off? In most instances of a DSW, I've always seen plywood top/bottom plates being used, with no mid point braces. so really, you'd only have to worry about window openings, which would be fairly straightforward.

I'm not a builder either, just trying to scope out the rationale behind the systems. I personally like the idea of having the utility space, for future modifications, without running the risk of damaging the air/vapour barrier, but I imagine there may have been a good reason to put it at that plane.. Time/labor/complication. Only conrad/peter would know that I guess.

Heh expertise is more at the "Big thick wall - check!" level. Hopefully Bob Heath (owner of Belgravia NetZero) notices this thread. He's more on top of this stuff.

Hello there:

I'm new here ... wonderful site - thanks so much for taking the time to document your experiences! It is so useful.

My wife and I are about to build a netZero-ready house near Ottawa. We are designing the wall cavities to be 15 inches wide. Problem I am having is finding a cellulose insulation specialist who is both knowledgeble and confident enough to install the insulation in a manner that is consistent with the density required. Does anyone out there have a spec-sheet on how the installers maintain 3+ lbs/cuft in the open cavity? The folks around here are nervous about installing outside a closed cavity such as 16 or 24 o.c. stud walls.

Alternatively, Conrad, would you be willing to share contact info for the installer who did this house - I would love to chat with him or her!

Many thanks - many more questions and posts to come.

Mississippi John
Mississippi Mills, Ontario

Hello again - I assume this is a very busy time given the seminar scheduled for tomorrow. However, if anyone else out there has information regarding insulation installation, I'd be pleased to consider it.


Mississippi John
Mississippi Mills, Ontario


Peter Amerongen, builder of the three Edmonton NetZero houses, uses L. L. Climatic Insulation Ltd.


Looks like more people are starting to ask about the deep wall option. Check out:

I like the innovation that is being done in Alberta with tight building envelopes, such as the net-zero project homes.

In BC we manufacturer solar hot wateer heating systems to compliment this type of green building design.

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