Double Stud wall confusion
nathansterkenburg
| Posted in Green Building Techniques on
Hi, I live in Alberta, Canada where in summer, the temp. can reach +35 Celsius in the summer and -40 in the winter. So obviously i need an efficient house. Still in the design process and choosing a wall system, it seems like a double stud wall with separate plates seems like a good option, and I was wondering if I did spray foam filling the outer stud wall and covering over the studs and plates, and then filling the rest of the cavity with cellulose, would that be a sufficient air/vapour barrier? or would I need some type of exterior barrier as well? Thanks
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Replies
Nathan,
Spray foam makes a good air barrier, as long as you pay attention to sealing all the usual leaks -- penetrations and transitions.
If you choose closed-cell spray foam, you'll also get a vapor barrier. Open-cell spray foam is vapor-permeable, so it's not a vapor barrier.
All walls need a water-resistive barrier (WRB), and obviously the spray foam between your studs doesn't qualify. Your WRB could be housewrap or asphalt felt.
Thanks Martin,
I had planned on a house wrap with some type of drainage plain behind the siding as well. Another concern I had was water getting trapped in the walls and causing mold which most builders here seem to think will happen with a 'too airtight of a house'. Would that be a concern in a double stud wall?
Nathan,
A well designed wall shouldn't have moisture or mold problems.
Most moisture problems in walls are caused by rain. The way to handle rain is to include good window flashing, door flashing, and penetration flashing, and to include a ventilated air gap between the sheathing and the siding.
The second most common cause of moisture problems in walls is condensation or moisture accumulation in sheathing due to air leakage. (Here's the mechanism: exfiltrating air carries interior moisture, which condenses or accululates on cold surfaces in your wall assembly.) The way to prevent this type of problem is to make sure that your wall is as airtight as possible.
Leaky homes are more likely to have moisture problems than well designed tight homes.