Should I be concerned re plywood sheathing sandwiched by 4″ XPS & Tyvek on exterior & 2″-2# spray foam on interior wall
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| Posted in Green Building Techniques on
Heating & ventilation will be natural gas forced air coupled with a HRV.
The exterior wall proposed raises my concern about sufficient exposure through the studs & drywall to draw any moist to the interior. The designer says that there will not be sufficient moisture to worry about in our climate 8 zone. Is this correct under continuous heat within the home?
Additionally, if the home is not heated for long periods during the winter (except for the utility room) will this effect the durability of the wall system?
I’ll wait to hear back and thanks in advance for your help!
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Replies
John,
You've asked a green building forum whether this wall system raises concerns, and what you describe doesn't sound especially green--lots of petrochemical plastic. You might take a moment to consider the character of these materials, and how they will ultimately be disposed of. In particular, high density sticky foam coating the framing will be problematic at the end of this building's lifespan.
As to your question, is it problematic to have such low-perm materials surrounding wood products? Not necessarily, but it's not an ideal design. Ideally there would be vapor permeability in at least one direction from any wood product. There are wall systems for very cold climates that maintain permeability, even both ways. Check out the wall system for this house in the very extreme northern Alaska climate: https://www.greenbuildingadvisor.com/community/forum/general-questions/19942/sunrise-home. Note the use of economical and "green friendly" materials in a very high-R thermal enclosure. The design is scalable for whatever R value your climate requires.
John,
I agree with Thomas. If your proposed wall assembly is assembled carefully, with attention to water management details, and if the plywood is kept dry during the construction phase of the project -- something that isn't always possible in a rainy climate -- then the risk of problems is low.
But why create a foam sandwich if you don't need to? I always advise builders who are planning to use foam to choose one place to put it: either on the interior or the exterior side of the sheathing, but not both.
In your case, I would advise using 4 inches of exterior polyiso (which is more friendly to the atmosphere than XPS, which is manufactured with blowing agents that have a very high global warming potential). Then you can fill your studs with dense-packed cellulose rather than spray foam.
John,
You can also consider replacing the exterior foam with Roxul Rockboard 80. 2 Layers of 2" at R4.5 per inch. Place it under a rainscreen with battens and It will be indestructible: It will resist mold, insects and bulk water while being highly permeable. Tape the exterior sheeting and place a WRB (tyvek or ??) over it and then tape the staples also.
I like it as a choice for your climate due to it's durability.
Jon,
My vote-- double walls dense-packed with cellulose.
Forget the foam.