Polystyrene insulation between studs and plywood sheathing?
Putting an addition on my home…thinking about upgrading insulation package to include 1″ polystyrene between 2″x6″ wood wall studs and 1/2″ plywood sheathing (as oppossed to the much used polystyrene over sheathing) in order to increase r-value and to act as a thermal barrier. Ive researched this and havent found much if any information on putting the polystyrene under the sheathing, but in an email exchange with Paul Fissette, professor at UMass/Amherst he said that my idea would be work. I initially had thought of using 1/2″ polyiso w/foil facing, but Dr. Fissette said that a foil face would trap moisture whereas polystyrene is semi-permiable and would allow for moisture to move out of the cavity. The insulation in the cavity would be blown celluose or fiberglass batts.
Would appreciate your thoughts………..
JR
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Replies
JR,
Be sure to have an engineer review your plans. With your plan, you won't be able to depend on the plywood for shear bracing, so you'll need to include diagonal let-in bracing or Simpson T-bracing.
As long as you're not in a seismic or high wind zone and meet the code minimum braced wall line standards (let-in wood, metal strap or T, plywood corners or well-fastened gypsum no more than 25' on center and every 35' of building length or width with no more than 4' wall line offsets), you're good to go.
I agree with Fisette's assessment of the need for a breatheable wall assembly, and I would strongly advise the cellulose over the (carcinogenic) fiberglass. It has a long list of advantages, including fire resistance, insect and rodent resistance, mold resistance, sound reduction, and moisture buffering ability, as well as being non-toxic and low-impact. Just be sure to use a material that has only borates for fire retardant - no ammonium sulfate, which is a corrosive to metal fasteners.
Martin
thank you for the reply, I just spoke with my architect and he echo'd your thoughts, so I will be putting the styrofoam over the plywood with a weather barrier/rain screen on top of that....
Robert
appreciate your reply, I do live in a high wind zone (110mph zone) so I need sheer bracing to meet Massachusetts checklist requirements. I have decided to go with cellulose for the reasons you submitted in your thread...
JR