Rigid foam over kneewall studs to eliminate thermal bridging?
Hello! I’ve got a 1926 bungalow in Portland, OR (zone 4C). The attic was finished in the 50’s, and is terribly leaky and not insulated. The previous owners left behind a boatload of fiberglass batts, unwrapped, with kraft paper backing. I was hoping to use them to insulate the kneewall, after air-sealing the gaps with some caulk.
However, since the joist bays are somewhat shallow (I believe ~3.5″), I was hoping to cover the outside of the studs with rigid foam insulation as well (XPS or ISO) to prevent thermal bridging, secured in place with a polyurethane expanding foam.
My concern is that this scenario would have a vapor retarder against the warm space (in the form of the kraft paper) and a vapor retarder/barrier in the form of the rigid foam. Do you think that this could lend itself to trapping moisture in the wall? We don’t have an upstairs bathroom, but Portland weather is all over the map, and we do have a fair bit of humidity.
Thanks!
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Replies
Bill,
Your plan will work fine, as long as your foam has a minimum R-value of R-2.5. (That's not hard, since 1-inch of XPS has an R-value of R-5.) It's better to aim higher, however; I recommend that you install at least 2 inches of rigid foam. Polyiso is more environmentally friendly than XPS.
Don't worry about the kraft-paper vapor retarder. It won't prevent your wall from drying to the interior. That's good. Kraft paper is a "smart" vapor retarder with variable permeance, not a vapor barrier.
Remember that it's essential to include blocking between the floor joists under the bottom plate of the kneewall, as well as blocking between the rafters above the top plate of the kneewall. Each piece of blocking needs to be carefully air-sealed.
For more information on insulating kneewalls -- and an explanation of why you need the blocking -- see Two ways to insulate attic kneewalls.
For more information on the thickness of exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thanks for the advice! One other question: The previous owners recently remodeled the attic space, and installed drywall ceilings with fiberglass batt insulation. However, they mashed 9.5" batts into the rafter bays, and the foam baffles seem mostly (if not completely) compressed.
Would you have any recommendations for boosting the ventilation in this scenario? I was thinking that I could perhaps push in some rigid pvc pipes in behind the baffles. It would compress the insulation further in places, but would help open up an air channel from the scuttle space to the ridgeline vent.
Bill,
One of the purposes of the ventilation baffle is to facilitate drying of the roof sheathing if the roof sheathing ever gets damp. So your PVC pipes won't work. You need a baffle that allows the air to flow over the underside of the roof sheathing.
It's going to be hard to fix this problem without opening up the ceiling, I'm afraid. But if you have access from above, you can experiment with inserting a rigid, home-made baffle with proper spacers on the sides to keep the baffle from collapsing onto the roof sheathing. It sounds like it would be hard to insert such a baffle from above, especially if the fiberglass batts are jammed in tightly.