Optimizing use of rigid foam for top plate/soffit?
I want to blow cellulose in my attic, but it is impossible to get a very good r-value in the tight space where the roof slopes down to the soffit. I’ve been thinking of strategically using a small amount of rigid foam to make up for this deficiency, and based some some rough calculations, I would use a triangular block of foam (layers stacked/glued) to completely fill up the insulate-able cavity for about the first 12 inches, then for a few more inches it would slope down, until it ends and the cellulose finally has enough space to provide the full R-value (I’m using R-70 just as a rough target for this calculation, don’t worry about the specific numbers too much).
My problem is that I haven’t seen any references to anyone else doing this. I’ve seen some mentions of using rigid foam over the top plate, but no specifics on how much to use or how to orient it or anything like that.
This block of insulation could also act as a partial baffle, and a shortened baffle could extend above it to the necessary length.
Am I over-thinking this? Are the losses at the top plate worth the trouble? I realize it would probably be a LOT of tedious foam cutting/fitting/gluing, but I’m willing to do it if it’s a technically sound concept.
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Replies
You could be setting up major issues with roof sheathing by blocking soffit vents. If you are sure you will still have a suficiently vented roof the higher r foam will help where the insulation is thin. The generally accepted wisdom is that 2" space between the roof sheathing and insulation must be maintained for ventilation, although some vented roof designs only have a 1 1/2" vent space.. The added foam could be the bottom of such a ventilation channel. If what you meant by "insulate able cavity" provided for ventilation, it's a good plan.
I will definitely be leaving space on top for venting. Thanks for the 2" figure, that's good to know.
Nick,
Other people have done what your propose. As long as you follow Jerry's advice to leave an air channel above the foam, the plan is a good one. It sounds like you have a reasonable understanding of the fussiness of the required work. Good luck.