How to get R38 in low/slope truss roof?
I have a dormer area in my current project where low slope trusses won’t allow for R38 in much of the celing area–near 6′ up from the eaves.
I had thought to use a high density batt–I like “Ecobatt” insulation–and then add a layer of rigid foam atop the sheathing, under the sheet membrane roof that will be installed.
There is an open eave, with strip vent atop all the birdblocks.
If there is insulation added above the sheathing, does the ventilation pose a problem or nullify the R value of the added foam? Where will vapor condense in that situation?
I’m familiar with a spray foam ‘hot roof’ with no ventialtion, but I don’t know if that’s applicable here and I don’t know if any of the insulation outfits in town are doing it yet…
Suggestions welcome, thank you.
Replies
Anyone?
where is here?
Eugene OR
To get R38 with solid foam, you would need 5.84" of foam... that doesn't leave much room for an air channel under the deck.
It would be easier to put 6" of rigid foam panels on top of your existing deck, then fur out a new vent channel on top of that (over the foam), and put a new deck on top of that to attach shingles to.
Then you would cover the exiting vent channels.
Insulation on top of the truss ... and the vented area from the eave ... does no good.
Consider using e.g. Thermax along the perimter just above the drywall. You can step it in thickness and achieve a result of higher R near the perimeter. We used to use this strategy when space was at a premium. The rest you do in the batts. You maintain the ventilation air flow and get higher R near the edge.
So good it bears repeating
Insulation on top of the truss ... and the vented area from the eave ... does no good
This is important. A vent space in between insulation layers damn near kills the value of the upper layer. You want all of your insulation right on top of the drywall. You do not want air between the drywall and insulation.
Exactly (what I said) ... or at least meant.