Solutions to inadequate insulation at truss heel
After recently reading “Prevent Ice Dams with air sealing and insulation” I know that I am not currently able to get the insulation I want at the heel of my standard trusses.
I will be building a double stud wall inside an existing structure, which will help increase the effective insulation at the inner top plate, but there will still only be approximately 9 inches of available space measured normal (perpendicular) to the roof sheathing where the trusses meet the (inner) top plate.
Zone 6a (Maine)
I have a number of thoughts:
First is a question: If one could guesstimate, is there a specific R-value at this critical heel at which ice dams become a notable risk, given: being in Maine, heating to approximately 65 degrees, air tightness being well addressed and the rest of the attic being loose fill cellulose at a target of r-60.
Secondly: In order to have, say, 2 inches for ventilation, I am left with only about 7 inches for insulation. I have read 2 inches for venting is preferable, but is that true still when it steals the inch from insulation?
Thirdly: What lengths should I go to to remedy this problem? My ideas being:
1) Install a different insulation at the heel that is perhaps more effective (rigid foam/spray foam) (rather than just continue the loose fill cellulose). I really don’t want to bother getting a sprayer in…
2) Build a drop down of sorts (tray ceiling) to increase insulating capacity at heel. Is this something people ever do it retrofits? Seems relatively simple and effective though certainly more sheet rock work.
3) Scab down the bottom of the trusses straight across to increase space (same idea as tray ceiling, but entire ceiling would be lower instead of just the corner). If I did this I’d be tempted to jack the entire roof up, which I kind of want to do anyways (I know, I know), but that’s irrelevant to the issue at hand…