fiberglass sandwiched between rigid foam in vaulted ceiling
sorry, initially posted this in construction techniques
I’m going to finish my bonus room above the garage of my cape cod. I’m trying to determine the most cost effective way of insulating the 2×6 roofline in the bedrooms. Any insight would be appreciated. I plan on having the assembly vented. My initial plan to create the 1″ air gap from the soffit to ridge vent was to use thin plywood nailed to 1×1 pieces of wood. However, it seems a better use of money would be to use 1 inch of rigid polyiso foam for the vent channel. The foam is more expensive, but at least it gives me an r value of 6. So to get my R value of 38 my assembly would be:
plywood roof underlayment
1″ air gap
1′ polyiso r6
fur roof joists out 2′ to accept r 19 fiberglass batts.
2″ rigid polyiso foam board r12
total r value of darn near 38
any thoughts? my concern is sandwiching the fiberglass between the 2 layers of foam
Replies
tg
in the event you don't get any or enough information or opinion here, ask over at Green Building Advisor.
there's a drawing and perhaps members only information in depth HERE
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thanks.
Insulation between and below rafters.
Air that is trapped in a space less than 19mm wide will provide
excellent insulation, as the space is too small to allow a
circulation to start, it does not allow transfer of heat from one
side to the other.
However, you seem to be indicating that the channel will be
vented to the outside, and of course this will then transfer
your heat outside at a high rate. Not a good idea!
A better way, is to fill the spaces between the rafters with
sheets of polystyrene or similar closed cell products.
A filling 6 inches thick with another 2 inch thick layer of
polystyrene sheet below the rafters, covered with dry wall, will
give you a very warm roof. 30 minute fire resistant ceiling.
The layer below the rafters will stop the rafters acting as
a heat bridge,
That air channel is outsid eht foam, not inside it. He would be creating foam chutes so the unconditioned air gap is between the roof deck and the top sid eof the foam board. Yes, he could fill the full 5.5" with rigid foam board, but that would be pricey and a pain.
Don't worry about where you posted. Except for the Tavern, pretty much everything gets read by the regulars.
No harm in sandwiching the fiberglass between two layers of foam like that.
(Leaving the foam exposed on the top side is a bad idea in brush fire country, but that's probably not you. But you need to cover the foam on the bottom side with drywall.)
Insulation - expence
Yes it may be expensive but, it will last the life of the building and it will improve all year round comfort - an added bonus is that he will have snow on his roof, long after everyone elses has gone.
An alternative, place the fibreglass batts between layers of polystyrene - this will improve the overall performance.
An alternative, place the fibreglass batts between layers of polystyrene - this will improve the overall performance.
That's exactly what he proposes, only using polyiso, and polyiso has a higher R value.