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fiberglass sandwiched between rigid foam in vaulted ceiling

tgmuller's picture

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 2x6 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 1x1 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

tg (post #207359, reply #1 of 6)

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Don't worry about where you (post #207359, reply #3 of 6)

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.)


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Insulation between and below rafters. (post #207359, reply #2 of 6)

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 (post #207359, reply #4 of 6)

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.

YAY!  I love WYSISYG editing!  And Spellcheck!

____________________________________________________

Insulation - expence (post #207359, reply #5 of 6)

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 (post #207359, reply #6 of 6)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville