closed cell insulation vs. fiberglass bats

ben5048

I am attempting to insulate a catherdral ceiling. While I know that closed cell insulation has a higher R value, the space being insulated has very limited space between the roof deck and the sheetrock, thereby forcing me to deal with the problem of heat transference. I've read that if I don't have enough thickness for the closed cell insulation, the added gain by the higher R value with closed cell will be lost due to this principle. If this is true, what is the minimum thickness i need? I have 5 inches to work with.

jmiller

You're going to have to use the CC foam, or fir out your rafters (12" ?) so you can fit the recommended R value of fiberglass, plus a two inch air space under the decking, plus baffles to protect that air flow, plus continuous soffit intake vents, and continuous ridge vent. In other words, forget about fiberglass.

The temp differential is what's forcing you to deal with the heat transference. I.e. it's cold out and you want to stay warm. You're dealing with that by insulating.

What you should worry about is vapor transference. The air space above fiberglass insulation is to carry any conditioned air vapor (moisture) that leaks from inside the house out the roof vents. CC foam is perfect for cathedral ceilings because it becomes an air barrier at one inch, and a vapor barrier at two inches. Any vapor that would normally go out the ceiling and into the attic is blocked by the foam, so that warm air can't condensate on the underside of the decking.

CC foam is R 6-7 per inch. Five inches would be about %75 of the recommended R value for attics in my region.

I'm not sure what you've been reading, but not the right things. Building Science is the best source of info on what their name suggests. Here's what they have to say about unvented attics: http://www.buildingscience.com/docum...r-all-climates

ben5048

good info. thanks

Gary in WA

Find the climate Zone for the closest local City in your State below the map; http://publicecodes.citation.com/ico...001_par001.htm

Compare the Zone for the ceiling R requirements; http://publicecodes.citation.com/ico..._11_sec002.htm

Follow the directions for the rigid foam board required for a non-vented; http://publicecodes.citation.com/ico...006_par003.htm

For a vented ceiling, read this and the two pages after “ventilation”; http://publicecodes.citation.com/ico...9_8_sec006.htm

Choose the fiberglass insulation wisely; http://www.diychatroom.com/f98/how-b...ulation-90438/

If you go with SPF, the permeability rating is 1.51 per inch of thickness, you’ll be fine. To have it act as a true vapor barrier(vapor impermeable 0.1 perm), it would need to be 8” thick; http://www.certainteed.com/resources...CertaSpray.pdf

Gary

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Harolde

you have read it right , 5 inches is not very applicable in insulating the heat , the sheet is min 10to12 inches . and the info provided by jmiller is very correct .
i am just wondering that from where i can have 5to7 inches sheet ???
i havent seen any in the market ???
Senior Apartments in Las Vegas

jmiller

Interesting links Gary, thanks for posting them and the correction about foam being a vapor retardant rather than barrier at < 8".

Are you suggesting he use foam board on top of the roof deck, or in the rafter bays, or both?

federer

good links! this is what i am looking to do right now