Home>Discussions>INSULATION & HVAC>confusion on correct way to insulate a catheral cieling in upstate new york
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pennyanguy
confusion on correct way to insulate a catheral cieling in upstate new york
pennyanguy

I am quite confused. We are located in Rochester, NY. We are building an extension above a garage that will have a cathedral ceiling, 2 X 12 framing and already has the roof deck , shingles, soffit vents and ridge vent installed. We were planning on using fiberglass bat insulation, but now are thinking spray foam would be a better choice. We understand from the code enforcement officer that he REQUIRES R-38 in the ceiling. There are no other roof penetrations, skylights or such in the ceiling and is fairly small at 20' X 15' with a 4/12 pitch. Heating for this room may be either in floor radiant or minisplit unit or a combination of both..we have yet to decide. the rest of the house is forced air heating and cooling.

The confusion stems from the option of either closed cell or open cell foam, and what different installers are saying is needed.

Some installers are saying 11" of open cell directly adhered to the deck, close off the soffit and ridge vent, and drywall directly on the studs.

Others are promoting 6" of closed cell directly adhered to the deck, close off the soffit and ridge vent, and drywall directly on the studs (leaving a 5" air space above the drywall?). these guys say the open cell will allow moisture to build up in the middle of the open cell (depending where the dew point is) and eventually cause rot issues...that is why they are recommending the closed cell. This seems very expensive as well.

Another web site seems to recommend to vent each joist bay, which would leave 9.5" of space to fill with open cell foam....which would obviously need to be furred down 1" to meet the r38 required, then a vapor retarder, then drywall...

I like this last concept, as this is what I was always taught...to vent under the deck and insulate as well as you could below..however, ALL the contractors I have spoken to say NOT to do this as you are moving the dew point to just below the vented area..promoting moisture to build up below the deck vents.

At this point I am very confused...please advise

keith3267
Re: confusion on correct way to insulate a catheral cieling in upstate new york
keith3267

You already have the soffit and ridge vents, so I would suggest that you use R-30 Fiberglass which is about 9.5" thick. You won't need a vapor barrier. This will leave you with 2" of air space under the deck. To meet the R-38 requirement, use 1.5" thick, 4x8 panels of closed cell foam screwed to the bottoms of the rafters. Then attach the sheet rock with long screws, 2.75" minimum, and use plenty of them.

Tape the seams of the foam boards with a solid tape so that the foam boards make up the vapor barrier as well. The foam boards will do more than just add an additional R-9, they will act as a thermal break to the rafters. A 2x12 rafter has a R value of around R-14, which is half of the fiberglass so it acts as a thermal bridge. The foam will reduce that. In the winter you won't see the snow melting right over the top of each rafter as you so often do on homes with cathedral ceilings.

Done this way, there will never be a dew point in your insulation. Your insulation will be kept very dry by the vents and any moisture that gets past the vapor barrier will be quickly absorbed into that dry air.

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