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Old 04-13-2011, 12:21 PM   #1
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Insulating Attic - Peaked Roof addition over Built-Up Roof


New to the forum. Found some great info here but nothing specific enough for my scenario.

I live in Ontario, Canada and minimum R-Values in attic are 45, and a vapor barrier is pretty much a necessity.

I bought a house that has a recent addition and new roof. The original building had a built-up roof (tar and gravel). When the addition was put on they put a traditional peaked roof over the entire structure. The peak has proper ventilation (eaves and peak). Where they cut the back of the old house off I can see into the cavity under the built-up roof and there is very little insulation and an iffy vapor retarder. There is the ceiling drywall, some 30+ year old paper backed (toward drywall) insulation with no indication of R value, but is only about 3" thick, 5" of air gap with no vent, then the built up portion.

Now for insulating this space... can I assume that the built-up roof will provide a decent vapor retarder? Then I could just fill above it with blown in insulation. I would still have to add some insulation in the cavity at the outside wall.

Or... fill the cavity between drywall and built-up with closed cell foam. Leave old paper backed batt in there?

Or... remove drywall, old batt, put in new batt, new poly vapor barrier then new drywall.

So many scenarios.

Thanks,

D.

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Old 04-16-2011, 11:27 AM   #2
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Insulating Attic - Peaked Roof addition over Built-Up Roof


Welcome to the forum!

"Now for insulating this space... can I assume that the built-up roof will provide a decent vapor retarder? Then I could just fill above it with blown in insulation. I would still have to add some insulation in the cavity at the outside wall." --- It will stop moisture unless there is air getting under the flat roof, which it sounds like. The insulation on stops heat when close to the source. Any air space is counter-productive; The "biggest loser" in fiberglass insulation....

"Or... fill the cavity between drywall and built-up with closed cell foam. Leave old paper backed batt in there? " --- Would only work if the ends are air tight.

"Or... remove drywall, old batt, put in new batt, new poly vapor barrier then new drywall." ---- Doubt you could get the 16" required in the old roof, though could add insulation above old roof also. Don't like the two v.b. idea for condensation for sure because old roof is impermeable.

Gary

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