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Community and Q&A

Unvented roof deck

GBA Editor | Posted in GBA Pro Help on

I am finishing an existing work shop with a low pitch vaulted ceiling. There is no practical way to vent the ridge. Since it is a shop, no code applies to r -values, but we want the most we can achieve.
Here is what I am working with:
Mixed climate ( Central Virginia)
2 x 12 rafters 24″ o.c. 5/8 osb roof deck, full coverage ice shield and 50yr shingles 2 1/2 / 12 pitch
~ 685 sq. ft. ceiling area
2 x 4 sidewalls 16″ o.c.
Concrete slab floor ( vapor barrier or insulation under is unknown)
Sheetrock is proposed for the final wall covering
Tight budget

Questions : Does it matter ( to a unvented roof assembly ) that there is no continious climate control ?
What is the most practical approach and products to use ?

The only methods I have used in the past is a flash and batt on exterior walls. I have yet to do non-vented roof assembly.
Thank you for your time, JTN

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  1. Riversong | | #1

    In a relatively unconditioned space with no moisture sources you can use any insulation you'd like in the roof assembly. I would recommend a very light-colored roofing to minimize solar radiant gain and avoid any non-breathable insulation under the roof deck since it is tightly sealed on top and needs to breathe in at least one direction in case it ever gets wet.

    You've got plenty of rafter depth, so cellulose may be your best option. It will also help fireproof your shop and keep the critters out.

  2. Christopher Briley | | #2

    Echo that. Pack it tight and you'll be fine. The only problem you may run into is that with an unvented roof you may void the warranty of the asphalt shingles. (the worry being that you will overheat them and age them quickly) Since you say they are 50yr, I'd wager they will have a "hot roof" warranty. We can thank the SIP industry for this development.

    I would pack the roof and walls tight with cellulose, with no poly vapor retarder (I was going to say "vapor barrier" but I just got done reading Martin's Vocabulary article). I might even consider strapping the interior studs before I apply the sheetrock. Once all the equipment is there and extra 1-1/2" of cellulose is probably pretty cheap.

  3. Christopher Briley | | #3

    ...but a quick internet search will tell you if the shingle company has a hot roof warranty.

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