Vented or unvented roof help

Old 04-10-23, 08:34 AM
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Vented or unvented roof help

Hi all. I'm in the process of renovating a sun room. The original builder insulated the walls and floor but not the ceiling which is vaulted. drywall right on the 2x12 trusses and nothing in between. There is currently no insulation and no venting top or bottom. My plan is two maybe 3 layers of rigid insulation on the underside then reinstall the drywall hanging below that.

Should I install venting in this situation?

My feeling is yes but I'd appreciate input?

What I'd do is use a bunch small pieces of furring strips on the underside of the roof beams to create a continuous air gap between the insulation board and the roof beams. Then soffit vents and either roof vents or gable vents up top.


Old 04-10-23, 08:57 AM
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We can't see what you have so "can you install venting" is the first question. I would try to avoid furring strips and instead make sure each bay had an air inlet at the bottom and outlet at the top. That could be done in a continuous strip with vented soffit and ridge vent or you can install individual vents.

You might want to also consider fiberglass or another insulation. It is very good at sealing against the rafters while it is difficult to get a air tight fit with rigid foam unless you use spray foam to seal the edges.
Old 04-10-23, 10:11 AM
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I would sit down and figure out what the job will cost you re venting and insulation etc<
Then see what the cost is to get it spray foamed.
I believe that with spray foam you do not need any venting so you save the venting cost and hassle installing it.
Old 04-10-23, 06:42 PM
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Hi all

I appreciate the replies. I did think about Fiberglass bat between the rafters but I don't like it that close to the roof. It would leave almost no air space in there and I worry about rot/mould. It is about half the cost of rigid insulation though.

Upon further inspection I realized that they put OSB over slats when they re-roofed last time so there is tons of continuous air space between all rafters so no furring strips needed anyways.

I installed 3 inches of rigid insulation two layers so the seams don't line up and taped all of it. Its the plastic lined stuff. I left enough space at all the edges to spray foam all around. I picked up enough sofit vent to run the whole length.

I still have to figure out the top vents. Its a single slope on the roof against the main house so continuous ridge vent I don't think will work. I've seen continuous vents mid slope but the roof is 13 years old and I worry messing with it that much I might tear some shingles and start a new project I don't have time for. I'll probably just cut in a couple roof vents. Its only 120 sq ft room so probably 2 or 3 will work.

I'm also hoping all this work cools the room a bit in summer but we will see.


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