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Old 09-06-2010, 03:27 PM   #1
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Venting a flat roof assembly

I really dont want to do this but I see no other way.

I've got an unvented flat roof made out of 2x8 rafters, 16" OC. Rim joists are also 2x8 (double thickness). Rafter plenums are currently filled with FG batts.
Ceiling is attached to rafter. No "mini attics" or any separation. No eaves/soffit/overhangs at all.

Roof membrane is directly on the decking. Roofer skipped putting rigid insulation above the deck so it is a cold deck.

The condensation is bad so I propose removing the batts, driill out all rafters + rim joist with approx 1-1/2 inch holes. The rim joist will need a hole at each plenum, every 16 inches. Those holes will be twice as deep as the rim joist is doubled up. Their is a mansard roof on the sides of the building so the holes wouldnt be exposed to the weather, nor look ugly.
There will be a fan running constant inside the mansard roof to help increase airflow through all the holes.

I'm worried this is too many holes, especially on the rim joists but at least the rim joist has bearing walls under it. The rafters I spose will have to be sistered first, then drilled out, 3-4 holes per 10 ft span.
I dunno, it just seems like too many damn holes.

Insulation layer will be placed only beneath the rafters.
Bummer, thats gonna lower my ceiling 8 inches.

Tell me this is crazy .....but then tell me a better way.

Not a big fan of SPF or cellulose or any solution requiring an HRV/dehumidifier. Those could be the only other viable options, though, besides reroofing and converting to warm deck construction.

Also a plastic VB on the warm side of the ceiling aint gonna cut it in this situation. Creates a double VB.

This is a tough problem to say the least.


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Old 09-07-2010, 09:07 PM   #2
Join Date: Aug 2010
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some pics? not sure what you are proposing-you are doing things from the inside??


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