[Greenbuilding] roof venting (Clarke Olsen)

Ross Elliott relliott at homesol.ca
Sat Jan 21 13:40:44 CST 2017


Actually, the more insulation in the cavity, the colder the mold food (sheathing & framing) and hence the greater likelihood of condensation. Since your  outer roof material will be impermeable to water, as well it should be, you basically have a vapour barrier on the wrong side, so without ventilation the roof cavity must be able to dry to the inside. You need to air seal the assembly 100%, to keep air leakage and water vapour out. Next, ideally you would put at least 2" of extruded polystyrene on top of the sheathing, and under the roofing - then steel or wood shingles on strapping, or asphalt shingles on plywood on top of strapping with a vented gap between the plywood and the XPS. 

But if you can't do the XPS on top because you already have a roof on, staple up continuous insulation baffles along the underside of the sheathing, ventilate top and bottom and add batts, NO POLY, and then latex-painted drywall. What you choose for the airtight layer is up to you, it just has to be continuous, I would use Tyvek in place of the usual poly under the drywall, or make the drywall itself the airtight layer.

Ross

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Message: 1
Date: Sat, 21 Jan 2017 13:02:05 -0500
From: Clarke Olsen <colsen at fairpoint.net>
To: Green Building <greenbuilding at lists.bioenergylists.org>
Subject: [Greenbuilding] roof venting
Message-ID: <960DC8C1-46C0-4FD9-BFDB-A1D0DA82F016 at fairpoint.net>
Content-Type: text/plain; charset="us-ascii"

I am building a 750 sqft building with two single plane (shed) roofs, using 12" I joists @ 3/12 pitch. 
Can we just stuff that 12" with Roxul, or is ventilation needed?
Clarke Olsen
clarkeolsendesign.com
373 route 203
Spencertown, NY 12165
USA
518-392-4640
colsen at taconic.net








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