[Greenbuilding] roof venting (Clarke Olsen)

[Jason Holstine]

Or, if you are avoiding use of foam, ventilate w/the baffles, R30 Roxul in the rafters, then nail in 1.5” thick Roxul ComfortBoard underside the rafters, and drywall on the boards. This addresses the thermal bridging.  See page 10-11 of this document: 
http://www.roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/Residential/ENGLISH/201604%20Thermal%20Applications%20Brochure_US%20ComfortBoard%2080%20revised.pdf



-- 
Jason Holstine
Amicus Green Building Center
“Sustain Your Space”
e: jason at amicusgreen.com
w: www.amicusgreen.com
p: 301-571-8590
f:  301-571-8597
tf: 866-587-9140


On 1/21/17, 2:40 PM, "Greenbuilding on behalf of Ross Elliott" <greenbuilding-bounces at lists.bioenergylists.org on behalf of relliott at homesol.ca> wrote:

    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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