[Greenbuilding] roof venting (Clarke Olsen)

[Clarke Olsen]

This is a new project, with T&G OSB, about to be taped: no roofing yet. That should constitute a barrier reducing the need for a 3/4" air space
under the roof. I would really not like to add 2" to the roof, as it would make the facias larger than anybody wants. I had wanted to add 1/2"
of foil-faced foam to the underside, with strapping for drywall, but realized that we need to let out moisture somehow..
Clarke

On Jan 21, 2017, at 2:40 PM, Ross Elliott <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 vapor 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 vapor 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
> 
> ----------------------------------------------------------------------
> 
> 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





> _______________________________________________
> Greenbuilding mailing list
> to Send a Message to the list, use the email address
> Greenbuilding at bioenergylists.org
> 
> to UNSUBSCRIBE or Change your List Settings use the web page
> http://lists.bioenergylists.org/mailman/listinfo/greenbuilding_lists.bioenergylists.org
>