Roof assembly in the pacific northwest
I’m looking for some advice for a friend that is in the process of building a house and they are currently in the design phase. The draft designs call for a number of monoslope roof pitches.
What would work best here? We’re on the coast in the Pacific Northwest.
I think the default around here is a vapor barrier at the ceiling (6 mil poly behind the drywall,) and then blown in insulation around and over any services (ducting, electrical, plumbing vents, etc.) Then a vented attic cavity with sheathing, roof underlay, and the roof material (this one will be metal.)
Some other ideas might be:
– Keep all else the same but put some insulation on top of the sheathing.
– Move the thermal barrier to the roof deck with breathable batts against the bottom of the sheathing and a few inches of external insulation on top. Do not have a vapor barrier on the inside (the roof underlay would be the vapor barrier.)
– Spray foam the underside of the roof deck.
What do you think? Is the roof assembly shown in the draft of the drawing likely going to be robust? Would you do something else with the assembly if we assume the design of the house (mono-slope roof pitches) is a constraint?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
canada_deck,
I don't see mono-pitched roofs as a restraint. They function just like the more common gabled ones.
The simplest is vented attic (or if the ceilings follow the roof porch vented rafters), which works well here as long as the roof pitches are over 2/12. The insulation can be cellulose or batts. Some incidental mold can occur on the underside of the sheathing in the PNW, but unless there are other causes fuelling it, it's not a worry. It's not a good idea to run ducts in the attic, even if well insulated.
In terms of the other options:
- You can't divide the insulation between the roof top and attic floor with either a vented or unvented roof.
- That works but is more complicated and expensive.
- The same is true of spray foam.
Thanks. Here is a typical section in the draft plan. So from what you are saying, it sounds like this should be OK but you would advise against running any HVAC ducting above the ceiling? There will still need to be exhaust ducting (bathroom and kitchen fans.)
canada_deck,
Looks like a good, workable roof assembly to me.
"OK but you would advise against running any HVAC ducting above the ceiling?"
I might not go that far. Certainly no HVAC equipment in the attic, but ducts should be fine if insulated well.
At the bottom of this blog you can find my solution to mounting bathroom fans in inaccessible roof spaces:
https://www.greenbuildingadvisor.com/article/a-curb-for-your-metal-chimney
I wonder if a thin layer of foam on the roof deck for a vented roof in Pacific Northwest would eliminate the incidental mold from night sky radiation… I dont know that it’s worth it but even a 1/2 layer would probably work
feyr_design,
I haven't heard if there was a follow-up to the RDH study which said that at least initially, anti fungal coating worked well. I would be wary of any solution that didn't leave a good drying path for the sheathing and made the venting ineffective.
Partly why I haven't pursued it is that my experience working on these vented roofs here is that although they may show some mold growth, as long as the only factor driving the high moisture content is the typical PNW climate, the problems aren't worth remedying.
https://www.rdh.com/wp-content/uploads/2017/08/RCI-2015-Problems-with-and-Solutions-or-Ventilated-Attics-GFINCH.pdf
Martin's blog on the subject:
https://www.greenbuildingadvisor.com/article/moldy-roof-sheathing-in-the-pacific-northwest