Why vent in an unvented roof design for snow region?
user-1085163
| Posted in Green Building Techniques on
In Joseph Lstiburek’s book ‘ Builder’s Guide TO Cold Climates ‘ ,in discussing unvented roof designs, he says ” In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to flush heat trapped due to insulating value of the snow…”
Why does this heating of the cladding require venting ? condensation ? ice dams ?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Brian,
The potential problem is ice damming. Deep, fluffy snow acts as insulation. If the snow is deep enough, it can allow melting of the bottom layer of snow; when the water flows down, it encounters the eaves, where the roofing is colder, and freezes. The ice at the eaves gets thicker and thicker, creating an ice dam.
If you include a ventilation channel between the top of the insulation and the roof sheathing, you cool the roof sheathing and the roofing, reducing the chance of ice dams.
For more information, see Prevent Ice Dams With Air Sealing and Insulation.
Moisture is a problem too here in the Adirondacks with anything except spray foam.
Martin has it right- you need a thermal bypass for the R-value of the snow to keep it from turning into an ice dam, if deep snowpacks on the roof are the norm.
I've had sections of my roof with as much as 4' of snow on it on the leeward side after a big storm, which has a fairly substantial R-value. In my case I have to shovel it down to something much less to limit the ice-dam risk after those events.
Thank you Martin and Dan for your prompt responses. I especially enjoyed the link "Prevent Ice Dams..." and the many blogs. I'm located in the Laurentians, north of Montreal ,zone 8, snow fall +4feet ( which is interspersed with layers of ice due to subsequent melting and freezing as the weather alternates ). Obviously roof venting is an absolute must .
Martin, why your preference for ridge vents as opposed to gable vents, as the latter seem to work just fine ? In my area most reroofing is replacing the ridge vents with maxi vents (ventilation maximum.com) as they work better in deep snow.
Just to mention as a sidenote that one of my more successful insulation jobs ,I think, was blowing 2 inches of closed cell foam on the attic floor, and topping that off with 16 inches of cellulose. This was done last spring, and this winter will tell the tale.
Cheers, keep up the good work.
Brian,
Research has shown that the best air flow patterns in a vented attic occur when soffit vents are paired with ridge vents. However, this broad statement needs to be qualified with a few other statements.
In areas of deep snow, it's possible that ridge vents will have reduced air flow when the ridge is covered with snow. Whether this temporary situation matters is an issue of debate.
In general, attic venting matters much less than most people think. The three most important things you can do to reduce ice dams are (1) remove all heat sources from your attic, (2) seal air leaks in your ceiling and thermal bypasses into your attic, and (3) install thick insulation on your attic floor.
To repeat: attic ventilation doesn't matter much.
According to building scientist William Rose, “Once you’ve sealed all of the openings that lead from below into the attic, corrected the ductwork, and installed a nice thick blanket of insulation in the attic, then one venting strategy is about as good as any other. Gable venting and ridge venting are both fine. Soffit venting with baffles is fine. Combinations are fine. If parts of the roof have a lot of venting and other parts have little or none, most would agree that that’s fine too. Power venting, however, is noisy and expensive.”
Hi Martin,
I could not agree with you more on all points. However, in high snow, these maximum vents, which sit 2 feet above the ridge, are quiet {no moving parts}, are cheaper, at $100 a pop, than building a ridge vent,and don't block up.
By the way, I'm not affiliated to this company in any way.