Roof ventilators for deep snow country
SteveInCalif
| Posted in General Questions on
We had an all-time record snow year in North Idaho (65 inches in Feb and first week of March, Climate Zone 5). All over town, roof ventilators were covered with snow and many houses had serious ice dams and water damage (including ours). Our roof ridge had 18″ of snow on it and our roof valleys had over 24″. I had the roof shoveled and the ice dams steamed off to stop the water damage (a huge expense!).
What are the best roof vents for deep snow country?
RIDGE VENT. I’ve seen claims that ridge vents will be kept clear of snow by warm air flowing out of the attic, but we are air-sealing the attic (closed cell foam, R14) with R46 blown fiberglass insulation on top of the foam (R60 total), so I expect the attic will be within 10 – 15 degrees of outdoor temperature. Therefore, counting on warm air melting the snow to keep the ridge vent clear just isn’t realistic. Also, some ridge vent manufacturers state that their vents must be kept clear of snow. I can’t see having the ridge shoveled 4 to 8 times per year.
LOW PROFILE, TURTLE & POWERED VENTS. Low profile vents have the same snow problem as ridge vents. I’m averse to powered ventilators because of noise and low profile getting covered with snow.
TALL CUPOLA-STYLE VENTS. I’ve found two companies that make tall, natural draft cupola-style roof vents, DuraFlo and Ventilation Maximum. Apparently these are now widely used in Quebec. Because these require a slight breeze to create air flow, the manufacturers recommend the louvers must stand higher than the ridge which detracts from curb appearance. See attached photos.
For our 34 foot wide roof (based on 1:300 vent area requirement with 40% roof / 60% soffit area split), I calculate I need four of the smallest Ventilation Maximum roof vents or five of the smallest DuraFlo vents. Is there a problem having individual ventilators spaced 6 to 8 feet apart?
Ventilation Maximum also makes a cathedral ceiling system that puts a wider plenum over the ridge that feeds individual ventilators. See photo. Would this be a better approach? The downside is this is even less attractive from the street.
We are going to apply a 2 inch closed cell foam layer over the ceiling sheetrock to air-seal between the living space and the attic. All bathroom fans exhaust outside. There is no evidence of mold or mildew in the attic. So I don’t believe we have had any moisture problems in the attic.
So, my questions are:
1. What do you think are the best roof ventilators for deep snow country?
2. Am I right to be worried about ridge vents and low roof vents being covered by snow?
3. Have you used the cupola-style ventilators? What’s your experience been with them?
4. How evenly do ventilators need to be distributed across a roof for good ventilation performance? Would four or five individual ventilators evenly spaced across a 34 foot wide roof be sufficient? Thanks in advance for any and all input and thoughts!
Steve
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Replies
The best vent in a snowy climate is a gable vent. It is also very important to make the ceiling as air tight as you possibly can. Gable vents are good because you can generally size them to slightly pressurize the attic to help prevent inside air from getting up there. Lastly you want as high an R-value above the top plate as you have space for.
The spinny vent things can actually make problems worse, they depressurize the attic and suck air from in the house into your attic.
Steve,
If you have a problem with ice dams, you can't solve your problem with ventilation. That approach never works.
Most ice dams are caused by air leaks that allow warm indoor air to reach the attic or the roof sheathing. A secondary cause (less important than air leaks) is insufficient insulation (that is, insulation with an R-value below minimum code requirements).
The most important first step to solving an ice dam problem is to perform air sealing work. Here are links to articles that can guide you:
"Prevent Ice Dams With Air Sealing and Insulation"
"Air Sealing an Attic"
"Blower-Door-Directed Air Sealing"
Martin, thanks. I wrote previously (a month ago or so) about all the steps we are doing. I know you answer hundreds of inquiries.
We are doing lots of energy improvements, not just the attic ventilation. The attic ventilation is one element of the overall program.
• Covering 35 recessed light fixtures with TenMat covers foamed in place
• Retrofitting LED lighting to recessed lights
• Air-sealing the attic sheetrock with two inches of closed cell foam (R14)
• Adding blown fiberglass to a total of R60
• Rebuilding all the rafter baffles (some old ones had collapsed and were plugged with insulation)
• Adding snow-proof roof ventilators in second floor attic
• Replacing the soffit vents that are 50% plugged with paint in places
• Changing to a closed combustion Power Direct Vent gas water heater so I don't dump all the utility room heat into the attic around the clock via the combustion air inlet.
Improving the roof ventilation is one important component of the overall program.
Thanks, Nathan. I'm concerned about gable vents because of the possibility of wind-driven snow and rain entry. The ridge is aligned east-west and the prevailing winds are off the prairie from the west. I think there's less chance of wind-driven snow and rain entry with the roof ventilators. With good attic air sealing, any negative pressure caused by the roof-top vents should induce air flow from the soffits, not from the house. We are going to air-seal the house-attic boundary with two inches of closed cell foam on the attic sheetrock. We are also going to spray foam on the two skylight tunnels and a couple of vertical sheetrock walls that separate the second floor living space from the first floor attic.
Hi Steve -
Check out this Building Science Corp reference on attic ventilation, including information/guidance on roof ventilation for high snow load areas:
https://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation
Peter