Ice dams in cold climates with heavy snow

[watts107]

How can I prevent ice dams in a northern climate with very heavy snow? My cabin has a cathedral ceiling with R-48 insulation and a very good vapor barrier. The roofing is 50 year asphalt shingles with ice and water shield a few feet above the wall line. It is typical to have 3 to 4 feet of snow on the roof for several months. The ice dams were about 8 inches thick this winter.

[Alaskan Son]

Re: Ice dams in cold climates with heavy snow

Cathedral ceilings are notorious for causing ice dams. There typically isn't enough ventilation supplied to the roof. The first thing you need to do is check to see that you have all the ventilation you can get. Make sure the soffits are ventilated and that the ridge is ventilated. In a nutshell, you need to keep the roof as cold as possible. If heat escaping from the house (which cannot be stopped) is allowed to melt the snow, the resulting runoff flows down the roof and refreezes once it reaches the colder eves. The idea behind the ventilation is that it will allow the escaping heat to leave the roof system before it causes the snow to melt.

[watts107]

Re: Ice dams in cold climates with heavy snow

Thanks for the reply. There is no ventilation in the roof. The entire cathedral type roof assembly is filled with blown-in, dense packed, fiber glass insulation. I have considered putting a "cold roof" over the existing roof so that I could get some ventilation and keep the roof colder. However, wouldn't the ridge vent get completely plugged when covered with 4 feet of ice and snow?

[Alaskan Son]

Re: Ice dams in cold climates with heavy snow

Possibly. It should still work better though as cold air should still move from eve to eve. There are ways of improving the roof though if you are seriously considering spending the money to upgrade (which is no small undertaking).

[Alaskan Son]

Re: Ice dams in cold climates with heavy snow

Where do you live?

[David Meiland]

Re: Ice dams in cold climates with heavy snow

My guess is that part of the problem is interior air leakage into the roof assembly. Is the ceiling drywall? Any can lights? More insulation and better air sealing will help. Installing a new roof with an air gap underneath would also help.

[Alaskan Son]

Re: Ice dams in cold climates with heavy snow

You may be right David. This is also a common problem in Alaska, and often shows up as a "leaky" roof. The moist air rises, condensates, and freezes especially around things like can lights, then when it warms up it all melts, and..

the roof is leaking.

[davenorthup]

Re: Ice dams in cold climates with heavy snow

I have never heard of such a thing........ :)

[fixitron]

Re: Ice dams in cold climates with heavy snow

My first thought is that the fiberglass is not dense-packed (DP) properly and is allowing air movement through it. An R-48 for DP fiberglass requires 11" of depth (2x12s) and requires careful attention to DP properly. If the rafters are framed 24" OC, the cavities would be more difficult to DP properly. My second thought is that if the ceiling is strapped or if the ceiling is T&G pine, that air can move through it (plastic is not considered an air barrier). My third thought is that there may not be a full R-48 out to the outer edge of the the exterior walls, depending on framing details. Of course if there are any recessed lights or bath fan/ducting or anything else in the rafters other than fiberglass, they could be the heat leak.
This year in northern Vermont (8,300 HDD) we had a week in January where the daytime temperatures were well above freezing (up to 50 degrees) with lots of snow. At night the temperatures dropped to the teens. This freeze-thaw cycle for several days contributed to the formation of ice dams on evey well insulated roofs that had not experienced ice in the past.
We can never really stop the flow of heat, only slow it down. A truly "cold" roof should only be considered after the ceiling has been insulated as well as possible, otherwise, you are just hiding the symptom of losing too much heat.

[watts107]

Re: Ice dams in cold climates with heavy snow

Where do you live?

The cabin is located in Island Park, Idaho, about 25 miles from Yellowstone National Park. The snow was very deep this year. About 150% of normal. There is still about 3 feet of snow on the ground.

[watts107]

Re: Ice dams in cold climates with heavy snow

My guess is that part of the problem is interior air leakage into the roof assembly. Is the ceiling drywall? Any can lights? More insulation and better air sealing will help. Installing a new roof with an air gap underneath would also help.

The ceiling is well sealed with a continuous vapor barrier. I have tongue and groove fir on the ceiling. I have some canned lights but the ice dam is persistent across the full length of the roof.

[Doug McEvers]

Re: Ice dams in cold climates with heavy snow

I have built many cathedral ceilings in MN, always vented and air-sealed. Insulation is always to code or above and insulation is full depth at the plate line. There is never a build up of ice on these roofs, the snow layer is uniform.

[watts107]

Re: Ice dams in cold climates with heavy snow

My first thought is that the fiberglass is not dense-packed (DP) properly and is allowing air movement through it. An R-48 for DP fiberglass requires 11" of depth (2x12s) and requires careful attention to DP properly. If the rafters are framed 24" OC, the cavities would be more difficult to DP properly. My second thought is that if the ceiling is strapped or if the ceiling is T&G pine, that air can move through it (plastic is not considered an air barrier). My third thought is that there may not be a full R-48 out to the outer edge of the the exterior walls, depending on framing details. Of course if there are any recessed lights or bath fan/ducting or anything else in the rafters other than fiberglass, they could be the heat leak.
This year in northern Vermont (8,300 HDD) we had a week in January where the daytime temperatures were well above freezing (up to 50 degrees) with lots of snow. At night the temperatures dropped to the teens. This freeze-thaw cycle for several days contributed to the formation of ice dams on evey well insulated roofs that had not experienced ice in the past.
We can never really stop the flow of heat, only slow it down. A truly "cold" roof should only be considered after the ceiling has been insulated as well as possible, otherwise, you are just hiding the symptom of losing too much heat.

Thanks for the reply. The insulation is 12" thick. I have 12" TJI's on 24" centers. The insulation was packed very firmly in the rafter cavities. I watch the entire installation of the insulation and vapor barrier.

I am a mechanical engineer. I did a very simple thermal analysis to get a handle on the roof temperature at the roof to snow interface. I assumed the outside temperature was 20 F and the inside ceiling temperature was 80 F (24' ceiling height). I further assumed that the snow on the roof was 48" deep and had an R value of 48 (1 R per inch). In this case the R value of the snow equals the R value of the roof insulation (R 48). In this case, half the temperature drop should occur in the insulation and half in the snow. The roof top temperature would be about 50 F (half the difference between the outside temperature of 20 F and the inside temperature of 80 F). I could double the insulation to R 96 and the roof top temperature would still be 40 F. The snow is a great insulator!

I recognize that this is a very simplified thermal analysis, but it shows that adding insulation may not be the answer.

[watts107]

Re: Ice dams in cold climates with heavy snow

You may be right David. This is also a common problem in Alaska, and often shows up as a "leaky" roof. The moist air rises, condensates, and freezes especially around things like can lights, then when it warms up it all melts, and..

the roof is leaking.

I have had no leaks inside the cabin. I have a wrap-around deck that is about 9 feet wide. I do get leaks on the deck.

[watts107]

Re: Ice dams in cold climates with heavy snow

I have built many cathedral ceilings in MN, always vented and air-sealed. Insulation is always to code or above and insulation is full depth at the plate line. There is never a build up of ice on these roofs, the snow layer is uniform.

This is good news. The plate line area was insulated and sealed with foam. I have full depth insulation to the plate line area. How do you vent the roofs that you construct? Doesn't the ice and snow simply plug the ridge vents?

My snow layer is very uniform. I do not see any area where heat loss is obvious.