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Just some food-for-thought.
* Igloos. Keeps many Eskimos warm and protected. So snow and ice can give you that environment, Bill. But they (igloos) do vent out the top!!
Merry Christmas!
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Hi G LaLonde
Your's is a question I have tusseled with for the past several years, and have come to the conclusion that deep snow and very high external temperatures are the true reasons for soffit-to-ridge venting: keeping the back side of the roof deck the same (or very nearly the same) temperature as the ambient outside air. In the former, ice dams will not (cannot) form. Lets assume that the temperature of the snow crystals sitting on your roof shingles under 20 inches of snow are 28 to 32 degrees....regardless of amibient air temperatures. It takes little warmth to cause them to melt, leading to ice daming. Thick insulation under (and in direct contact with) the roof deck will slow, but not eliminate thermal transferance from the living spaces below. Perhaps I'm missing something...but in my simple minded way, this seems to be straightforward logic....and it has definitely worked for me :-)
Best
Bruce M
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For anyone interested, the k value of snow (= 1/R) is 0.34 for freshly fallen snow, or 1.3 for packed snow at 32 degrees. The latter is the same for ice. Absorbtion of solar radiation for fresh snow is 0.13, for white snow it is 0.45. For most roofing materials it is greater than 0.8.
*Fred, Thanks for replying to my question. I always enjoy reading your thoughts on things. I'm a true believer in your construction measures but haven't yet tried the no-vent option. Theories and formulas aside, I'm not convinced until I see something work with my own two eyes, so I guess it's time I tried this out! By the way, what the heck is going on in Vermont?? Are you a victim of global warming??....
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Hi Gang, I just have to add this. Check a ski area that made snow before the ground froze and dig down to the ground, you'll find warm ground. Snow in ski area here melts all season even if if stays below freezing all winter. Our metal roof has r30 and r38 with a ridge vent, No snow melt at all until it warms up outside and the ridge clears. When this happens heating begins, and ice forms as the snow will start melting. The color of this roof is forest green and draws that sun uv and ir to begin heating and nightly cooling. With a 45 degree roof it is danger to be below, not to mention neat ice that looks like daggers waiting for us. I've seen a few of them penetrate the ground 8 inches........
Hope this helps, oh, we have stairs on the side of the cabin to get out when it's like this.
Cameron
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Fred,
Am I getting this straight? I've been thinking about this all day while I've been shoveling the extra sunshine off the front walk. I ran the AC in the truck today.
For the sake of argument let's say the indoor temperature is 60 degrees F. (I don't do metrics.) The out door temperature is 0 degrees F. There is a foot of snow on the roof or about 30#/sq.ft. The snow has an insulation value of less than R-1, but it has a specific heat of about .5 BTU/lb. and it has a latent heat of about 145 BTU/lb.
The attic insulation is effectively R-20. If the attic is unvented the thermal mass of the snow keeps the attic temperature at 32 degrees so that the heat loss from the inside(assumed to be 62 degrees) is 1.5 BTU/ sq ft./hour. Thus, it would take almost 100 hours to melt 1# of snow. But, this 32 degree snow is losing heat to the 0 degree outside much faster than that. So it stays frozen.
If the attic is vented the attic temperature drops to
0 degrees i.e. equalized with the outside and the heat loss through the attic floor insulation goes up to 3.1 BTU/sq ft/hour. Effectively double.
Do I have this right?
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Fred...Gene...other proponents of non-vented attics.........As I sit here looking at the 12+ inches of snow already on my roof, I'm wondering about the R-value of all that white stuff and the effect it would have on an unvented attic. I have a very well insulated and sealed home with a vented attic (soffit & ridge vents). I have no snow melt or ice dam problems with this system. As I recall, the R-value of a foot of snow is roughly comparable to 3 1/2" of fiberglass and once again I'm on my way to a very well insulated roof. My question is: Isn't it possible that with all this insulating value on top of the shingles, that an unvented attic (no matter how well insulated&sealed) , would eventually warm to the point where it would start to melt the snow at the shingle line and contribute to ice dam problems?? Or am I missing something?? Hoping everyones attic functions well in the New Year!
*If the R value of snow is what you suggest, does that mean I can have a toasty 70 degree interior with just 4' or 5' of snow between me and -20 degree exterior for the rest of the winter? (And are you using the label R value or the actual R value of fibreglass?)
*Bill, You may not believe this, but snow does have a considerable insulating value. I was an engineering student up at Michigan Tech in the Upper Peninsula where the snow comes heavy and early, and you could go out and dig a hole in the ground almost any place that was covered with early snow and undisturbed. We dug soil boring and testing pits all winter long in these areas and the ground was never frozen, even though the outside temperatures commonly get to 15 or 20 degrees below zero during the winter.
*Just some food-for-thought.* Igloos. Keeps many Eskimos warm and protected. So snow and ice can give you that environment, Bill. But they (igloos) do vent out the top!!Merry Christmas!
*I understand it has some insulating value but I doubt the igloos are a toasty 70 degrees. I'm just questioning applying a straight line r value to snow - or anything for that matter.But I go back to the observation that when I go out and look at snow melt on roofs, the vented roof bays seem to be the first place it melts, unvented bays and over rafters later.From my observations, when all other things are equal, the "vented" roof snow melts faster. Try blocking half you vents and look where the snow melts first.
*Sorry to throw cold water on the idea that snow can make good insulation, but heat loss is dependent on other things than simple R-value. By definition thermal resistance (R) is proportional to temperature differential. Liquid and solid water have considerable thermal mass, and even greater heat capacity at the point of phase change (32 degrees). So heat loss to 31 degree snow (or ice) will be considerably more than to colder air which has very little heat capacity. If you don't believe this here is an experiment. Go outside naked in 0 degree air. Stay there for ten minutes. Now jump into 34 degree water stay there for ten minutes. Now examine your self. After the ten minutes in the air you will be very uncomfortable. After the ten minutes in the water, you will be dead. Come to think of it DON'T TRY THIS AT ALL.