Does snow on a roof simulate insulation?

Mike T., Swampeast MO

While I don't claim to truly understand I've been taught and have read that the roof will always "see" 32F when covered with snow regardless of the air temp outside.

D107

Since I'm planning a ventless attic

with polyicinine foam in the 8" joist bays under the eaves, I thought a good 8" snow (dense pack I'd hope) would give me some idea of the insulation I'd get. Of course the gable walls could not be part of this test, so I guess the value would be limited.

Thanks,

David

jp_2

south facing?

I've seen south facing shingled roofs with no venting curl in a couple of years.

snow will insulate, I've looked and looked but have not found the insulating factor i wanted to see. i've wonder the same about ice.

Mark Hunt

No


If the heat gets that far David, it is of no use to you anyway.

Once the heat leaves the thermal envelope, it is lost.

About the only thing that snow on the roof does is: 1) Indicate that you have little heat loss in that direction
2) Reflect any solar gain away from that direction.

I could argue that snow accumulation on a roof is bad and good at the same time.

Confused yet?

Mark H



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jp_2

snow is water......

remember, snow is water, and how many btu's to change that water 1F? so it has buffer like qualities.

fluffy snow, say 20F, will act like fiberglass insulation, little air pockets.

ice and snow will get as cold as the surrounds, but again with a time delay dependant on either mass or 'fluffiness'.
so sleeping in a snowbank during the night will help keep the night lows away from you.

'they' say a snow cave will keep you 'warmer than'.

but i have not found any numbers to the above.....sorry

in canada we kept dead cows covered in snow so they wouldn't freeze before we butchered them. I was totally amazed that this worked, left covered for several days and they did not freeze, easy to skin.


added:

according to wikipedia, snow has a R1 rating per inch

http://en.wikipedia.org/wiki/R-value_(insulation)

so your 8 inches would increase total seen by house, an increase of R8.

D107

'once heat leaves thermal envelope'

well I was hoping that snow would be the edge of that envelope. In unvented attic, envelope becomes the foam under the eaves; with snow it would be pushed outward 8-10"....I guess I was thinking of the igloo--but of course those are probably constructed with a special technique by the Eskimos.

As for it being good and bad, you gave the two reasons. If you had others in mind, I'd say that being snow it has by definition a sub-freezing temperature which will be conducted downward into the house, while at the same time it may insulate against transmission losses upward from house.

(To answeer JP's question, house eaves face NE and SW.)

Thanks,

David

realolman

I wonder if you would mind explaining a little more about what you're doing. I'm not getting it.

I have an old house and I installed insulation everywhere, One of the best things I did was to tear off all the existing soffit and facia and drill big holes in the old wooden soffit underneath and then re-install the aluminum with vents.... and a ridge vent... along with over a foot of insulation in the attic

Up in the attic the rafters were at non standard spacing so I made things from sheathing to hold the insulation down from the roof deck.

I used to have icicles 15' long. Now I got a nice covering of snow and no icicles. It's one of the few things about the wintertime that makes me feel good every time I see it.

Anyway.. I'm not understanding what you're doing . I wonder if you would explain it in some more detail. Thanks

D107

Check buildingscience.com

It's called an unvented attic. that website has a lot of free pdf files (and books for sale) that show how to construct both a vented and unvented attic. Frequent wall visitor Constantin has constructed same kind of attic with no apparent problems.

In my situation I have a nice attic space (10ft peak) that I don't want to lose by dumping cellulose a foot high on the attic floor--above the 6" or so existing old fiberglass batts which is the only way I would get a decent level of insulation.

So spraying foam between the eave joists and the gable walls will give both a good seal and insulation to the attic, making it a conditioned space. When I install a new roof, it is recommended to put some rigid insulation above eaves as well to lessen any possible heat damage to shingles. Building Science claims damage to shingles from reflected heat is minimal if it's done right. Probably good to have an HRV installed at same time to ventilate newly tightened house.

jp_2

sorry but,

this statement does not make much sense:

"""When I install a new roof, it is recommended to put some rigid insulation above eaves as well to lessen any possible heat damage to shingles. Building Science claims damage to shingles from reflected heat is minimal if it's done right."""

adding insulation 'under' a shingle will damage more than protect. it will help keep heat within the shingle. reduce ability for the shingle to shed heat.

also: from where is this 'reflected heat' coming from?

sounds like you better do some more research?

D107

I am sending you privately the pdf off the building science

website since it's probably copyrighted material. Though my wording may be inexact, I believe the article will sustain the general idea. In an unvented roof, as I understand it, the additional heat comes from the insulation under the sheathing that does not permit the heat to dissipate as it would ordinarily with a vented roof. There you generally have a soffit vent leading air under sheathing up to a ridge vent and out. Again, Building Science claims that if done right, this additional heat should not be a significant factor in shingle life.

D107

I am sending you privately the pdf off the building science

website since it's probably copyrighted material. Though my wording may be inexact, I believe the article will sustain the general idea. In an unvented roof, as I understand it, the additional heat comes from the insulation under the sheathing that does not permit the heat to dissipate as it would ordinarily with a vented roof. There you generally have a soffit vent leading air under sheathing up to a ridge vent and out. Again, Building Science claims that if done right, this additional heat should not be a significant factor in shingle life.

Thanks,

David