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Home > Archive > Alternative Power sources > July 2005 > Re: DANGER: RADON
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| On Mon, 18 Jul 2005 12:59:48 -0300, Derek Broughton
<news@pointerstop.ca> wrote:
quote:
>Robert Morein wrote:
>
>
>Why would it? Ice build-up is caused by the melting, not freezing. If your
>roof stays cold, you won't get ice damming.
The snow is self-insulating, so the lower layers tend to melt due to
heat escaping from the house. The problem is worse in geographical
areas (and in normal thawing seasons) where wide ambient temperature
swings occur during the 24hr day. In continental areas, this swing can
easily be 15degC (37degF), daily, any time of year.
Melted snow doesn't run off so much as bead and compact or creep in a
slurry, to be caught by the next temperature dip. Ice is a lousy
insulator, as the air content is removed - so a sort of equilibrium
occurs - thick snow forming ice to the level where the original
insulating effect is no longer sufficient to melt the snow.
Roof slopes tend to show little regard to geography, nowadays. Its
fashion, convenience and low cost that sells.
RL
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| On Tue, 19 Jul 2005 20:47:33 GMT, "daestrom"
<daestrom@NO_SPAM_HEREtwcny.rr.com> wrote:
>
>Actually, ice build up is *not* caused by daily temperature swings
>above/below freezing. Take it from someone that gets 160 inches a year
>average snowfall. Ice forms when heat in the attic melts the bottom layer
>of snow, and the water runs down out over the eaves where the outdoor
>temperature underneath the eave is below freezing. When the outdoor
>temperature is *above* freezing, the ice at the eaves melts and any water
>melted under the main roof doesn't re-freeze before dripping off the eaves.
>
>The key to preventing ice dam build up is to keep the attic air space as
>*cold* as possible. Lots of soffit vents, and either ridge or gable vents
>(or both) to keep the attic air space as close to outdoor temperature as
>possible. If the home has 'cathederal' style ceilings, we put a lot of
>insulation in the ceiling, but leave a 2 to 4 inch gap between the top of
>the insulation and the roofing. This allows a cold channel under the
>roofing to prevent heat leaking through the insulation from melting the snow
>on the roof.
>
>If part of the roofing gets exposed to the sun, and it is a cold day (below
>freezing), this can also build up some ice. The sun on dark shingles will
>melt some snow around the edges of the exposed shingles and the resulting
>water will re-freeze as it runs down the roof towards the eaves. But in
>heavy snow country this isn't much of a problem since very little of the
>roof is exposed to the sun :-(
>
>In deep snow country, the pitch of the roof and wind exposure are issues
>with regard to how much snow accumulates. Surprisingly, sometimes a low
>pitch roof (such as 5/12) accumulates fewer drifts than a steep (12/12)
>roof. While the windward side of both roofs are swept clean of snow by fair
>winds, the lee side of a steep roof often accumulates much deeper drifts
>because of the swirls/eddys as the wind trys to rise up over the windward
>side then 'detaches' from the ridge. On the shallow roof, the wind can flow
>more smoothly over the lee side and less snow is deposited in the form of
>drifts. Of course, this depends on lot on how the local wind flows
>over/around the house.
>
I find that shoveling the snow, to maintain less than six inches of
coverage, goes a long way towards reducing icing at my place.
'Lake effect' is non-sequitur, but then I guess ice dams were too.
RL
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