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Old 07-23-2009, 05:18 PM   #1
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Reducing Skylight Heat Loss?


Hi to all from the new guy.

We just bought a house that has some ice damming issues. I'm pretty sure the ice damming results from heat loss through our 2 skylights. We live in an area that gets deep winter snow (northern Canada). House was built in 1996, and skylights are good double pane Velux units, but there was a horrendous amount of ice below them last winter. We get deep snow here.

The skylights are situated above (1) the stairs, and (2) the bathroom. Both skylights are at the top of tunnels which extend from the ceiling about 7' up to our 5 12 pitch roof. Unfortunately, the bathroom skylight is only about 20" away from the top of a gable valley, where the major ice problems occurred.

I have heard that one way to reduce heat loss up these 'tunnels' is to frame in an acryllic 'window' at the bottom of the tunnel, to prevent heat/steam from escaping up the tunnel and melting snow on top of the glass skylight. Has anybody attempted this, and can you advise if it has been effective in minimizing winter-time heat loss?

Our roof also has very little exhaust venting, so I'm planning to have continuous ridge venting installed to allow any heat to escape.

suggestions?

thanks!

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Old 07-23-2009, 05:46 PM   #2
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Reducing Skylight Heat Loss?


My attic fan is wired so that I can override the fan 'stat and turn it on in the winter to prevent ice dams. Never had any more ice dam weather so never tested it.

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Old 07-23-2009, 06:05 PM   #3
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Reducing Skylight Heat Loss?


Are they venting skylights?
If so you want the layer removeable

One apt I did install some heavy glass as a barrier
Anything that prevents heat from going up the trunnel will help
Also check to make sure around the tunnel was insulated
And of course the attic...or is it a cathedral ceiling?

Any lights nearby? Incandescent? Change them to CFL
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Old 07-23-2009, 06:41 PM   #4
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Reducing Skylight Heat Loss?


attic floors are very well insulated, and the skylight has wrap around silver stuff (fiberglass mats I think). In terms of 'venting' skylights, they don't open up, they are fixed. Is that what you mean?

6 pot lights in the opposite end of the house and we're changing them to CFLs.

re: the fan, I can see how this would help, but I am going to go the non-powered route first, doing things such as insulating and ventilating, to see how far that takes us.

thanks for your feedback
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Old 07-23-2009, 06:46 PM   #5
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Reducing Skylight Heat Loss?


If they are fixed then I would make a permanent lower glass block
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Old 07-23-2009, 08:35 PM   #6
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thanks SD,

any reason you suggest glass instead of acryllic?
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Old 07-23-2009, 08:46 PM   #7
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Reducing Skylight Heat Loss?


Acrylic will discolor over time
You can get some glass cut thicker
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Old 07-24-2009, 01:10 AM   #8
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thanks SD.

sounds like a much more fun project than wrapping fiberglass insulation bats around the skylight tunnels up in the attic. I'm putting that one off until cooler fall weather.

I have some further questions about the most appropriate roof venting for this house, will post soon.

great forum, thanks for your help.

cheers
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Old 07-24-2009, 03:44 PM   #9
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I would put up a plastic obscure panel before glass. I frame skylight trunks with maximum flare at both ends, some finish out at 2'x8'. That would be one big, heavy piece of glass, especially if safety glazed. I would be worried of the liability if it ever fell from an earthquake, poor installation, etc.

Skylights may be tempered, heat-strengthened, laminated, wired, or approved rigid plastics- IRC 308.6.2

After you close off the trunk at the ceiling level, you may still get condensation in the 7' high trunk. This will probably mildew or mold, so keep an eye on it. The sun is warming the whole trunk with a heating- cooling cycle, without the ventilation it had before the panel. The paint will start to peel and discolor, the curb frame start to cup or twist. Been there, done that, until I removed the whole thing. Once you close it off, it will get even hotter, putting even more heat into the attic, much worse than before. Be safe, G
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Old 07-24-2009, 03:50 PM   #10
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Reducing Skylight Heat Loss?


Good point on the flared tunnel

Possibly remove the glass in warmer weather?
Or have it framed out/hinged so you can open it
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Old 07-24-2009, 04:38 PM   #11
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Reducing Skylight Heat Loss?


Good point!

Our skylights are good double-paned Velux units, and I've never noticed condensation up there yet. As you mention, his could change if I close the unit off, but I don't have any other options at this point, other than improving ventilation and blocking off the tunnel at the ceiling.

The main problem is that in winter, when there is a 30 degree celsius (sorry for the Canadian units) difference between indoor and outdoor temps, the snow on top of and around the skylight glass melts away and turns to ice.

If the skylight remains problematic in a year or two, would replacing this skylight with a solar tube beat my problems? How expensive are those things?
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Old 07-24-2009, 08:39 PM   #12
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Reducing Skylight Heat Loss?


Hi,

Has the ice caused a problem?
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Old 07-25-2009, 12:32 AM   #13
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well, the ice damming in this location has worn out an area of 25-year shingle roof in 13 years, and last year water leaked under some shingles and resulted in an insurance claim for the previous owner. We are shelling out for a new roof, and I'd ideally like to prevent the skylights and associated heat loss from causing any more damage.

plus, as my wife would say, I'm anal retentive
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Old 07-25-2009, 03:11 AM   #14
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Reducing Skylight Heat Loss?


You will still have heat trapped in the rafter bays of the particular rafters that align with the skylights, IF this is a vaulted/cathedral ceiling that you are talking about.

If so, then read this and get the brochure from the link.

Pay attention to the very last page.

Download a brochure from the Air Vent website, entitled, "Venting Hip Roofs" and on the last page of the brochure, it shows a method to ventilate the rafters below and above the Skylight that is installed in the path of air flow, which impedes the direct eave intake to roof top exhaust pathway.

Forget it. Here is the link:

http://www.airvent.com/pdf/literatur...s_HipRoofs.pdf

Ed
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Old 07-25-2009, 08:11 AM   #15
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Hi,

There is a 7' tunnel. It is not a cathedral ceiling.

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