# Heat loss through skylight

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• posted on December 28, 2005, 10:50 pm

Nope. Average days.
Nick
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• posted on December 27, 2005, 9:42 pm

You are wrong on all counts. Try numbers :-)
One more time: 460 Btu/ft^2 of sun falls on the ground on an average 26.7 F December day in Chicago. Unlike insulation, any skylight with more than 2 layers of glazing will gain more solar heat than it loses... 6 is optimal:
20 FOR N=1 TO 8'layers of glazing 30 GAINF0*.9^N-24*(65-26.7)/N'Btu-ft^2-day 40 PRINT 100+N;"'";GAIN 50 NEXT
layers net gain
1 -505.2 Btu/ft^2-day 2 -87 3 28.93997 4 72.00598 5 87.78537 6 91.26282 7 88.70224 8 83.11487
A skylight with a south tilt or a shutter or a reflector can gain more.
Insulation can only LOSE heat.
Nick
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• posted on December 25, 2005, 9:13 pm
snipped-for-privacy@yahoo.com wrote:

Step #1, check to see that you are actually losing heat....climb up to the top and see what the temperature is. On sunny day it will most likely be *way* hotter than your house and you are gaining; probably will lose at night. Maybe they'll balance each other out.
dadiOH's dandies v3.06... ...a help file of info about MP3s, recording from LP/cassette and tips & tricks on this and that. Get it at http://mysite.verizon.net/xico
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• posted on December 25, 2005, 9:38 pm

The highest point will always be the hottest, no? The skylight loses heat all of the time. The room will gain some heat during the day, dependent on orientation and weather, due to solar gain.
It's possible that the solar gain may outweigh the heat loss, but unless there's some heat storage mechanism, and the skylit rooms have their own thermostats, the room temperature will fluctuate between too hot and too cold.
Adding another gasketed pane of glass or plastic to seal off the bottom of the skylight or skylight shaft will go a long way to help minimize heat loss. Ceiling fans will help mix the air and help keep the heat more evenly distributed throught the height of the room - it will keep more heat where people live - 6' and lower.
On the flip side, a retractable awning/shade will help keep some of the excess solar gain from warming the room and its contents during the summer.
R
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• posted on December 25, 2005, 9:30 pm

It could also be that those rooms just don't get as much heat anyway. While your conclusion may or may not be correct, the reasoning is wrong.
If the skylights don't have some sort of double pane or storm panel, add one. Watch for excessive heat buildup though, when the sun shines
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<%-name%>
• posted on December 25, 2005, 9:43 pm

Why is there air?
Since you provide absolutely no information on your construction/type of skylights and climate, I refer you back to my questions.
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• posted on December 26, 2005, 5:49 am
SQLit wrote:

Sorry...here are some details: The skylights are flat (not the bubble type). The brand, I believe, is Velux. I live in Chicago. I have thought about getting plexiglass installed to the inside frame but now concerned that it may not be a good idea since during summer, the 'pocket' between the plexiglass and the skylight might get too hot.
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• posted on December 26, 2005, 6:09 am
I just installed a sheet of tempered glass in a clients skylight. Her's is more like 85 years old. Built like those old casement windows. As soon as we dropped it in place you could feel the draft stop. The air shaft to this one was more like 5-6' deep. It was in Evanston, by the way. You could also use polycarbonate or that multi channel stuff they use for greenhouses. That would certainly take any temperature you could throw at it. Richard
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• posted on December 26, 2005, 3:07 pm
Velux is the leading and quality brand of skylights. Ones that are 12 years old should be reasonably energy efficient, provided they are still intact and have not loss the seal between panes. I agree with those that have said the skylights may not be your problem. There are lots of other factors at work. How the heating systems is set up and balanced is one. Another is any other differences in rooms. For example, skylights are often used in rooms with cathedral ceilings. Those areas take more heat because of the high ceiling, plus frequently these ceilings have less insulation than one with an attic.
I have two good size Velux in my family room and don't have any problem with the room being colder. Plus, you have to balance what you are achieving. If you save some energy by putting up something more to shield the windows and it looks like hell or blocks the light, is it worth it?
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• posted on December 26, 2005, 2:21 pm

December is the worst-case month for solar house heating in Chicago, when 460 Btu/ft^2 falls on the ground and 740 falls on a south wall on an average 26.6 F day with a 34.0 daily max, so 1 ft^2 of single of R1 horizontal glazing with 90% transmission would gain 414 Btu and lose 24h(65-26.6)1ft^2/R1 = 922 for a net loss of 508 Btu per day.

Good idea. That might reduce the gain to 373 and reduce the loss to 461 for a net loss of 88 vs 508.

In full sun (say 250 Btu/h-ft^2) in Chicago, 225 might pass through the first glazing and 203 might pass through the second, a difference of 22. If it's 83.7 F indoors and outdoors (the average daily max in July), R1 resistors to that indoor and outdoor temp might make the temp between the glazings 83.7+22xR0.5 = 94.7 F, which seems OK.
I once made a hinged Styrofoam shutter for a west-facing skylight with a 1:1 slope and several layers of glazing and painted the skylight side black to make it warmer in wintertime, if the shutter were closed most of the time. It was, but after the dead of winter, the Styrofoam melted into a black mountain range with pink crevasses and peaks around the screws that held the foam to the plywood backing. It got nice reviews in an art show :-)
An R2 skylight might collect 0.81x1940 = 1571 Btu/ft^2 of unwelcome solar heat on an average July day in Chicago. A reflector hinged at the north edge of the glazing might eliminate most of that and enhance winter collection. Sun elevations in Chicago (N lat 41.8) are 90-41.8+/-23.5 = 71.7 and 24.8 degrees above the horizon at noon on 12/21 and 6/21, so the upper edge of a reflector can block all the direct noon summer sun if it's on a 71.7 degree elevation line up from the south edge of the glazing, like this, viewed in a fixed font: . - - -
. . |
<-- north 1.5' . . 1.12'
. . |
. . 48.2 degrees 71.7 degrees ............----------------------.................. 1'
If the glazing is 1' wide in the north-south direction, we can bounce all the noon winter sun into it if the reflector edge has a 90-41.8 = 41.2 degree elevation angle from the north edge of the glazing, which makes the upper edge 1tan(48.2) = 1.12' above the south glazing edge, with a sqrt(1+1.12^2) = 1.5' slant height, so 1 ft^2 of skylight would collect about 0.9x0.81x740 = 539 Btu and lose 461, for a net gain of 79 Btu/ft^2 per day.
If we replace the skylight glazing with hinged R10 foil-foamboard and use 1.12' of R1 vertical glazing and raise the board up to 48.2 degrees during the day, we might collect 0.9x0.9x740 = 599 Btu and lose 6h(70-30)1.12ft^2/R1 = 269 during the day and 18h(65-26.7)1ft^2/R10 = 69 at night, for a net gain of 261Btu/ft^2 per day.
If we leave the R2 glazing in place and use 1.12' of R1 vertical glazing, we might collect 482 Btu and lose about 6h(70-30)1ft^2/R3 = 80 during the day and 57 at night, for a net gain of 344.
Nick
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• posted on December 26, 2005, 3:06 pm
snipped-for-privacy@ece.villanova.edu wrote:

Snipped the rest.
Your reference to R1 and R2 skylights and glazing may or may not have been understood. You should indicate how those designations correspond to a typical skylight.
That homemade skylight shutter that had the foam that melted - what was the purpose of painting the top side of the insulating foam black to gain heat? That's just going to absorb the heat and raise the heat inside the foam - pretty much exactly the opposite of what should be done. The melted plastic is also a safety issue - it releases dioxins. The foam itself is required by code to be covered by 1/2" drywall, or equivalent, for reasons of fire safety.
It's also a little drastic to totally block skylights when the added light and skyview is the reason the skylight was installed in the first place. You're just trying to block the heat flow, not necessarily the visible light.
R
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• posted on December 26, 2005, 5:00 pm

Have you considered your idea as seasonal? Plexi or plastic will warp and absorb a lot of light. A lot cheaper for one season UV light eats the plastics over time and they will turn yellowish.
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<%-name%>
• posted on December 26, 2005, 6:15 pm
I have 2 baths with Velox Lo E skylights (about 4 square feet each) and motorized vent flaps (no windows in either). I haven't noticed any "cooler" problems as long as the vent is closed. These 2 baths are not that big. However, even if open, the air tends to go out, so it may produce a draft of moving air, but not cold outside air. I also have 2 of these same units in the kitchen, however, as the kitchen is open to 3 other rooms, I probably wouldn't notice. BTW, also in the Chicago Burbs.
SQLit wrote:

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• posted on December 26, 2005, 6:34 pm
Hi Art,
You said you have "motorized vent flaps" on your skylights. Did you get these installed (as an addition)? If so, where can I buy them? Would appreciate a contact number for the company where you bought them from and also the installer.
I called Velux and they said they have retractable shutters (motorized or non-motorized, but basically to block sunshine). Are the "motorized vent flaps" same as the retractable shutters? I am thinking of having Velux installed retractable shutters as the two skylights in the vaulted family room bring in sun directly on to my TV, making it almost impossible to see anything on the screen (during non-winter months).
TIA.
Art Todesco wrote:

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• posted on December 26, 2005, 11:32 pm
The Velux units I have are about 26" x ~24". They fits between a 24" on-center roof truss. They came with a small hinged wood flap at the top, which can be operated by cords or by a "stick with a hook." Check out http://www.veluxusa.com/products/skylights/fixedWithVentFlap / They offered an optional motor and controller to open and close the flap. When I bought the last 2 of them, they were about \$80 or \$90 each. The controller was very pricey (~\$200), so I engineered my own. The motors run on DC, one polarity to open and one to close. They have built-in limit switches. I used a transformer with a full wave rectifier. A rocker switch applies the + DC in one position and - DC in the other. BTW, the last ones were purchased about 8 - 10 years ago. I couldn't find the motors on their website, but maybe I just didn't dig far enough.
snipped-for-privacy@yahoo.com wrote:

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• posted on December 27, 2005, 5:41 pm
On 25 Dec 2005 10:11:52 -0800, snipped-for-privacy@yahoo.com wrote:

Maybe you should look for another source of heat loss?
I have two (velux) skylights and one 3050 window in my upstairs office. The room next to it is a bedroom and has no skylights but two 3050 windows. The glass area is about the same. The heat pump duct area supplying the rooms is the same. The area and volume of the rooms are the same.
The room with the skylights stays about 4-5 degrees warmer year round. My conclusion is that I'm picking up enough sun load to offset any heat loss caused by the glass area in the skylights. I live in the mid south, but sunload is sunload on a relative basis.
In the winter that 4-5 degrees is ok. In the summer when it can get to 85F in that room (with AC) we shade the skylights to lower the temp and help out the AC.
Frank