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
40 PRINT 100+N;"'";GAIN
layers net gain
1 -505.2 Btu/ft^2-day
A skylight with a south tilt or a shutter or a reflector can gain more.
Insulation can only LOSE heat.
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.
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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
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
Sorry...here are some details:
The skylights are flat (not the bubble type). The brand, I believe, is
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.
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
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.
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
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
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.
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
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
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
Art Todesco wrote:
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
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
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.
On 25 Dec 2005 10:11:52 -0800, email@example.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.
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