Do exterior solar screens provide radiant barrier to heat loss?
I live in Baltimore. I have been thinking about maintaining high solar heat gain in my windows for winter benefit. In the summer, when I need to put up insect screening anyway, my thought is to use one of those dual purpose insect screen and solar screen (e.g. like Linay type). I don’t see any measured SHGC on the solar screen websites but there are apparently 70-90% solar radiant heat reduction options. I like the idea of cutting down the radiant heat before it reaches the window pane to reduce cooling load. Now I am wondering whether these external solar screens offer any value in the winter. If they were rollable, I could keep them up during the day to allow heat gain. At night, when I go to draw the curtains, I could also lower the solar screen. Would this reflect radiant heat escaping the house at night (improve the nighttime U-value)? Would it keep the exterior window pane warmer and thus also cut down on convectional heat loss within a multi-glazed window unit and possibly window condensation?
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Replies
Richard,
I don't see why not.
While I'm not aware that anyone has measured thermal performance improvements attributable to exterior window screens, what you say makes sense.
There are two problems, though:
1. Most people don't want to go to the trouble of adjusting their window screens every time the sun goes down. That chore gets tired fast.
2. If you forget to raise the screens in the morning, you forgo the solar heat gain when the sun comes up — a big disadvantage.
Martin,
Thanks. That answers my question. I know what you mean. I'd just as soon put up the screens in late Spring and take them down in Fall and forget about them otherwise. Making them rollable/retractable is an added cost. This is nice though in Spring to have really clear windows when the windows are shut and roll down the screen when you want to open them. I don't think that's enough of a benefit, though to go rollable. However, if they would also give you added wintertime heat retention at night, things get more interesting. I close the curtains anyway, so adding a step to pull down the screen at the same time is no problem for me (especially if I know I can save money if I do). I would have to remember to pull them up when I open the curtains, as you say.
Ordinary window screens can block as much as 50% of the sunlight, while solar screens are generally more restrictive and either reflective or absorbent of visible light.
They are not designed to block far infra-red (heat), as the sun's rays arrive in a different part of the spectrum and do not turn into far infra-red until they are absorbed and re-emitted at room temperatures.
So there is likely little or no radiant barrier quality to window screens. To the extent that they reduce air flow at the outer glass surface, they may slightly reduce heat loss.
Robert, you're right. I read the websites more closely. They say they can reject 90% of the heat of the sun, but it seems this is from reduction in visible light spectrum, UV light and cutting down wind convection. "Like having a large shade tree" is the way one site puts it. The benefit in winter is from cutting down on the wind reaching the glass. Several sites seem to quote a 15% improved window thermal performance in winter.
This is an interesting subject to me because windows are tested and simulated without screens. Screens affect both SHGC and U. So the question of how screens affect a building's energy efficiency can be nuanced.
I recall a study done in a Solar Test facility in Toronto about 15 years ago on this topic. I'm trying to find someone with a copy.
Around here screens are non-optional in the summer. Many people remove them in the winter for asthetic reasons. The question the study tried to answer was based on energy concerns alone, is it better to leave screens in place or remove them for the winte?. To be clear, it was a seasonal cycle question rather than a diurnal cycle question.
Assuming my memory serves me correctly, which is an increasingly debatable proposition....
With 'standard' (non solar blocking) screens;
- Exterior screens should be removed in the winter
- their increase in insulating value was more than offset by diminished solar gains to the room
- Interior screens should be left in the winter
- they offered a sllight insulating value without affecting solar gains to the room
Hope that helps
Stephen
That doesn't make sense to me. If anything, the insulating value from a screen would be prominent in the exterior position since it's primary function would be to reduce windwash. The ASHRAE assumption for windows is a 15 mph average exterior wind speed for an exterior air film R-0.17, while the interior still air film offers R-0.68.
I suspect that the loss of solar heat gain would be much greater with exterior screens than with interior screens, and perhaps the recommendation would be for that reason.
i just put up coolaroo material over my two west facing windows in the hopes of lessening heat build-up on my patio. the heat is extreme here in south florida but, frankly, i have not noted any lessening of the heat in this room - i have lined all my verticals on the south facing windows and now have planted a large mehoe tree on the south side. what is the story with heat and coolaroo fabric - thank you
Anonymous, I'm curious what you concluded on the Coolaroo fabric? I just added the Coolaroo screens to the exterior of the west side of my house and am now wondering if I'm reducing or gaining heat on the west. My Windows are reaching temps over 100 degrees despite the screens which obviously give me the impression they are of no value in reducing heat gain. And may actually be increasing it. is there any info or testing on the Coolaroo fabric?
The Coolaroo fabric should be working unless it's touching the window. Of course, it must be installed outside the window. 100 degrees isn't too surprising if it's 80-90F ambient. 10% of the solar heat is still reaching the window.
Do you have two similar windows that you can do a comparison test with?
Regarding external screens:
a) The most helpful study may be one said to be in progress at LBNL. They are doing a "technical evaluation of window attachments". If external screens are included, we may gain insight on the economics as well as the intangibles associated with external devices. I'm told we may see an initial report at the end of summer, 2014. In the Chicago area I believe it's more important to allow the winter sun to heat the home than to benefit from the insulation value of an externally screened window (no data supporting this opinion, however.)
b) I have some experience with solar grates. The grates provide savings similar to solar screens. While the screens are 80% closed, the grates are 80% open. Screens darken the room some while maintaining a view. They are recommended for permanent installation to avoid the labor and handling damage and they appear most commonly in warmer climates where the winter heat loss is accepted. The appearance from the curb is changed.
Grates are designed for seasonal use. They require virtually no maintenance. Grates operate using selective reflectivity such that impinging sunlight causes the infrared portion to be greatly absorbed while the visible fraction is generally reflected into the building to keep it well lighted. A view is maintained through the grates with the best view being head-on. The appearance from the curb is changed. Our grates are installed once the a/c season begins and are removed once the a/c is no longer needed. It's "install and forget". (If the grates were integrated with shutters they could be stored in place.) With the grates stored in winter, the window is bathed in whatever sunlight is available. To boost the R value of the window from conductive and convective loss, I install an acrylic insert on the interior. The acrylic is in place year around.
As regards insect screens, we have casement windows with the screen inside. We select which windows we want to designate for occasional ventilation and leave those screens in place all year. If we had external insect screens the grates would go outboard of those screens during summers.