Blog Post

How Window Screens Affect Winter Fuel Use in Heating Climates

The Midcoast Green Collaborative is a Maine-based public nonprofit with the wide mission of socially responsible economic growth for their region. There's also an associated good blog with a high percentage of building-based posts — lots of nice, accessible, generally bite-sized, hands-on observational science. One swell example begins, "Do you leave the screens in your windows and storm windows during the winter? We have been telling clients to remove them, but didn't have a number for amount of energy saved. Searching the web, I found nothing, so I decided to do a little testing..." Excerpt:
Next, I placed three pieces of dark soapstone (12 x 12 x 1/2 tiles) near a window facing south, on a sunny March day around noon. One piece was placed in the shade, one in direct sunlight (through the window), and one with the screen in the window. I came back and measured the temperatures of the three stones with an infrared thermometer from a distance of about 1 foot. The shaded stone measured 76.7ºF, the one in the screened window 88.7ºF, and the one in the unscreened window 96.7ºF.
From this it would appear that leaving screens in windows or storms during the winter block about 30-40% of the heat that would otherwise enter through the windows. Audits have the amount of heat obtained from passive solar through windows ranging from 10-25% of the fuel based heating. So, window screens represent somewhere between 3-10% of total fuel.
Of course, in climates and seasons where solar gain isn't wanted, there's an inverse corollary. There are interior and exterior window shades listed in GreenSpec. The section introduction says:
Window shades, blinds, and other treatments can control daylight penetration and significantly reduce heat loss or heat gain through windows. In commercial buildings, engineered window shading installations can be part of an integrated design strategy addressing glare, heat gain, and solar penetration. In residential buildings, insulating window blinds and quilts may be appropriate retrofits for older, leaky windows, when window replacement can't be justified. In new construction or when replacement can be justified, installing high-performance windows is usually a better option than investing in energy-conserving blinds or shades.

Published January 20, 2009

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Comments

January 22, 2009 - 8:59 am

From the description, I'm assuming that the window screen was on the exterior of the glazing. I'm interested in the performance of interior screens as typical with casement windows.

If the sunlight passes through the glazing and then is partially converted to heat at the dark colored screen, what happens to that heat? How much is lost back out the glazing and how much is transmitted through the screen mesh?

Time for another experiment?

Jim

January 28, 2009 - 4:37 am

John F -

While high SHGC windows appear to be a good thing for passive solar gain, the high SHGC glass also usually has low U-values, leading to an overall larger loss of energy during the winter months, unless skies are always sunny, and the high SHGC windows are only on the south. Typical average window glass is roughly either double clear at U-0.5, SHGC-0.6, or double Low E at U-0.33, SHGC-0.33. If someone is willing to manage insulating shades on a passive solar design in a variably sunny/cloudy climate, double clear can be a winner (only on the south side, that is), but I think most people don't want to worry about such things, and therefore Double Low E windows are the overall winner in heating climates.

Also, orientation specific glazing requires more forethought in construction - both in ordering and installation; selecting different window dimensions for the different windows increases the likelihood they get ordered and installed in the right way.

Among the other advantages of casement windows, having interior screens at least means that solar gain is blocked inside the window, as opposed to outside.

Casements also leave the possibility of adding a 3rd layer of glass/film into the location for the window screens. Home Depot & Lowes both sell inexpensive screen frame material onto which you can affix the shrink film window product (and foam gasketing for air sealing to minimize condensation issues), turning your double pane casements into something akin to triple panes.

January 26, 2009 - 11:00 am

I am thrilled to see the data on the window screen reflectivity problem. I have believed this to be true for 30 years. I have had the same issue with the National EPA energy Star program apporving only windows with a low SHGC (Solar Heat Gain Coefficient) in Upstate New York. A low SHGC is just wrong for New York where the heating season is 10 times more expensive than the cooling season. I would like to find an Energy Star rated window with a huge heat gain for my climate. Any heated houses in New York need high SHGC windows with no screens!! thanks for the data.

January 23, 2009 - 6:29 am

Jim,

The test was done with the screens on the inside, but that shouldn't affect the results too much. Indoor air temperature change was not accounted for however. The math assumes that screens would be on the outside.

For inside screens the effect is less, since the intercepted light is turned to interior heat. However, that screen is now warming the glass, which increases conductive heat loss though
it. Not as big a win, but still worth doing I suspect. No math for that situation however as it gets a little tricky. Do you have a suggestion for how do the experiment?

January 23, 2009 - 3:45 am

Jim,
The absorbed heat of the screen on the inside of the window would be given up by convection to the room air, or reradiated in infrared frequencies short and long, most of which would be reflected back into the room by the window glass.

January 23, 2009 - 3:42 am

It's true that the bug screens provide some shading of the solar radiation, and therefore have a very slight passive cooling affect during warm weather, and a very slight passive cooling affect in cool weather as well - if they are mounted on the exterior side of the window.

Regarding whether they help warm the room if mounted on the inside of the window, it technically depends on (1) color - and since most screens are dark gray, then they don't reflect too much light back out of the window, and (2) what suface is next ... the desirable sun's heat could be absorbed by the floor, furniture, or occupant - which may be preferable from a comfort or an energy storage perspective.

Screens do not have an impact on the U-Factor (insulation) of the window surface.

So, they can provide very slight shading that is desirable only in warm weather. More effective shading devices provide serious energy-efficient improvements, and can be dynamic - rolled up manually or automatically as conditions change.

February 19, 2009 - 8:05 am

Greetings John,

While a soft coat lo-e double might have U - 0.25 and SHGC - 0.39 (all values for center of glass)
A hard coat lo-e double might have U - 0.29 and SHGC - 0.71
And a hard cost lo-e triple might be U - 0.15 and SHGC - 0.51

Forget Energy star. Look at the Canadian Energy rating (ER). Or better yet find the best windows for your particular situation.