Shading Windows to Cut Cooling Costs

Window shading saves cooling energy by stopping sunlight before it becomes heat inside the house. The closer to the outside the sunlight is stopped, the less heat gets in.

With standard double-pane windows, about 76% of the sunlight that enters the glass becomes heat inside the home, according to the U.S. Department of Energy study Energy Savings from Window Attachments. The same study puts window heat losses and gains at about 25% of building HVAC energy use.

This concept decides three choices: what kind of shade to buy, which side of the glass to mount it on, and which windows to treat first.

Solar Heat Gain Coefficient (SHGC) is the share of solar heat that a window lets through. The NFRC consumer guide rates SHGC on a scale of 0.00 to 1.00. A lower number means less heat gain. ENERGY STAR states that window SHGC ratings typically range from 0.15 to 0.60.

A shade changes how much heat reaches the room. Its position decides how much it can change:

• Exterior shade. The shade intercepts sunlight in open air. The heat stays outside the glass.
• Interior shade. Sunlight has already passed through the glass. The shade must reflect it back out through the window. This is physically harder than blocking it.
• Film on the glass. The film sits on the pane. One industry source, EZ-Snap Direct, states that the pane heats up and re-radiates some heat inward.

The Building America Solution Center puts it this way: interior attachments are less effective than comparable exterior shading "because they do not block direct sunlight until after it has already passed through the window."

Orientation sets how much sunlight arrives in the first place. The Building America Solution Center shading guide gives these figures for midsummer in the contiguous U.S.:

• East- and west-facing windows receive up to 2 times the solar heat per square foot of south-facing windows.
• One square foot of roof or skylight receives 3 to 4 times the solar radiation of one square foot of north- or south-facing wall.
• North-facing windows need minimal shading, except at very low latitudes.

Insect screens, same material, two positions. A 1999 study (Brunger et al.) found that exterior screens reduced solar heat gain by 46%. Interior screens reduced it by 14%. The screen material was the same. Only the position changed. Source: Building America Solution Center.

The 2021 Portland heat wave. Researchers (Rempel et al. 2022) modeled homes without air conditioning. The reduction in dangerous heat-index hours was:

• Interior roller shades: about 18%
• Insulating interior shades: about 23%
• Exterior shades: about 55%

Awnings by orientation. Consumer Reports, citing DOE figures, reports that awnings reduce solar heat gain by up to 65% on south-facing windows and up to 77% on west-facing windows. The same source gives figures for interior coverings:

• Highly reflective blinds, fully closed: about 45%
• Medium-colored draperies with white plastic backings: about 33%

Cellular (honeycomb) shades. These are interior shades with air-filled cells. The DOE Energy Saver page states that a tight installation can cut unwanted solar heat gain by up to 60%. The older DOE study gives up to 80%. The two DOE sources do not agree, so treat 60% as the more conservative figure. Shades mounted on side tracks seal best.

Whole-home cooling. DOE field studies (Cort et al. 2018; Hunt and Cort 2020) measured summertime cooling savings of 15% to 25% from optimized use of high-efficiency interior and exterior shades combined.

The 13% figure is from ENERGY STAR. The point that shading and SHGC can substitute for each other is from the Efficient Windows Collaborative state fact sheets.

1. Find each window's orientation. If a window faces east or west, it gets the most summer heat. Treat it first.
2. Prefer exterior shading where it is practical. In every measured comparison above, the exterior option outperformed the interior one.
3. Size a south overhang to the window. In the Building America example for Boulder, Colorado, a 36 in (0.91 m) tall window needs an overhang about 19¼ in (49 cm) deep. The overhang sits about 11½ in (29 cm) above the top of the glass. On each side, the overhang should extend past the window by at least the height from the window bottom to the overhang bottom.
4. If a window faces east or west, a simple overhang is not enough. Low sun angles pass under it. Building America advises deeper cover (a porch roof or pergola) or vertical shading such as trees, trellises or solar screens.
5. Check AERC ratings. The Attachments Energy Rating Council rates 12 interior and 7 exterior attachment types. DOE recommends comparing products by these ratings.
6. If you are buying windows, match SHGC to your shading. If cooling is a significant concern and no shading is available, the state fact sheets advise SHGC below 0.32. In homes with high A/C costs or overheating, they advise 0.25 or less. ENERGY STAR advises 0.23 or lower in southern, hot climates.
7. Weigh winter. Low-SHGC glass also cuts useful winter solar gain. Building America recommends fixed shading (overhangs) combined with operable shading (blinds, screens) for performance across seasons.

UK homes. A UK industry source, Artistic Blinds, flags wind exposure as a critical issue for external blinds at coastal or elevated sites. It names film as an option for listed buildings where the exterior cannot be altered. The sources used here do not cover UK building regulations on overheating. Check those separately before building.