Do Window Awnings Reduce Heat? What Portland Homeowners Should Know

65%

Possible cut in solar heat gain, south-facing windows (DOE)

77%

Possible cut, west-facing windows (DOE)

68°

Height of Portland’s noon sun in late June

21°

Height of Portland’s noon sun in late December

On June 28, 2021, Portland hit 116 degrees, the hottest temperature ever recorded here, as OPB recounted on the fifth anniversary. Plenty of houses in this city were built for mild summers, with big west windows and no air conditioning. A window awning is one of the oldest ways to fix that, and it still works for a simple reason.

Why shading outside beats shading inside

Once sunlight passes through glass, it turns into heat in the room, and an interior blind can only slow it down. Shading on the outside stops it first. The Department of Energy’s Building America Solution Center puts it plainly: exterior window attachments “block sunlight before it passes through the window, so they are typically more effective at reducing solar heat gain.”

The same guide cites research on ordinary insect screens: mounted outside, they reduced solar heat gain by 46%; mounted inside, by 14%. Same material, different side of the glass.

Why awnings work so well at Portland’s latitude

Portland sits at about 45.5° north. At solar noon in late June, the sun stands about 68° above the horizon; in late December, only about 21°. That spread is what makes a fixed window awning on a south wall so effective.

Here’s the arithmetic. The shadow an awning casts down the wall is roughly its projection times the tangent of the sun’s height. An awning projecting 30 inches casts about 74 inches of shade down a south wall at noon in June, enough to cover a typical window. In December, the same awning shades only about 11 inches, so the low winter sun still reaches most of the glass.

East and west windows are different. Morning and afternoon sun comes in low, under an awning’s edge, which is why west windows benefit most from deeper awnings, side panels or exterior solar shades.

Which windows to shade first

1. West windowsLate-afternoon sun arrives when the house is already warm. The DOE’s 77% figure is for west-facing glass.
2. South windowsHigh summer sun, easily blocked by a fixed awning, while winter sun still gets in.
3. East windowsMorning sun is lower and gentler, but east-facing bedrooms heat up early.
4. Upstairs roomsHeat rises. Shading second-floor glass often makes the biggest difference to how a house sleeps.

Fabric or aluminum?

Both shade the glass the same way. The choice usually comes down to the house: fabric suits Tudors, cottages and Craftsmans that want color; aluminum suits a lower-maintenance or more modern look. See both on our window and door awnings page, or compare fixed frame types in stationary awnings explained.

Door awnings

Over a door, an awning’s first job is rain. A small dome or traditional awning keeps the doorstep dry while you find your keys, and shades a south- or west-facing door that would otherwise bake. In Portland, window awnings no more than 54 inches deep, supported by an exterior wall, outside a design zone and not projecting past the property line generally don’t need a building permit, according to Portland Permitting & Development. Commercial awnings fall under City Code Chapter 32.52.

The honest downsides

• They can block part of the view from the top of a window, especially with a deep drop.
• Side-angle sun can slip past an awning that’s no wider than the window, a limitation the DOE’s guide notes too. Making the awning wider helps.
• Fabric needs care. Clean it now and then, and plan on a recover someday; the frame usually outlasts the cover. Our awning fabric replacement guide covers what that involves.

Are window awnings outdated? Not functionally. The physics hasn’t changed, and with heat waves now part of Portland summers, shading glass from the outside is one of the simplest upgrades a house can get.