Exterior vs. Interior Shades: What the Research Shows

Blinds and curtains block the light, but by then the sun's heat is already inside. Shading the window from the outside stops most of that heat before it reaches the glass.

Solar heat gain on a 90 degree day: no window covering 80 percent and about 86 degrees inside, interior covering 50 percent and about 80 degrees, exterior covering 15 percent and about 73 degrees — Illustration: what 80%, 50% and 15% solar heat gain can feel like inside on a 90° day.

Why outside beats inside

Stop the heat before it gets in

Interior blinds trap the heat

Sunlight passes through the glass, hits the blind and turns to heat inside your room. The blind blocks the glare, but the heat stays in.

Exterior shades keep it outside

An exterior screen or awning catches the sun before it reaches the window. The heat it absorbs stays outdoors, so the room behind it stays cooler.

The U.S. Department of Energy's Building America Solution Center makes the same point: because exterior attachments block sunlight before it passes through the window, they are typically more effective at reducing solar heat gain than interior ones.

Measured here in Washington

What the PNNL study found

Pacific Northwest National Laboratory, working with Lawrence Berkeley National Laboratory and funded by the U.S. Department of Energy, tested motorized exterior fabric shades in side-by-side lab homes in Richland, WA, then in three occupied homes.

~20%less air-conditioning energy than windows with no coverings

~10%less air-conditioning energy than interior vinyl blinds

2–3°Fcooler rooms than rooms with interior blinds

~9°Fcooler at the afternoon peak in a test with the AC turned off

Source: Hunt, W.E. and Cort, K.A., Evaluation of Exterior Shades at PNNL Lab Homes and Occupied Field Sites, PNNL-30536, Pacific Northwest National Laboratory, 2020. Sponsored by the U.S. Department of Energy Building Technologies Office.

Put it to work

Exterior shade for your home

Exterior screens

Motorized fabric screens, the same kind of shade PNNL tested. Block heat and glare while keeping the view.

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Window awnings

Shade individual windows from above to cut the heat in the rooms behind them.

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Retractable awnings

Shade the deck and the glass doors behind it, then retract when you want the sun.

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Find out where shade would help most

At a free consultation we'll look at which windows take the most sun and recommend the simplest way to keep the heat out.

Schedule a free consultation Call 425-372-7766