Horizontal window overhang calculator


Aid to sizing window overhangs for optimal energy savings

If your windows aren’t working for you, they’re working against you.

This is true whether you spend most of the year running an air conditioner, a furnace, or an even split. It’s true because windows – even the most expensive gas-filled triple-paned sort – have paltry insulation values compared to walls. Which is to say, windows let outside heat in or let inside heat out much faster than your walls.

Of course, we have windows for other reasons too: natural light, views, and emergency exits from bedrooms.

But how can your windows be made to work for you, thermally speaking? With properly designed overhangs or awnings!

With right-sized overhangs on the side of the house facing the equator, you can let in the low winter sun and block the high summer sun, reducing your heating and cooling bills and increasing your comfort. This is low-tech green at its finest.

Calculating the right size for overhangs for your latitude, given the size and height of the window and roof can seem daunting, especially if your window isn’t facing true south. SketchUp includes great tools for visualizing the sun’s angles at different times of year, but building a model of your entire structure might be overkill for smaller projects. SketchUp also renders very slowly if your computer doesn’t have a lot of free space.

That’s where this web-based calculator comes in. Whether you’re building from the ground up or want to tweak the thermal performance of an existing window, just enter your measurements and this tool generates a graphic representing your window and the shadow of your overhang. Sliders allow you to change the time of day and the time of year, and the graphic responds quickly.

The relevant dates are the solstices (roughly June 21 and Dec 21) and equinoxes (roughly March 21 and Sept 21). Generally, you want to design an overhang that provides complete shade to south-facing windows for the summer solstice, and complete sun for the winter solstice. Since it’s often still cold in March and still hot in September, you then decide which side of the equinoxes you want to compromise on – whether you’d rather get less heat from your window in the spring or deal with overheating in the fall. Shade fins and/or shutters can help minimize excess fall heat.

There’s more to know about the ideal ratio of window glass to floor space, and how thermal mass plays into the equation, and there are many great books on passive solar design theory, but when it comes to putting it all into practice, a calculator like this is indispensable.

-- Reanna Alder 10/27/15

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