DIY Project: Beat The Heat with Reflective Window Shields
Reflect sunlight to stay cool. Easier to put up and take down. *Not* for insulating.
If you need to keep your house cool in the summer, sizing and cutting some reflective window shields is a quick and easy project you can do yourself.
But if you hate the idea of carefully taping and removing tinfoil, there is a better way - foil-faced bubble wrap. It’s easy to cut, easy to re-use, and quick to install or remove. You can cut shields for your windows for less than $60.
The Big Picture: Reflection, Shade, and Layers
To keep your house cool long-term, there are many layers of strategy:
🌳 Plant trees. Create shade and reduce the ambient temperature around your home.
☂️ Add outdoor blinds and shades. Stop the sunlight before it starts - prevent it from hitting your house in the first place.
🏠 Install a metal roof (expensive). If you can afford it (I can't) - using a metal roof or other materials that are not asphalt shingle will reflect more sunlight and absorb less heat.
🎨 Use a light-colored roof. A white or light color for your roof makes the most difference in lowering temperature. Even if you are stuck with asphalt, using a lighter color can make a difference of 20 degrees.
💨 Air seal your attic. Prevent heat from getting in by stopping air movement. Do this before you insulate.
💨🚪 Air seal your attic access hatch. This tiny door can let most of your heat in or out. You want flat insulation. Seal this hatch.
💨⭕Air seal everywhere else.
🧤 Add insulation. Slow the transfer of heat so it stays outside longer. Adding both wall insulation and especially more attic insulation can help keep the heat out of your house.
🔲 Install quality windows (also expensive). Windows with a reflective coating, or double-paned and triple-paned windows will help to reflect UV and keep more heat out.
☀️ Add solar panels (also, also expensive). Another cost prohibitive item, but if you happen to be adding solar panels - they will keep hot sunlight off of your roof, and provide your house with more shade. If you're choosing between mounting solar panels on your house vs the garage - consider whether putting them on the house would help to keep you cool (and still provide an optimal orientation).
🌂 Add indoor blinds and curtains. Inside your house you can lower the heat by using honeycomb blinds and thick curtains. This also helps to keep heat inside in winter.
However - if can't do all of these layers or just need a quick fix - reflective window shields may be a cheaper and easier solution! These films claim to reflect up to 97% of sunlight.
How To Do It
Measure Your Windows. Using a tape measure - write down the width and height of all windows in your house.
If you prefer to only fix a few windows - tackle the south-facing and west-facing ones. These will get the most heat and sunlight during long summer hours.
If you are in the Southern Hemisphere you of course want the opposite - those pesky north-facing and west-facing windows instead!
Count each sheet slightly larger - an inch or two bigger on each side, to better fit and stay inside the window frame. So err on the side of larger and more material.
Calculate the total area. Multiply width x height and get the square area for windows you want to cover. Then add all of the windows together to get the total sum.
Figure out how big of a sheet you need. I'm looking at products like this on Home Depot. This is basically bubble wrap with tinfoil glued onto it. You _could_ use plain old tinfoil, but I find it difficult to repeatedly set up and take down, fold, and store without ripping. With a roll of bubble-wrap-foil - it's easy to set up and take down over and over without damaging.
I have heard good things about EcoShield - that it is made from less toxic materials and is less dangerous in a fire. But I am not sure where to buy it.
Buy the roll. Get up early and head to the hardware store while it is still cool outside. Then cut and place before it gets hot.
Measure and cut sheets to size. Using your math from earlier - use a tape measure and marker to note where to cut, and draw some markings or lines. You may need to plan out the cuts to make the most use of the sheet.
Shields up! Once cut simply place the reflective foil into the window box! If needed you can use tape - or simply pull down the blind - to keep the shield in place.
Enjoy staying cool! I found this made a big difference in the feel and temperature of our house, especially on the top floor.
Note: Bubble-Wrap-Foil Is *NOT* For Insulating
You may find some claims that using bubble wrap foil is a good insulator, and that you should place it everywhere - around your food cooler, in your vehicle, etc. Unfortunately this is *not* true. The bubble wrap at most provides insulation around R1 - and only then when it has an inch or more of air space behind it to actually do the insulating. This product is *not* a good insulator, and should not be used for insulating.
What bubble-wrap-foil *is* great at is: reflecting sunlight! So stick to that.
Will Reflecting Sunlight Damage My Windows?
I have found no evidence of this. I called three different window companies, and each of their salesmen told me: reflecting light with tinfoil will not damage the windows. There are a few guides and anecdotal sources online that also claim no damage. If you find a better reference, please let me know.
Stay cool in there!
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Edit June 2023: Add info + links on sealing your attic hatch.
References
Bubble-wrap-tinfoil is *not* a great insulator:
Energy Vanguard - “not insulation … R1”
BuildItSolar - “Bubble wrap increases the R value from about R1 to about R2”.
Window films
Efficient Window Coverings - window film or coating can block 95% - 99% of UV light.
Cool Roofs
Attic Access Hatches
Cellular Shades
EcoSmart Shades - “The fabric forms air pockets (cells), which trap heat and cold. When surrounded by the right material, air pockets are a tremendous insulator”
Treehozz - "Honeycomb shades can block heat about as well as a 1/8 inch sheet of insulated foam. Their R-value (or, their ability to insulate against heat flow) ranges between 2 and 5."