Do impact windows lower your electric bill?

In Miami the air conditioner runs most of the year, and it's usually one of the biggest things on your electric bill. Impact windows can lower that bill by letting in less of the sun's heat and less humid outside air, so the AC has less to do. How much depends on your home. Results vary from house to house, and nobody can honestly promise you a number before they've seen yours.

How heat gets past a window

Two things are going on. The first is sun through the glass. When afternoon light hits a west-facing wall of plain glass, that heat lands on your floors and furniture, and the AC has to remove it. The second is air. Older windows that no longer close tight let humid air slip in around the sash, and the AC has to cool that air and pull the moisture out of it.

John M. described both in his review. He wrote that his old windows "did not seal well nor filter the sun’s rays at any level." Afterward, he wrote that he had "high quality windows that seal properly, filter a good portion of the sun’s rays". Notice what he's describing: how the windows behave. He didn't give a number, and we won't either.

An impact window is not automatically an energy window

This surprises people. What makes a window an impact window is laminated glass, which is there to keep the opening closed in a storm. That doesn't make it good at stopping the sun all by itself. The energy side comes from the options you pick:

• Low-E coating. A thin, nearly invisible coating that helps deflect the sun's heat while still letting in daylight. For a Miami house this is usually the first option to ask about.
• Insulated glass. Layers of glass with a sealed space between them, which slows heat moving through the window. Ask how it changes the weight of the window and the price.
• Frame. Aluminum moves heat more easily than vinyl, though the frame is a small share of the window. Our comparison of aluminum and vinyl impact window frames covers the other reasons to pick one.
• Weatherstripping and fit. A sash that closes tight against good weatherstripping keeps the cooled air where you paid to put it.

When you compare quotes, ask for the solar heat gain coefficient, or SHGC, of the exact glass being quoted. Lower means less of the sun's heat comes through. In a place where you cool far more than you heat, that's the figure to look at first. If the person quoting you doesn't know it, ask them to find out and put it in writing.

Where new windows help most

Not every opening matters equally. The ones worth your attention take direct afternoon sun with no overhang or tree to shade them, or they leak air now. On a hot, breezy day, hold your hand near the edges of a closed window. If you can feel moving air or see daylight at the edges, that opening isn't helping your AC.

Style plays a part too. A fixed window has no moving sash, so there's less to leak, which makes it a good match for a big sunny opening you never need to open. Our guide to which impact window goes in which room goes through those choices.

When the bill may barely move

New windows may not change much if:

• Your AC is old or badly sized, your ducts leak, or your attic is thinly insulated. Those can matter more than the windows, and they usually cost less to fix.
• Your house already has deep overhangs, heavy shade or few windows.
• Your current windows close tight, and the new ones have no Low-E coating.

If the electric bill is your only reason for buying, have the AC and the attic checked first. Impact windows are mainly a storm decision, and a cooler house is a side benefit. Whether the whole project makes sense for you is a bigger question, and we take it up in are impact windows worth it in Miami?

The install counts as much as the glass

A good window set badly still leaks. The joint between the window and the opening is where air and water find their way in, so ask how it gets sealed. We use commercial-grade caulking and sealants approved for home exteriors, and we caulk each screw hole so the fasteners don't turn into small leaks. Nabi T. wrote about a door job that "they made sure every crack was sealed." Hold any installer to that, on every opening. The frames and glass we offer are on our impact windows page.

How to measure it yourself

If you go ahead, you can check the result without taking anyone's word for it.

1. Before the job, save your last 12 months of electric bills, or download them from your utility account.
2. Write down the kilowatt-hours you used each month, not the dollars. Rates change, so dollars can mislead you.
3. After the job, compare each month with the same month a year earlier. Give it a full year, because one hot month proves little.
4. Note anything else that changed: a new AC, a different thermostat setting, more people at home, a pool pump. Any of those can move the bill.

Weather changes from year to year, so read the result as a rough guide. You may notice the first change somewhere other than the bill. A room that was hard to keep cool in the afternoon may feel more even.