Update, July 2017: After two summers of experimenting, I am a firm believer in closing windows and blinds to keep the house cool when it is hot outside. But many people will resist. They can’t tolerate closed windows and blinds when it’s a beautiful sunny day. There's evidence of this when I'm riding the bus. The bus is air conditioned, but people insist on opening the windows, which actually makes the bus warmer. But I admit that there's some pleasure in experiencing a breeze on a sunny day, even if that breeze is warm.
The original post: The assumption here is that you want to keep your house as cool as possible in hot weather without resorting to air conditioning. I checked* a couple of dozen Internet entries for stances on whether windows should be open or closed to achieve a cooler house when the weather is hot. There were many entries to pick from; lots of people have reflected on this. The consensus is, it depends.
Generally, if your house is decently insulated, keep the windows and blinds closed when the sun is shining. Open the windows in the evening and at night. More specifically, keep the windows closed when the outside temperature is hotter than it is inside, and open the windows when it’s cooler outside than inside.
This doesn’t work if your house is not well insulated and was illustrated for me at a house in my past. We had a stand-alone garage that wasn’t insulated, and it got quite hot in there in the summer if the doors and window were closed. Our adjacent house didn’t get nearly as hot.
Chad Skelton of the Vancouver Sun has written my favourite article on the open-close issue. He went to the trouble of using a thermometer and confirmed that it is indeed a good idea to keep windows and blinds closed on a hot day. There’s more to it than that, though. Here’s an excerpt from his blog, posted in July 2012:
“But it’s important to make a distinction between how hot the air feels and how hot the air actually is.
“Opening the window on a breezy warm day may make you feel like you’re cooling off your house, but it’s an illusion. In fact, while the breeze feels nice, you’re actually letting loads of hotter air into your home.
“The better strategy, in my view, is to keep your windows closed to lock in the cool air and then, if necessary, use a fan to move the air around and make yourself feel cooler.”
On southern Vancouver Island, it almost always cools off at night. That’s not the case on the Prairies, at least not when I lived there. It often stayed miserably hot all night. Same experience when I visited relatives in Ontario.
We can take advantage of the Island's cool night air. Hasten the cooling of your house by having a fan blow the cooler air inside. I have tried this, and it works. I put a fan out on the deck, and it blows cool air in through the patio screen.
Open windows, of course, can bring security concerns. You’ll need to gauge which windows are safe to leave open in the evening and night, if at all.
Still, despite the logic, there are reasons to reject the wisdom of closing your windows and blinds during the day:
The cat likes to lounge in sunshine.
The cat likes to have the windows open.
Having a darkened house depresses you.
You need to see what’s going on outside.
You have a great view that you paid a lot of money for.
You crave an outdoor breeze; the artificial breeze of a fan is not good enough.
It’s summer, you want to embrace the warmth.
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For your basic stay-cool arsenal, you'll need an indoor thermometer, an outdoor thermometer that's in the shade and a fan. Plus the discipline to keep the windows and blinds closed even if it makes your house feel like a dungeon. There are more cooling suggestions in the links below, like awnings, overhangs, plenty of insulation, and strategically placed vegetation.
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* Academics write papers all the time based on surveys of the literature on a topic. They do minimal or no original research. Instead, they review the research that has already been done and then offer their conclusions. That’s sort of what I’m doing here, though with substantially less rigour than what an academic journal might require. For instance, a journal might look askance at the way I found my literature. I typed variations of the phrase “windows closed or open in hot weather” into the DuckDuckGo and Google search engines.
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Articles that caught my eye:
A discussion at reddit.com: Is it better to have windows open or closed on a hot day?
StarTribune.com: Summer comfort is an open and shut case
Fixit columnist answers this question: When a house isn't air-conditioned, is it better to keep the windows open or closed? A friend said it's best to keep the windows closed to keep the heat out. I think the opposite. The windows should be open to allow the house to cool off. Which is correct?
Chad Skelton’s Curious Dad blog: On a hot day, should you keep your windows open or closed?
B.C. Hydro: Power Smart tips to help you stay cool and save
A rambling discussion at answers.yahoo.com, where a few people favour keeping the windows open during the day.
theconversation.com: How to keep your house cool in a heatwave
BuildingGreen.com: Simple strategies for keeping cool
University of Nebraska extension: Keeping cool this summer
ibpsa.org (PDF): A preliminary model of user behaviour regarding the manual control of windows in office buildings
Among the observations: people like to open windows on summer days moreso than in winter; window opening and closing tends to happen when a person arrives.
Warm House Cool House, a book summary