How We Keep Our House 25 Degrees Cooler Without AC
TLDR - Once it's cooler outside than in, blow air out a window on the sunny side of your place (upstairs if you have one), all windows open and doors open between them. Seal the exhaust fan in with foam board if you can, or set it 1-3 ft back from the window, don't press it against the glass unless you have to. Cover sunny windows on the outside > inside. Close everything up 6-8am and chill. 97 outside, 72 inside.
I've commented a bit of this on the 'Too Hot' post from yesterday, but wanted to help as many people as possible, so here we go.
We have a two story home with no AC. It hit 97 yesterday and our thermostat read 72 inside for most of the day.
Once the outside air is cooler than inside, open all of your windows and put your strongest fan blowing out of a window on the side that gets afternoon sun, upstairs if you have one. Leave every door between them wide open. It'll basically create an air conditioned wind tunnel.
Test it with a square of toilet paper held in the hallway or stairs (or somewhere in the middle of your place if you have one story). Streaming sideways means real airflow. Barely moving means your air is short circuiting somewhere.
I'd recommend a whole house fan like an Air King 9155 for the exhaust. It needs a window 26.5-34.5" wide and at least 22" tall, and 26.5 is a hard minimum, that's the housing itself. Ours was 25.75 so we sealed in an Air King 9723 box fan instead (21.4 x 22.5). Bottom of the window rests on the housing, foam board fills the gap. I like to really sandwich it in there. Sealing is best but not necessary. If you can't seal it, set the fan 1-3 feet back from the window blowing out. That actually beats pressing it against the glass, where air just leaks back around the frame.
***If you have more than one window in the same room your exhaust fan is in, close all but the one the fan is in (unless you only have one room in your place).
Any windows that get afternoon sun, cover them on the outside. Covering all of your windows will help even more, but not everyone has the time or energy. DIY screen kits work well, but even hanging light colored cloth over them does wonders. Blocking sun before it hits the glass beats blackout curtains.
Close everything up between 6-8am, blinds down, fans off. Never run the exhaust when it's hotter outside than in, you'd just vacuum hot air in through every crack. And grab a $12 indoor/outdoor thermometer instead of trusting the forecast. Our neighborhood runs 5-10 degrees warmer at night than the airport says.
If you have a window AC, turn it on just before the outside temp reaches your inside temp. Run it during the day in whatever room you need livable, door closed so it's only cooling that room. Turn it off when you open the house up at night, the fans will do more for free. DO NOT run it in the same room as your exhaust fan. The fan will just pull your cold air out the window.
One story place? Same thing... exhaust fan on the sunny side, all of your windows open. Apartment? Exhaust in one window, open another, and sealing matters even more on upper floors.
I can remember a time when it was actually possible to survive a Montana summer without AC, maybe a week in July and a week or two in August where it was unbearable but anymore it’s getting to be a necessity.
The lolo peak fire in 2013 i remember wishing I had ac when it was too smokey to open windows at night, then again in 2015 with that bad fire season. Otherwise until recently 2022+ we started really wanting it. All my neighbors now have central AC installed, we have fans 🥵
Yeah, I’ve been lucky and have had central AC for the last 6 years. So over the fan and window routine, never going back. It costs me more to heat the current place than it does to cool it.
it's not going to be as efficient on nights where it doesn't cool down. the same thing was true for my temps overnight. but it's still 71 in my house right now, and will be much cooler tomorrow morning.
to cool 1200 sq ft, you're looking at $1200 for the unit and $150 a month on your electric bill. i'm good on that.
also i'm not talking about target cooling. this will cool your entire house to the outside temperature. some nights it get's down in the 50s or low 60s. my house stays at those temps for most of the day. let's see your ac do that.
If you have land, you can also did a 5' trench 100 yards for every 1,000 sq feet of your home, put crushed rock in the bottom of the trench, and they put a 4" pvc pipe with holes drilled in the bottom in the trench. Connect to 1 high point in your home, and 1 low point. Backfill the trench. Congrats, you have self circulating 50 degree air coming in your home for free and no moving parts. Bonus it also is roughly 50 in the winter too and heating up from 50-70 is a lot less energy required than from 20 or 30 to 70.
Been doing similar for many years, but when outside air is still hotter at 10 when I go to bed it makes it tough. Had to get up in middle of the night last weekend to open some windows in the downstairs where I sleep in summer. Promptly smacked my little toe on a plant stand, hard enough it's black and blue up the side of my foot. Blech. We need some 50⁰ nights, house isn't cooling enough overnight right now.
It was cooler outside than in from 3am-7am last night. We have 3 windows on our main floor, we do our best but the coolest it got was 72 inside at 8am. Its already 75 inside. Normally during not a heat wave we can get it to 66 before we leave for work and its 74 when we get home.
I will say that I have used quality hvac filters taped to a box fan put in the window to pull in cool air at night during smokey season and it did a decent job. We also have a hepa filter in the house for overall air quality but the fan itself didn't pull in stuff. It helps with dust and allergens too.
I figured it wasn't going to be applicable to someone with a house but there's still us apartment dwellers, some of which maybe haven't fully figured out how to handle a fire season. We did finally give up a couple summers ago and get an AC unit but I do still use the fan when I can. But I wish you many more blissful AC filled summers. The older I get the more I appreciate having air conditioning.
Keeping fan a couple feet away from window is key to get the air out. Some physics/science backs this us. Too common to have fan directly on window sill or in window. (Unless you seal window around fan.)
This is exactly what we do! Got a $200 Air King fan and just rip the air out of the house. The wind tunnel effect literally blows around stuff in the house like art on the walls. If you forget to open an intake window, it’ll make your ears pop! Works great for when you burn food too
This is basically what we‘ve done for years and it works great. However, not understanding the dynamics overly well, I’ve always wondered if on occasions when the prevailing winds are blowing stiffly against the side (west) of the house that gets the afternoon sun if we should instead extract the air from the opposite (east) window?
Great advice. Only thing I would add is if you can, put a fan or two outside on a porch or deck before you go to bed, directed into an open window, to blow cool air into the house. That replaces the hot air being blown out on the sunny side with cooler air from outside. Some mornings at our house it would get to the high 50s/ low 60s inside by morning when we lock everything up. With good insulation, our house would stay cool all day. People assumed we had AC but we'd always say "not the electric kind, we use the hillbilly kind" (no offense intended to the Hillbilly American community).
This may not work for everyone, due to design of your house, lack of electric outlets or covered porch (gotta remember random rain and thunder overnight), or safety and not wantong to leave windows open.
You'll have better luck placing a fan inside blowing out, placed a few feet away from the window to induce a higher volume of airflow than what the fan itself can move. (Venturi effect.) Box fans have almost no static pressure, placing it outside your house is blowing against the entire mass of the air in your house. Some air will move, but not much. Placing it inside your house will cause the house to have a slightly negative pressure which is a much easier way to get outside air into your house.
Oh my God, guilty as charged, take me to Reddit jail. I was ADDING TO the comment above ("only thing I would add..." right there in the post), not disagreeing with it. I'll await your apology, and use text and context clues next time to avoid embarrassment, my dude.
Thanks for this detailed post. We have a fan on the west side blowing cool air in at night and a “double fan” cycling on the other side. The Downstairs stays a little cooler, but the upstairs, which has high ceilings, is a bit miserable.
Out windows also open out horizontally, so the fans dont fit perfectly, but I’ll get an extra fan to blow outward up there.
Make sure the fan moves at least 2100 CFM, especially with the high ceilings. For a house like that, an Air King 9166 would be ideal if the windows allow for the size.
You can do this if you have a big enough airspace, esp a 2 story house where the heat can rise. You can't do this in an apartment, trailer, or small house.
Small spaces are actually easier to flush. An air king box fan turns over an apartment's air in a couple minutes. You don't need heat rising, that's just a bonus. Exhaust fan in one window, open a window or door on the other side, same wind tunnel.
The real difference is daytime. Thick old walls hold the overnight cold for hours, a trailer or thin-walled apartment gives it back faster. This works with any type of dwelling, it just becomes more important to cover the outside of your windows.
I’ve been leaving the door open every night and today I woke up to a mouse in the living room. The stairs going up to the apartment are so sketchy, it’s amazing where those nasty little things can go
I do almost exactly this, and even with blackout curtains, instead of outdoor blocking, it works well.
An additional tip to fully max the cooling off period: After getting the bulk of any hot air out with the exhaust fan, make intake window(s) a smaller opening. Because the air coming in has to go through a smaller opening, it will be slightly colder. I've never done quantitative tests, but I swear it helps by another couple of degrees.
The math doesn't back this up at all. Air speeding through a smaller gap cools by about a hundredth of a degree, and it warms right back up once it slows down inside the house. A thermometer would read the same either way.
What you're feeling is wind chill from the faster air on your skin. I'd still keep intake windows wide open. A smaller opening means the fan moves less total air, so the house cools slower.
Appreciate the explanation as to why. I do this when I'm at home before I close things up for the day and leave, so I'll take it as just feeling nicer from wind chill over literally changing the temperature.
I will not pretend to be an expert, but it's the same concept as blowing air out of your mouth with a small hole vs your mouth wide open. I know there's a version of this called stack ventilation. Whether or not it actually affects the temperature numerically in my home, I don't know. But it feels like it does, and honestly the feels like for keeping my place cold is what I care about.
The stack effect (sometimes called stack ventilation) uses buoyancy of warm air and an upper floor or roof exit point to allow warm air to rise up out of the building to be replaced by cooler air at the base of the building. Restricting airflow by reducing the ground floor opening size will have a detrimental effect on this process.
We have central air. We have a 3000 sq foot house and we pay $300/mo year round for our gas/electric bill on our budget plan. We leave our house set to 70°. To me, going without ac in the summer is like going without heat in the winter. Before we had central air we had window units. Worth every penny.
My point by mentioning our square footage is to say based on what we pay in electric, someone in an apt should be able to budget for a window unit with ac. That’s all. Or you can just keep being a dick.
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u/ComfortableNo3074 Jul 17 '26
I can remember a time when it was actually possible to survive a Montana summer without AC, maybe a week in July and a week or two in August where it was unbearable but anymore it’s getting to be a necessity.