Why won't our room cool?

September 10, 2022 9:31 AM

Our bedroom does not cool when windows are open. Last night, at bed it was 24C in our room. We had 3 windows open. 2 on the north side, one on the south side. One north wimdow faces the south window dead on. The other north window is in the ensuite bath. Bathroom door open.

We had one fan in a window, in the north side facing out. And one fan in the door drawing air into the room. Windows on main floor open, one north one south. A ceiling fan in the stair well on, drawing air up. And our bedroom ceiling fan on, again drawing air up.

Outside was 12C in the morning. Our room was still at 22C.

Husband and I disagree about fan placement. The above is his method. Mine would be fan in window drawing air in, and door fan blowing air out

But either way...why on earth does our room not cool at night, especially give the 10C difference??

In the winter, we can have windows open and still have same effect. Except when the wind is from north, and then the temp drops fast. But it had to blowing fairly significantly

posted by Ftsqg to Home & Garden (17 answers total)

What is above and below your bedroom? If you have a nice hot unvented attic above you it will be hard to cool your bedroom.
posted by sciencegeek at 9:52 AM on September 10, 2022 [1 favorite]

It seems to me that the fan in the window blowing out is actively preventing cooler air from entering through the window. Yes, it may be drawing up *some* cooler air from downstairs, but that’s a much slower and less direct cooling route than simply bringing the cold outside air directly into your bedroom.

Your bodies are producing heat all night, so if the rate of cooling of the fans is slower than the rate of heat production of your bodies, the room won’t cool.
posted by mekily at 9:53 AM on September 10, 2022 [10 favorites]

There's a couple things that help - one is having low point windows and high point windows. Double hung windows can help with this by dropping the top window pane and opening a window that is lower than that. That helps cold air come in and move through the space and push the hot air out. And you want ones that are not just across from each other but diagonal across the room to get more area flushed out. Obviously, that isn't always possible with existing window configurations.

I also seem to recall a tip of big openings for the cold air to come in and small openings for the hot air to leave. And I think the simplest thing to do is overnight just open all the windows in the space you are trying to cool and make sure that there's a high point for the hot air to exit. Maybe just put one fan facing out one night so it's just drawing all night and see how that goes.
posted by amanda at 9:54 AM on September 10, 2022 [2 favorites]

Sciencegeek brings up a good point, if your attic isn't properly ventilated, it may be storing a lot of excess heat. Examine how it is vented (eaves? mushroom vents? ridge vents?) and if it appears to be properly vented, check that the vents were not covered with insulation accidentally.
posted by amanda at 9:56 AM on September 10, 2022 [1 favorite]

It sounds like you are pulling air from the rest of your house (which is presumably warmer than outside) into your bedroom. If you reverse the direction of your fans (pull air out of the room into the rest of the house and pull air from the outside into the bedroom), your bedroom should cool down faster. In other words, you are correct.

You want to maximize the airflow through the room and running your ceiling fan is likely to disrupt that by creating more turbulent airflow. Try turning it off.
posted by ssg at 10:00 AM on September 10, 2022 [4 favorites]

Also: Why are you only trying the way that your husband has suggested? Try both ways and measure temperature differences. The way that works better is the better way for your situation.
posted by sciencegeek at 10:06 AM on September 10, 2022 [10 favorites]

I think you should get a window fan or box fan that you can "seal" into a window downstairs - foamboard will do, you just want the fan to be able to suck as efficiently as possible - to draw air out of the house, through maybe just one (you'll have to experiment) upstairs window. Leave the ceiling fan off during the ventilation process; you can turn it back on once the room cools, but it's definitely not going to help with what you're trying to do.

You can try the alternate method - one fan sucking out from an enclosed window exposure, downstairs one or more windows/doors open - to see if that works better or is more secure than a downstairs fan.

If you own this house and have somewhere to install it, consider a whole house fan. If your bedroom or bathroom closet ceiling has attic above it, that would be an ideal location, because it'll directly be pulling cool outside air into the room. It generally takes well under half an hour to completely turn over the air in your house with a whole house fan.
posted by Lyn Never at 10:11 AM on September 10, 2022 [1 favorite]

I have a well-insulated house, so it doesn't heat up quite as much during the day, but it's also slower to cool at night, FWIW.

I don't run the ceiling fans when I'm trying to move cool air through, I think it just disrupts the flow and mixes in hot air from the ceiling. I have high ceilings, the ceiling can be hot, whatever. I have a single fan blowing cold air in from a window.

I grew up in a hot, humid place without AC, can confirm that whole house fans are good stuff if that's an option for you.
posted by momus_window at 10:14 AM on September 10, 2022

I, an internet stranger, think your preferred placement makes more sense. The fan in the bedroom window should be pulling cool air in all night. I, like your spouse, liked to use the fan to blow the hot indoor air out, but it's not as effective when the temp difference is so large.

I used to live in Southern California where daytime temps outside were often 30C or higher and nighttime temps were around 19C. We put one box fan in the bedroom window to pull cooler air in, kept the bedroom door open, and another window fan in the living room to pull cooler air in. The bedroom cooled off fast, within an hour of sunset (thank you low humidity/desert climate).
posted by spamandkimchi at 10:18 AM on September 10, 2022 [2 favorites]

This YouTube video suggests that placing a fan a foot or two from the window is much more effective at moving air out the window--includes actual data as measured by an anemometer.

If you think of a fan as pushing the air in front of it, rather than pulling air through it, there's a logic to this.
posted by idb at 10:32 AM on September 10, 2022 [6 favorites]

2nd-floor bedroom? I'd close curtains and windows during the day to keep in any coolness, and open at least 1 window, with a fan pointing out, in midafternoon, to exhaust rising heat. My 2nd floor bedroom can trap heat, so I choose whether windows should face out or in based on outdoor temps.

Also, a fan pointed at you, or creating a breeze near you, will cool you. On very warm days, I have fans in both windows, and one pointed at the bed.

Also, on quite warm days, I sometimes hose off the roof, which cools the 2nd floor by a couple degrees. My area is humid, but this still works on hot days.
posted by theora55 at 11:35 AM on September 10, 2022 [1 favorite]

I came to post what idb said: the fan should be NEAR and NOT IN the window for best air motion. See the youtube for illustration of how big a difference that makes.
posted by anadem at 12:04 PM on September 10, 2022

To get cool outside air into a house, you want to be running the house at negative pressure with respect to the outside. The way to establish that negative pressure is with fans that shift the hottest air inside the house to the outside.

If you're using ordinary room fans or box fans for this, it's best to place them some distance back from a window blowing outward, as Matthias Wandel's video linked above demonstrates, so that all of the air in the stream that the fan blades make originates from inside the room. Having a strong stream blowing outward through the centre of an opening will also entrain more air to move out through the opening along with it (this is the principle behind Dyson's "blade" fans), so the entire window opening becomes a place through which hot air is leaving the interior. And since all that air being removed has to get replaced from somewhere, any other window you open anywhere else in the house will wholly draw outside air inward.

If your bedroom is the main room you want to cool, close all the house windows except for the ones with fans blowing out through them and the intake window on the coolest side of the bedroom. You want the outward fans located in rooms as far away from your bedroom as possible (and upstairs, if you have an upstairs) otherwise you'll just end up with warm air from elsewhere in the interior drifting in through the bedroom door and mixing with what you're drawing in from outside.

Exhaust fans in ceilings are good as long as the ceiling void is well vented to outside. Ceiling exhaust fans positively pressurize the ceiling void, so if you've got multiple ceiling exhaust fans you need to be running all of them so that you don't get hot ceiling air flowing back into the negative pressure zone below the ceiling through an exhaust fan that isn't running.

Interior ceiling fans will do very little good in this instance. Their main purpose, on their summer setting where they blow ceiling air downward under the fan, is to create air movement that helps sweat evaporate; this compensates somewhat for unavailability of actually cool air. But they do nothing to help with whole-house air exchange, which is what you're trying to achieve here.
posted by flabdablet at 12:26 PM on September 10, 2022 [3 favorites]

In addition to checking the space over the bedroom, each time before you start the fan, check for any existing airflow, and be sure the fan reinforces it. Or look at the wind direction in your hourly forecast. Fans work.
posted by diodotos at 4:53 PM on September 10, 2022

Response by poster: Thanks for all the ideas. Our main floor cools easily with the two windows open. The other 3 bedrooms cool. Ours does not. It seems in most houses that if you crack a window you can feel the cool air move into the room. That rarely happens in our bedroom. But does in the rest of the house.

So while all the suggestions (better insulation, whole home fans, curtain closed) etc all make sense, and we do most of that, I just can't figure out the dead zone that is our room. We have tried all various configurations of fans. This morning, our bedroom was again 22. The nest monitor in the hall just outside our bedroom read 17. The main floor was 15. The house is cool. The bedroom is not. Outside was 13.
posted by Ftsqg at 11:06 AM on September 13, 2022

Our main floor cools easily with the two windows open. The other 3 bedrooms cool. Ours does not. It seems in most houses that if you crack a window you can feel the cool air move into the room. That rarely happens in our bedroom. But does in the rest of the house.

It might be that in order to maintain enough negative pressure in your interior that your bedroom actually sees some, you'll need to restrict incoming airflow in the other bedrooms to a greater extent than you're currently doing.

What happens if you set up a fan blowing out through a window somewhere else in the house (or turn on all the exhaust fans) and then close all the other windows except for one window in your bedroom?

How do things change if you also close enough internal doors that air must come in your bedroom window and out through your bedroom door in order to reach the room(s) where fans are blowing it out of the house?

Another way to collect useful information about this kind of thing is to wander slowly around with a stick of incense and see which way the smoke drifts. If there's some huge air leak that's spoiling the negative pressure before your bedroom sees it, you should be able to find it that way.
posted by flabdablet at 12:01 PM on September 13, 2022

We had 3 windows open. 2 on the north side, one on the south side. One north wimdow faces the south window dead on. The other north window is in the ensuite bath.

If your bedroom is at the western end of the house and has a windowless west wall made of brick or stone or concrete and not shaded by trees, you might be surprised by how much heat it will absorb from solar radiation over the course of an afternoon, how well it holds that heat through the night, and how effectively it radiates such heat as it doesn't hold into the interior.

If that's what's going on, your cheapest fix will be to grow some kind of vine all over the outside of that wall. Passionfruit will grow quickly and make pretty flowers that smell good and supply you with bonus fruit for quite a lot of the year.
posted by flabdablet at 12:10 PM on September 13, 2022