Open Skylight: Hot or Not?
July 7, 2014 9:15 AM   Subscribe

My apartment is on the top floor of a single family home. My landlord has advised that we open the skylights, one in the bathroom and one in the stairwell, to help keep the house cool. We have a window AC unit going in one of the rooms (a bedroom next to the bathroom with the skylight). It seems like opening skylights would let the cooler air out, like any window. What is the right move here to keep the apartment cool?

Priority 1 is Keeping the Bedroom Cool. Priority 2 is cooling the rest of the house. Priority 3 is saving money (argh).

Please show your work. Posting simply "Hot air rises duh" will result in me banning you. (Note: I have no power to ban you).
posted by Potomac Avenue to Science & Nature (11 answers total) 1 user marked this as a favorite
I would not only not open the skylights, I would cover them to provide insulation. Oh look, here's a mefi question on that.
posted by desjardins at 9:21 AM on July 7

I think we'd need more info on the overall airflow. Does the bathroom with the skylight only open to the bedroom, or is there another doorway? Can the bedroom be closed off entirely? Is your AC powered such that it could cool the entire apartment if you had enough airflow between areas, or is it really only strong enough to effectively cool the one room (note that once the outside temp is at 90, this is the case for most window AC units)?

My strategy is to keep the bedroom door closed and the AC in the bedroom on to maintain a cool space for sleeping, but maximize airflow elsewhere in the apartment during the cooler evening hours. In your case, that would mean keeping the bedroom closed and the AC on, but opening both windows and the skylights at night to promote airflow through the house via the stack effect. During the day, close the windows, including the skylights, to keep the warmer daytime air out. The right window coverings may also cut down on solar energy entering your house during the day.

If the air outside your apartment is miserable at night, this strategy doesn't work well. But if it gets down to even 75 at night, this can keep the rest of the apartment livable.
posted by pie ninja at 9:37 AM on July 7 [4 favorites]

You will lose the cooler air, either immediately or eventually and I don't know much about air mixing dynamics so I couldn't say for sure.

If you want to go the AC route, you want to make your apartment a sealed environment. Are there vents that lead downstairs? Can you put something on the outside of the window that will block the light or insulate it?

If you want to go the airflow method: You need a way to draw cool air in while dumping hotter air out and preferably up (which you already have via the skylights). I'm lucky, in the townhouse I rent in that I can open my fourth story windows, put a fan in my bedroom door (facing the four story, mostly enclosed staircase that opens up into the cooler basement), crank it to max power, and sit in relative comfort. I don't know if you would have a similar set up: a mostly closed off, small-ish 'tunnel' that connects you to cooler air beneath you.
posted by Slackermagee at 9:43 AM on July 7

Does your apartment get hotter than the outside air? (This is pretty common in top-floor apartments where the roof absorbs a lot of heat and it's transferred into the apartment.) Basically if your apartment is getting hotter than the outside air, yes, at least open the stairwell skylight and let out that extra-hot air. But if your apartment stays cooler than the outside air, cover up the skylight (and any other windows) and seal the apartment up.

Right now I live on the first floor and it stays cooler than the outside air in my apartment as long as I keep the windows closed and the shades down during the day (I open everything up at night). When I used to live in an attic I would come home to melted soap if I left the windows closed - that roof was an impressive machine for converting sunlight into heat.

What you want to do with the AC-cooled room depends on whether you have the AC on all day or only at night when you're trying to sleep, and how well-insulated the room is.
posted by mskyle at 9:48 AM on July 7 [2 favorites]

When it's cooler outside than it is inside, you open the skylights and windows. The hot air goes up through the skylight and it sucks the cooler air from the outside into the house.

When it's cooler inside, everything stays shut and the skylight is covered with something to keep the sun out.

Science: I'm cheap and hate running my A/C
posted by the young rope-rider at 9:56 AM on July 7 [7 favorites]

Does the bathroom with the skylight only open to the bedroom

It opens into a hallway right outside the bedroom.

Can the bedroom be closed off entirely?

We can close the door. That's the current solution to keep just the bedroom cold. But then the rest of the house is wicked hot. Given that we will open the bedroom door to let the cold air seep into the rest of the house sometimes, especially when we have guests, the question is, open skylights or closed?

Is your AC powered such that it could cool the entire apartment if you had enough airflow between areas, or is it really only strong enough to effectively cool the one room

I have no idea. :(

Does your apartment get hotter than the outside air?

It's relatively shady, and somewhat breezy, so I'm going to say it's a wash. About the same.

If the air outside your apartment is miserable at night

I live in New York City a fetid swamp next to a crusty mud pond.
posted by Potomac Avenue at 10:11 AM on July 7 [1 favorite]

Close the skylights.

It's not as simple as "hot air rises, duh". Your landlord is thinking of the stack effect but is not accounting for the fact that it reverses in summer, because of the AC you're running.
posted by Dashy at 10:34 AM on July 7

If your house is the same temp as outside or cooler (this might change at night) then the skylight should be closed all the time. And you should cover it with anything that will keep the light out because direct sunlight is bad.

Your A/C will likely do a decent job of keeping things cool if you keep everything sealed and shut.

I'm currently keeping a 4-bedroom place below 80 with one, sometimes two window units.

posted by the young rope-rider at 10:44 AM on July 7

Since you mention living in a (metafiltorical) swamp I would amend the recommendation to keep everything shut unless it's cooler outside than inside to say: only open up to let out hot air/pull in cooler air when it's cooler outside AND the dew point is below 60. When the dew point is higher than that, you will be replacing your nice dry inside air with humid outside air, which can cause mold/mildew problems and can actually increase the load on your AC the next day.
posted by drlith at 11:54 AM on July 7 [1 favorite]

Have you tried using an IR thermometer to objectify or quantify any of the methods you have tried?
posted by buzzman at 1:54 PM on July 7

Top floors of New England single family homes are often ovens. Assuming your building is as poorly insulated, and black-roofed, as the home I grew up in, opening the sky-lights will let out the searing air near the ceiling.

As an experiment, I'd try standing with your head near the ceiling in your apartment on a day when the AC is running. Is it hot up there? Open a skylight.

Is it cool up there? Keep it closed.
posted by zippy at 2:22 PM on July 7

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