I have a two story apartment with only one room upstairs and an access to a terrasse (it's kind of a closed mezzanine). I noticed that even when every window in the apartment is open, the room upstairs still feels stuffy. I believe it might be because there is only one window upstairs and it's not even in that room so the air upstairs just doesn't circulate well.

Hence my idea to install a vent/extractor (bathroom style, maybe more silent) blowing air from that room into the attic in order to get some air flowing upstairs. I know it's bad to blow humid air from a bathroom into an attic but this is just a regular room so I wonder if it would be okay (attic is well ventilated, we even asked the contractor to add more vents up there last time they did the roofing). I would probably only use it in the summer since the point is to ventilate better instead of using the AC. We get cold winters here (Montreal) so I imagine that blowing air in the winter would create condensation which would be bad.

Here is what the apartment looks like. The orange square is a patio door, green rectangles are roof vents and the red arrow is where I want to put my fan.

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what do you guys think? Stupid idea? Anybody has a better idea?

  • Have you checked the HVAC venting? Jun 8, 2017 at 3:09
  • 1
    What you describe is the principle of a whole house ventilation fan. They typically move around 20 to 100 times the air volume of a bathroom fan. I wouldn't expect a bathroom fan to move enough air to make a noticeable difference.
    – fixer1234
    Jun 8, 2017 at 8:18
  • The rest of the apartment is well ventilated, it's only that particular room upstairs with no window that has an issue. The idea is to extract air from that room to make way for fresher air from downstairs. Whole house fans are expensive and this is really a small room. Does a whole house fan need to blow air outside or can it just extract into the attic (which has vents)? I was actually worried that blowing air into the attic would create condensation. Or would it instead help in turn ventilate the attic by pushing hot air through the roof vents?
    – EvilMorty
    Jun 8, 2017 at 23:29
  • @fixer1234 not sure I'm explaining well so here is what the apartment looks like link the orange square is a patio door, green rectangles are roof vents and the red arrow is where I want to put my fan.
    – EvilMorty
    Jun 8, 2017 at 23:53

3 Answers 3


Bathroom fans move a trivial amount of air. It's enough to exhaust some of the highly humid (or "aromatic") air from a tiny room like a bathroom, and it's low enough for the air to be replaced through general air leakage (and you typically don't run the fan for long). If all you're trying to do is turn over the room air a little so it doesn't seem stuffy, you will probably want to size it equivalent to what a bathroom fan does in a bathroom.

The general guideline for that sizing is to replace the room air at a rate of 8 times per hour. For an 8 ft. ceiling, that's 1 CFM per square foot of room. That's length x width x height / 8 = minimum fan CFM rating.

There are a couple of considerations for condensation. It isn't a summer issue, but if you put this in, you should design around the possibility that it could be used in the winter.

In the winter, the normal inside air is likely to be a lot more humid than the outside air in Montreal, so excess humidity may condense. Hopefully, any condensation will be in the form of a cloud, like the moisture from your breath outside, and the air current will dissipate it and/or exhaust it through the roof vents before it collects.

I assume you aren't in a position to install a dedicated roof vent. I would still duct it to the general vicinity of the close-by roof vent, and just secure the end of the duct so that it exhausts near the vent.

The other consideration is something I ran into with my own house in Virginia, and our winter temperatures aren't anywhere as low as Montreal's. In an unusually cold period, humidity from a bathroom fan condensed at the roof vent (which is metal). It collected as water, flowed back down the duct, and dripped out of the bathroom fan (a form of Chinese water torture for anyone sitting on the throne). If you use an insulated duct, you aren't likely to run into that.

  • Great detailed answer with just the details I needed! Thanks a lot. I'll check the attic again to see if I can run an insulated duct there. Might not even be necessary as I believe there is a vent very close to where I want to put my fan. Might be a fairly easy project after all.
    – EvilMorty
    Jun 9, 2017 at 2:32

Your idea should work. I cut a hole in my garage ceiling, installed a 20"X 30" air return grill and filter, built a 48" tall box above the hole and installed a fan blowing positive presure into the attic. My idea was to save money on my power bill during hot Arizona summers. I have not seen savings for the Air Conditioner, but the fan draws air through a window in the garage and keeps it from becoming hotter than outside temperatures. I know the fan is helping because it is running a lot and the cost of it has been offset by the reduced AC load. The AC unit is not running as much, and the blower unit in the attic is not as hot, both of which I think are long term beneifts. The biggest benefit however is the fresh air and cooling of the garage. I have discovered other benefits not related to this question, making me wonder why this is not standard practice.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Jun 30, 2020 at 16:14

I'm not sure an electric exhaust fan installed in that room will do what you want it to do.

Currently, the problem is that when you open the windows the air has no way to blow into and through the room. It's sort of like blowing into a straw with your finger over the other end. In order to have fresh air blow into the room, you need a way for stale air to blow out of the room.

Likewise, when you install an exhaust fan in the room, you need "make-up" air in order for it to bring in fresh air. In order for the exhaust fan to work, you'd need to open a window too.

All houses have positive and negative sides to the building, depending on which way the wind is blowing. Adding a relief grille (with backdraft damper) will work when the wind is blowing the right way. When the prevailing wind changes and blows the opposite direction, it won't work so well because the relief grille will close and try to keep the air out.

Maybe a window is better and will provide a means of egress if it's being used as a bedroom.

  • It's not a house, it's actually a condo in a small apartment building so I can't add a window and I can't do any major modification to the building. The main floor of the apartment is easy to cool and ventilate as there are plenty of windows on two opposite sides of the building. But you're right, that room upstairs provides no way for the air to escape. It's really kind of a cul-de-sac. The idea of the exhaust fan was to allow fresh air to come in into that room from the rest of the apartment. My question was more: is it okay to blow air into the attic or may it cause condensation problems?
    – EvilMorty
    Jun 8, 2017 at 23:19
  • Condensation is caused when warm moist air reaches its dew point. Putting warm summer room air into the attic will not cause condensation because the attic ventilation should vent the area. If possible, I'd peek into the attic to make sure the vents are "clear" and working before cutting the hole and installing the vent.
    – Lee Sam
    Jun 9, 2017 at 0:03
  • That's a good point. I'll definitely make sure I check the attic first to make sure the vents are clear. thanks
    – EvilMorty
    Jun 9, 2017 at 2:24

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