Bathroom Air Flow - Window open or shut when showering with extractor fan on?

BobT36
BobT36 Posts: 515
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Hi all, been wondering about this for a while:
When showing with the extractor fan running, is it better to have the bathroom window open or closed? 
This specifically applies to winter, when it's cold outside and all other windows in the house are shut (with just the bleed vents open). 

Also, does whatever answer also apply with this bathroom layout? (regardless of season):

The window is a long one and stretches over to where the toilet is, but only the shaded bit (blue) opens (top hinged). The extractor fan is above the bath on the left (grey). A lot of moisture often hangs around on the right side (above the toilet), both before and after extractor installation.


I was just wondering this as with the window open and it being cold out, that cool the tiles, causing moisture to stick more. However the extractor needs air to move (especially with all other windows shut), so presumed it would be better to have it open?
However with it open, I presume that's just creating a channel of air from the window at the left to the extractor on the left. Would it be better to leave the window closed so the fan pulls air from under the door, drawing more moisture from the right bit of the room? (where it currently settles).

The extractor does a decent job, I've just always wondered whether to also bother having the window open or not.
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  • grumbler
    grumbler Posts: 58,629
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    edited 3 December 2023 at 1:56PM
    An open window can make extraction only better, not worse.
    If the whole house is perfectly sealed (that is usually not the case), then you need at least one window (partly) open for any extractor fan to work.
    Cold air getting inside not only cools the tiles, but replaces moist air and its relative humidity drops significantly when it warms up.

  • markin
    markin Posts: 3,723
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    edited 16 October 2023 at 7:38AM
    If the fan is powerful then ideally you want warm air from under the door, the golden rule is that all steem is gone in under 15min.

    If the air flow is better with the door cracked open then trimming the door or cracking the window a little will boost the flow.

  • Postik
    Postik Posts: 403
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    I've always opened the window (where possible) with the extractor running, as I believe the fan needs to "pull" the replacement air from somewhere.  I assume it will have a much easier job pulling said air from an open window than it will from underneath a closed door.

    With that said, in the middle of freezing winter I can't always stand to have the window open whilst showering.  We also don't have the window open when showing at night with the light on because of moths and insects.  In those instances I have the extractor running but then open the window at the end when I leave the room.
  • BobT36
    BobT36 Posts: 515
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    grumbler said:
    I the whole house is perfectly sealed (that is usually not the case), then you need at least one window (partly) open for any extractor fan to work.
    Yup, I get that a fan only 'moves' air, therefore it needs somewhere to move it from, to replace what it's extracting. Think the bleed vents will be enough? In the rest of the house, (If I were to keep the Bathroom window shut).

    Postik said:
    I've always opened the window (where possible) with the extractor running, as I believe the fan needs to "pull" the replacement air from somewhere.  I assume it will have a much easier job pulling said air from an open window than it will from underneath a closed door.
    Agreed. However it's the 'air flow' I was considering with this layout here, in that wouldn't the window open create a channel from the window (at the left) up to the extractor (above the bath, at the left), potentially leaving a void at the right side of the room? I often see a wet patch on the ceiling above the toilet for a good while afterwards. That's why I was wondering with this specific layout whether having the window shut might be more optimum (so the air flows right to left, rather than left to left). Good point about cool, dryer air replacing the warm, moist air being the main point, though.


    Regarding insects, I bought one of those cheap window meshes from AliExpress, and it's been working wonders. You cut two magnetic strips to size and stick one around your window. Then the other one you stick to a mesh sheet (also cut to size), stick a plastic trim over the other side, then just attach the mesh via the magnets. Cleanable and removable, and it stops 99% of the insects and moths getting in. Worked really well. As it's magnetic it's easy enough to just lift up the bottom part to close / open the window.
  • Eldi_Dos
    Eldi_Dos Posts: 1,533
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    If moths and other flying insects are bothering you while washing in the evening try leaving a outside light on, they are usually attracted to light, especially if light has white cover.
  • Chickereeeee
    Chickereeeee Posts: 1,169
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    edited 17 October 2023 at 5:28PM
    I know exactly what you mean, and pondered it when installing my extractor fans. 

    For our en-suite, I added a closable trickle vent over the window, (which is at the opposite end of the room to the shower). This (plus gaps round the door) provides enough airflow to keep the room clear when showering. When finished, I leave the door ajar while the fan runs on for 20 minutes, to fully dry the room.

    Seems to work well. No moths,  no danger of leaving window open, vent can be closed if REALLY cold (otherwise provides 24hr ventilation).
  • BobT36
    BobT36 Posts: 515
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    Yeah it's the opposite end of the room thing that I was wondering about, since the openable bit of the window is pretty much right next to the bath / shower. Was terrible before we got the fan installed (rented), as all the moisture would just collect at the other end. 

    Still does a bit now (though not as much), tis why I was wondering about keeping the window shut and creating the right > left channel of air than left > left. 
  • Chickereeeee
    Chickereeeee Posts: 1,169
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    BobT36 said:
    Yeah it's the opposite end of the room thing that I was wondering about, since the openable bit of the window is pretty much right next to the bath / shower. Was terrible before we got the fan installed (rented), as all the moisture would just collect at the other end. 

    Still does a bit now (though not as much), tis why I was wondering about keeping the window shut and creating the right > left channel of air than left > left. 
    My way (above)  uses the trickle vent to restrict the airflow, meaning a lot still comes from around the door even though the vent is closer. So, when shower in use, BOTH ends of the room have airflow.

    Depends if you can fit a trickle vent I guess.
  • BobT36
    BobT36 Posts: 515
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    The window has a trickle vent. :) 

    Might try it with the window fully closed (but trickle vent open of course) and see if it makes a difference. 
  • Chickereeeee
    Chickereeeee Posts: 1,169
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    BobT36 said:
    The window has a trickle vent. :) 

    Might try it with the window fully closed (but trickle vent open of course) and see if it makes a difference. 
    With the extractor on, feel around the trickle vent and the closed door (usually the bottom).  You should feel significant airflow from both.  If not, either your door doesn't have enough of a gap OR your extractor fan is not moving enough air.

    I recommend this type (depending on room size)

    https://www.internet-electrical.co.uk/ventilation/in-line-bathroom-toilet-fans/manrose-mixflo-in-line-mixed-flow-fans/manrose-mf100t-mixflo-ipx4-100mm-3-speed-mixed-flow-inline-duct-fan-with-adjustable-timer-black?gclid=CjwKCAjwp8OpBhAFEiwAG7NaEs2PlRv2-Dq1XDW7y9RljP6YrQtiQ25vjEcvqi80fI4W3ox5sYwG8hoC1QsQAvD_BwE
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