bathroom skylight venting for cooling purposes

homeboundJuly 8, 2012

I am reposting this to the HVAC forum. Thanks for your help on this. Here goes:


(Originally posted by FMS13 on the Remodeling Forum):

Would top floor bathroom roof ventilation fan help or hurt AC efforts? I have a top floor bathroom with "sunroof" passive louvers type vent. Two glass window flaps are at the ceiling of the bathroom and then it goes out to the roof and has vent slats on the sides to allow ventilation. Problem is whenever I open the windows, dirt and leaves collect there and fall into the bathroom.

If I take out the vent slates and use a fan instead so that I don't get dirt and leaves in when I open the window - would this be good or bad for my central AC? One person says it will suck the cool air out of your house, and that the passive louver system is best. Another says closing the louvers and putting in a solar fan with a temp gauge to turn on and off to let out hot air will keep the house cool during the day when you turn down the AC.

So, which is the best to do? This is completely separate from the attic, which is just crawl space and insulated.


reply by worthy:

The purpose of the bathroom vent fan is to remove excessive humidity from the air to avoid rot and mould problems. Passive ventilation doesn't do the job. To avoid removing an excessive amount of conditioned air, you can put a timer on the fan; some controls allow for a choice of pre-programmed exhaust times, automatic operation when the room is occupied and even humidity sensing controls.


reply by homebound:

I know the OP, so let me clarify their situation.

The house is circa 1920 and the bathroom has a tall, louvered skylight (original) which extends upward roughly 4' above ceiling plane). The skylight is opened and closed by way of a pair of hinged glass panes that fold open (vertical) or closed (horizontal) near the ceiling. (The louvers are permanently open).

The OP's intention is to permanently close and seal the louvers (to prevent dust and leaves), replace/install a vent fan in the skylight, and use the vent fan for longer periods to vent and cool the house (instead of using the central AC all the time).

This house is a typical two-story townhome (row houses) in Washington DC which has a hot, humid summer season most of the time.

I will reserve my opinion at this time, but suggested that they post here to get other opinions. They have been told that installing a vent fan into the skylight on a thermostat is a good idea (much like an attic vent fan).

Thank you.

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It is humid in that area. I have a hard time believing that the opening the house with the AC running is going to make it more efficient. I have a hard time believing that opening the house alternately with running the AC is going to make it more efficient.

    Bookmark   July 9, 2012 at 2:17PM
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I'll this one a try...

This structural feature sounds like a pre-AC way to get some air movement and to exhaust hot air from the top floor, in a town with very muggy weather. It's no longer needed, you've got AC, so close it up (and keep your windows closed when it's hot or humid outside).

Second topic - moisture in a bathroom needs to be removed by a fan or window, and a fan is better. Get one installed and use it.

    Bookmark   July 9, 2012 at 5:44PM
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Thanks for the comments.

My thought on leaving the vents open (and closing the skylight with only the hinged window panes) was to avoid creating a super-heated bubble of air in the bathroom. I once tiled the walls of a skylight in this area during the summer and the temp was well over 140 up in that thing. It was radiating into the room.

If a vent fan were installed in the skylight, what are your thoughts on having it on a thermostat switch? Does that conflict with central AC?

    Bookmark   July 9, 2012 at 6:37PM
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Good summary with appropriate historical emphasis, snidely.

Homebound, this only has one fighting chance of working. That is if you have an air inlet as well as outlet in that bathroom. Don't lose sight of the fact that air must come in to replace the air leaving the envelope.

If you don't do that, the best entrance for air might be at the lowest, coolest, driest levels. You can suck warm humid air into the comfortable part of the house. More complicated, with central AC, if the blower is pressurizing the area with the vent, air will be sucked into depressurized areas of the house actively. Either way you are subverting the mechanical cooling. You might also be causing condensation problems when the hot-humid air hits cool surfaces.

    Bookmark   July 9, 2012 at 6:52PM
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Using a fan to ventilate a hot shaft doesn't seem like a good idea because of the need to suck in humid replacement air as ionized correctly points out. It's like opening a window, which I'm sure you don't do this time of year.

If your current structure radiates heat into the room and it's not essential as an architecural feature, have it removed and build over the roof and ceiling to cover the hole. Or, shorten and narrow the shaft, put in a modern skylight, and insulate the vertical portions well.

Skylights are notorious for radiating heat in summer and cold in winter.

As I tried to suggest in the earlier posting, it seems like you have two issues to deal with, I don't think there's a two birds with one stone opportunity.

    Bookmark   July 9, 2012 at 7:22PM
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I suspect that removing the skylight won't be an acceptable option since they are accustomed to the light it provides during the day....even though the dirt and leaves are the issue at the moment. (frankly, it doesn't seem to be that big an issue from my observation, but it's not my home.)

I take your points but other than removing it (or replacing with a shorter, narrow one), are there any other options? Leave it as is, maybe?

But to get back to the OP'sm original intent, it seems to make little or no sense to vent/cool a house with a modified skylight (when the central AC is turned off) , right? At least we're clear on that, I think.

    Bookmark   July 9, 2012 at 8:52PM
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Skylights are surely a place for radiant heat gain and for air leaks, but they don't have to be. Replacing it with a modern skylight might be a big money-saver.

    Bookmark   July 10, 2012 at 4:50PM
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