Cover or keep old whole-house fan?

Question

I have a question: does it make sense to have a whole-house fan, the ones that are in the ceiling in a home, when you have air-conditioning .. or is it just a big cool-air leaking hole, that causes your temperature in the home to go up? We had one already in the house when we bought it. There was no central air in the house, but we installed it later. We never use it, but my husband wants to keep it for just in case.

Woodbridge Environmental Tiptophouse.com · Answer

The central whole house fan or poor mans AC as my dad called it has some benefits but also can cause all sorts of issues as well.

Normally there are two locations where the fan sits, one directly on the ceiling with a large louver located somewhere on the end of the house, or located on the end of the house with a louver located on the ceiling inside that opens when the fan operates.

In either case the louver that is mounted on the ceiling that opens and closes with the fan does not seal properly. This allows both heat and cool air to move freely through and around it. If your not using this at all, and only want it for those emergencies when the AC fails then simply construct a 2 inch thick foam box and place it over the fan or louver system in the attic.  Just remember to remove it before you decide to use the fan again. Ideally is should be removed. But most folks keep them for those spring or fall days when the air in the house is stale but its cool out.

Issues with these fans however can be serious.  If you do not have a large enough opening in which to allow the air that enters into the attic a way to escape, or you operate this during periods of high humidity during the day and cool evenings the fan can cause condensation to form in the attic below the roof. This results in mold development. If you do use this fan during this time, you need to be sure that your attic is properly ventilated. If not the moisture will become trapped.  A sign that this is occurring is black stains around the nails where they come through the plywood that holds the shingles on. If your seeing these stains, your focus should be on ventilation in the attic and not so much on the fan. But do not use the fan until this issue is solved or you will speed up the mold development within the attic space. You can also look on the top of the insulation in the attic for signs of moisture condensation. Your looking for water stains on the insulation that resulted in water dripping down from the roof above.

The last thing I would offer is in regards to the location of the AC system.  If its located within the attic, you should consider having a home energy audit performed. Attic placed AC systems typically are over sized by at least one ton or 12,000 btu's  This is done to overcome the heat load applied to the system because the duct system in effect is located outside of the conditioned space. Making the system work harder to overcome the heat of the attic before delivering the cool air to the rooms below. A good audit will show you how to fix this and make your home more energy efficient.

Karin W · Answer

Thank you so much for your advice! The foam box to place on top of the fan/louver sounds like a great idea. 
I don't think we ever used it enough to cause moisture issues in the attic. And we also recently had our roof renewed. 
In your last paragraph I assume you are talking about the regular air conditioner. This unit is outside, not in the attic. 
However, I think an audit is always a  good idea and I would love to have one performed because the temperature in our home is always allot higher than what we set it on. We might need a larger unit, also. Right now its all a money issue. 
Thank you again for your respond.

Woodbridge Environmental Tiptophouse.com · Answer

I would not be so sure about the unit being undersized.  Lots of air conditioning complaints because of all the heat were having all over the U.S.  Remember most design considerations are for a 20 degree temp drop over outside. Meaning if its 90 out the house should be able to come down to 70 inside. But for every degree it goes over 90 the house warms up one as well.  I have received a lot of calls telling me that their AC is not cooling the house, but when its 98 outside if the system is sized properly your inside temp will not be much less the 78.  But because of the correct sizing the house is still cold because of the moisture removal.

With air leaks coming out of the top of the house your home is wasting energy. So the foam cover will help a lot. Also check around the insulation for dark spots.  This is a sign of air movement through the insulation and its acting like a filter which is why it gets dark and dirty looking. Somewhere where you see this dark staining is a hole in the ceiling leading down to the room below or through a wall.  Seal any hole in the attic where wires, electrical boxes and pipes move though. Not an expensive thing to do but you will see a good savings on energy and comfort when doing so. Air sealing both the attic and basement/crawl spaces below can save you upwards of 45% overall on energy use in the home. Paybacks for doing this is normally less then one season. Increased comfort is right away.

Karin W · Answer

Very interesting .. what about vents in the bathroom ceiling .. do they leak cool air as well?  And can you tell me exactly what kind of foam to use for the foam box to cover the whole-house fan?  Thank you for your time  and help!

Woodbridge Environmental Tiptophouse.com · Answer

You can purchase the foam boards at HD or Lowe's stores. The foam normally is blue with a foil covering on one side.  It comes in various thickness. You want the 2 inch thick boards.  You will not use the white foam or EPS boards. 
Using spray foam and foil tape you can cut the boards with a saw into the pieces needed to make a box, then using a small amount of foam on the edges connect each side to the top. Once its connected then use the foil tape to hold it together. Let it sit for several hours to allow the spray foam to harden. This should make a strong box to use.  Just remember you need to construct this in the attic, as it may not fit up the stairs or through the hatch access.

Karin W · Answer

We actually only have a tiny crawl space up there, so it will probably be hard. But I will give it a try. My husband thinks he can just put something between the louver and the fan to keep the air out. What do you think about that?

Woodbridge Environmental Tiptophouse.com · Answer

As long as you can seal it and prevent heat loss from occurring that is ok, but I doubt you have enough space to get anything between the louver and the fan housing. Unless you simply pull the fan off and to the side then cover the louver. Then if in an emergency you need the fan remove the insulation then put the fan back into position to use.  Just be sure to unplug or disconnect the fan to prevent it from being turned on once it has been moved.

Kayleigh · Answer

I have some questions about a house we bought with a whole house fan.

Cover or keep old whole-house fan?

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Cover or keep old whole-house fan?

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