One of my fall chores is to insulate the whole house fan in our house.  Previously I built a box in the attic out of foam boards, but using it is a pain because I have to empty out a closet to get to the attic door and then  crawl through deep blown-in insulation to get to the box. 

In our house, the fan comes in through a hall ceiling. It would save a lot of trouble if I could put an insulation cover on the fan from the hallway and avoid the trip to the attic.  A quick search on the internet shows some nice looking products, such as http://www.batticdoor.com/WholeHouseFanCover.htm.  The ones I found cost at least $40 after S+H and consist of a thin flexible insulation blanket.  I wanted to spend less money and have more insulation.

Step 1: Get the Materials

Measure your fan opening.  Mine was 36" by 31".
Buy a sheet of 1 inch foam insulation.  I had to buy a complete 4x8 foot sheet for a little less than $20, but maybe in a larger town with better DIY stores you could find a 4x4 sheet.  Hopefully I can find someone who can use the left over foam.

Cut the insulation to size.  I cut mine to 36" by 32" to take advantage of the factory scribe at 32".  Fairly clean cuts can be made by scribing with a utility knife and snap using a straight-edge.  The picture shows my first cut which is not so nice.  It cleaned up nicely with a Dremel tool however.  This edge faces the wall so it is not very visible anyway.

Finally, buy a box of sticky back Velcro.    Amazon lists a box of 15 feet of 3/4 inch Velcro for about $15. I had a box with 20', but you only need about 12 feet.  Generic versions of Velcro might be cheaper; I didn't check since I had some on hand.
Spray panting foam will melt it. I would recommend a coat of latex paint (if it will bind). I've found simple window weather sealing kits to be pretty effective.
<p>The newer ducted style whole house fans like those from CentricAir at www.centricair.com have an insulated damper door that automatically seals off the attic from the living space whenever the whole house fan is not in use.</p>
Very nice solution! Much better than crawling around in the attic!

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