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.

Step 2: Apply the Velcro

I applied the 'hook' part (the scratchy side) to the fan. Attaching it nearer the inside of the box allowed me to avoid the four screws.
The screw heads might be a problem by preventing the panels from closing together so I cut out small divots with a knife to make room for them.
A utility knife allows you to carefully cut the Velcro at the corners to make them fit nice and snug.
Go ahead and stick the 'loop' side (fuzzy size) onto the hook side and make sure it's attached firmly.

Step 3: Finishing Touches

Now pull the sticky back off the fuzzy side.
Carefully position the foam board below the fan, and when it's in just the right location push the board up onto the Velcro sticky back. Then spend some time going around the perimeter pushing up firmly to ensure good adhesive connection to the foam board.

According to the instructions on the Velcro box, it takes a day for the adhesive to reach full strength. So I did not immediately try to pull the board off, fearing the adhesive would give way before the Velcro gave way. I don't know if this would happen or not, it's just a theory.

Style points...
I may apply some white spray paint next spring to make the cover the same color as the ceiling. Or I may not - the baby blue color isn't too bad. For those with more refined tastes, one could apply spray can texture but I don't think I'll go that far.

How's it work?
The layer of Velcro around the perimeter seems to effectively stop any cold air leaking out from the fan. Before putting on the cover, cold air coming down off the box was quite noticeable. But with the cover on, no cold air can be felt anywhere.

Update: This is a good time to make small marks on the ceiling so you know where the panel edges should be. This will be helpful when it's time to re-install it in the fall. See the final step for additional comments and modifications!

Summary
Total cost would be about $35, which is not that much cheaper than the cheapest commercial unit I saw.
However, this has better insulation, and cost would be reduced considerably if you can buy a 4'x4' sheet of foam somewhere or share the cost with someone else.

Step 4:

UPGRADES:
After having this in place for a few weeks I made several changes:

1. I noticed a small bit of cold air was coming out from between the ceiling and the metal trim. I added some caulking to stop this. See photo

2. Since I only used half of my 4x8 sheet of foam, I decided to use the other half to double my insulation. A single 1" layer of foam has an R value of 5, so by gluing on a second layer I could get an R value of 10. I used Elmer's white glue, which has the advantage that everybody has some laying around somewhere. The disadvantage is the need to wait over night for the glue to dry.

3. Once my modified insulation panel was ready to be reattached, I found it was surprisingly difficult to align the panel Velcro with the matching Velcro on the fan trim. The main problem is that the Velcro is out of sight and there are no visual cues telling you where to place the panel so the Velcro strips line up exactly. If the Velcro is not aligned properly the panel could drop off the ceiling or at the very least gaps may form allowing cold air to seep out.

The first modification I made was to mark the top side of the panel with "E" and "W" (the direction of our hallway) so that I would always install the panel with the original orientation (see photo). Theoretically it shouldn't matter if the panel were flipped around, but theory is not always the same as practice.

The second modification I made was to put two white screws into the ceiling to guide me while installing the panel. By holding the panel against the screws I am assured that the Velcro strips on the east and west edges will match correctly. A person could also just make some small marks on the ceiling to do this.

Here's how to mark the screw locations on the ceiling: on the panel measure the distance, X, from the Velcro to the edge of the panel. Now shift your attention to the Velcro on the ceiling. Measure X inches out from the Velcro and make a small mark on the ceiling. This is where the edge of the panel should be located, so the screw should be placed such that its *edge* is lined up with this mark.
Note: it is much easier to mark the edges of the panel when it's first installed onto the ceiling. That's when the alignment is perfect by definition. Once you pull the panel off it's much more of a pain to find the exact edge locations!

Finally, I put two long screws into the foam panels on the sides against the wall. For my case, these stick out about 1.25 inches from each edge and come within about a quarter inch of the wall. The screws force correct alignment in the N-S direction when raising the panel into position. Screws in foam are not very strong, but with care they should last a long time.