In the summer, we use our whole house fan to stay cool — it draws cool, fresh evening air through the house making us comfortable enough that we never used an air conditioner this past summer. Our electricity bill was great.
But now that it’s fall, we might as well call it a “house hole” instead :-)
We have a 32″ square hole in our attic. We had an old mattress cover that was about the right size and we tossed it over the top every fall thinking, “close enough”. Then we had our energy audit last Spring, and this is what we found: the picture on the left is of the louvers that cover the fan opening when it’s not on; the picture on the right is an infrared photo of the same area taken (with our mattress cover installed). Blue is cold, and cold is bad.You can also see some un-insulated areas along the top of the window, as well as around the fan itself. But that dark blue area is right in the middle.
Blue is bad.
Since the energy audit, we have had the house insulation filled in where the first contractor messed up, and topped off the insulation in the attic. But I still needed to improve on the mattress cover.
How To Make A Whole House Fan CoverOne word: rigid Styrofoam insulating panels.
You can get them at an lumber yard, or home store like Home Depot or Lowes — I bought 3 pink panels, 2 feet (24″) wide, 8 feet long and 1 inch thick. But first I measured.
Oh, and a roll of duct tape, wide if you can find it. Total cost for three panels and tape was about $20.
The trick is to make a box with one face open. This isn’t precision engineering, so give yourself and extra inch. My fan opening is around 32 inches square, and about 14 inches off the floor to clear the motor. These are the inside dimensions — you’ll need to add an inch to the length of the sides to account for the thickness of the panels. Start with the 4 side panels.
You can mark the measurements on the board — I used a pen and a 4″ carpenter’s level. If you don’t have the level, any straight-edge will do.
There are a couple of ways to cut this insulation. If you have a sharp blade on what used to be called a “Stanley knife” (before 9/11, now “box-cutter”) you can make a nice clean cut — just pull the blade smoothly. With 1″ board, the blade will cut almost all the way through — I just snapped off the remaining part. If you have thicker board you can cut in from both sides and snap it. If you have power tools, you could also use a jigsaw or sabre-saw if you wanted, or for that matter a circular saw. A carpenter’s finish saw would work, too. In a related job, I used a hacksaw blade mounted in a frame designed for cutting in tight places, which lets the blade extend out. I would not recommend a chainsaw. Bottom line: this doesn’t need to be finish carpentry.
Cut the long way first for the box sides, then out of the two long pieces, cut four sides to length, in my case, 33″ x 14″. Overlap one edge with the other and tape the first two sides together. Try to align the tongue-and-groove edge the same way, and make sure what will be the top edge of the box is about the same height so the top will fit on flat. Keep working around until you have all four sides taped together.
Now for the top. Lay down an un-cut board over the box edges, and tape two of the sides temporarily — aligning the two edges of the board with the box sides will make everything nice and square. Use the underside of the opposite edge to mark the length of the board and cut it (you can take it off the box sides). The other half of the remaining board should fit, tongue-and-groove and make a nice seal — mark the undersides, and cut them.Now just assemble the remaining pieces with duct tape, and voila — a box.
I am still experimenting with methods for creating a good seal between the box and the attic floor. I bought a can of spray foam, which might do the job. Or maybe just duct tape — my attic floor is just rough-cut boards, so that poses a little challenge. I am thinking of creating a permanent flat surface with left-over rigid panel that sits on the floor and seals tightly against the fan frame — this might be a good idea in my case — if your attic has a nice plywood floor then you’re probably all set.
In any case, this is an air sealing job with an insulation component. The duct tape will do a good job with air-sealing the box itself. The insulation value of the 1″ rigid board isn’t all that great — R 2.5, I think, but I think this will be a far better outcome than the old mattress cover.
This job took me about a half hour once I had the board home from the lumber yard.
How To Do It Right the First Time
All of this insulating was needed because I bought the cheapest possible fan from Home Depot. It’s loud, was difficult to install, and despite the gaping maw I hacked in my ceiling, does only a pretty good job of sucking air through the house.
If I had it to do again, I would get a top quality, quiet, self-insulating whole house fan that didn’t require cutting a ceiling joist, since it is narrow and designed to fit a standard 16″ width between joists. This model has two smaller, quieter fans; when on, the doors open up to let air flow; when off, then close down and have a thick layer of insulation on top, built right in.
Oh well, live and learn.
Our energy audit found two other gaping holes in our house, and the rigid pink Styrofoam panels are a good solution for those, as well.
More adventures in air sealing to come :-)