Five Percent: Conserve Energy

Climate Change Is Important: Energy Conservation is the First Step

September 21, 2009

Insulate Your Whole House Fan for $20

Category: Energy Audit,Household,Save Fuel,Tips – Tom Harrison – 10:13 am

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.

House Hole (click for full size image)

Whole House Fan, or House Hole (click for full size image)

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 Cover



One 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

tamrack-self-insulating-whole-house-fanAll 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 :-)


  1. Hello All:

    Great thread.

    I am looking into the whole house fan idea right now and wanted to get the experts take on it. (That’s you guys!)

    I live in the SoCal desert where summer temps can reach 120f. Hot! Winters are mild, in the 70s. Coldest it ever gets is in the 40s at 3am in the dead of winter. (If you can call 40s “dead”.) :-)

    This whole idea started when I considered insulating my uninsulated garage. Quote was $411. Instead of doing that, I’m now thinking of having the whole-house attic fan installed. I got a quote of $550. I need to check the type and quality of fan to see if it’s a good price or not, of course.

    My roof is concrete tiles, typical southwest style.

    If I’m going to get a fan, I want my 91 pound dog sticking to the ceiling, like Jeff suggested his cats would be. I don’t want horribly loud noise of course. So, I get that it should be mounted OFF the floor, correct?

    House is just over 3,000 square feet.

    Could someone recommend a fan brand and model for this application? And possibly give any advice for my situation.

    Also, is 1 fan enough for me? The attic–in the summer–is so hot, I have seen the devil himself up there sharpening his tail.

    Thanks so much!

    Comment by Eric — June 14, 2012 @ 6:16 pm

  2. Eric….

    My fan is made by Air Vent. They have a very comprehensive site with all their products and a whole slew of PDF’s about proper fan sizing and attic ventilation. The information there is invaluable if you are considering whole house ventiilation. I think it’s a good place to start since there are additional things to consider when planning for a whole house fan. At 3000 sq. ft., you will likely need a 36″. Whole house fans need to have enough “free air space” (attic vents) to the outside in order to freely move the air up and out of your attic. You may or may not be aware of this and it’s really common sense. But you may find yourself in a situation like I was in where I had to increase the attic ventilation in order to accomodate the air flow of my new fan. It was on Air Vent’s site that I discovered that my attic was not properly ventilated in the first place. I had a ridge vent and nothing else. I actually found out that my ridge vent was not doing anything for my attic ventilation. Seems like in the old days (back in the 50’s), the mindset was to seal off the attic rather than ventilate it.

    In any case, even if your attic does have proper ventilation now, you will need to make sure it can handle the addition of a WHF. Air Vent’s site is a pretty good tool and I highly recommend checking it out.

    Comment by Jeff — June 14, 2012 @ 9:02 pm

  3. But I also wanted to add that after I added the much needed soffit vents to make my ridge vent actually do something, that the ambient temperatures in my attic have dropped considerably…even with my whole house fan off.

    Comment by Jeff — June 14, 2012 @ 9:05 pm

  4. Eric —

    First, I think the aerodynamics of cats and dogs differ sufficiently as to mitigate your safety concern. I would be slightly concerned about uncaged birds (unless they were also 91 pounds, in which case I would be concerned about the fan). :-)

    So here are some things to consider:

    • The Tamrack fan I linked above is expensive, but quiet, but one is not likely to be sufficient for your case
    • The inexpensive fan I bought at Home Depot does the trick, but it’s not quiet; because it has a larger motor and is mounted to the framing of the house, you know when it’s on. We still love it, but could do better.
    • I have seen fans that mount inside the attic itself, near the exhaust window; when on they open the louvers and draw air through the hallway ceiling and out; they can be powerful and very quiet.
    • Keeping the attic itself properly exhausted is different than a whole-house fan — keep the attic as cool as possible and your home will not get as hot. The whole-house fan draws cooler air from outside through open windows to cool the house … which only works if it’s cool enough outside. 120°F seems hot to me (just sayin’) — maybe it gets cooler at night?
    • The downside of a whole-house fan is that you are cutting a hole in the house “envelope”, reducing the air sealing and insulation; this is why I built my box (for the winter), but it’s a bit of a trade-off in the summer, since we only run the whole-house fan in the evenings

    Hope I haven’t muddled things.

    Comment by Tom Harrison — June 14, 2012 @ 9:32 pm

  5. Tom,

    What you’ve done is EXACTLY what I’ve been planning for years. My only different idea would be leaving the two top panels of pink insulation (in your picture) “hinged” on one side with duct and/or foil tape so that the “doors” would opened by pressure when the fan was on and then fall back into place when the fan is switched off. In central Texas, there really isn’t aren’t clear seasonal times to ‘remove the box’ or ‘install the box’

    Any thoughts or suggestions?

    Thanks. Great post!


    Comment by Stewart — August 28, 2012 @ 2:15 pm

  6. Hey guys,
    I’m planning on building a “winter” box but wanted to through in my lessons learned to either help you of spark some additional ideas.
    I installed a WHF about 10 years ago for the same reasons; fresh air, cooling the house w/o using A/C, reducing costs, etc.. All things considered, I have no regrets. I chose a 30″ unit because at the time I had to consider a) ease of installation and b) square footage (about 1500). I tried to locate the fan closest to the center of the house so that the distance from each bedroom was approx equal and there was minimal obstruction for the air to travel from the downstairs to the exhaust port. So upstairs in the hall, its located in such a manner that the air flow is relatively balanced flowing from all directions, although I do have to close a door a bit from the nearest bedroom. I also built a plenum (wooden inlet duct) to extend the distance the air has to travel before in reaches the fan blades. Thus the frame of the fan assy rests approx 12″ above the ceiling. (suggested by the manufacturer)
    Since I wasn’t too anxious on cutting a joise, I located the fan so that the joise is aligned along the centerline of the louvered grate. Anyhow it looks and performs great.
    To cover for the winter, I saved the cardboard box the fan assy was packaged in and reused the lid. I sprayed contact adhesive to all exterior side surfaces of the box. Then cut bat insulation and wrapped the box lids sides to provide approx 3″ of insulation. I also wrapped duct tape bands around the perimeter to secure the pats in case they separated due to gravity. I then added insulation to the top.
    In the spring, I just removed the box from the fan and set it aside until the fall.
    Now I think I have to do something a little better because as I walked in the hall the other day, I couldf feel a draft.
    I’m planning to use either 2″ foam board (R15) with additional insulation added or use styrafoam (cheap boards) and bat to build insulated panels. I then would construct a box. I estimate the R value would approach R-20. I’ll make that decision when I check availability and cost of materials.
    The idea about the panels on top closing and opening is a great idea but way too complicated for this application. I agree, you don’t want to cause any additional back pressure that would affect airflow . And using actuators requiring power, wiring, switches is only going to add cost and most likely headaches. Why not install counterweights on the top panels that allow the panels to open when the fan is on and close when it’s off? Sort of like the louvers on the grate.
    Again, I think it’s a good idea and I just may give it some additional thought because when it is extreamely hot during the summer and the A/C is on, It would help keep the upstairs cool.

    Comment by Alan — January 7, 2013 @ 6:45 pm

  7. Alan —

    While R-value is important, in my installation it was getting the air sealing that turned out to be the real key.

    After my first pass, which I showed in the photo here, I realized there was a lot of air leaking through the attic floor boards and the various connections to the plenum leading into the house.

    I put down a nice flat surface of pink styrofoam board and sealed it with spray foam. The spray foam is great because you can get it in all the cracks and crevices — and at least in my installation there are a lot of paths for air (including outside of the fan box and frame). I backfilled the open areas around the box with fiberglass insulation. Air sealing solves the more significant problem with cutting a hole in the envelope of your house.

    Anyway, I sealed everything up and now, every fall just drop the box shown in the picture on top of the flat surface and tape box to flat platform with duct tape. In the spring, remove box and all is well.

    I agree that the self-insulating fans are probably not worth the effort in many climates — here in Boston, it’s pretty much either cold or not. In some areas, the insulation when closed is mainly for when it’s hot during the day and cool enough at night to run the fan.

    Comment by Tom Harrison — January 9, 2013 @ 9:09 pm

  8. I guess I should have followed up earlier, but here it is…
    I made a box constructed of 2″ foil faced styrafoam board purchased from Lowes. The foiled surface faces the inside and the “panels” and joined with foil taped, inside & out. The box encloses the fan/motor assembly and rests on the face where the plenum meets the fan base. Upon initial fit, I “pounded” the box to ensure a relatively tight fit is obtained and any obstructions or interference cause by the centering blocks that were used during initial installation were impressed into the foam edge of the box. I then removed the box and removed a little foam material and covered the fan assy. The fit is great and if there is any crevises, O can’t detect them nor feel any drafts when checked.
    Total cost was about $35. I’m very happy with this and hopefully should last another 15+ years. When I walk under the fan in the hallway, I no longer feel a draft or the cold. So far so good!
    I’m not sure how to post pics otherwise I would.

    Comment by Alan — March 27, 2013 @ 11:47 am

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