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Whole-house fan with blown-in cellulose insulation

josher668 | Posted in Energy Efficiency and Durability on

I have been working on installing attic baffles in my attic in preparation to blow in cellulose insulation to a height of 18″ to achieve ~R60 for the majority of the attic. Per my calculations and manufacturer specs, my attic has 14 square feet of net free exhaust area 50 feet of ridge vent and 18 soffit vents.

I would like to install a Triangle Engineering whole house fan that requires 11 square feet of exhaust venting. My concern is whether or not the whole house fan will interact with the blown in insulation? Most websites state that it should not be an issue, but I saw a posting here at GBA saying otherwise.

If I frame around the fan with plywood/rigid foam and meet the exhaust requirement, do I have to worry about the blown cellulose going into the ridge vent? If I run the fan while I’m up there and observe dust and cellulose being kicked up, what are my options?



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  1. GBA Editor
    Martin Holladay | | #1

    You should be OK as long as:

    1. Your insulation dam (at the rectangular opening where you mount the whole-house fan) is significantly higher than the top of the cellulose insulation, and

    2. Your ventilation openings are generously sized (so that air can freely leave the attic when the fan is operating).

    That said, whole-house fans have one significant drawback: The fan opening is often poorly insulated and leaky. If the fan opening leaks air during the winter, your energy bills can be high.

    -- Martin Holladay

  2. charlie_sullivan | | #2

    Whole house fans also tend to bring in humidity, which in some scenarios might then need to be removed at a higher energy cost than was saved by using the fan in the first place. Are you in climate where that's likely to be an issue?

  3. Jon_R | | #3

    If you are sure that you want one, I'd get the two speed model. Fully variable speed (eg, VFD with a 3 phase motor) would be even better.

  4. josher668 | | #4

    Thank you for the responses.

    Martin, is there an exact value for "significantly higher"? Would 4 inches be enough? An cover the blown cellulose with a thin layer of fiberglass batts to prevent the cellulose nearby from moving?

    My ventilation openings are the 18 soffit vents that are .75" by 1.25" and the 50ft of ridge vent at the top of the roof, so I feel the attic can handle it. I have a total of 50 rafter bays and each one now has an accuvent attic baffle installed so air can flow up or down these chutes to the soffit. Is this good?

    Jon, I was looking at the Triangle Engineering CC3023 found here The fan does have a high and low speed and I was going to wire it with a timer 15 minutes - 4 hours along with a locking toggle switch. I have not seen any fully variable speed whole house fans.

    Charlie and Martin, I've read a lot about the "gaping energy hole in the ceiling" caused by a whole house fan and am planning on building a box around it that has a hinged top piece that I can close (hopefully from below by reaching up through the fan) and instead of the grill in the ceiling, use cabinet doors that are held shut by magnets (with foam along the edge). I can then also stuff some bags filled with fiberglass insulation into the cavity between the doors and the actual fan to help stop the energy loss. Or I might attach rigid foam panels (could do 3" worth" to the inside of the doors.

    I live the chicagoland suburbs and what's happening is that on a nice, 76 degree F day, the upstairs of my house can easily reach 6 to 8 degrees hotter. I know this is largely due to the lack of insulation in the attic (currently only 2 inches of fiberglass batts), but I absolutely HATE running the air after work from 4 pm to 9 pm to try and cool the house down. I've had moderate success with box fans in windows, which leads me to think a more powerful whole house fan is the answer. I know that when it's a really hot day I will have to use AC, but the spring and the fall seem like a good time to use a whole house fan, as long as I can insulate the opening and not blow insulation into my ridge vent.

  5. GBA Editor
    Martin Holladay | | #5

    Only you can make the decision on whether you want to install this fan. Charlie is correct that such a fan can bring in lots of humidity, so you don't want to operate the whole-house fan at night and then try to run the air conditioner during the day. You need to swear off using the air conditioner for several weeks at a time for this approach to work.

    Making your dams 4 inches higher than the insulation is probably good enough, especially if you install some fiberglass batts on top of the nearby cellulose.

    I'm not going to double-check your math on the ventilation opening sizes. The formulas you need to follow are provided in my article, Fans in the Attic.

    So you need to do the math. If you are worried about swirling cellulose, it never hurts to size your ventilation openings to be larger than the formula requires.

    -- Martin Holladay

  6. josher668 | | #6

    Thank you for all of the responses and items for me to think about. Last spring and fall there were weeks at a time where we didn't use the AC, relying on box fans so I think we can get by with the improved air circulation from the whole house fan.

    Martin, I used Triangle Engineering's Net Free Exhaust computation page found here to confirm that my attic can handle the fan. Their ratings and values align with your information on your Fans in the Attic article.

    Jon, that's quite the article you've found! Many of the computations are a bit beyond me, but if I can get the temps to the mid to low 70's to match the temperature outside without using AC, that's a win for me. I realize the whole house fan won't stop me from using the AC on those 90 deg Chicagoland summers, but it will help a lot on the cool spring and fall evenings.

    Thanks for all of your help and I'll let you know how this project goes!


  7. Expert Member
    Dana Dorsett | | #7

    Latent loads in Chicagoland can be pretty high, and nighttime ventilation can be a lousy strategy if targeting 50% RH @ 75F (dew point = 55F) for the indoor conditions.

    If you have a reliable local weather station, look up the current dew point, not just the outdoor temp before making the decision to go with the whole house fan vs. air conditioning. If the dew point is much above 55F you might want to consider letting the AC run, even if it's cool enough outside to get decent sensible cooling out of the whole-house fan.

  8. Jon_R | | #8

    Several issues to consider: Are you comfortable with the conditions? Do you avoid enough use of AC to make up for the moisture load that you gain (perhaps $.50). For sustained use (not nightly with AC during the day), mold growth can be a concern.

  9. Jon_R | | #9

    I tried to get a rough idea of the cross over point where moisture gained outweighed cooling saved. Lots of variables, but my rough conclusion is that it may be financially worth letting in humid outside air at night whenever it gets sufficiently cool - perhaps < 70F. Perhaps someone knows of a more targeted study.

  10. brendanalbano | | #10

    Chicago's a tough climate. The impression I get is that none of the non-AC cooling techniques (whole house fans, swamp coolers, etc.) are particularly great here because of the high humidity and relatively small day-night temperature difference.

    A quick google to find average dewpoints led me to this site:

    Which shows the average dew points in June, July and August all being higher than the 55F Dana referenced.

    You might be better off with conventional AC, and trying to limit usage by setting the thermostat as high as is comfortable and using ceiling fans to create a bit of air movement which can increase comfort even though it doesn't change the temperature.

  11. Lisbeth99 | | #11

    Jon I live in Florida and it doesn't get much more humid than that. We bought a house last year that already had an attic fan. We couldn't be happier. We love to turn off the a/c and this makes it a reality. The fan keeps the air moving in the house during the fall and winter when we can turn off the a/c. I would never want to be without it again.

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