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Will positive pressure work to vent an attic (i.e., blowing air in)? Attics exhaust fans are a bad idea. But what about blowing air in (using positive pressure).

Erica Downs | Posted in Energy Efficiency and Durability on

My father is interested in removing heat from his attic to try to keep the upstairs of his house somewhat cooler. I have advised him that generally speaking, attic fans are a bad idea, which he has come to terms with. However, with that understanding that creating negative pressure in the attic can cause all kinds of problems (sucking out air from the conditioned space, possible back-drafting of combustion appliances, etc.), he has come up with the idea to blow air into the attic instead (i.e., creating positive pressure) to force the hot air out. I have already discussed with him that it is the structure that is heating up, and radiating heat into the attic, so an attic fan will not likely help much. So that aside, since he is determined, will he actually be able to create any air flow out of the attic by using positive pressure? Is there a fan powerful enough, and if he finds and installs it, what are the pros/cons that we may not be thinking of yet?

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Replies

  1. John Brooks | | #1

    When a House is Air conditioned...the absolute pressure in the upper portion of the house will be negative with respect to the outside(WRTO).
    If there are any leaks in the air barrier between the attic and the house...hot attic air will "spilll" into the house.
    Pressurizing the attic would only make things worse.
    The best money spent would be to make the attic floor airtight and add ample insulation above.

  2. Erica Downs | | #2

    Thanks John -- that's what I pointed out when he was considering an EXHAUST FAN (hence the "bad idea" part of my original question). But in reference to your response, I'm not sure how creating POSITIVE pressure in the attic (via a fan blowing IN) would allow conditioned air to "spill" into the attic. This seems like a different issue to me.

    My feeling was he would have trouble finding a fan powerful enough to create an air flow through 1,000 sf attic space. And if he did find a fan that worked, it still would not resolve the heat issue, and there may be some unexpected consequences.

    Just some further background info -- the house is in the Boston area, so hot, humid summers. There is no central air conditioning, just a single portable AC unit. He has been diligently working at air sealing in the attic.

    Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Erica,
    When you install a fan to pressurize the attic, the attic will become pressurized with respect to the outdoors and with respect to the conditioned house below. The pressurized air in the attic will force its way through ceiling cracks into the conditioned house.

    In other words, the fan will make the ceiling leak at a higher rate than it otherwise would. You'll end up with more hot air leaking through your ceiling during the summer than if you had no fan.

  4. John Brooks | | #4

    Hi Erica,
    I didn't say conditioned air would spill into the house
    I said Attic Air would Spill into the House
    (if the house were air-conditioned and it is hot outside)

    If you pressurize the attic...
    the hot air will flow/spill into the house even faster

  5. John Brooks | | #5

    thanks Martin...I was typing while you answered

  6. Erica Downs | | #6

    Thank you both -
    (John, apologies, I just realized I misread your response!)
    Will pass along the info to my father...
    Erica

  7. Sandy McLardy | | #7

    I realize this is an old post....but it pops up. The idea that attic fans are a bad idea is nonsense. And so was the study from the school in Florida. Removing hot air from an attic works. I would like to know who on this site is familiar with manual j load calculations? You can get confused with radiation, convection, and conductance, but the issue boils down to temperature Difference and resistance (r value/ u value). If your attic is 140 or 90 that will make a huge difference! If you are worried about co gas made sure you also remove your bathroom fan, stove vent etc. A little information is dangerous and wasteful. Cheers

  8. GBA Editor
    Martin Holladay | | #8

    Sandy,
    If you have developed an easy way to lower the temperature of your attic from 140°F to 90°F, that method would lower cooling costs -- but only if the method used to lower the attic temperature required less energy than the amount of energy you expect to save, and only if the method doesn't have negative ancillary effects (like depressurizing the house or sucking conditioned air through ceiling cracks).

    Researchers haven't yet discovered a way to lower attic temperatures that is inexpensive enough to justify widespread implementation. For an article discussing all of the drawbacks of powered attic ventilators, see Fans in the Attic: Do They Help or Do They Hurt?

    I assume that you were being ironic when you suggested that readers should "make sure you also remove your bathroom fan [and] stove vent" -- but if irony was your intention, your point escapes me.

    Q. "I would like to know who on this site is familiar with Manual J load calculations?"

    A. Many long-term readers and contributors to GBA are familiar with Manual J load calculations. If you are interested in learning more (rather than merely implying that this site's readers need to study more), you may be interested in reading these four articles:

    Saving Energy With Manual J and Manual D

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Calculating Cooling Loads

  9. Expert Member
    Malcolm Taylor | | #9

    Sandy,
    I'm interested in any argument on a subject that overturns what is accepted wisdom - but you really haven't provided any basis on which we could change our minds. Why not flesh out your reasons for using attic fans so we could see why they are a good idea?

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