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Roof venting doesn’t decrease AC load

bobbomax | Posted in Energy Efficiency and Durability on

Martin, in Fine Homebuilding, you wrote that roof venting doesn’t decrease air conditioning load (http://www.finehomebuilding.com/2017/07/11/roof-venting-doesnt-affect-cooling-loads#comment-13202 ).

Which leads me to ask what roof venting IS good for. Humidity control? Presumably, venting doesn’t increase heating load in the winter.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Bingo!

  2. GBA Editor
    Martin Holladay | | #2

    Robert,
    You should read a GBA article titled All About Attic Venting.

    In that article, I wrote:

    "Here are the four most common reasons people suggest to explain the practice of venting attics:

    • To reduce the chance of moisture build-up in the attic or condensation on the underside of the roof sheathing.
    • To make roofing shingles last longer.
    • To lower cooling bills during the summer.
    • To reduce the chance of ice dams.

    Although attic ventilation is sometimes able to contribute in a very small way to addressing the problems on this list, there are much better solutions to all four problems than ventilation."

    For a full explanation of the reasoning behind these sentences, read the article.

  3. Anon3 | | #3

    Roof venting does decrease AC load, in some houses by a lot actually. Those solar attic fans are amazing.

    Have you ever been to a room that is closed off for ages? Your attic/house will smell and feel like that if it's not vented.

  4. Expert Member
    Dana Dorsett | | #4

    Anon3:: Really? Show me the data!

    A review the published literature of actual measurements shows attic venting in hot climates usually increases cooling energy use (with but a few exceptional cases to prove the rule.):

    http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

    Best case with one of those "amazing" solar fans is yields only a low single digit percentage in energy savings:

    http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/

  5. Anon3 | | #5

    Show the data? I installed one recently, the difference it made was huge. I should have gotten the bigger version in hindsight. Oh well, on my next house I'll get the bigger version.

  6. user-2890856 | | #6

    How is it that an attic fan made a difference Anon3 ? Was the difference in a sensible feeling or in energy used ?

  7. Anon3 | | #7

    The AC would struggle when it's 90f outside. With the solar fan, it's now fine into 100f. I was getting massive radiant heat before, now it's mostly gone.

    Note this is a older house.

  8. GBA Editor
    Martin Holladay | | #8

    Anon3,
    I've asked you before: Can you tell us your name? And perhaps you can tell us whether you work for a product distributor? Many of your posts seem to promote one or two products.

    The Fine Homebuilding column that Robert Fankhauser referred to in his original question concerned cathedral ceilings. Anyone who attempts to install a solar-powered attic fan in the ventilation channel of a cathedral ceiling is nuts. Such a fan would undoubtedly pull indoor air into the ventilation channel through ceiling cracks, raising the home's energy bills.

    Anon3 has apparently brought up a new topic, concerning attics instead of cathedral ceilings.

    Suffice it to say that solar-powered attic fans have the potential to cause more problems than they solve. They can occasionally be useful, but only in attics that (a) have insufficient insulation, (b) have ductwork located in the attic, and (c) have a very, very airtight ceiling. Such homes are rare.

    If you have an older home with insufficient insulation and ductwork in the attic, the solution to your problem is (a) perform air sealing work on the attic floor; (b) seal duct seams; (c) improve duct insulation, if possible, and (d) add more R-value to the existing attic insulation.

    If you have a big bank account, an even better solution is to convert the vented unconditioned attic into an unvented conditioned attic.

    The steps I have described make much more sense than installing a powered attic ventilator.

  9. Anon3 | | #9

    Dude I should have installed the attic fan years ago, but thanks to forum like this I was mislead. Oh well, you learn something everyday.

    There's another benefit, the incredibly nasty attic air is now almost fresh smelling!

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Anon,
    What difference does it make how your attic air smells unless that smell permeates the house below? In other threads you have suggested that the attic fan will make the whole house smell better, but if the attic air is causing odours below, the solution is to air-seal the ceiling, not to draw indoor air through the leaks into the attic.

    I wish you would engage in some discussion of the advice you suggest, rather than moving on to a different discussion and just repeating. If you are right, that's how you could convince us. Just making an assertion and leaving it hanging doesn't do that.

  11. EricPadgett | | #11

    I don't get how ANON3 has not been banned. He consistently posts conflicting posts that are confusing and incorrect. I'm sure you can handle the $16 hit to improve the quality and consistency of content. There are several reasons he doesn't want to post his name. Trolls have no interest in helping others but rather to amuse themselves. I've also not found a troll that wasn't a coward hiding behind internet anonymity. He will smile at this post knowing he made somebody post, but I hope it's the last post he can write.

  12. Expert Member
    Dana Dorsett | | #12

    Eric: This forum is not (and should not be) about promoting homogeneous thinking of some orthodoxy. As long as participants are willing to support assertions with something other than more unsupported assertions, it's useful to have real world situations to dissect & analyse.

  13. GBA Editor
    Martin Holladay | | #13

    Dana,
    As you know, I've never had a problem with the posting of conflicting opinions, as long as the comments section isn't used to insult the personalities of other readers or authors.

    When a frequent poster shows signs of promoting products, however, and seems unable to back up assertions with valid data, it's only reasonable for a web forum administrator to ask the person in question to identify himself or herself, and to let us know whether these product recommendations are motivated by business interests. If such a person refuses to identify himself or herself, it would be within the rights of the web site administrators to block future comments.

  14. Jon_R | | #14

    I expect that air sealing without correcting pressure differentials (often attic->interior in the summer) would reduce but not stop odor movement.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    The distinction I'd make is between those asking and those answering questions. If the information being tendered to people coming here for advice isn't correct, the forum loses it's usefulness to all of us.

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