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  1. #1
    Join Date
    Aug 2012
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    1

    Default Warm climate attic ventilation

    I'm struggling a little to understand and improve my attic ventilation. I want to avoid mechanical methods (electric/solar fan, "whirlybird", etc) and just use the natural buoyancy of hot air rising to do the work.

    I have about 1,800 SF of attic floor space which means I need 12 SF of total net free air (NFA) ventilation. This is using the 150 SF of attic floor space to 1 SF of total venting as my guide which seems to be consistently referenced as the standard. I also have a fairly low pitched roof meaning less total attic airspace, but Im not sure how that factors in.

    Question 1: I see some recommendations that 50-80% should be high mount ventilation (ridge vent or dormers high towards the peek) and the rest low mount (soffit or louvers at the base of the rafters which is what I have.) Other recommendations say 60% low and the rest high. Which is correct? Does it matter? Should there be more top vents or more bottom vents or should it be equal? Im trying to understand the science around the correct answer and get the maximum air flow effect. For example, does more venting at the top create a vacuum effect causing more air to be drawn in the bottom and out the top or does that vacuum effect restrict the amount of air transfer there would be with more lower venting?

    Question 2: Most of my research seems to address cold climates where snow buildup and ice dams are a problem. Given those climates, it seems a certain level of heat (above the outside temp) is needed in the attic to avoid these issues so there is a maximum amount of venting youd want. Im in a warm climate in California. No snow and the coldest it gets in the winter is 45-50 degrees F and we maybe get 2-3 days of temps approaching freezing on an off year. Given this warmer climate, why would there be ANY limit to the amount of venting Id want? Why wouldnt I want my attic temp to equal the outside temp? In summer, it means my attic insulation (and A/C) would have to work less and there would be less heat impact to my attic structure and shingles Ive measured temps over 130 degrees F in my attic 20-30 degrees above the outside temp. In the winter it would be the opposite. My insulation would have to work harder (along with my heater), however, Im typically only trying to raise the temp 5-15 degrees F in the winter anyway. Also, the variance in attic temp to outside temp in winter is a lot less normally so the actual impact I'd experience would be less. Overall it seems to be more efficient in my case to improve my summer performance with additional venting (getting the attic temp as close to outside temp as possible) while offsetting the summer benefits with the winter impact.

    Any thoughts? Am I missing something here?

  2. #2
    Join Date
    Aug 2012
    Location
    new york
    Posts
    6

    Default Warm climate attic ventilation

    When it comes to warm climate attic ventilation it is really a beneficial system that could be installed in a house. Perhaps any one could be able to have a favorable environment.
    .

  3. #3
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,360

    Default Re: Warm climate attic ventilation

    Continuous ridge and soffit venting.
    Last edited by HoustonRemodeler; 08-30-2012 at 08:19 AM.

  4. #4
    Join Date
    Aug 2008
    Posts
    66

    Default Re: Warm climate attic ventilation

    You have quite a sizable area of attic square footage; the other recommendations for ridge/soffit vents are good; it will take a sizable # of intake vents below and exhaust vents above to cool off such a large enclosed space with the sun beating down on a hot day; do the calculations at the sites below to get the proper # of venting square footage.

    Refer to the videos below by Iris Communication to calculate the amount of attic ventilation sq footage needed for your particular home; if the video links don't work, Google "how to calculate and install attic ventilation Pt 1" and "how to calculate and install attic ventilation Pt 2".

    The calculations in the video take into account the total attic area, the height of the attic, and the vertical distance between the lower intake vents and the upper exhaust vents, as well as the existence of a vapor barrier (kraft paper or plastic) under the attic insulation, and use the 1 to 150 rule or the 1 to 300 rule to calculate the total square footage of venting needed.

    They advise to place any roof venting fixtures on an inconspicuous section of the roof so that they are not visible from the street, or the front entrance.

    Your summer attic temp of 130 degrees is too high---you have to get it down to below 100 on hot days; place a permanent large thermometer near inside attic ridge visible from living quarters at the attic door, so you can periodically check the attic temp, especially on hot days; recommended attic floor insulation of R40 with a vapor barrier will isolate the living quarters from attic heat and attic cold in the winter months.

    Strongly recommend you consider a 3-speed whole house fan mounted on a 3' X 2' stainless steel or aluminum screened louver in an inconspicuous gable end with 3-speed switch control in living quarters to double as an attic vent on hot days as well as a living quarters vent to cool down the house on hot days (high speed during day/low speed overnight) which will eliminate the need for expensive AC on many hot days & not so hot days/nites; whole house gable fan has to avoid a gable location proximity to any nearby neighbor to avoid the noise issues associated with some less expensive whole house fans; the more blades a fan has the better; 4 blades is more efficient and makes less noise.

    Learn to work with Mother Nature; she sends us hot, humid weather sometimes, but she also often sends us free-for-the-taking low humidity cooler temps and breezes, often overnight, where a fan system is much better than AC.


    http://www.youtube.com/watch?v=vShj1l0PnF0
    http://www.youtube.com/watch?v=GFLTho7nLOE
    Last edited by dodsworth; 08-30-2012 at 05:17 AM.

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