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  1. #1
    Join Date
    Jul 2013

    Default Attic ventilation: positive pressure from gable fan to improve ridge vents?

    I'm trying to keep my attic cooler! I have a small house with no soffits (no overhang or eaves, so no place to install them!) but gable vents at both ends and a gable fan blowing outward. Recently, ridge vents were installed - which I know is a very questionable decision in combination with powered ventilation.

    Without powered ventilation, the attic gets much too hot for my taste - over 40F above ambient, and it's really making keeping the house cool. But I'm wondering what the best way to get the gable fan and the ridge vent to work together. Would it be better to reverse the flow on the gable fan, so it pulls air in, to force air out at the ridge vents?


  2. #2
    Join Date
    Jul 2010

    Default Re: Attic ventilation: positive pressure from gable fan to improve ridge vents?

    What's missing in your equation are lower vents (soffit or eave). You need to find a way to install them.

    With no soffits or eaves, how is the building finished? Can you upload a photo or two?

  3. #3
    Join Date
    Feb 2008

    Default Re: Attic ventilation: positive pressure from gable fan to improve ridge vents?

    I agree with dj1, but also do some calculations as to the rate the fan is exhausting hot air (check the fan housing for its rating of cfm/cubic feet per minute) in conjunction with the size of the intake vent (square inches) on the opposite gable to make sure it is large enough to allow new outside changes of air into the attic so the fan can exhaust the heated air----remember, passive attic vents are calculated in square INCHES, while attic fans are usually calculated at cubic FEET per minute of air exhaust; thus, your exhaust fan might be starving for more air to exhaust, since you may not have a large enough passive air intake vent at the opposite gable to get a good air exchange in the attic.

    It is an absolute MUST that all passive & fan-driven vents have insect screening to keep all the insects out that are constantly trying to make a home of your attic; screening cuts down somewhat on vent capacity, but it is well worth it, and this is noted on the square inch rating usually stamped on the vent housing.

    It is strongly preferred that your INTAKE vent(s) for the fan be located on the NORTH or EAST side of the house (to suck in cool air from those less sunny locations), and the fan exhaust be located on the WEST or SOUTH side of the house----this allows the cool air (north/east), (note usual sun positions), to be sucked into the attic & the hot air to be exhausted to the south/west side of the house; there should be an automatic thermostat on the exhaust fan to activate it when the attic temp exceeds 100 degrees; I recommend a large thermostat be posted permanently in the attic, visible from the lower floor, and an overide on/off switch to be located in the living quarters to disable the fan if the house will be empty for any period of time to minimize a possible fire danger from an overheated fan, especially one that's been in service for several years.

    Google "How to calculate and install attic ventilation Pt. 1 you tube video" by Iris Communications to get an extensive video rundown on sizing attic ventilation---it is noted by Iris that there are many places & ways to install passive or fan driven vents into an attic system to obtain good ventilation; they recommend first calculating the total sq. footage of the attic & then applying the 1 to 150 rule or the 1 to 300 rule (ventilation vents rules) to determine the # of passive intake & exhaust vents; sometimes electrically-driven fans are not needed at all (they burn lots of electricity, & can possibly cause an attic fire if the windings overheat), or a solar vent fan can be substituted (see site below), which are less likely to present a possible fire hazard.

    Sometimes rigid foam insulation placed between the rafter joists is very helpful in attics that are difficult to cool.

    Last edited by Dobbs; 07-28-2013 at 10:34 PM.

  4. #4
    Join Date
    Aug 2007
    Portland, Oregon, formerly of Chicago

    Default Re: Attic ventilation: positive pressure from gable fan to improve ridge vents?

    There is one more concern when utilizing powered attic fans when the house is air-conditioned: If the powered fan creates a negative pressure in the attic, air will be drawn into the attic from every gap available, including conditioned air from the house. The ceiling plain of tthe average house leaks air badly. Every opening for cannister lights, bath fans, kitchen fans, holes made to pass plumbing lines and electrical lines, etc. is a potential air leak to the attic sucking cool air out of the house living envelope.

    If a house is properly ventilated, no additional powered venting should be neccessary.

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