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Thread: Attic Vent Fan

  1. #1
    DIY Member Phillbo's Avatar
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    Default Attic Vent Fan

    I live in Arizona so my attic will get extremely hot during the summer. I'm curious how much benefit I get out of using the existing Attic Vent Fan. Do they really make a difference or are they just a marketing gimmick?

    My gut tells me that the electrical savings I will gain by dropping the attic temp a few degrees will not be greater than the electricity used to run the fan.

    Any thoughts ?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It would somewhat depend on how good the ventillation is in the attic on its own. Do you have enough surface area for the soffit vents? What about gable vents? A ridge vent?

    One thing that surprised me as to its effectiveness was a radiant barrier stapled to the underside of the roof joists...it lowered the attic temperature about 30-degrees in my NH townhome. This is simply a craftpaper covered with aluminum foil. The version I have has fiberglass filaments to reinforce it. After putting this up, the upstairs ceiling was the same temp as an interior wall whereas before at the end of a sunny day, it was decidedly hot.

    In the winter, snow sits on the roof 4-5 day longer than my neighbors, since it also reflects interior heat back into the room. Being a condo, I couldn't put a hole in the roof to install a fan...this seems to work, and it is entirely passive. Worth a try.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Senior Robin Hood Guy Ian Gills's Avatar
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    I am going to try the radiant barrier idea.

  4. #4
    Homeowner Thatguy's Avatar
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    Quote Originally Posted by jadnashua View Post
    One thing that surprised me as to its effectiveness was a radiant barrier stapled to the underside of the roof joists

    craftpaper covered with aluminum foil.

    it also reflects interior heat back into the room.
    ?
    Shiny side facing outward or inward. . . ?

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    It really doesn't matter! Lots of places to buy this on the web...but, because it is fairly heavy and long (typically comes in 4'x250' rolls, but you may find smaller ones), shipping can be pricey. Some have foil on both sides, but from what I read, it really doesn't matter. Any radiant barrier needs at least 1/2" of free air space, or it becomes effectively a conductor and can't do its job. The IR will reflect off the surface, regardless of which side is there. Double-sided theoretically may have a minor benefit summer/winter, verses summer.

    As an aside, a friend needed a new roof and installed an aluminium wood-shake look-alike. He said that the house immediately felt warmer (it was late fall- he lived on a mountainside) with the thermostat set at the same value. Same idea...it added a radiant barrier. Note, not all metal roofs will do this, this one had 1/2-3/4" of free air space underneath it to the roof deck.

    Some places, primarily in the south, carry it in the store. I actually bought mine in FL, and carried it back as luggage when I came home.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Homeowner Thatguy's Avatar
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    You also added 3-1/2" of dead air space to allow your attic vents to do their job; maybe that had the most influence. Thick paper by itself might have worked.
    I'm now wondering if the roof shingle temp. is elevated because they don't have the cooling effect of attic air on the underside of the roof sheathing so the shingle life is shortened. I know poorly ventilated attics can void the warranty on shingles and dead air space between the rafters is like a poorly ventilated attic.
    I'll have to research this one further.
    Last edited by Thatguy; 08-03-2009 at 02:47 PM.

  7. #7
    Senior Robin Hood Guy Ian Gills's Avatar
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    I was worried about the shingle temperature too when I examined how this works.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    Assuming you install it on the bottoms of the joists, you have the full joist depth to allow air to circulate. In my case, the soffit vents open into that area and go up to the ridge vent. I've not noticed any difference in wear to the roofing materials with those units next to mine.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    In the trades Dana's Avatar
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    Attic fans have been studied to death, and the benefit is marginal at best, and often negative. If it's a self-powered solar version and you have an excellent air seal between conditioned space & attic, studies at Texas A & M and the Florida Solar Energy Center say you can save as much as 6% in cooling electricity. See:

    http://www.fsec.ucf.edu/en/publicati...SEC-GP-171-00/

    But if you don't have a good air seal between conditioned space and the attic an attic fan can even INCREASE the power used, since it'll be pulling air-conditioned air in to the attic (cooling it nicely), from the living space below, which pulls in hot air from outside. (If you have non-air-tight recessed lights penetrating the attic floor, open plumbing chase, etc. an attic fan will have a net-negative performance.)

    Radiant barriers have also been studied to death- they work, but with diminishing returns with increased R value in your insulation. The Oak Ridge National Lab has done a reasonable financial analysis placing a present-value on a 25 year return (the most you should pay to break even after 25 years) based on the location/climate, R-value of the insulation and whether you have ducts in the attic above the insulation or not. See:

    http://www.ornl.gov/sci/roofs+walls/...es.html#table4

    So, looking at the table if you live in Phoenix and have R19 attic insulation, you should pay no more than $0.17/ft^2 if none of your ducts are in the attic, $0.23/ft^2 with ducts. If you have R38 up there, that drops to $0.12/ft^2 with ducts, $0.08/ft^2 without.

    The values drop a bit if the RB is attached to the roof deck vs. under the rafters:

    http://www.ornl.gov/sci/roofs+walls/...es.html#table5

    Most of the time additional insulation is a better deal:

    http://www.ornl.gov/sci/roofs+walls/...es.html#table6

    Shingle temps will run hotter, but not 25F hotter (more like 10-15F). But the solar reflectance & infra-red emissivity of the roofing and the pitch of the roof makes a bigger difference in shingle temp than whether there's radiant-barrier below or not. Steeply pitched roofs convection cool from the the exterior nicely, whereas flat roofs stagnate at high temps, etc.

    If the attic joists are exposed the joist-tops absorb radiated heat from the roof deck all too well, and the R-value of the joist is far lower than that of the insulation between them. Open-blow cellulose to cover the joist tops by 3" or more is usually a significant improvement in reducing the heat gain, since it puts ~R12 thermal break over the thermal-short circuit of the joist.

    Fiberglass works for this too, but being slightly translucent to infra-red it passes some, and absorbs some in the upper few inches, making the layer of insulation an inch or two below the top the hottest place in the attic (significantly hotter than the ambient air.) You'll need 5.5" (R19) or more over the top to get the full effect or 3.75" high-density (R15) "cathedral ceiling batting. Cellulose absorbs the radiated heat a the surface then re-radiates the heat back to the attic rather than cooking from within, so in thin-layers it'll measurably outperform fiberglass. Above R25 or so the differences are inconsequential, and difficult or impossible to measure.

    But from a fit point of view, blown insulations fill all the gaps & voids. At even modest insulation levels the gaps, thin spots and voids will dominate the heat gain, so making the fit as-perfect as possible with blown insulation is usually worth a ~15% performance improvement even at identical R-values.

    If you only have R19 batting in the attic, a 6" overblow of cellulose that covers the joist tops & fills in the gaps will cut the heat-gain by more than half. A radiant barrier perfectly installed might net you a ~30% reduction (best case.) Above R40, even a very hot attic is unlikely to be your largest heat-gain. (South facing windows, even when shaded from direct get about 50% of the heat gain as they would in direct sun, primarily from scattered light. Exterior window shades/shutters take that down immensely. Shading exterior walls on the E, S, & W sides also makes measurable differences in the cooling load.)

    If you're doing an insulation overblow, take the time to air-seal the space as well as you possibly can, foam sealing all electrical & plumbing penetrations, weather stripping the access doors/hatches, etc. Replacing recessed light fixtures with gasketed air-tight insulation-contact types is also advisable. Do NOT insulate over a fixture not rated for insulation contact, (even if you're using high-efficiency LED or compact fluorescent bulbs.) Building a fire-resistant air-tight box over them with 3" clearances can work, but it's time consuming- it must be installed in an air tight fashion to stop convection induced by a super-heated attic from pulling conditioned air up from the living space (adding to the cooling load.)

  10. #10
    DIY Member Bill MD's Avatar
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    Default Solar Attic Fan

    I'm considering installing the Sunrise FB850FT in my attic. From what I can find out it seems to be a good choice. But I have one concern. The solar panel is a flexible module. Is there any great disadvantage to this? Some solar panels are tempered glass. Is that better? The FB850FT is hail and extreme weather resistant.

    Thanks

  11. #11

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    That's the same model I am considering. I have to admit my main deterrent at this time is that it only moves 850CFM. To have any effect on cooling my attic here in Louisiana according to calculations I have been given I would need to be able to move 8000+ CFM. Obviously at that point it becomes cost prohibitive and negates timely potential energy savings. On the flipside is extended roof shingle life which is an important factor.

    I've also been told by people who have them around me that another issue is that being solar-powered they obviously stop working/slow down as the sun lowers. Attic heat lingers well after the sun has set.

    I am sure there are larger solar fans available and maybe even dual-purpose setups (solar switching to battery maybe) but as yet haven't found any. Or rather I should say I haven't found any that make it any cheaper, i.e. purchasing 7 fans designed to move 1250CFM is no cheaper than 10 fans designed to move 850CFM.
    Last edited by RDB1958; 08-18-2009 at 07:57 AM.

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    Computer Programmer Bill Arden's Avatar
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    I put in a 2 foot wide powered attic fan and it did very little, I then put lawn sprinklers up on the roof and the temps dropped from around 120f to 95F.

    I would not recommend doing this to a house you planing on keeping due to the moss it grows. I'm only doing it since I don't have any insulation in the ceilings after the fire. All I have is plastic I stapled up to make the place livable till winter.

    FYI: My new house will have white or silver shingles.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by Bill Arden View Post
    FYI: My new house will have white or silver shingles.
    If you can afford it, consider a metal roof. A Kynar coated aluminum roof can be considered a life-time roof, decrease you insurance (lower fire threat and resistance to high winds), and may (depending on design) act as a radiant barrier.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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