Help needed about attic fan!!

Discussion in 'HVAC Heating & Cooling' started by joseph gerard, Mar 14, 2010.

  1. joseph gerard

    joseph gerard New Member

    Messages:
    1
    Location:
    California
    Hi there,
    I need some suggestions about attic fans. I have a attic having space 800 sq. feet. I want to put an attic fan for a good ventilation but don't know anything about what type and rating would be good for that space. Also I want to know that would Solar Attic Fan be effective in this case or not.

    Joseph
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    You should go to a supply house and get a dedicated attic fan, because codes will require a thermostat with hi temp cutoff, etc.
    Solar would be ok if it is dedicated attic, but typically the air flow might be low.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    Being in CA, you may have a fair amount of sunshine, so solar has some benefits. They don't move all that much air, so you may need more than one. There are at least a few manufacturers. One I'm familiar with is www.solatube.com which originated the tubular skylights. Their home office (for the USA) is in CA, if that makes any difference. Keep in mind that you must have sufficient surface area for your air inlets if you want to move the air out of the attic, and those areas may need updating to work well.
  4. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    Effective toward what end? Humidity control in the attic? Reduced heating load? Extending shingle-life?

    Unless you have very open ventilation in the attic and a good air seal at the attic floor/conditioned-space boundary attic fans (solar or otherwise) draw conditioned air into the attic, cooling the attic, but increasing the net cooling load on the building. With a very good air seal at the attic floor, according to testing done by the Florida Solar Energy Center they're only provide a net decrease in whole house cooling load in attics with relatively low insulation levels and low natural ventilation (like R-19 batts + radiant barrier, but no ridge ventiltaion) Even then the decreased load as measured by power consumption was 6%. It would likely be higher savings for even crummier insulation levels. See:

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

    From a performance per dollar point of view you're better off spending the money on more insulation &/or radiant barrier, skipping active ventilation in attics. If you already have R30 insulation, radiant barrier, and both soffit & ridge ventilation the solar fan is little more than a roof ornament.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    When I added a radiant barrier to my attic, the attic temp dropped over 30-degrees on a hot summer day. Prior to this, the insulation got heat-soaked (about R35) and the ceiling was hot by the end of the day. After, the ceiling was the same temperature as an interior wall. Really impressive for a sheet of paper with some foil facing. It's just stapled to the underside of the roof rafters.
  6. Dana

    Dana In the trades

    Messages:
    2,915
    Location:
    01609
    Fiberglass insulation, I presume?

    Fiberglass is somewhat translucent to infra-red radation, taking several inches if depth to absorb it to 90% level. The result under a high radiant heat flux is that the insulation heats up in internal layers, and typically hottest place in the insulation is 1-2" below the upper surface, making it 10s of degrees hotter than the attic air on a hot summer day, so instead of R35 insulating against say a 130F attic, you have ~R30 insulating against 140-150F insulation. The upper layer of the insulation itself transfers that heat to the attic air via convection loops in the upper inch or so of insulation.

    Cellulose is opaque to infra-red, and while it's upper surface heats under high radiant flux, the surface temp is closer to the attic air temp, and you have benefit the full R between the attic air and conditioned space.

    But with a radiant barrier both types of insulaion run cooler when the radiant heat transfer between 150F roof and attic floor interrupted with reflective insulation, but the effect is more pronounced with fiberglass than with cellulose. (There's tons of research on this available on the web from Texas A & M archives, as well as the Florida Solar Energy Center, and multiple other sources.)

    If the joist tops are exposed, wood is very absorptive of the radiated heat, and has far less R-value. This too can be mitigated with reflective insulation, but even 3" of cellulose over the joist tops as a thermal break can be more effective in reducing the total heat flux into conditioned space, and has much bigger heating-season benefits than radiant barrier, which often increases the heating load during the shoulder seasons in heating dominated climates, since it runs the attic cooler on sunny spring/fall days too, not just winter. The roof deck is an unglazed solar collector with signficant benefit during the shoulder seasons.

    In all but the hottest, sunniest cooling dominated or mixed climates radiant barrier isn't typically cost effective if you already have more than R30 of cellulose that includes thermal breaks on the joist tops. It'll cut your peak cooling load, but not enough to make it pay. But if you have only R19 (as is common in coastal CA) it's cost effective, but not as cost effective annually for many/most as bringing the total R to R30-40.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    My attic has about 7" of cellulose with a layer of unfaced R-19 on top of it. Both winter and summer, the rooms are more comfortable with the radiant barrier. The material is relatively inexpensive, and at least from a comfort side, I felt it was well worth it. It's been up so long, I don't have before and after in energy use, which wasn't as important as the improvement in comfort.
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