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  1. #40
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304

    More on how things work

    Several different points:

    1) I have also been told that attic insulation *can* store BTUs, by no means do I consider that I know enough on this topic. Since it is a solid material, it seems clear to me insulation must be able to store *some* heat. Perhaps if I had some idea of how the specific heat or other qualities of that insulation (how many BTUs to raise its temperature 1 degree) that would make the truth clearer.

    2) On whether a power ventilator sucks interior air from the house, it seems to me probable that is unavoidable unless you have *no* leakage between the attic and the ceiling. For example if you have 50 sq.in. leak at the ceiling (easily obtainable) and 450 sq.in. soffit vents, then one would expect the leakage to be 90% from outside air, and 10% from the house interior. Does that make sense?

    3) Has anyone referred to standards of ventilation area for attic soffit and attic ridge? I tried to follow this thread but don't remember that. I have seen a recommendation of 1 sq.ft. of "net free area" per 100 sq.ft. of ceiling area, along with the idea that oversizing that vent area is a good thing (assuming you won't go with the conditioned attic design).

    Best wishes -- P.Student

  2. #41
    Join Date
    Jul 2005
    Posts
    717
    Rule of thumb;A minimum of 1 sq ft of vent for every 300 sq ft of attic floor area .This total is split 50/50 between the (low vents) soffit, and the (high near peak vents)or ridge vents. In your case; based on 1800 sq ft= 6 sq ft= 3 sq ft for low vents and 3 sq ft for high vents.
    Remember this is " free" area. Some vents may have an opening of 144 sq inches (1 sq ft), but with the screen attached to it may only be approx 72 sq inches (1/2 sq ft.

  3. #42
    Join Date
    Nov 2004
    Location
    Thibodaux, LA
    Posts
    1,170
    I used to believe in attic fans also. but know after going to an e-star school I have to agree with ulkra.

    Putting a fan in an attic is just an easy way to suck conditioned air from the living space.

    I would prefer to you install a radiant barrier on your roof rafters or have it sprayed with foam.

    If you still want to use the fan setup make sure your attic is sealed like previously stated.

    Attic temperatures that are too high will also lead to condensation problems and lose of capacity on your a/c equipment.

    I recently installed a 2-speed variable speed fan setup in a customers home with a metal roof. His attic reaches temps of 130 degrees plus. He also likes his a/c set on 70 in the middle of the day. Add all that up and you have serious condensation issues. Even with a trunk and branch system rated at R-8.

    Less heat in the attic is a benefit...
    "Football Season again finally"

  4. #43
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    Good points-

    Uktra- I am agreeing with you again, but you have put your blinders back on and are refusing to see outside the box.

    After reading the material at the links you provided, I read that it has been proven the insulation can and does store heat. If it can slow down the convection in or through it, it will also slow down the convection OUT of it. More insulation is not ALWAYS better.

    You are correct, and I agree, sealing obvious holes and keeping the cool air you have is the MOST cost effective way of saving money.

    ALSO- once that has reached it's peak benefit, and more steps ARE necessary to ensure comfort (NOT cost efficiency), there is no other option but to bring attic temperatures down someway, somehow. Radiant, vents, anything.

    You still have not denied that you are arguing to ignore attic temperatures completely. I am not putting words in your post, it's just that is your obvious point, unless you clarify otherwise, please.

  5. #44
    Join Date
    Aug 2003
    Posts
    626
    Doc holiday--I have been involved in sealing many attics on the homes I help build and I have never been able to get one air tight yet. I can tell you for sure yours is not either. But even if it was, you are only eliminating convective heat transfer to the rooms below. The pav still uses electricity, and the majority of heat tranfer to the rooms below is still radiant not conductive. You are only lowering the conductive transfer (from 120 to 130 to outside temp.)not eliminating it. Pavs have no effect on radiant.

    P student--You have been told wrong. Heat always flows from hot to cold, while insulation slows this process, the flow contiues.

    Bluetooth751--I would be surprised to see condensation on R-8 ducts if there is no leakage. Generally the surface of the duct is above the dew point.

  6. #45
    Join Date
    Aug 2003
    Posts
    626
    I don't have any blinders on. I never said conductive heat transfer from the attic to rooms below doesn't matter. If one has invested his money in all the other ways to lower heat transfer that I have mentioned, and stll has not been able to reach set point, then adding additional PASSIVE ventilation will help. Adding a pav not only will not help, but will probably increase load due to higher infiltration rates.

  7. #46
    Join Date
    Jun 2004
    Location
    Dallas
    Posts
    179
    Originally posted by uktra
    I don't have any blinders on. I never said conductive heat transfer from the attic to rooms below doesn't matter. If one has invested his money in all the other ways to lower heat transfer that I have mentioned, and stll has not been able to reach set point, then adding additional PASSIVE ventilation will help. Adding a pav not only will not help, but will probably increase load due to higher infiltration rates.
    OK- we're cool. Thank you.

  8. #47
    Join Date
    Aug 2003
    Posts
    626
    You are most welcome--have a great 4th.

  9. #48
    Join Date
    Apr 2002
    Posts
    11,808
    I always thought that venting the attics was first off a northern thing, to stop condensation in the winter.

    I can see a house in the summer, with poor ceiling insualtion benefitting from a power venter, however sometimes they can depressurize the whole home.

    I do not have to worry about the cold weather where I am now, and I think it is better not to vent the attic as it just allows more humidity in.

    We had a guy from some icynene (sp?) down here a while back and they promote insulating the whole pitch of the roof and sealing the top of the walls to the roof. May only end up with a 100 degree attic this way. It also seems to rely on leaky supply ducts for additional cooling in the attic.

    Seems like a good way to go in the hot humid environment, insulate the pitch with something that does not trap moisture and keep the humid air out.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  10. #49
    Join Date
    Aug 2003
    Posts
    626
    Carnak--you are correct about vented attic due to prevention of ice dams. Vented roofs for cooling climates are fine as LONG AS THE DUCTS & AIRHANDLER are NOT IN THE ATTIC. Then like you say insulating the roof inder the sheathing is the way to go.

  11. #50
    Join Date
    Apr 2002
    Posts
    11,808
    It is unfortunate that you get forced to put air handlers and duct work in humid attics in order to maximize floor space and ceiling heights.

    Then everyone gets dissapointed when these cold systems inevitably do what comes natural to them and they sweat.

    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #51
    Join Date
    Aug 2003
    Posts
    626
    One of the ways I get around the codes is to put foam chutes between the roof rafters and then net and dense pack cellulose. This turns the attic into conditioned space and yet still meets codes.

  13. #52
    Join Date
    Jun 2003
    Location
    GA
    Posts
    395

    Red face Re: crappy fans not doing their job

    Originally posted by whos on call
    uktra.
    This is my first reply. Uktra your not making sense. Are you saying that a 100 cfm fan pulls more than what it is rated for? If so then I need to go back to school and complain to my teachers that I was taught incorrectly. I want my money back!! The fan will pull the air that is closest to it, and if that vent allows for 100 cfm or greater, then thats where the fan is going to pull the air..period. Thats not my opinion, thats the laws of physics. You can't argue with reality.
    "go back to schoo", yes that's a good idea. As if there are any scool in HVAC worth more than the price of the text book.

    uktra is the only one not talking trash in this thread. I could direct you to some good reading that I'm sure uktra is well aware of, but I'd be wasting my time.

    md

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