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  1. #14
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,416
    And further, are you trying to imply that finishing an attic and heating/cooling it could represent a 10-30% reduction in cost to the HO?
    [/QUOTE]

    In initial cost no, in operating cost yes.
    The thermal barrier is at the roof line not at the ceiling line.
    no heat loss/gain for the ducting.

  2. #15
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    Quote Originally Posted by Nubicon View Post
    That's interesting. Around here if you seal an attic and don't install the proper vents (soffit, ridge, sidewall) the house will fail inspection. But I get your concept...only I wonder how the HO would feel about spending fuel money to heat an attic so the HVAC guy didn't have to insulate the duct?
    You posted this in response to Carnak's icynene response. Thus, the conversation turned to a sealed attic and the fact that's cheaper in the long run for the consumer.

    If your inspection department would fail a sealed attic, then ask them what they think about cathedral ceilings.
    Perhaps you should have read the instructions before calling.

  3. #16
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by Nubicon View Post
    That's interesting. Around here if you seal an attic and don't install the proper vents (soffit, ridge, sidewall) the house will fail inspection. But I get your concept...only I wonder how the HO would feel about spending fuel money to heat an attic so the HVAC guy didn't have to insulate the duct?
    You do not vent a sealed attic.

    You add a minimal amount of air and it is conditioned space, therefore the new stringent rules of increased insualtion do not apply.

    I am not saying do not insulate. I am saying you do not have to go the extra yard for the R-8.

    Uninsulated ducts in a conditioned space are fine until something goes wrong. So like I said, if you seal an attic, cutting edge technology in some climates, and add minimal supply air to it, it is a conditioned space and the rules for unconditioned spaces do not apply.

  4. #17
    Join Date
    Apr 2002
    Posts
    11,808
    COnsider a sealed attic over a 1000 sqaure foot home, 50 CFM of supply air

    Attic will be possibly 80F in the summer.

    Now consider you do not seal it. Have as much insulation as you want blown in. Underside of roof deck is 160F, radiating down at all your blown in insulation, attic air heated to 130 F, would be even higher except for the vents. Vents by the way, which are there so that you do not get icylces and ice dams on your roof in winter.


    So now which attic do you think is the bigger energy penalty, one you can keep at 80F with a minimal amount of supply air, or one which is in essence a superheated sauna, that allows hot humid air to infitrate in around pot lights?

  5. #18
    Join Date
    May 2006
    Posts
    281
    R8 flex, 3" Duct Wrap and 2" Duct Liner here in Georgia is required on any permit pulled after 1-1-2008 in "unconditioned space". Because so many of the supply houses in my area sell to both GA and Al contractors, several are still stocking both R6 and R8 as well as both 2 & 3 wrap and 3 thickness's of liner.

    Some counties in Alabama still allow use of 2" wrap and R6 Flex, some have adopted Ga Code, supposedly Alabama state code will change Jan 2009 and become unified as Georgia is.

    Martin
    whaaaaa, you let all the smoke out

  6. #19
    Join Date
    Feb 2007
    Posts
    36
    Quote Originally Posted by jrbenny View Post
    You posted this in response to Carnak's icynene response. Thus, the conversation turned to a sealed attic and the fact that's cheaper in the long run for the consumer.

    If your inspection department would fail a sealed attic, then ask them what they think about cathedral ceilings.
    They would call it a cathedral ceiling, as it in no longer an attic? And when you do have a cathedral here you are required to install styrovent in the bays near the soffit and then insulate the roofline with
    R-38.

    And I can think of a hundred reasons why a HO would want to leave their attic unfinished. I'd like to see it worked out on paper how adding square footage is more efficient? All you succeed in doing with sealing the attic space is changing where the roof heat load is. From between the rooms below and the attic floor to the finished attic ceiling and the roof itself.

  7. #20
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,189
    Maybe carnak will share some of his numbers. He's done it on his own place..
    Perhaps you should have read the instructions before calling.

  8. #21
    Join Date
    Apr 2002
    Posts
    11,808
    I have a sealed attic, roof insulated with a total of about R7.

    I compared numbers with jrbenny, I would say it works as well as R30 under a vented attic. That was before I added any supply air up there. It averaged about 81F and 50% RH as an unconditioned space.

    Heat transfer then is what 81F can drive through a a sheet rock ceiling to air beliow the ceiling, heat rises so keeping my place at 78F would mean about 79F at the ceiling. 81-79 equals 2F driving heat transfer through sheet rock and two air films.

    My wife is storing some heirlooms in the attic, so I added about 40 CFM of supply air to guarantee a low humidity. With the minimal supply air, my attic averages about about 79F and humidity ion the 40s.

    So now the air in the attic above the ceiling is the same as the air below the ceiling and there is no heat transfer. A small amount of humidity wicks through the ceiling plane as the space below is a little dryer

    An extremely wealthy woman here, remodelled the top floor of an office building she owns here and made it into her penthouse town apartment, it exceeds 8000 square feet.

    Because of what happened here in a hurricane, wind driven mositure entering in through soffit vents saturating and ruining ceilings before a roof even failed, she wanted no vents.

    I was hired by her attorney to advise what to do. So you have to be pretty careful when dealing with wealthy people and their lawyers. I ran the numbers and by sealing the attic and blowing R19 icynene under the pitch of her dark sloping roof, it would take about 4 tons to keep that attic at 80F, which was roughly eaqual to what the heat gain would have been through R19 layed flat above the ceiling plane.

    But what you also need to consider is that numerous fan coils and insulated duct work in the attic are not being radiated by hight temperature heat and losing 10% of their cooling capacity. So in this case, less heat gain by conditioning the attic.

    My weasel clause advise on sealing up the attic to this attorney, "Add dehumidifiers as sensible heat gain will be reduced", especially since all the AC was already installed, designed around heat coming through the ceiling.

    Another thing to consider is in many climates it is extremely difficult to have an attic vapour barrier on the warm side of the insulation. Vapour barriers also stop infiltration. All it takes is a slight pressure differential and attic heat and humidity will pass right through fibre glass insulation just like return air through a filter.

    So in some climates, the sealed attic eliminates all the heat and humidity bypassing the insulation.

    Go through Building Science sometimes, they will call a sealed attic with 25 CFM a conditioned space. Big difference between conditioned and finished.

  9. #22
    Join Date
    Apr 2002
    Posts
    11,808
    The sealed attic is in the face of anyone who does light commercial work. It is the same thing as the ceiling space in a shopping mall with a flat roof.

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