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  1. #131
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889

    Joe Lstiburek Building Science Corporation

    Joe has a Phd in building science. If you ever have a chance to attend one of his presentations you owe it to yourself, your industry and your customers to do so.

    http://www.buildingscience.com/bsc/topten/south.htm
    http://www.buildingscience.com/bsc/topten/north.htm
    Last edited by adrianf; 05-14-2008 at 11:14 AM.

  2. #132
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,326
    Quote Originally Posted by adrianf View Post
    Joe has a Phd in building science. If you ever have a chance to attend one of his presentations you owe it to yourself, your industry and your customers to do so.

    http://www.buildingscience.com/bsc/topten/south.htm
    http://www.buildingscience.com/bsc/topten/north.htm

    I enjoy his monthly contributions to the ASHRAE Journal. Always informative and enlightening.

  3. #133
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,164
    he gave several classes here in 06 bringing information for us to build and withstand the hurricanes, most of his info was based on the studies that building science did in Florida who had two hurricanes the year before we had our two.
    one of the biggest things that will allow homes to withstand hurricanes is to stop venting attics as a vented attic has much more uplift than an unvented.
    I really enjoyed his talks..very entertaining man..and has common sense to go with thue
    education..plus a sense of humor.
    Anyone who has a chance to go to his classes should go...imo.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #134
    Join Date
    Mar 2008
    Location
    Long Beach, CA
    Posts
    3,379
    [QUOTE=shophound;1860171][COLOR=red]

    An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.


    I misunderstood the 70/30% statement. I thought you meant radiant heat from the roof was directly penetrating the attic insulation and making it perform at 30%. Radiant heat does superheat a surface and makes temperatures go sky high, but are you also saying that an attic would not rise above ambient on an overcast day if the ventilation was inadequate? Wouldn’t an interior of a car parked in the shade rise above ambient?

    Radiant heat is definitely a major cause of attic heat, just like color of shingles are, but poor ventilation allows that air to sit there and bake. Constantly recycle that air with turbines or a low speed/power PAV and the result of radiant heat has been diminished.

    But with all that said, radiant barriers are very logical.

    Brian

  5. #135
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,326
    [quote=Brian GC;1861209]
    Quote Originally Posted by shophound View Post

    [color=red]An overcast day is not an effective illustration to compare RB against PAV ventilation. A hot, sunny day would be more appropriate, since these are the conditions both RB and PAV's are designed to work against. My 70/30% ratio, admittedly rough, was a statement of breaking down total heat gain to an attic (heat gain to the house is a separate but related subject). 70% is from radiant heat gain, 30% is from the air in the attic itself becoming heated, which occurs primarily due to radiant heat! The air itself would not heat over outdoor ambient if it was not overwhelmed by radiant heat from structural members that have no radiant barrier in place.


    I misunderstood the 70/30% statement. I thought you meant radiant heat from the roof was directly penetrating the attic insulation and making it perform at 30%.
    I went back to see where that 70/30 figure came from...it was from adrianf's post a page or two back, where he said either FSEC or Advanced Energy stated that 80% of heat gain to attic floor insulation is due to radiant heating, with 20% from convective currents from the roof deck.

    The more I study this subject, the more "cathedralized" attic systems make sense, whereby the roof deck is insulated and the attic is sealed from the outdoors. The attic could then be considered conditioned space, and fed a small amount of supply air to keep it positively pressurized. Or...it could have supply and return, IMO, in order to keep humidity under control. As a service technician, working in such an attic would be a dream. 105 outside, 80 to 85 in the attic. Sure beats 130 or worse!

    Practically, however, most homeowners are not economically positioned to retrofit their attics to a cathedralized approach. Therefore the discussion returns to what's the best approach for conventional attic systems with attic floor insulation, soffit vents, and either ridge vents, whirlybirds, or static vents. I still maintain increasing attic floor insulation to recommended R value for the region and installing roof deck RB is the way to go. That and getting penetrations between attic and house interior sealed, as much as possible. For PAV die-hards, do all of the above and then purchase a solar powered PAV.

    Radiant heat does superheat a surface and makes temperatures go sky high, but are you also saying that an attic would not rise above ambient on an overcast day if the ventilation was inadequate? Wouldn’t an interior of a car parked in the shade rise above ambient?
    The rate of heat transfer would be significantly reduced on an overcast day to an attic due to diffused line of sight between roof deck and the sun. I have been in many attics on overcast days...they do not heat appreciably until either the cloud deck thins to allow muted sunlight through, or the clouds disappear altogether.

    Radiant heat is definitely a major cause of attic heat, just like color of shingles are, but poor ventilation allows that air to sit there and bake. Constantly recycle that air with turbines or a low speed/power PAV and the result of radiant heat has been diminished.
    Heating of attic floor insulation via roof deck and structural member radiation is not significantly reduced by ventilation. In order for this to be true (and logical), the airflow volume would have to be quite high, enough to keep the actual roof deck cool enough to reduce radiant heat transmission to the attic floor.

    Concurrently, unless the attic is to be cathedralized/sealed, it needs to be ventilated. It's a Catch 22. Don't ventilate the attic and use no RB, the attic bakes like an oven. Ventilate it naturally with no RB, the attic floor insulation loads up with radiant and convective heat during the day, reradiating and conducting that heat into the dwelling all night long. Ventilate with PAV and no RB, the PAV depressurizes the attic AND the house, whereby air conditioned air is lost to the attic. Install RB and don't ventilate, the amount of heat build-up is greater than if the attic is ventilated. Install RB and naturally ventilate nets reduced heat gain to attic floor insulation, and reduces transfer of air through ceiling penetrations to attic. Install RB and use a PAV concurrently, loss of conditioned air through ceiling penetrations is increased, possibly offsetting gains this approach is supposed to provide.

    But with all that said, radiant barriers are very logical.
    They are. Even more logical, for new construction, would be to cathedralize the attic so heat gains to cooling equipment and ductwork is minimal. Or...if a conventional attic MUST be used, get the equipment and ductwork OUT of the ATTIC! Use a little forethought, fur down the hallways, and run the duct there (why do hallways need stratospheric ceiling heights like the rest of the house must have these days?). Sacrifice some needless feature of the house and put the air handler in a mechanical room/closet where it belongs. Keep all that stuff out of the attic!

  6. #136
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    If anyone is interested I have HOBO data logger recordings of my semi-conditioned (formally known as attic) space in Excel format that I can email.

  7. #137
    Join Date
    Jan 2005
    Location
    USA
    Posts
    1,450
    I put in a large PAV when I redid my roof and it sucks parden the pun. I will be putting in a ridge vent soon. My question is there a temperature that having both be beneficial.
    "It's always controls"

  8. #138
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,326
    Quote Originally Posted by adrianf View Post
    If anyone is interested I have HOBO data logger recordings of my semi-conditioned (formally known as attic) space in Excel format that I can email.
    Of course I'm interested. Email addy is in my profile.

  9. #139
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,371
    I am also interested in viewing the data. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  10. #140
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,164
    I'd like to see it also..I'll make sure my email address is available also.
    And...thanks!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  11. #141
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    I'd like to see it, email is in my profile.
    If more government is the answer, then it's a really stupid question.

  12. #142

    Confused

    I live in Lousiana and during the summer my AC runs constantly on hot days. This year I had the AC system checked and all checked OK except for the fact that my attic is very hot (over 130) even though I have ridge vents and soffit vents. I think that my soffits vents area is sufficient but suspect that the ridge vent area is lacking because my home has four hip roofs(short ridges). I was considering installing a power vent (recomended by one AC contractor)but after reading here I have the feeling that installing wind turbines may be a better option. I have seen some homes in my area with a combination of ridge vents and wind turbines. I am also not sure how many turbines I should install. Can there be to many? Any thoughts and help would be appreciated?

  13. #143
    Join Date
    Apr 2008
    Location
    Spokane
    Posts
    16

    power attic fans help

    Mevil, sounds like you have alot of vents, good. I would definitely install the attic fan. I installed just one 1350 cfm gable fan for my 1250 square foot attic on the west end of the house. It has a thermostat, so it will turn itself on at 90 degrees, and off at 90 degrees. Alot of air movement up there. It works great, and the cooling bill showed it.

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