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  1. #1
    Join Date
    Feb 2005
    Posts
    30

    Attic Insulation Question

    Geothermal a/c with air handlers in the attic. Metal roof with ridge vents. system is struggling to produce adequate a/c.

    HVAC contractor believes the attic insulation is insufficient (system designed assuming R-38 on attic floor but R-30 was installed in error).

    Builder is offering to put rigid insulation on attic floor and recover with plywood (so it would be R-30 fiberglass that is in the attic floor now covered with plywood, a sheet of rigid insultation, then another sheet of plywood). the reason they dont want to pull up the existing plywood and install R-38 is because they are worried about the stress this might put on the plaster ceilings below.

    Q: would you do the above OR would you just get the spaces in the roof rafters sprayed with icenyne to get the attic temperatures down?

  2. #2
    Join Date
    Apr 2002
    Posts
    11,808
    The heat radiating at your air handler and insulated duct work could be robbing you of 10% of the cooling and is a lot more significant than R30 vs R38. Maybe ask if an allownace was made for the heat picked up by installing HVAC equipment in a hot solar collector.

    You need a complete load calcualtion done and you need to make sure that the attic duct work is not leaking. The HVAC guy should be verifying the capacity of the equipment as installed. Measuring the room air temperature/humidity entering grilles, the temp humidity of the air entering the unit, leaving the unit and coming out of the supply diffusers needs to be looked at.

    Icynene is great in the right climate but is meant to work with a sealed attic

  3. #3
    Join Date
    Feb 2005
    Posts
    30
    Would you pls explain what you mean by "hot solar collector"? are you speaking about my metal roof?

  4. #4
    Join Date
    Apr 2002
    Posts
    11,808
    Your attic gets hotter than the outside air temperature when the sun beats down on it.

    It is a solar collector. People do not like to give up floor space so they install equipment in a hot humid attic, then get upset later if it does what it naturally does-- sweats.

    My own home I have a white metal roof, insulation beneath it,the attic is sealed, it works great. Before I started up my AC, during the heat of day the attic was about 1 degree hotter than the outside air.

    Maybe your attic heats up to 130F or more. The metal roofing will be radiating heat directly at the equipment installed in the attic. Attics are a piss poor place to install HVAC equipment, but just a fact of life as people want high ceilings and do not want to give up floor space for essential mechanical systems.

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,347
    Your best option short of those more drastic measures you mentioned (which may not give much bang for the buck) is to have radiant barrier installed underneath the roof decking. Spray-on or foil type, take your pick. Foil type performs better but is more difficult to install. Spray-on highly effective, will reduce heat gain to ducts in attic, and a/c equipment in attic. Make the attic cooler, the house and the a/c equipment cooling the house will be cooler.

    Safe bet that none of the supply boots penetrating the ceiling from the attic are sealed where the boot and plaster meet. Safe bet some aspects of your ductwork are leaky. These are typical things so often overlooked in most new and remodel construction.

  6. #6
    Join Date
    Jun 2008
    Posts
    63
    I would be concerned that the rigid board insulation would create a vapor barrier trapping moisture between the ceiling and the existing insulation.

    What is your local code about fire protection? Foam boards usually require a covering as a fire barrier, like gypsum wallboard. Some building codes do not require an additional fire barrier for certain metal-faced, laminated foam products.

  7. #7
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by jax1 View Post
    I would be concerned that the rigid board insulation would create a vapor barrier trapping moisture between the ceiling and the existing insulation.

    What is your local code about fire protection? Foam boards usually require a covering as a fire barrier, like gypsum wallboard. Some building codes do not require an additional fire barrier for certain metal-faced, laminated foam products.
    In your jurisdiction single family dwelling units require fire rated ceilings? Do the wood trusses require to layers of type X?

  8. #8
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    With a heatload calculation it would be very easy to to compare the change in heat gain from R38 to R30.

  9. #9
    Join Date
    Jun 2008
    Posts
    63
    Quote Originally Posted by Carnak View Post
    In your jurisdiction single family dwelling units require fire rated ceilings? Do the wood trusses require to layers of type X?
    No, but there is a concern about toxic gases released when some foam burns.

    It may or may not apply to the OP, but it's free to ask.

  10. #10
    Join Date
    Jun 2008
    Location
    Cincinnati, Ohio
    Posts
    2,176
    Also, if you compress batted or blown-in insulation you lose R-value.

  11. #11
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,188
    "Builder is offering to put rigid insulation on attic floor and recover with plywood (so it would be R-30 fiberglass that is in the attic floor now covered with plywood, a sheet of rigid insultation, then another sheet of plywood). the reason they dont want to pull up the existing plywood and install R-38 is because they are worried about the stress this might put on the plaster ceilings below."

    Certainly sounds like a whole lot of work!!

    Where are you located?
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
    Join Date
    Aug 2003
    Location
    Spring Hill, Florida
    Posts
    213
    Why not have Icynene sprayed below the roof deck between the truss system. They would need to seal off all your attic ventilation but it would the best way to solve your problem. The only other issue to that is the heat pumps would need to have fresh air ducts ran to them if they haven't had that done already.

  13. #13
    Join Date
    Jul 2008
    Location
    Rydal,Ga
    Posts
    84

    jax1 is right !!!!!!!!!!!!!

    That was the first thing that came to mind, ,,,,disaster.... especially in humid climates...

    You could have someone spray a couple inches of closed cell foam on the roof deck ..... would help a bunch....
    But depending your budget.... It is always a slam dunk to seal & condition any attic.... 4.5" Closed Cell R-25 or for for a little less 8" Open Cell Foam (Icynene)

    NOTE "Icynene" is a name brand... from Canada it is a Open Cell 1/2 lb....

    And it is over priced $$$$$ !!!

    Most all PSF manufactures sales a 1/2 # product...
    Foamedix, NCFI, Demalack...... Even a couple of the fiberglass guys are make spray foam now

    As mention several times... find and PAY a "HERS Rater" to do a comparisons of what you have.... to a "Seal Conditioned Attic" And have a Manual J done as well
    REMRATE (HERS Software) will tell you your Load Calc. perimeters.

    We spray both foams and do HVAC and have several hundred sealed conditioned attic.... all are very satisfied customers !!!!

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