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attic insulation Post New Topic | Post Reply
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Posted: Jun 18, 2010 08:18 AM
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attic insulation
mason,,,hog buildings,,,lots of moisture,,, 1:4 pitch,,,flat,,like georgeous georges head... 2x6 between interior lid and exterior roof deck,, both metal... filterglass insulation in cavity r19 aint workin well,,go figure,,and the exterior deck is rustin,, owner would like to remove exterior deck,,,remove filterglass and have spf applied to the interior lid from the top side,,, he would like to keep the spf app to 2" and maybe top with more filterglass to keep "costs down"... i encouraged him to allow me to apply 3" minimum,,4 would be better,,and gave him a good quote,,,(and i am confident he will find one of the new hillbilly's to apply the 2",,,eeggad their everywhere) i told him to leave the airspace,,keep the costs down,,,screw the filterglass above the spf... ??? would this be a good app??? 3"-3.5" to lid from above??? ??? will him putting filterglass above the spf be worthwhile,,,or more importantly,,be detrimental to the performance of the lid or will it add additional rvalue to the whole lid system and some performance enhancement?? ???should the subsequent airspace above the spf be vented in places to allow moisture vapor to be exhausted,,(there is no perfect vapor retarding membrane,,,and this is a high moisture environment)since the metal roof deck should be a vapor retarding membrane in and of itself??? thanx for your time & consideration... 'dude |
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mason
Posted: Jun 18, 2010 11:45 AM
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You will still have convection currents from the air space and air infiltration from the roof deck causing havoc with the fiberglass. So, as you fear, the fiberglass installed on top of the foam will be not be performing efficiently. Depending on the outside temperature (hot and cold) the fiberglass would be at 50% efficiency. (I presume a blown in fiberglass.) I would add fiberglass only to comply with ignition barrier requirements or to achieve R value code levels. But, as you suggested, I would want to put enough sprayfoam to give you the energy efficiency required for the building. |
