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Community and Q&A

Roof insulation

Muskogee | Posted in GBA Pro Help on

Home location in zone 2A Central Florida (Not on the coast). Having 2,053 SqFt living space w/above ceiling blown in insulation R-30 and an additional 1,027 uninsulated space. Fully vented at the soffet with minimal ridge vent. Soon to install 2 solar powered thermostat controlled 1600cfm roof vents. The wall construction is CMU block w/stucco finish. The roof is 6:12 pitch, Arch shingles over 30lb felt paper and OSB. The ceiling plane is tight.
I desire to reduce the heat load from the attic space by installing PolyIso insulation on outer side of the existing 1/2″OSB deck. This would be accomplised by removing existing shingles. Install new 30 lb felt paper, single ply. Install at parimeter -treated 2×6 blocking on the flat. laying in 1.6″thick 4×8 PolyIso planks R-9.1. Closing this with new 1/2″ decking, properly fastened to truses w/appropriate screws and washers and proper nailing pattern for local building code compliance. Applying new fire retardent paper w/19″ overlap. Finished with new Arch shingles and acessories. This will build a insulated roof deck system. The ceiling and attic space is unaltered. Will there be a moisture entrapment concern between the wood decks? Do you offer any further design modifications or concerns I need to address? Thank You, I look forward to your reply.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I strongly advise you not to install or use any powered attic ventilators (fans). Using these devices usually increases rather than decreases your energy bills. For more information on this topic, see Fans in the Attic: Do They Help or Do They Hurt?

    The second point revolves around the question of where you want to establish your insulation layer. You have two choices: to install your insulation on the attic floor -- an option which makes your attic unconditioned -- or to seal all of your attic vents and to install the insulation along the roof plane -- an option which would create a conditioned attic.

    If most of your insulation is on your attic floor, then adding rigid foam insulation above your roof sheathing is a total waste of money.

    You don't mention whether you have any ducts in your attic; if you do, you should take the money that you would have spent on the rigid foam insulation for your roof and use the money instead to improve the insulation around your ducts.

    For more information, see Creating a Conditioned Attic.

  2. Muskogee | | #2

    Thank You Martin for your reply - However I think you missed my point. 1. The attic ventilators are solor powered and thermostat controlled (NO electric cost here). 2."My efforts are to minimize the heat load out of the attic space." The mentioned ventilators should move the hot air to the outside. There is no way to create a negative air space in the attic because the entire parimeter is assembled with a vented soffit material. The PolyIso would be used to reduce the heat permiating through the existing (typical construction) shingled roof. This will remove a considerable amount of heat from the HVAC ductwork (again typical flex tube) that is located in the attic space. All of which is in good working order. The conditioned space still remains in its present location, below the attic floor. This will turn the attic space from a hot space into a warm environmen.t The Question remains - Will I be creating a condition for moisture to accumilate between the decking materials? I do not want to cause a moisture rot, mold ar mildew environment. Thanks

  3. GBA Editor
    Martin Holladay | | #3

    My answer remains unchanged. I urge you to disable the powered attic ventilators.

    The polyiso that you intend to install above the roof sheathing is a waste of money, but it won't cause any moisture problems.

    The money would be better spent improving your duct insulation.

  4. charlie_sullivan | | #4

    I agree with Martin about the insulation. Insulate the ducts, which will be cheaper and more effective for what you want to accomplish. Seriously insulate them. You can insulate them to about R80 for what your roof insulation would cost.

    As for the attic fans, they might be OK if you are really sure there are absolutely no leaks in your ducts or your ceiling, but that seems unlikely, so I'm afraid I agree with Martin there too.

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