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I want to insulate my attic that has my furnace ac coil and duct work in my attic and I would like to lower my heating bill because I use propane as my heating fuel so I need to know what would be best to insulate my ceiling or floors in my attic.

  • Are there insulation baffles around the perimeter of the attic to keep air flow at the soffit? If so, blown in insulation over the duct work to insulate them further as well would be good. I personally have added roll insulation in clients homes across the joists since the original insulation was to the tops of the joists. Doing it this way kept the soffit vents clear and heavy winds will move the insulation around. R-42 finish depth is a good depth to achieve although R-48 or more would be ideal. I would have made this an answer but the question of the baffles need to be answered, blown best. – Jack Nov 27 '16 at 17:56
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You wouldn't put a heating duct outside your house, so why would you put one in a vented attic?

Since your furnace and ducts are in your attic -- I see only one sane option here, and that's to insulate the roof, forming an unvented, conditioned attic. Don't skimp on the insulation here -- I would use lots of R-value, R-50 at a minimum, to keep any snow that may get on the roof from melting and refreezing into an ice dam.

In general, this is most often done with open cell spray foam applied to the underside of the deck, but this can also be done by applying at least two layers of foamboard with the joints staggered in both directions to the roof deck and then putting an overdeck over the top of the foamboard, creating an assembly much like a commercial compact roof, as in the illustration below (from the Building Science folks).

detail of a sloped compact insulated residential roof

Furthermore, I would make sure that there's air exchange between the attic and the rest of the system, so the attic doesn't turn into a hot, wet, condensing mess during the summer.

See BSI-046: Dam Ice Dam and BSI-077: Cool Hand Luke Meets Attics for more details.

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