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Community and Q&A

Should we insulate the attic floor or roof where attic contains AC system only?

Douglas Forman | Posted in Energy Efficiency and Durability on

We have an uninhabited attic space (roof slope too low) that has a high velocity AC system air handler and distribution in attic space with blown in insulation between attic floor joists. Heating is a conventional hot water system with boiler in the basement. The attic is vented with both soffit and ridge vents; attic temperatures currently max out around 125 degrees where we live in southeast Michigan. Here’s the dilemma: air sealing is needed between the second floor ceiling and the attic but the combination of ductwork on top of the attic floor joists, low roof slope, blown in insulation and a fair amount of plywood flooring make access difficult. Plus, the attic needs more insulation since it only has 6″ of cellulose and of course the AC is working against the hot attic air. Given our location, is it better and/or more economical to seal penetrations and insulate the attic floor as best as possible and let the AC system deal with the attic heat, or to insulate the roof and improve attic temperatures?

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  1. GBA Editor
    Martin Holladay | | #1

    If you have an air handler and ductwork in the attic, then the correct location for your insulation is between the rafters. I advise you to convert your attic into an unvented conditioned attic.

    Here are two articles to explain why and how:

    Keeping Ducts Indoors

    Creating a Conditioned Attic

  2. Douglas Forman | | #2

    I've thought about your recommendation to create a conditioned attic but there are some concerns that I hope you can address. First, I'm not confident that the work can be done properly given the low headroom in our attic and the difficulty of moving around and working toward the eaves because of ductwork present in the attic. I installed two new bathroom ventilation fans from the attic last year and the contortions that were necessary were brutal. Second, as a result of the first concern, it seems like the insulation would need to be sprayed directly on the underside of the roof deck and rafters versus leaving an air space. If insulation is applied directly, would the ridge vent need to be removed and replaced with cap shingles and the ridge opening closed up? Third, will high attic temperatures reduce the life span of an attic mounted air handler (we have a Unico unit) or are they designed for that environment? Seems that prematurely replacing an expensive air handler should be part of the pay back equation.

  3. GBA Editor
    Martin Holladay | | #3

    Adding insulation between your rafters will make your attic cooler, not warmer, during the summer.

    Fixing construction problems often requires work in awkward areas. That's unfortunate, but it's part of the routine work performed by many remodelers. If you don't want to do the work yourself, you can start looking for a contractor.

    If your attic really is too tight for human access, it's possible (although expensive) to insulate a roof from above using rigid foam. Of course, that means you will have to replace your roofing.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Douglas, I like the plan at this thread. The only better plan is to put rigid foam exterior to the roof sheathing. Has to also be done right and timed with need for new roofing. The access is good though and no fear of bad foam.

  5. Douglas Forman | | #5

    I only wish my roof hadn't been replaced two years ago otherwise rigid insulation on the roof sheathing would have been a great solution. It appears that foam directly on the underside of my roof sheathing is the only practical way to go given my attic conditions; it sounds like an air space would be preferable but at least I've got a new roof. I do understand that roof insulation will cool my attic in summer; sorry if that wasn't clear. My question about payback would've been better stated as does high attic temperatures in an attic reduce the lifespan of mechanical equipment because if so, it seems that early replacement cost would be part of the payback equation to make the attic cooler? Thanks again for your thoughts.

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