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Community and Q&A

Vented attic or conditioned attic?

user-5924747 | Posted in Energy Efficiency and Durability on

I am building a new home in Arizona , in summer gets to about 115 and winter it can still sometimes get down to freezing , is it better to have insulation on top of ceiling or underside of roof , my thinking is that if I have a vented attic with a radiant barrier to the rafters this provides a large ” air ” thermal break and has the insulation directly above the ceiling for the winter as well , I have places ( false beams , feature ceilings etc ) where I can run my ducting so that it is still within the conditioned space , is it worth doing

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  1. GBA Editor
    Martin Holladay | | #1

    If you can keep all of your ducts and all of your HVAC equipment inside your conditioned space -- not in your attic -- then it is usually easier to create an unconditioned attic with the insulation on the attic floor, not a conditioned attic.

    In your climate, for new construction, it makes sense to use radiant barrier roof sheathing. (Remember, the shiny side of the roof sheathing faces down.)

  2. user-5924747 | | #2

    ok thank you , I think I will at least use this method on my south side ( I have flat roof in the middle ) what kind of R values will I be saving doing this method , my mechanical guy says its not worth it as you will always get leaks through the ceilings etc , thx again

  3. Dana1 | | #3

    A flat roof will have higher peak temperatures than a sloped roof, due to the lack of convective cooling on the exterior. Using radiant barrier roof sheathing on that section, reduces the heat transfer to the attic floor.

    A CRRC rated high SRI cool-roof finish will lower the roof deck temperatures directly (and substantially) by reducing the solar gain while retaining significant radiant cooling capacity.

  4. user-5924747 | | #4

    Yes , I was going to paint the flat part ( and have my solar panels for shade ) and use a cool roof tile on the east / south and west sloping parts , the mechanical guy still recommends spraying the insulation to the underside tho , common sense would tell me to leave an air gap between the radiant barrier and the insulation , I have built many houses in the UK but none here , we only do heating not HVAC so its a bit new for me

    many thx

  5. GBA Editor
    Martin Holladay | | #5

    For information on insulating a flat (low-slope) roof, see Insulating Low-Slope Residential Roofs.

  6. Dana1 | | #6

    Without an air gap radiant barrier sheathing provides no benefit.

    Splitting insulation with some at the sheathing, some at the ceiling with an air gap between undercuts performance of the insulation. It's either a vented roof with a gap between the roof deck and insulation (in which case radiant barrier sheathing provides some benefit), or it's all on the underside of the roof deck, and sealed. All ducts & air handlers need to be fully inside the thermal envelope.

    The thermal mass of tile roofing makes it inherently "cool roof" in any color, since it delays and lowers the peak temperatures at the roof deck, and is inherently ventilated for better convective cooling. But it doesn't hurt to take the next step with a higher direct solar reflective index (SRI) finish.

    Arizona spans US climate zones 2B through 4B (more at higher elevation points). A more precise location would be useful for refining the specifications for the assembly. Do you have a ZIP code for that location?

  7. user-5924747 | | #7

    Hi , thanks for that , my new house will be in zip 85331 Cave creek AZ ,

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