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5 year old house - finished attic. Permanent stairs, carpet, heat, ceilings, drywall - completely finished. The insulation is spray foam (open cell) directly on the underside of the roof deck (min - R19) and on the inside of the exterior walls. There was ridge vent originally installed, but sealed up with spray foam (closed cell). What I am trying to explain here is that the entire attic is within the building envelope - with the exception of a sloped ceiling, it looks like any other room in the house. I had the building envelope tested (blower door test) and there was very minimal air leakage anywhere in the house and what was detected was remediated.

There are knee walls and a dropped ceiling after the underside of the roof joists pass 9' (roof peak is at 13'), but these are just studs and drywall - no isulation. The spaces they create are still inside the building envelope (insulation is on the exterior walls only). The space behind the knee walls are accessible via access panels (used for storage) and there is no appreciable temperature difference in summer or winter in these spaces. The area above the ceiling is closed off and I cannot measure.

The problem is, the attic gets really hot in the summer. I can feel a temperature gradient as I walk up the stairs and have measured the bedrooms on the floor below at 75 (at the ceiling) and the attic ceiling at 85 degrees. My thought is to get a bathroom exhaust (or 3) inthe ceiling and vent the space that way. Any pros / cons / other suggestions?

The attic is 800 sq ft and the finished space 5600 cu ft.

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  • You need more air changing from the HVAC to handle the additional heating load of the attic space, simple as that. Is there a return vent? Increase return capacity? End of long run.. install booster fan on supply? Not enough vents? Add additional duct work? Supplement with secondary HVAC like portable unit or mini split?
    – Damon
    Nov 13, 2016 at 9:14

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Spaces are inside or outside, never both at the same time!

What you need is not venting as this attic is conditioned space -- it's inside the house control envelope, in other words. What you have is likely an issue with improper ductwork that isn't returning hot air from the attic efficiently -- the temperature buildup is caused by a combination of stack effects (hot air rises) and the inability to get this hot air back down to the air conditioner to be cooled. If there are returns from the upper attic down to the HVAC system, check to make sure they are not closed off by a damper, and that the HVAC system is properly sized for the load as per Manual J. (HVAC sizing is like Goldilocks -- not too big, and not too small, but just right for the house you have.)

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  • It should also be noted that if this was an "addition", the HVAC may not be sized properly for the additional load.
    – Tester101
    Nov 13, 2016 at 2:14
  • @Tester101 -- verily true.
    – ThreePhaseEel
    Nov 13, 2016 at 2:42

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