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Insulating a Low-Slope Roof in a Hot-Humid Climate

11458890624 | Posted in General Questions on

I own a 2400 sq. ft. single story house in Miami built in 1957 with a 3000 sq. ft. asphalt low slope flat roof. Just had a Zehnder CA550, Mitsubishi ductless mini splits, and a Santa Fe Ultra120V dehumidifier installed.

It has vented exterior soffits around 80% of the perimeter. No current insulation besides paper-backed foil radiant barrier in some areas. About 30 years ago HVAC flex duct was built into interior soffits and a dropped ceiling in one bedroom. That ductwork has been removed and the Zehnder ComfoTube is now routed through this unconditioned space. Doing this with no attic was a major undertaking.

Recommendation I received to prevent the ComfoTubes from creating excess condensation is to hot roof by removing and sealing up the soffit vents then either blow in insulation or insulate from the top of the roof with Iso, EPS, XPS, or VIP. As there’s no attic nor access across the trusses, blown insulation will require about 100 holes to be cut (multiple along each stud bay) in addition to the ones already made to route the Zehnder ComfoTubes.

The roof is 10 years old and I was quoted $6K to do silicone treatment sometime in the near future (air pockets are starting to form). Quotes I have so far for blown insulation (including cutting and patch-up of about 100 holes) are around $8-9K total. This brings me to about $15K so I am considering roof-on-roof instead with 2-4” of polyiso which I may be able to get done permitted for about $20-25K.

Any recommendations on which route to take? How about regarding materials?

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  1. 11458890624 | | #1

    As any update to the above, I am now considering Rockwool boards instead of polyiso which I understand is done fairly regularly with flat roofs like mine. Any thoughts on this?

  2. GBA Editor
    Kiley Jacques | | #2

    I’m giving your question a bump. While you wait for folks to weigh in, read through this Q&A thread, which asks essentially the same question. And be sure to read the article Martin references.

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