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Florida Hot/Humid – R-Value on drawing vs Contractor

sthrndrw | Posted in General Questions on

New construction insulation at roof and floor deck.
Building located in Florida – Climate Zone 2 > waterfront, raised building.


I need to find a middle ground for what was on the drawings and what the contractor bid with respect to performance and cost to clients.

Florida Building Code and IECC Minimum R-Values
Floor – R13
Walls – R13
Ceiling – R38

Contractor/subcontractor anticipated all open-cell
Subfloor – R11.1 (3”)
Walls – R14.8 (4”)
Roof Deck – R20.35 (5.5”)


The subcontractor is responsible for both the HVAC and thermal insulation. His opinion: using open-cell on raised subfloor and applying fluid-applied membrane would be best. Reason being, the building will move and with closed-cell being so rigid, air gaps/crack could develop..


Drawings called for R32 closed-cell to bottom of subfloor, contractor priced R11.1 (3”) open-cell. What R-Value do you consider middle ground? 

The finished ceiling (exterior, bottom 2×12 joist) – one layer 5/8” Type-X gyp. for fire rating (parking below) plus Hardie panel for maintenance. Ideally I would like to have the vapor barrier farthest to exterior.  Do you take issue to the air space between the closed-cell insulation at deck and air barrier created with finished ceiling?

Roof assembly is 24” wood trusses, peel & stick membrane with metal panel roof straight to deck, interior ceiling is tongue and groove wood. No attics, no ventilation other than makeup/fresh air. Contractor plans on R20.35 (5”) open-cell to bottom roof deck noting the Florida Building Code Prescriptive method.


I think we need to require a minimum of R30 open-cell to roof deck, but the sub (mind you also responsible for HVAC install) stands behind R20…thoughts? The building has complete fluid-applied exterior membrane and dual-speed HVAC with supplemental dehumidification. Should we duct a little conditioned air into the truss space or something from the dehumidifier?


 Always confused on minimum Ceiling R-Value vs insulation applied to deck.

Thanks in advance

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  1. walta100 | | #1

    I see large amounts of spray foam insulation in new construction a huge red flag for poor design by a lazy designer who has no regard for you budget.

    The way I see it there is always a better lower cost way without spray foam but it will require hard work and creativity from the designer.

    Stick to your guns and except nothing less than code minimum that seems likely to be almost optimal given your climate.


  2. Expert Member
    BILL WICHERS | | #2

    This is easy: your contractor is quoting you less than code minimum insulation. That's not going to fly, regardless of the type of insulation used. Do NOT be fooled by that "spray foam R value is better than other insulation R value", that is NOT true. R value is R value, period. You need to install AT LEAST code minimum R value in all areas. There is no "middle ground" here - - you have to go with at least code minimum. You can put as much more as you want, but you can't go with less.

    All that said, I agree with Walta that using spray foam everywhere isn't usually a good option. I prefer to use batts in the walls myself. There are various ways to do other areas, and if you can use a vented attic, loose fill cellulose or fiberglass works just fine.


  3. Expert Member
    Akos | | #3

    Same here. If the R value on the spray foam tag is not code min, the inspector won't pass it. No hand wavy arguments about better sealing of spray foam will work. R value is R value.

    Since folks love to install HVAC into the attic down south, spray foam in this case is a good idea. Open cell foam roof would normally need conditioning but in your case with trusses and T&G, you'll get enough drying to the inside.

  4. user-1072251 | | #4

    there has been a lot of publicity about how open cell foam - which is permeable - rots roofs; especially in your climate. Yes, it is cheaper than closed cell, but that's before adding the cost of replacing your roof. Figure out how to use batts and ventilate properly. & Read up on open cell problems.

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