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Florida Shed With A/C, Insulation, Vapor Barrier – Please help!

Ron Taylor | Posted in General Questions on

We are planning to buy a 12 x 24 pre-fab portable building like the Lofted Barn at

We live in southwest Florida and need to air condition it 365/yr (pretty much). Going to use the building for our handmade soap workshop and humidity is very important to our soap curing process so we will be using a portable dehumidifier in the space in addition to a wall a/c. Hence, we are very concerned about how to insulate and what to do about the vapor barrier.

The pre-fab sheds don’t have any sheathing and the siding already comes attached to the frame – it’s not easy to take off either so that’s not really an option.

We can’t afford to do spray foam right now.
Avoiding the foil bubble insulation and radiant barrier foil too.

My wife would like to use pallet planks for the interior walls because it’s free and she wants her She Shed to be a nice creative space – prefer not to use drywall if we can avoid it.

Best options for us to insulate and deal with moisture? And ridge vent or no ridge vent? We haven’t ordered the building yet so want to get that right too.

Thanks so much!

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  1. Stephen Sheehy | | #1

    If you haven't ordered the shed yet, you might look into having a local carpenter build it. Might be cheaper and you can get exactly what you want. A simple 12' x24' shed is pretty simple to build, especially since you need not worry about frost.

  2. Steve Knapp CZ 3A Georgia | | #2

    If you follow Stephen's suggestion, you also could install exterior rigid foam insulation. Outside insulation (perhaps in combination with air permeable insulation) would make it easier to address your climate control and humidity issues. Building a tight structure with enough insulation would also help you reduce the size of your HVAC system and ongoing energy costs.

    The Lofted Barn sounds like the wrong approach for what you are trying to achieve.

  3. Expert Member
    Dana Dorsett | | #3

    An un-perforated aluminized fabric type radiant barrier caulked to the framing inside each framing bay, leaving a 1" air gap to the siding, and compressed unfaced batts with a housewrap air barrier under the rough-plank interior should work for both the roof & walls. Alternatively, cut'n'cobbled 1" foil faced rigid polyiso foam with the 1" air gap and split batts or kraft faced R8s (if 2x4) on the interior side. There's no point to adding a ridge vent unless you also supply soffit venting. The details of how that might work (if at all) will vary depending on how the shed is framed/constructed.

    You need the vapor retardency of the radiant barrier/foil facer to really control the humidity well in a location as damp as FL.

    The Rolls-Royce of humidity control & air conditioning would be a 3/4 ton Daikin Quaternity mini-split, which will dehumidify to a %RH setpoint, and can even dehumidify when there is no sensible cooling load. You may still need a room dehumidifier to manage it during some months, or during processes that are dumping a lot of humidity into the room air.

    Or you could use anybody's 1/2-3/4 ton PTHP for cooling & heating along with a room dehumidifier, which may work fine most of the time- depends on just how tightly you need to control the humidity.

  4. Ron Taylor | | #4

    Stephen - thanks for your reply. I wouldn't mind getting a local carpenter to build the shed but everyone is so busy right now that finding someone who has the time is a challenge. Super busy building season right now. I'm keeping that option open if I can find someone to do it though.

    Steve - Thanks for your reply. I could really use the lofted barn storage space and I was afraid that it might not be efficient enough. The one I was looking at wasn't a huge lofted space - only 11 ft ceilings. The standard 7' walls just don't give the storage/work space I need. :(

    Dana - Thanks for your reply. So, you're saying that the cut n' cobble rigid foam or non-perf aluminized fabric radiant barrier inside each framing bay is my alternative to the exterior house wrap, right? Since I can't take the siding off and apply it in large sheets. So, I shouldn't worry about vapor diffusion through the studs, right? Just get the spaces between the studs sealed up with foam or radiant barrier with the 1" space and that's good enough for vapor retarding?

    And the housewrap air barrier under the interior rough-plank walls is ok because it's permeable right? Just want to understand correctly because I read to be very careful about moisture barriers on the interior in my area.

    I'll look into the Daikin too. Thanks for clarifying the ridge vent soffit connection.

  5. Ron Taylor | | #5

    Oh man, I just remembered that I should probably do something about a vapor barrier on the floor too.

    Would just taping some 6 mil poly over the existing decking and adding another layer of decking over that be my best bet?

  6. GBA Editor
    Martin Holladay | | #6

    You don't need a vapor barrier on the floor.

    Your biggest challenge is air sealing. Every conditioned building needs an air barrier. The air barrier needs to surround the entire building, including the floor, walls, and ceiling or roof. You don't want any gaps, cracks, or holes that leak air.

    Here is a link to an article that discusses insulating buildings that lack any wall sheathing: Insulating Walls in an Old House With No Sheathing.

  7. D Dorsett | | #7

    Martin has it right- this is first & foremost about air tightness, not vapor diffusion. The concerns about vapor retardency of the walls & floor are unfounded.

    But the walls stay drier if there is capillary break between the siding & inner layers, and wood siding will last longer and hold PAINT better if it has the ability to dry in both directions. The air gap provides both functions. A foil facer or radiant barrier facing the gap provides a modest amount of thermal benefit. The gap need not be fully 1", even 3/8" is good enough. At 3/8" you'd be less than a half-millimeter shy of the 10mm code minimum rainscreen depth for western British Columbia, much of which is practically temperate rain forest. SW FL receives it's copious annual rainfall in big gulps, followed by sunnier drier breaks which offer longer better drying for the siding than in foggy-dew western B.C. Limiting the gap to 3.8" would give you the space for another R2.5-R3 of center-cavity-R, and lower thermal bridging through the framing which doesn't hurt, only helps the thermal performance of the assembly.

  8. Ron Taylor | | #8

    Thank you Martin and D Dorsett.

    Change of Plans...

    I was able a Florida shed builder that uses an exterior vapor barrier under vinyl siding. I understand the need for air tightness supercedes the vapor barrier, but if I can get the vapor barrier, I'll take it.

    The builder uses a radiant foil as a vapor barrier on the exterior side of the sheathing under the vinyl siding.

    How would this change the suggested insulation methods above?

    Please keep in mind that my wife and I are diyers and have never insulated anything before. We just want to get it right. Thanks so much for bearing with us.

  9. GBA Editor
    Martin Holladay | | #9

    I'll repeat: The issues we have raised have nothing to do with vapor barriers (or with radiant barriers, for that matter).

    Some radiant barriers are vapor barriers. Others are perforated to make them vapor-permeable. I doubt whether the type of builder you describe would install a radiant barrier in an airtight manner.

    Even in Florida, you really don't want to have a vapor barrier between your siding and your sheathing. In cold weather, it would be a wrong-side vapor barrier.

    If this wall has OSB or plywood sheathing, the easiest way to improve its airtightness would be to tape the OSB seams (or the plywood seams).

    -- Martin Holladay

  10. Expert Member
    Dana Dorsett | | #10

    The amount of moisture diffusion you get through dry OSB sheathing is pretty miniscule, even without a vapor barrier.

    A radiant barrier foil under vinyl siding adds about R0.5-R1 to the thermal performance of the assembly, but it's usually a pretty expensive R1 compared to other methods.

    I'll disagree with Martin on one point: In FL there is NO increased risk to putting a vapor barrier on the exterior of the sheathing, as long as you don't make a moisture trap by installing low-permeance interior finishes (such as vinyl or foil wall paper, or alkyd paints- standard interior latex paint is fine.) It's simply not cold enough long enough in a FL winter for significant moisture accumulation to build up in cold sheathing. Even in the "Florida Arctic Zone" way up north around Tallahassee the mean January temperature is north of 50F. Accumulating significant moisture through latex paint even in 50F sheathing would require sustained interior humidity levels north of 65% RH @ 72F, a level high enough to be a mold hazard even in the conditioned space. Wintertime outdoor dew points in Tallahassee average in the low 40s F. You would use a dehumidifier &/or ventilation to manage the peak humidity levels sufficiently to keep the mold from growing in the work space, and that alone would be bring the humidity levels low enough to protect the sheathing from excessive wintertime moisture accumulation.

    Caulking the framing to the OSB with polyurethane caulk inside every stud bay, and putting a bead of caulk between any doubled up framing such as top plates, window headers, etc, and a bead between the bottom plate and the subfloor can be as air tight or tighter than taping the seams. With a powered caulking gun it's even pretty quick & easy.

  11. GBA Editor
    Martin Holladay | | #11

    Fair enough. I accept your point about exterior vapor barriers in Florida, and stand corrected.

    -- Martin Holladay

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