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Do I need a vapour barrier on the ceiling (warm) side of the unconditioned garage in a hot humid climate?

Do I need a vapour barrier on the ceiling (warm) side of the unconditioned garage in a hot humid climate? Garage is below conditioned space.

I am building in a hot humid climate (Bahamas) and I have a lower level Garage that is below an air-conditioned upper level bedroom. The floor structure is PT wood joists with Rockwool insulation in the cavity. On top of the floor joists we have plywood , a water shield membrane and durock board below the ceramic tile floor finish. There will be hot, humid UN conditioned air in the garage. The Entire buiding is built of ICF and roof deck is spray foamed therefore is is quite airtight when doors and windows are closed. Because it is a vacation rental house I expect that the A/c will not be at full capacity and windows and doors will be open much of the time therefore the Delta T will be lower.

Asked by Cynthia Zahoruk
Posted Thu, 09/05/2013 - 09:08
Edited Thu, 09/05/2013 - 13:05

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2 Answers

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Cynthia,
No, you don't need a vapor barrier on your garage ceiling. The assembly already has a vapor barrier -- the layer under the Durock that you describe as a "water shield membrane." Since your plywood subflooring will be chilled by your air conditioner, it's possible that the subfloor will accumulate moisture when your house is air conditioned. That's unfortunate, but hard to avoid.

As long as your garage ceiling is vapor-permeable (for example, if it is finished with a layer of gypsum drywall), the accumulated moisture in the ceiling assembly will dry downwards during the months when your air conditioner is turned off. Whatever you do, don't put up a vapor-impermeable layer like polyethylene on the garage side of the assembly.

If you will be bringing a spray foam contractor to the job site to insulate the roof assembly, you might consider using spray foam to insulate the garage ceiling too. If you install closed-cell spray foam there, the insulation will do a better job of protecting your cold subfloor than the rock-wool batts that you are planning to use.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 09/05/2013 - 13:12

2.
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Thank you. The garage is finished with Type X GB. unfortuanely the spray insulation is applied fromkits (not trucks) and VERY expensive to purchase this way. we are trying to keep costs down.

Answered by Cynthia Zahoruk
Posted Thu, 09/05/2013 - 15:22

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