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How should moisture/vapor wetting and drying be considered in this situation?

Geoff Summa | Posted in GBA Pro Help on

Details:
– Climate Zone 6, Marine setting
– Stand alone garage(1970’s) to be converted to an overflow guest house w/ full bath no kitchen.
– Guest house to be used in summer only.
– Existing structure on a floating slab w/ no vapor barrier beneath, sitting poorly drained soils.
– 20′ by 16′ area w/ cathedral ceilings, 2×4 walls and 2×8 rafters
– 20 mil vapor barrier to be installed over slab (seams taped), may not be able to extend under existing perimeter walls.
-2×4 sleepers laid flat over 20 mil VB
-1 1/2″ xps fitted between sleepers
-1/2″ subfloor fastened to sleepers w/ engineered finished floor
– Walls/Ceilings insulated w/ rockwool batts
– 1/2/ drywall for interior cladding painted w/ water based paint
– Bath to be vented to exterior
– Existing exterior cedar shingle siding to remain, asphalt roof shingles to remain.
– Gutters and downspouts added to move bulk water away from slab perimeter
Questions:
– If VB cannot be extended under perimeter walls and there exists no capillary break between slab and bottom plates will moisture getting into wall cavities present a problem?
– Should vapor barrier extend along walls/ceilings to force slab moisture getting into walls to dry to the outside or should a smart membrane be used to to allow drying to both inside & out?
– Can someone walk through the building science concerns I need to consider here?
Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Geoff,
    You accidentally posted this question twice. Your question has already been answered. Here is a link to the other Q&A page where your question was answered: Goff Summa's post with answers.

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