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Moisture Control Measures for Garage Slab

smrtk | Posted in General Questions on

Hello, hope you are all having a great day.

I am renovating a small home (30′ x 24′) in Southeast MI (Zone 5A) and want to convert the garage into livable space (entry, utility and storage mainly and I know that this is not always looked kindly upon but it’s a small house, I have lot’s of stuff and I really need the room!).

The dimensions of the existing garage are roughly 14′ x 24′ and it comes off of the house forming a tee on the 30′ wall. The house was built in 1930 (lake cottage) and the garage was added in 1951. The garage slab is flat and in great condition (no cracks) and has a footing. Almost always dry except during heavy rains (needs bulk water management: gutters and grading). My plan so far (pending approvals and some more research) is as follows:

1. Remove existing garage structure
2. Frame a deck floating over the existing foundation tying it back into the house deck (same plane)
3. Frame walls and roof tying back to main house

I am going to insulate the roof with polyiso on top of the deck (framing for existing is all 2×4 including roof rafters) with fiberglass in the existing cavities, unvented. The new construction for the garage will be 2×6 all the way around with polyiso on top as well. Also not vented.

My main concern here is the deck and concrete slab. There is no curbing (better term?) on top of the slab to prevent water from coming in (bulk or otherwise). The deck framing will be 2×8 due to the height of the garage floor relative to the existing house floor. I am also concerned that by closing all of this up and insulating I will have a high probability of moisture condensation on the slab and the eventual effects that will have on the framing.

I am going to handle bulk water with the gutters and grading. Should I build up curbing around the perimeter (PT sill plate and PT rim joist) taped, caulked, wrapped, etc.?
Should I try to encapsulate or otherwise seal the concrete or maybe condition the space under the deck using the forced air HVAC system?

Any thoughts or pointing in the right direction appreciated.

Thank you in advance,
(Hopefully not) Damp in Detroit

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  1. Jon_R | | #1

    Sounds very similar to a crawlspace - there is lots of info about how to control moisture in them.

  2. smrtk | | #2

    That is where I was heading as well, just wanted to make sure. Insulate and encapsulate then? Condition as well?

    Thank you, David

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