Hello GBA! First-time user here, looking for a quick reality check on some unusual wall and floor assemblies… any input appreciated!

 

Goal:

I have a 600sf adobe-brick garage in Zone 3 (West Texas high plains; hot and dry with some winter freezes). I want to convert it into a conditioned guest suite: a bedroom, bathroom, and small artist’s studio. 

 

Proposed Approach:

Frame inside of the existing structure, insulating and sealing in a way that allows the enclosed space to be reasonably energy-efficient, and the framing not to be too vulnerable to moisture issues, while allowing the adobe to dry to the inside or outside as needed. (If this sounds a little weird, note that the artist’s studio needs stud walls for hanging heavy work, and my hope is that the approach will provide a more energy-efficient envelope than just stuccoing the interior and cutting channels through adobe bricks for electric).

 

Existing conditions:

No water damage except in 1 poorly-graded exterior corner where bricks show some erosion (fixing that)

No evidence of termites, though records show the house was treated at some point

(Note: main house is c.1940 wood/stucco with a 1997 heat pump that works fine summer and winter)

 

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Here’s a description of the assemblies as I’ve been planning them, plus questions for each. Any and all feedback/guidance would be much appreciated!:

 

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FLOOR ASSEMBLY (bottom-up):

Existing concrete slab

Shims

Sill gasket (from conservationtechnology.com)

Treated 2×4 sill, anchored to slab with tapcons or bolts

Joist gasket

Treated 2×6 joists

Rigid foam insulation inside rim joists

Subflooring

 

Questions:

F1. Do I need a (smart?) vapor barrier below the sill, and if so should it continue up the wall?

F2. Should/can I add insulation between joists (if so, what kind)?

F3. Should I add continuous beads of acoustical sealant at sill/joists?

F4. Should I add a termite shield?

F5. A 3” PVC bathroom drain line will run ~10ft under the floor, above the slab. Should I insulate it? How?

 

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WALL ASSEMBLY (outside-in):

Existing Stucco-covered adobe

1/2” Rigid foam insulation (adhered to framing and taped, but touching adobe in places)

2×4 framing with R15 fiberglass batts

1/2” Gypsum drywall

 

W3. Should the foam sit on the slab or on the outside edge of the sill? 

 

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CEILING ASSEMBLY (bottom-up):

 

1/2” gypsum drywall

Existing 2×6 ceiling joists, with

2 layers of R-19 foam insulation

 

Questions:

C1. The ceiling joists and roof rafters rest on top of the adobe wall; the metal roof lays directly on the 48oc rafters. So it’s very open up there, but 70+ years on it’s seems to be in decent shape; the wood is a little stained in places but strong. I’m planning to replace the roof, and will add solid decking at that point; anything else I need to do to protect the ceiling assembly without breaking what’s not broken?

 

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That’s the plan… at least for now. What do you think? I’d be super grateful for any guidance or answers to specific questions above. Happy to provide any clarifications if needed.

 

Thanks very much! — Miles