Garage floor to living space.

[Wireslinger]

planning to convert part of an attached garage into living space possible laundry this forum seems like a good place to bounce ideas of others.
First thing to attack will be raising the floor about 18" from the current concrete slab to match closer existing floor. My plan is 6mil vapor barrier then 1 or 2" of xps foam followed by pt 2x4 layed flat 12"o.c. Tapconed through foam to concrete then shimed 2x? Run perpendicular to the 2x4 and a second run of 2x? Perpendicular to that finished off with 3/4 tongue and groove subfloor and sheet vinyl.
Am I missing anything should I throw rock wool insulation between one of the joist levels?

 

[Cacher_Chick]

If the It would be a lot less trouble to frame in prefabricated I-joists to bring up your height and subfloor over that. I would hesitate to use any insulation other than foam board unless you are 100% confident the garage floor will not carry water in. Dont forget that you must meet the requirements for fire containment between what you build and the garage space.

 

[Sponsor]

 

[Wireslinger]

I opted for 2x joist for ease of availability but will look into the I joist. More r value between the floor and slab would be nice so perhaps more xps between the joist. I haven't gotten far enough in my head to be thinking about the fire wall between garage and this space my gut reaction is a 2x6 wall with fiberglass insulation paper facing in, paneling or .5 drywall on interior and 5/8 drywall (fire code?) on garage side. Maybe I should put 1" xps in there somewhere.

 

[Dana]

planning to convert part of an attached garage into living space possible laundry this forum seems like a good place to bounce ideas of others.
First thing to attack will be raising the floor about 18" from the current concrete slab to match closer existing floor. My plan is 6mil vapor barrier then 1 or 2" of xps foam followed by pt 2x4 layed flat 12"o.c. Tapconed through foam to concrete then shimed 2x? Run perpendicular to the 2x4 and a second run of 2x? Perpendicular to that finished off with 3/4 tongue and groove subfloor and sheet vinyl.
Am I missing anything should I throw rock wool insulation between one of the joist levels?

A few things...

XPS is the least-green insulation material in common use today due to it's HFC blowning agents and high polymer/R. The only reason to use in in lieu of EPS (blown with low impact pentane) would be for water vapor control, but you don't need that since you have a 6 mil vapor barrier between the ground moisture and your new floor.

An IRC 2018 code minimum floor in US climate zones 4A (in yellow, below) would be R19 between joists, and in 5A (in green) it would be R30. per TABLE N1102.1.2. Alternatively the floor could be insulated at the slab level with continuous insulation to U0.047 (R21 "whole assembly) for zone 4, U0.33 (R30 "whole assembly") in zone 5. If one used reclaimed roofing polyiso foam (often cheaper than batts that would take a minimum of 3.75" of foam in zone 4, or 5.25" of foam in zone 5. Tape the seams with housewrap tape- multiple layers are fine.

Do NOT insulate the slab with fiber insulation, but fiber between joists if fine.

If doing it with batts, cheap R19 batts aren't air-retardent enough to actually perform at R19 without an air barrier on both sides of the batt, but R21 fiberglass and R23 rock wool are. If the floor is framed with 2x6 it would work for zone 4 as long as you install something to keep the batts from falling down in an earthquake (or a polka party with 250lb Czechs dancing with kegs on one shoulder. ) With 2x8 joists R30 rock wool would fit, but it would take 2x10s to hit R30 with fiberglass.

Don't forget to air seal and insulate the ends of the joist bays and the band joists and any remaining wall (all the way down to the floor foam if insulating with rigid foam above the slab)

If installing short piers and beams supported by the slab is likely going to be necessary for to span the full width of a garage.

The ends of the joists can be supported by ledgers fastened to the wall framing with lag screws.

Reclaimed foam can often be had for cheap from Repurposed Materials Inc. in Marcus Hook PA:


But they're not the only player in the region.

 

[Dana]

I opted for 2x joist for ease of availability but will look into the I joist. More r value between the floor and slab would be nice so perhaps more xps between the joist. I haven't gotten far enough in my head to be thinking about the fire wall between garage and this space my gut reaction is a 2x6 wall with fiberglass insulation paper facing in, paneling or .5 drywall on interior and 5/8 drywall (fire code?) on garage side. Maybe I should put 1" xps in there somewhere.

Adding foam insulation in the wall increases rather than decreases the fire hazard. Polystyrene (EPS, XPS) is even worse than polyisocyanurate or polyurethane, since polystyrene melts and spreads while burning, whereas polyurethane & polyiso char in place while burning, and have a higher kindling temperature.

Paper facers on batts are their own fire hazard, and need to be covered by gypsum board.

 

[Wireslinger]

I'll look into the eps, I've not worked with it before and always just grab xps because its available locally without ordering. A quick search didn't tell me its compressive strength so not sure it'll work with a sleeper floor on top. I'm trying to avoid digging piers and the two existing walls are hollow block (new room will be in a corner so spanning whole garage won't work) supporting the floor on them would require extra effort.
Thanks for the help, sounds like I'm on the right track other than eps/xps decision.

 

[Dana]

I'll look into the eps, I've not worked with it before and always just grab xps because its available locally without ordering. A quick search didn't tell me its compressive strength so not sure it'll work with a sleeper floor on top. I'm trying to avoid digging piers and the two existing walls are hollow block (new room will be in a corner so spanning whole garage won't work) supporting the floor on them would require extra effort.
Thanks for the help, sounds like I'm on the right track other than eps/xps decision.

XPS won't work under a sleeper for a load bearing structure without running the math on it either. You may be able to get away with it in a residential floor application though.

Most surplus roofing EPS is Type-VIII (1.25 lbs per cubic foot nominal density) good for about 13 psi at 10% elastic deformation, and most roofing polyiso it 2lbs density at about 20 psi, both of which are good enough for "walkable" roofs, but not something to put under a load bearing element. Even 1lb density wall sheathing polyiso is good for ~15 psi. Typical EPS compressive strengths and densities are found here.

Designing for the dynamic weighting of a sleeper supporting a floor takes some engineering math but a 2x6 or 2x8 sleeper under a 2x timber serving as a center supporting beam for the longer spans would almost certainly work. How much span are we talking between beams?

Also note, if insulating at the slab level even 2" of floor foam is good enough (IRC code doesn't demand any, if the crawlspace between slab & floor is unvented), as long as the perimeter walls of the micro-crawlspace has at least 3" of polyiso, or 4" of EPS. If using polyiso on the walls, be sure that the cut bottom edge is on top of the vapor barrier, not in direct contact with the slab, since it can potentially wick ground moisture from a damp slab. (EPS doesn't have that issue.)

 

[Wireslinger]

. How much span are we talking between beams?

The room will be under 10x10 with just the flooring built on the foam. The two new walls will go to slab with only vapor barrier under them. Two existing block walls will get rigid insulation then framed.

 

[Dana]

The room will be under 10x10 with just the flooring built on the foam. The two new walls will go to slab with only vapor barrier under them. Two existing block walls will get rigid insulation then framed.

Hanging ledgers on the CMU wall and the framed walls you'd have comfortable capacity margin with 2 x 10 joists (even 2 x 8s would do for 40lbs per square foot live load at 16" on center) and nothing resting on the foam. Play around with this calculator a bit to get comfortable with that.

Are you in US climate zone 4 or is it zone 5? In either zone a 3/4" foil faced polyiso with a batt insulated 2x4 wall tight up to it is sufficient to meet IRC code minimums, and that isn't too much of a moment-arm for through-screwing a ledger or band-joist to the CMU with 4.5"-5" masonry screws or anchors secured with epoxy. (You may have to order them online, though HD carries 1/2" x 6" Tapcons that would work, if extreme overkill and quite expensive.) Quikrete High Strength Anchoring Epoxy is available at box stores, which uses a standard caulk gun. Don't get the "fast setting" version unless you have it all pre-drilled and you are able to work super-fast at gluing & screwing. The slow-setting version gels in about 20 minutes, which gives you some breathing time, but let it go at least a few hours before hanging the joists on the ledger.

^^(foam not shown)^^

With the R5-ish foam and foil facer between the wood and CMU the wood is sufficiently protected from damp masonry that it need not be pressure treated. Insulating from the ledger on down to the slab-foam can be done with 1.5" polyiso, and the interior facing side of the ledger/band joist can be insulated with R13-R15 batt overlapping the wall foam below. If the 2x4 framed wall above is insulated with R13-R15 fiber (unfaced or kraft faced) there is sufficient dew point control at the foam/fiber boundary to not accumulate wintertime moisture in the studwall from interior moisture drives even in zone 5, using just standard latex paint on wallboard as the interior side vapor barrier (no polyethylene wanted or needed, but it needs to be reasonably air tight to the interior.)

Most box stores carry 3/4" foil faced polyiso. If for supply reasons you have to do it with EPS or XPS it needs to be 1.5" thick in zone 5. The R5 labeling of 1" XPS notwithstanding, XPS loses performance over time as the HFCs diffuse out and would only be R4.2 @ 1" at full depletion), but in zone 4 any amount- even 1/2" will do, though 3/4" or 1" would be better. Many box stores carry 1" foil faced or plastic faced Type 1 EPS, which would be fine in this application.