Converting a Garage into a Cabin- need advice on floor joists
Hi Group,
– I have a place Up North, that has a 28’x46′ garage that was partially converted to a cabin by the previous owner. It’s enclosed, 4″ slab, even has a line to the septic tank and water line.
– This portion of the building has a row of cinder blocks cemented to the slabs, and 2×4 outer shell construction (Please see attached sketch).
– I am looking for input on what would be the best way to run the floor joists.
– I want the plywood sub-flooring to rest on the Mud Sill level to give a full 8′ ceiling.
– There is roughly 9.75 inches from floor to top of Mud Sill to work with installing floor joists.
– How to best put the floor joists in? What size Floor Joists? (2×8? 2×10? could technically do 2×6 as all load will be transferred to floor…..)
– All of the load will be transferred to the 4″ thick slab concrete floor.
– Another idea (not in the sketch – 1/2″ treated plywood down, with 2×10’s as floor joists would bring me up close to the mud sill… shim to level..
– I’m a noob. So please be gentle -but appreciate any good ideas.
Thanks,
Joe
PS-please see sketch which is drawn to scale
Replies
if you heart is set on that level,
how about 2 inches of XPS, 2x4 sleeper (not treated) 2x6 at right angles with 3/4 inch stock or 3.4 inch plywood between as small shims.
I'd run rim joists all the way around, at the height you want, then hang the joists off that with joist hangers. Once you have the rim joists up and level, everything after that will also be level. I'd probably add blocking mid span, between the joists and slab, to minimize any deflection. You also should consider some kind of venting for that enclose area,
Shep - Great ideas.
- I know a good idea - when I steal it.
- Need to figure out the HVAC on this.. but this is getting close to a "Do-Able" and affordable design.
- Thinking 6 mil vapor barrier, then figure out the HVAC (Big Issue), Propane, Plumbing/PEX, insulation side of it...
Thanks again for the input!
Joe
XPS as in eXpanded Poly Styrene Insulation ?
I would think that it would be fairly compressible, don't you?
But it would get me dimensionally at the 9.75" (with 3/4" shim)
2x6's are entirely possible if I transfer the load to the floor at regular intervals, and the spans are short (which they are).
Although- I like the idea of additional insulation (I am considering shooting expanding foam in the entire cavity ( I know - expensive ) or at least form a 3' insulation around the perimeter...
https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/EIS/10015703/10015703-FOAMULAR-SI-and-I-P-Units-for-Selected-Properties-Tech.-Bulletin.pdf
Depends on the type.
250 has a compressive strength of 25 PSI, laying a 2x4 sleeper would be good for 1,000 pounds per foot of length. (you would want to derate to be conservative)
You can also get the high strength stuff. A little compression or give will not be a huge issue for most things. Are you going to build internal walls on this floor, or install a hot tub or large format stone or tile floors?
I would guess if you put the 2x6 on 16 inch centers, and cross-laid sleepers on 16 inch centers, you would be fine. You could put the sleepers every foot if you were worried.
XPS would be a fine choice for inside the perimeter blocks. What kind of insulation are in the walls and attic?
This is good advice. I built my entire garage on 250 XPS - It's compressive strength is higher than the bearing capacity the IRC assumes for untested soils.
I am just finishing up a similar project, and one thing to consider is vapor transmission from the floor.
The way I went about this is 2x10 treated joists which I did have to “shim” with a treated block nailed to the concrete floor and then attached the joists to that. Then sprayed 5” of closed cell foam for vapor control and thermal layer. Topped it off with 1-1/8” plywood subfloor glued and screwed. This gave me an amazingly solid floor that is quiet and stiff.
Hi SilverLineHomes. I would love to see pictures of what you did- it sounds interesting.
- As this is going over a concrete slab... I thought I would put down 6 mil vapor barrier over the entire surface, and carefully tape the seams.
- I hope to shoot closed cell foam insulation in the cavity - after everything is in place... at least 3" over the entire floor // maybe fill it ((( but it is expensive ))) - so dunno yet.
- Definitely around the Perimeter/Rim - I would like to fill it from floor to top of floor joist with closed cell.. maybe in 3' if not the whole base (( I don't want to be 80 and having my feet freezing up there ))
- Right now - 2x10's are almost double the cost of 2x8's... if I transfer the load to the slab - I see no reason not to go with 2x6's (and far less costly) -- See Ver003 sketch attached.
- I think that the blocking every 16" would be more than needed, don't you?
- Not sure what the spacing on blocking should be.....
Thanks for the input. I appreciate the ideas.
Joe
Have you considered a floating floor? This is what I intend to do in my own garage workshop:
Use a good vapor barrier on the slab and then rest XPS rigid insulation on top. Then two layers of plywood staggering the seams and screw the two layers of plywood to each other. The plywood floats on the insulation.
The compressive strength of the rigid insulation is more than enough.
If you felt the need for sleepers you can go that route. Check out this article in Fine Wood working where the author uses rigid plus sleepers:
https://issuu.com/pornsakh/docs/fine-woodworking--216-annual-issue---tools---shops
Hi SawDust Steve, and Thank You for your input.
- That is not a bad idea either - but I do intend on building interior walls - and making the cabin a 2 bedroom, 2 bath cabin. 1300 sq ft.
- Therefore, there would be interior walls, plumbing, HVAC, etc. .. so a little different in scope than an all-open workshop (although I like the idea of full insulation coverage on the floor of the work shop.....)
Thanks again,
Joe
Version 4 - 9.5 inch TJI Engineered I-Beam Proposal.
- This group certainly has a lot of experience and good ideas!
- After going through a few cost studies, I am now leaning toward 9.5 inch TJI Engineered I-beams forthe joists.
- 6 mil vapor barrier over the concrete (carefully overlapped and taped).
- 2x10 Treated Rim Joist
- Steel hangers (easier than nailing into I-beam directly to rim joist)
- etc.
Please see attached sketch - and please feel free to suggest better methods.
- BTW - surprisingly to me - going with the engineered I-beams would be about 1/4 of the cost of doing the construction with 2x10's - stronger, and less prone to squeaks....
Thanks again, Group,
Joe
Attached are some of the photos I took through progress.
Most of the space I was able to get 9’ walls but a small section was 8’ where the house and garage meet. I utilized this area for a bathroom and used the space below to get plumbing to meet grade so I didn’t have to install a grinder pump. I was able to leave the ceiling up and not deal with the blown in insulation. I did have to fur down the ceilings though (very tedious). I will attach more photos as I am limited out. I ran a Lennox ducted mini Split unit for heat and A/C.
Nice job! I really like TJIs for floor framing- straight, uniform, light. They make framing a good, solid floor much easier.
Additional photos. I did my best effort to make it look as seamless as possible. Hard to tell there was a garage anymore. Wonderful project. Was 23x23 garage now it is 2 bunk rooms, single bedroom and a full bath (sleeps 10 conservatively comfortably).
I wouldn't use spray foam here. A much more cost effective method would be to use rigid foam cut to fit. A much much more cost effective method would be to use used rigid foam. There are recyclers who sell the foam that is taken off commercial roofs. You can find them on the internets.
"But what about the air sealing of spray foam?!?!" Not much of a benefit here, and you can always seal the ends where any air infiltration could possibly maybe but probably won't happen.
NICE WORK!!!! Thank You for uploading the pictures. That really helps clarify things!
That looks like a wonderful way to do it. I see that you used a thin layer of spray foam as a vapor barrier.
Happy to hear it was helpful!
I did 5 inches, 2” for the vapor but I wanted the floors warm and I got a really good deal from my contractor. For me It was more cost effect this way. Just at material cost it was cheaper and then throw in my labor to tediously cut and install all of that rigid in between.
I had originally wanted to spray foam the entire cavity as you did. That stuff is not cheap. I may lay down some XPS between floor joists and seal the whole thing with a thin layer of spray foam. Spray foam also gives mechanical rigidity, too.
I spoke with my County Inspector. He said that they want R 30 horizontally from the cinder block in to the structure 4'.
- He stated that he felt very confident that it would keep the floor warm - even in to the middle of the room.
- I'm getting closer to having solved this puzzle.. I will post updated sketches over the weekend.
- I will check into spray foam (that was my original plan, but spray foam is expensive here) again......
- Thank You - group - you are all wonderful to work with and have great input and I especially like the pictures of the work in progress.
i once lost a few sheets of XPS insulation off of my truck at a stoplight in a busy part of the city. I didn't get back to retrieve it for about fifteen minutes, and it was still in the street where traffic was driving over it. I managed to pull it out of the way and found it virtually undamaged. There were a few tire tracks , but none so deep or consistent enough to match to any tire. That was my lesson on the practical compression strength of XPS. Since that experience I now and then use it as a very lightweight load support, and I have parked on it. It would easily support a floor for a living space, and even a garage if you had a couple sheets of plywood over it.