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Old 02-28-2011, 01:01 PM   #1
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Insulating Split-Level Ground Floor Concrete Slab


Hello,

This is my first post to DIY Chatroom. I am looking forward to hearing some informed opinions on my questions.

I have a split-level 1979 raised bungalow home in cold Quebec that I am doing a lot of energy-efficiency retrofits to. One problem area was the ground level entry room that led to the stairs up and down, as well as the garage.

I gutted the area. There was no insulation in the walls between the living area and garage. I built up the 2x4's with 2x2 so that I could put R-19 fiberglass batt with a vapour barrier, and I changed out the door to the garage. Vapour barrier wrapped all electrical outlets and spray foamed and tuck taped.

The last problem area is the persistently cold floors. I ripped out the existing tiles (which came off without breaking with minimal effort, horrible installation job). Below were the original tiles which required much more effort. This was thin set on 1/4" plywood on top of 1/2" plywood. Despite the poor installation techniques, the tiles were in good shape. The floor was solid and did not creak or crack.

Now that it's all exposed I see that the floor is a concrete slab with 2x4 "joists" (I use that term loosely with 2x4s) spanning the width. The 2x4's are supported on either end by raised concrete and a 2x4 laid on the slabs. As you can see in the pictures there are 2x4's placed on the slabs directly that I presume were used for support and the shims used for leveling.

Now that my life story is out of the way, I am debating how to go about insulating the floor. There were never any moisture problems (at least during my ownership over the last 6 years).

My initial thought process is to lay down XPS foamboard, tuck taping the seams. Unfortunately I'll have to cut down 2x8 sheets to 1x8s to fit them under the "joists". I'll then spray foam the small header all around and call it a day.

However I have questions regarding what material to use. Should I lay the foamboard on top of a membrane like Delta-FL to allow for he slab to "breathe," or can I just lay the board right on the floor.

Another question, can I save a couple bucks by using expanded polystyrene instead of extruded? I know EPS isn't as good a vapour barrier as XPS. So from cheapest to most expensive, should I go EPS only? EPS with Delta-FL? XPS? or XPS with Delta-FL?


And since I'm posting, might as well pose my next question... best underlayment system for the tiles? OSB? ply? How thick? How many layers? Is Ditra (or similar) necessary for a 'dry' room (quite pricey here in Canada).


Thanks for your help!!!
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Old 02-28-2011, 01:03 PM   #2
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Insulating Split-Level Ground Floor Concrete Slab


Oh, and one last question... R-5? R-7.5? I believe I only have clearance for R-5 or R-7.5 (1" or 1.5" board)

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Old 03-01-2011, 10:33 AM   #3
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Insulating Split-Level Ground Floor Concrete Slab


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Old 03-01-2011, 11:06 AM   #4
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Insulating Split-Level Ground Floor Concrete Slab


Why not take out 3 of the 2x4's so you can insert full sheets of what ever thickness XPS will fit, and yes I would use XPS, and then fill the joist caity with cellulose insulation.

While you are at it you might what to replace your spacer/shims with pressure treated lumber, which it should have been to begin with.

Underlayment for tile, ceramic? if you are using ceramic, the you might also want to consider a heating grid, under the tile.

3/4" sub floor, cement board secured with screws and thin set, then tile.

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Old 03-02-2011, 12:52 AM   #5
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Insulating Split-Level Ground Floor Concrete Slab


XPS/EPS: it makes not a stink. They are very similar all the way around, and in fact many builders here are moving to EPS sub-grade because it works just fine and is cheaper. I used to say XPS, too, but that has been dis-proven to me by extensive research. If you can get (and want to pay for) higher density EPS, you can likely lay the 2x4 "joists" right on it instead of the "shimmers" on the concrete. Just be aware that the advertised "60 psi", or whatever, is for a 10% deformation, which is beyond the elastic limit of the material. Somewhere around 23 psi (approx 1/3 of 10% deformation rating), with the 60 psi stuff, you'll be at 1% deformation, well w/in the elastic limit. (That means it will spring back to shape after someone steps on it.) If you are set on putting tile back, that is all a moot point, as the flexing may be too much. However, your floor will be warmer. I'd also put visqueen and/or DryLoc on the concrete, in case there is none below it. Concrete wicks moisture. As for cellulose, I'd be a little concerned about water vapor or liquid water getting to it in an entry way. I'd stick w/ closed cell foam. Cellulose should have gone in your walls, and likely no vapor barrier, but an airtight system instead. It sounds like you air sealed well, so the poly will likely not be a problem. The FG is OK, but not the best insulation.
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Old 03-02-2011, 05:03 PM   #6
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Insulating Split-Level Ground Floor Concrete Slab


Thanks guys! Great feedback that is very much appreciated!

@jklingel - If I got with R-7.5 (1.5") then I'll still have clearance below the joists across the whole span. In order to distribute any weight any prevent any flex when shimming should I shim over spare pieces of cut out ply? I obviously can't have any flex whatsoever. This way the force of the shims is spread out over a much larger area.

I was checking into the compressive strength of the EPS and XPS that I have available at the big box stores here. The EPS they carry is called Isolofoam HD and the XPS is the DOW brand, either Cladmate or Wallmate or SM. The SM XPS from DOW has a minimum compressive strength of 30PSI while the EPS and Clad/Wallmates are closer to 15psi.

I'm thinking I will go with the XPS R-7.5 1.5" 15psi material... would you approve? I'll pick up some DryLoc or similar if they have any as well
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Old 03-02-2011, 05:04 PM   #7
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@ Jackofall1 - good idea about removing every other 2x4. I think I will do that when laying down the foam board. Being an entrance though I don't think I will opt for the in-floor radiant heating as this will just incur additional heating bills without much added comfort.
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Old 03-02-2011, 09:26 PM   #8
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First, I am not an engineer. If you feel comfortable with these rough calcs, cool. Assume that even a 300 lb person can't flex a 16" 2x4 on edge, when just walking on it. That gives you 16*1.5=24 sqr inches of bearing. 300 lb/ 24 = 12.5 psi. Your 30 psi XPS is 30 at what deformation? I don't know that, but the web site will have it. If you get foam with 1% deformation and 12.5 psi, you could support a 300 pound person, barely. The 300 lbs should be distributed to more than the one 2x4, and likely more than 16" of that one, so my GUESS is that that scenario would work. Plywood plates will give you even more room for error. Jam 'em in tight, and glue/toe nail/whatever so they won't wiggle out. Then tape and/or goo between the foam sheets to air seal, and at edges, and I bet you have a far warmer place. Good luck. j

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