Going to tile but first need floor insulated - best product

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J Blow

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In finishing a 1300 sq ft basement I decided to insulate the concrete floor with 1" EPS and then a layer of 5/8" OSB on top. Almost everywhere will have carpet or laminate so OSB seems ok there. The bathroom will be tiled - floor and walls. I'd like the floor to be insulated in the bathroom as well but wondered if there was something that was recommended in particular. I have never tiled before but I suspect you can't tile over OSB on top of EPS, correct? Could I add a backer board to the EPS/OSB and have a it work out or is there an easier way? Seems I saw a product that was basically XPS with backer board on it but didn't have success locating it in any of the stores I went to. I already have 1" XPS on all the concrete walls and will frame out with fiberglass insulation if that matters at all.

Thanks in advance for any help offered!
 

Dana

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OSB subfloor is a fine substrate for ceramic tile, it's done all the time. The EPS under the OSB makes it even less springy than if you had installed the subfloor on joists spaced 16" o.c. since it is supported everywhere by the foam.

Your foundation wall insulation plan may need some adjustment if you are in a climate colder than US climate zone 5. The R5 may not be sufficient exterior-side R for dew point control with R13-R15 batts on the above grade section, which would result in some amount of chronic wintertime condensation at the foam/fiber boundary, with some potential of getting mold going there. The safer bet would be to put at least the IRC chapter 7 recommended R for wood-sheathed above ground walls for your climate zone, and go with only unfaced batts in the studwall (rock-wool or high density fiberglass preferred, so R15 in a 2x4 wall), and NO INTERIOR SIDE VAPOR BARRIERS any tighter than standard latex paint on wallboard.

That way the ground moisture that makes it through the wall can still dry toward the interior rather than accumulate in the studs & insulation, while keeping moisture from the interior from accumulating in the cavity during the cold winter months.

climate_zones.jpg


If it turns out you are in zone 6 it's worth adding 1" EPS inside the XPS bringing the foam-R to about R9, which would give you plenty of margin for zone 6 (and probably OK even in zone 7 if you back off to R13 on the batts). Tape the seams of the XPS with housewrap tape (or better yet, seal them with duct mastic) and use dabs of foam-board construction adhesive to put up the EPS, staggering it's seams with those of the XPS.

With R9 foam and 2x4 6" o.c. and R15 batts that would bring the "whole wall" R to about R21-R22 after factoring in the thermal bridging, which is about right for zone 6. (See table 2, p10 of this document.)
 
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J Blow

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Great information. Since I've posted this I've found tons of people saying that the tile on OSB over foam won't work but your explanation of why it will makes much more sense to me.

Adding an extra inch of foam to the concrete walls makes sense, too. The basement is a walkout with one sidewall being about half above grade. They had already insulated with unfaced batts in the 2x4s above the grade. For some reason they have both a thin (clear) and thick (black) vapor barrier over the insulation. I put 3 inches of foam in front of that to bring it out to the concrete with foam level. I'll add in more unfaced batts in that wall when built. That will give that wall either concrete and 2 inches of foam plus unfaced batts below grade and unfaced batts (with 2 vapor barriers), 3 inches foam, unfaced batts in new wall above grade.

Here's where I'm wondering about dealing with the wall that's entirely above grade. They also insulated the 2x4 frame with unfaced batts and did the double plastic thing there, too - again not sure why but I don't see it matters much. What I'm not sure of is how much more insulation is necessary. It would be really easy to sheetrock that existing structure but probably isn't enough insulation I'm guessing. I suppose 2 inches of foam and then a studwall with batts is the right answer....or is that overkill? Seems there could be a way to not do another entire stud wall since one already exists but that presents a couple problems - amount of insulation and hanging drywall if I just add the foam.
 

Dana

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What's the total material stackup from the exterior paint to the interior paint on the part that is entirely above grade?
 

J Blow

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I believe it's exactly like an exterior wall upstairs...except unfinished. Hardboard siding, a house wrap, osb or similar appropriate wall material, studded wall with what I believe is r-13 or so in it. The only covering on that unfaced insulation is two layers of plastic.
 

Dana

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I believe it's exactly like an exterior wall upstairs...except unfinished. Hardboard siding, a house wrap, osb or similar appropriate wall material, studded wall with what I believe is r-13 or so in it. The only covering on that unfaced insulation is two layers of plastic.


So from outside to in you have:

siding | Tyvek | OSB | R13 fiberglass | double polyethylene | sheet rock (or air, in the basement)

Are you in fact located in climate zone 6? (I'm only guessing due to the "Mount Rushmore State" avatar pic.)

In climate zone 6 with R13 outside the polyethylene vapor barrier you can install up to R23 as fiber insulation on the interior side of the vapor barrier without condensation issues. Instead of 3" of foam board to bring it flush with the concrete you could install R23 rock wool or R21 fiberglass batts butted together side by side, overhanging the concrete by ~2". Install 2" EPS (R8) up against the concrete, and hang a sheet of housewrap over the rock wool as a vapor-permeable air barrier, sealing at the top & edges with caulk/housewrap tape/mastic/whatever and to the foam with duct mastic. When you put up the studwall it will compress the batts a bit, since it's hanging a half-inch or so proud of the foam board, but that's fine. Then insulate the studwall only up to where it overlaps the sandwiched batts by about 3-6". Don't cheap out and use low-density R19s in there- they are too floppy and highly air-permeable. R23 rock wool or R21 fiberglass won't sag, and offer far superior resistance to any infiltration or convection air flow.

For half-filled stud bays it's sometimes important to have an air-barrier on the top edge of the batt to avoid loss of performance due to convection, but if you use R15s the density and air retardency is high enough that it's acceptable to skip it. If you use R13s or R11s definitely put horizontal blocking at the top of the half-batt.

If you go back to plan-A and shim it out flush with the concrete with foam, you only need to go ~6" above the top of the concrete with the 2" foam you would still need between the studwall and concrete, and only batt the studwall part way. That's makes for a fairly expensive wall though, since it's 3-4x the amount of foam. Be sure to seal the seams & edges of any rigid foam well.
 

J Blow

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It looks like I will have the expensive wall solution as I have a lot of that done. I'm a little confused what walls you are speaking of unless you aren't addressing the wall completely above grade at all. On the walls that are 100% above grade (fully studded exterior wall) could I leave it at just r13 if I make the concrete just r5? If the concrete has the r5, the r8 foam, and then the unfaced r13 in the walls, what should I do with the above grade wall assuming it needs more than just the wrap, r13, and drywall. If I would just drywall and leave it like that, does that mean I need to make the concrete less insulated? I'm gathering a problem comes in with a difference between the 2.

Funny thing about this is that I'm sure some people just throw up some studs against the concrete and add some drywall and call it good....which I figured is all I would need to do (maybe some fiberglass) before all this research. Ugh. I'm glad it's getting done correctly...but dang...
 

Jadnashua

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The hassle with any slab floor is that it often isn't as flat as you think it is! But, there are panels that are designed to be bonded to a slab that are foam - Wedi is one, and comes in 1" (and thicker) versions. You thinset it down, then treat the seams then you can tile it. While not officially specified for use on a floor, Schluter's Kerdiboard would also probably work (call them first for confirmation). It also is attached like a big tile in this application. That gives you both the insulation and the tile compatibility. The thinset underneath it ensures 100% support (tile hates movement, and particularly vertical movement). I know Kerdiboard is available up to 2" thickness...I'm not certain on Wedi. Neither is particularly inexpensive, but at least the Wedi is designed specifically for this purpose, and you'd have their warranty. Since it is in the bathroom only, the cost shouldn't be horrible.
 

Dana

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It looks like I will have the expensive wall solution as I have a lot of that done. I'm a little confused what walls you are speaking of unless you aren't addressing the wall completely above grade at all. On the walls that are 100% above grade (fully studded exterior wall) could I leave it at just r13 if I make the concrete just r5? If the concrete has the r5, the r8 foam, and then the unfaced r13 in the walls, what should I do with the above grade wall assuming it needs more than just the wrap, r13, and drywall. If I would just drywall and leave it like that, does that mean I need to make the concrete less insulated? I'm gathering a problem comes in with a difference between the 2.

Funny thing about this is that I'm sure some people just throw up some studs against the concrete and add some drywall and call it good....which I figured is all I would need to do (maybe some fiberglass) before all this research. Ugh. I'm glad it's getting done correctly...but dang...

Where is the R5 in your stackup for the above grade concrete? If the existing concrete already has R5 XPS applied you only need to add R4 EPS to it for dew point control. (Somehow I thought we were talking about a section of wall that still had no foam.) That would make the 2" of foam nominally R9 instead of R8, which is fine.

An R13 studwall with no foam performs at about R9.5-R10 due to the thermal bridging of the studs, which is way below cost-effective in a US climate zone 6 climate. IRC code min for zone 6 is R20+ 5 (R20 cavity fill in 2x6 studs + R5 continuous insulation, such as foam to thermally break the studs) or R13 +10 (R13 2x4 + R10 continuous insulation.) With the thermal break over the studs those solutions come in at about R20 "whole-wall" after factoring in the thermal bridging. What I was recommending would end up better than code for the portion of the wall with the existing studs, and nearly-code for above-grade concrete part.

Existing studwall stackup after build out:

siding | Tyvek | OSB | R13 fiberglass | double polyethylene | R20-ish batt | Tyvek | empty studs | sheet-rock

The 5" of R20-ish continuous batt + Tyvek would be cheaper than 3" of rigid foam to shim it out to make it co-planer with the concrete, and would out-perform the 3" foam to boot. With R20-ish batt that section would perform at about R30 after factoring in the thermal bridging. R30 is not an insane whole wall value for a zone 6 climate. See Table 2, p10, but read the whole first chapter. If you are heating with propane, heating oil, or electric resitance heating, this is TOTALLY cost-effective. If natural gas it's a fairly long payout, but since it's cheaper than the foam shim-out solution what the heck? You could stuff R13 batts in there to shim it out and go floor to ceiling with your R8 foam behind the new studs, but that's probably going to be more expensive (more foam) than doing thicker batts + Tyvek there, for about the same whole-wall performance.

Existing above-grade concrete after build out would have a stackup of :

concrete | R8 or R9 foam | R15 batt | sheet-rock

That is essentially an R15+ 8 or 9 stackup in lieu of an R13 +R10, which would perform about the same as R13+ 10 code-min- slightly less if you used R13s, but not enough to matter given that the studwall above it is at better than code.

R5 is not enough to protect against moisture accumulation in the studwall over the winter in your climate- R7.5 would, but R8 EPS is cheaper than R7.5 XPS, and unlike XPS it won't lose performance over time. (The higher R/inch from XPS is due to the HFC134a blowing agent, which bleeds out over a few decades, after which the material's thermal performance is identical to EPS of equal density & thickness.)
 

J Blow

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I really appreciate all the time you are taking here - great information. By the way, yes zone 6 and yes I already have r-5 XPS up on the concrete. Almost all the concrete is below grade except about 5-10 or so feet above grade where the sidewall meets the walkout wall.

Due to my basement stub outs locations, I am really trying to avoid adding anything besides 2x4 wall. That area is 100% below grade. The recommendation is adding more foam for dew point control but that would move the framed wall out even further. Can I frame the wall and insulate with the foam between the studs or does that defeat the entire purpose of dew point control if the studs are really only protected to r-5 where they are touching the current XPS?

This has been invaluable information and the links are great. Thanks, again.
 

Dana

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If you cut'n'cobble rigid foam between the studs it reduces the amount of water that accumulates in the wall cavity since the temp at the foam fiber interface is now higher, but but lowers the temperature at the stud edges a bit. Using a 2-D model (not super-accurate for estimating, but for illustration purposes), the 3.5" of stud has an R-value of about R4.2, so with R5 of foam between the stud edge and wall it should be OK, since most of the R is on the outside of the foam/wood boundary. In a 3-D reality it's a bit different from that, but not enough to matter.

But since the studwall is not structural there is no problem with turning the stud sideways or using 2x3 studs. A 2x3 stud will still allow you to make the additional inch of foam continuous, not cut up by the studs. If you compress an R13 batt into the 2x3 stud cavity it will perform at about R10:

60610d1354245933-compressed-insulation-r-values-compressed-fiberglass.jpg


That would mean you have R9 foam out of a center-cavity R of R19, a 47% foam-R/total-R ratio which has HUGE dew point margin for zone 6. It would be less work to install too. The "whole wall" R after thermal bridging would come in around R17-ish, which isn't quite up to code-min, but it's not terrible if most of that part of the wall is below grade.

The biggest problem with 2x3 framing is the studs are more apt to be bowed or twisted than 2x4 studs (which can detract a bit from the flatness of the finished wall (unless you use finger-jointed 2x3s, which are truer and stronger than typical milled lumber). If you buy 20-30% more studs than you actually need (and buy them bundled & banded from a lumber yard, not out of a picked-over rack at the box store) you can chuck the real corkscrews & dog-legs of the bunch and usually have enough that are "straight enough". It's the same amount of board-feet as buying 2x4s in the exact quantity that you need, so the overage is not a huge cost-adder.
 
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