Rigid Foam Location: Above or Below Floor Joists
The question below did not receive any responses. I am trying to rephrase here…
I don’t know how common it is to use rigid foam to break/mitigate thermal bridging in floors. In my case, I need to frame a new floor above an existing tiled (over concrete) floor, so as to make two rooms’ floors flush with each other. The room where the new floor will be framed is 23’x15′ approximately. I have 7″ from the existing tiles to the other room’s underfloor. Which of the following two approaches seems sounder to people?
A) Lay a layer of rigid foam (1.5″ 25 psi), and 2×6 joists over the foam (using joist hangers); maybe a 1″ of spray foam over the layer of rigid foam (to air seal); fill the empty cavity between the foam and the underfloor with fluffy insulation. (Incidentally, I just came across some 100 psi foam, which I could buy from someone: would the extra compression strength have any considerable additional advantage, or is 25 psi enough?)
B) Regular framing, with 2×6 joists sitting on 2×4 or 2×6 sleepers (still using joist hangers); 1.5″ foam between the sleepers; maybe 1″ spray foam over the rigid foam and sleepers (again, for air sealing); fluffy insulation in the cavity, but a layer of rigid foam (1″?) over the joists; then the underfloor screwed with longer screws.
A slight slope of 1″ in the existing floor might make B) a better option? Personally, A) strikes me as more solid. If so, I’ll just have to understand what is the best way to compensate for the slope, given that the first layer, against the existing tiled floor will be made of foam; I guess something I don’t quite know is how shimming could be done OVER RIGID FOAM: could a sleeper perpendicular to the joists be run under the joists, hence pressed between the joists and the foam? In general, maybe something I do not quite understand is how much the joists will be compressing on the foam, and how much, instead, the work of holding up the floor, with a 15′ span, will be done by the joist hangers?
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A sketch would be helpful; it sounds like you have an unusual situation and as they say, a picture is worth a thousand words.
25 psi foam on 1.5" joists is about 3x more than you need for normal loading, not including a "creep" factor of 3X. Including a creep factor, 25 psi foam is just right. If you're concerned you could use 40 psi foam. 100 psi is incredibly dense.
Floors are prone to developing squeaks; I would be cautious about adding a layer of foam between the subfloor and the framing.
Thanks, Michael!
Definitely, let's abandon the option where the foam is right under the subfloor.
Unfortunately, I don't have any 40 psi foam. The foams I have are:
1.5" 25 psi
2" 15 psi
2.5" 15 psi
The 1.5" thickness makes it easier to use 2x6 joists.
The problem arises with the shimming. The contractor seems to prefer to not rip every single joist. I am enclosing a drawing I just made, to better illustrate my primary question: How can I properly compensate for the slope in the existing floor? As the drawing indicates, at one end there are going to be 7" between the existing, tiled floor and the subfloor; at the opposite end (that is, some 14.5' feet away), there are going to be approximately 8".
A secondary question has to do with insulation and air sealing: Is having an inch of spray foam over the rigid foam an overshoot? The alternative, for me, would be to just have a couple of inches of spray foam along the floor's rim joists.
Thanks!
(By the way, I very much enjoyed the podcast on spray foam, which was recently posted, and in which you participated.)
I see, I've done similar assemblies several times when turning garages into living spaces. The typical approach is to rip each joist, and it's not a big deal--start with getting the joists on each end correct, stretch stringlines between them and measure for the remaining joists.
When there is enough height, which often isn't the case but it looks like you have, I like to run a continuous layer of foam above the concrete then rip joists to sit above the foam, then fill between the joists with fluffy insulation, as you have sketched.
An alternative method that would avoid ripping each joist would be to run joists of the smallest dimension you need and then shim under each one at multiple locations, but that's more work than just ripping the joists.
I'm glad you like the BS+Beer on spray foam!
Thanks, Michael!
I'll see if the contractor, once his team is actually on the job, realizes that ripping the joists is not too much work after all (and less work than shimming with blocks). Unfortunately, with a span of almost 15', I suspect that 2x6 joists is the minimum width I can go for. Otherwise (maybe along the lines of what Akos mentions below?), I wonder, I could have had 2x4 joists, with a couple of 2x4's (or 1x3's?) on the flat, perpendicular to the joists, as shims, to give more support; provided, of course, that there be joist hangers as well. Yet, I don't think 2x4's are acceptable for a 15' span, are they?
Speaking of spray foam, given that I will have to have the unvented roof spray foamed, is it worth adding a 1" layer on top of the floor's rigid foam, for air sealing, or is that a waste, and spray foaming just the rim joists will be enough in your view?
What you can also do is lay down the rigid and tapcon on top 2x4 on flat sleepers to spread the load.
You can rip your floor joist and toe nail it to the these sleepers. Since the joist are resting directly on the ground, there is no need for any joist hangers.
When doing these types of insulation projects one thing to make sure you don't have any water issues. If you can see any dark spots after heavy rain or snow melt you need to deal with the water outside the structure first.
Thank you very much, Akos!
When you say "tapcon," are you referring to how the sleepers will be fastened to the existing, tiled floor? That's something I've been wondering about: it seems that sleepers above the rigid foam would have to be secured somehow, but that securing them to a tiled floor, through the 1.5" rigid foam, would very tough. Or would you trust them to just be squeezed between the joists and the rigid foam, and maybe toenailed them here and there?
I will definitely watch for possible water issues––thanks for the recommendation!
Depending on the tile, drilling trough the tile won't be fun. Get a good rotary hammer drill and spare bits, make sure to keep them nice and cool. If you can mark it out so you can hit grout lines it will make the job much easier. I would try a couple of holes first before committing, you might have to pull up the tile if it is hard to drill through.
Securing to concrete through foam + sleeper is not too bad once you get through the tile. I have only done this directly into concrete and was no problem.
This way the rigid is continuous under all the framing which is the best to keep moisture out of the framed assembly.
Thanks, Akos!
I wonder whether securing the rigid foam with screws will really be needed in my situation. Let me explain: I absolutely want to have a continuous layer of rigid foam under all and every part of the joists. Yet, I was hoping that such a layer could be laid down against the tiles after the rim joists have been installed. In other words, I was hoping that the four rim joists could be considered as forming a box of sorts, within which to slide the panels of rigid foam (maybe with a bit of adhesive on them, to further secure them to the tiles?), tape the foam panels at the seams (and maybe at the rim joists, though, there, I'd like to have a layer of spray foam anyway, after the joists have been installed); and then install the joists, ripped or with under them some boards as shims, over the rigid foam; then maybe apply a 1" layer of spray foam over the entire floor (though I suspect that having spray foam just at the rim joists will be sufficient). Would such an assembly be sound you think?
The fact is that I am seriously concerned that my contractor might think I am asking him to unreasonably add to the agreed-upon labor (which was just that required to frame a floor over the existing floor). Of course, I could step in myself, and work on that drilling and securing the rigid foam with screws myself, while the builders work on something else; yet, I fear I might end up being on their way. All things considered, if the foam could be placed under the joists (but not under the rim joists) with no screws and no drilling, that would be greatly preferable. What do you think?
The screws are not for securing the foam. They are there to hold the 2x4s on flat down. They indirectly secure the foam as well but that is not really needed as gravity will keep it in place anyways.
If you are installing 2x4s on flat strapping over the foam, the rim band is not really needed as the floor joist now bear directly on the strapping. I would add some construction adhesive between the strapping and the joist just in case to avoid squeaks.
I would install the rigid and strapping and have your contractor do the joists, batts and subfloor. They can still install a rim joist around the perimeter that now sits on the strapping bellow.
@Akos, Thank you! That makes perfect sense now (and, frankly, I feel I should I have understood it the first time, that the screws were for the strapping).
What you say about the band joist is very interesting. I did not even know that floor could be framed without a band joist. That is, I thought ALL framed floors would always have rectangle––made by the rim joists––surrounding it.
In any event, I keep thinking that ripping all the joists would be easier than adding the straps, to compensate for the slope. I think that whole floor will have 18-19 joists in total; those are not that many boards to rip.
A follow-up question, if I may: how important is to try to mitigate thermal bridging at the perimeter of the floor? Is that why you're suggesting that the rigid foam be under the rim joists? Should I insist, with the contractor, on that, or let him rest the rim joists on the tiled floor if he prefers?