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Insulation for thin concrete overpour

rossn
rossn Member Posts: 76
Uninsulated slab, 4' below grade, is getting vapor barrier + rigid foam insulation + 1/2" pex + 1-1/4+" concrete pour.

Is 25psi 1" EPS (R4.4) or XPS (R5) sufficient for load? I can get higher PSI, but the local foam rep says costs jumps up a fair bit on the 40 and 60 psi.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,168
    Not really quite enough information. What sort of loads are going to be put on the overpour? Will there be any reinforcement (steel or glass fiber) in the concrete?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    rossn
  • rossn
    rossn Member Posts: 76
    Thanks. This is residential basement... no higher than average loads anticipated. One concrete guy I spoke with did mention using 4000-4500 psi concrete with 1/4" pea gravel 30% aggregate, 70% sand, 7 or 8 sac cement, and 1.5-2lb glass fiber per yd. I understand for this application the glass fiber would be good.
  • hot_rod
    hot_rod Member Posts: 22,022
    Good advice from the concrete guy.

    25 psi foam is plenty for a basement slab, even a residential garage slab.

    You want 1-1/2 times the aggregate size over the tube, so the pea gravel works well. At least 2 lbs glass fibers per yard on thin pours. I've been told 3 lbs, you only get one chance.
    There are other additives based on the conditions when you pour and if it will be a pumped mix.

    Hopefully the concrete overpour is not the final finish? The glass fibers will show unless you power trowel and burn them smooth :) And you may still get some "road map" shrinkage cracking
    Be sure the tube is fastened well, you don't want floaters in a thin pour.
    .
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rossn
  • Jamie Hall
    Jamie Hall Member Posts: 23,168
    Your concrete guy is OK. The combination of higher strength concrete properly mixed, and the glass fibers will give you adequate bearing strength for most residential loads (which can be remarkably high, by the way). You should be good for a point load -- like the foot of an most appliances or furniture -- up to about 200 pounds (that's per foot -- so an 800 pound total weight on four feet). Heavier objects (such as a piano!) should have the feet placed on a board to spread the load.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rossn
    rossn Member Posts: 76
    edited May 2023
    Ok, thanks. Sounds like 25 PSI is OK on the foam... I have heard I can expect some fracturing, and wasn't sure of foam strength in that case - so thanks.

    Yes, would be pumped. Curious about the other additives that would apply in that case.

    Being below grade, finished floor would be laying vinyl plank directly on the concrete. Final flooring won't be down for a while... are those exposed fibers an inhalation hazard? I would assume they'll break off with normal traffic.

    1" of foam (R4.4-5) generally sufficient for zone 6A? My calcs show < 1% heat loss difference going from R5 to R6. Hopefully that is right.
  • Jamie Hall
    Jamie Hall Member Posts: 23,168
    The glass fibers aren't usually considered to be a hazard, but as @hot_rod suggested, if you power trowel the floor they're not a problem at all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    rossn
  • rossn
    rossn Member Posts: 76
    Ok, thanks Jamie - I'll just put that as a requirement.

    I have to say, this is such an amazing, positive forum due to the people. I really appreciate you all chiming in!
  • rossn
    rossn Member Posts: 76
    I should have asked before.... do you all use foam tape or something similar around the perimeter, when there is only framing present (on exterior walls... not running to the stem wall)? I do have one room that will get some sound treatment, and will have to untangle how that works with the pour... I'm thinking it is probably putting some of that roll foam in place, then backfilling the top of that joint with some acoustic sealant.
  • hot_rod
    hot_rod Member Posts: 22,022
    The surface will look like dog hair on the floor that doesn't vacuum up :) No harm, just not a pretty finished surface. Sealers are not a bad idea as they hide/ seal down some of the hairs and can help the cure, especially on hot days, or hot and windy days.
    Many finishers spray those seal coats right after they finish the final trowel. The latex ones are not so stinky. It kind of looks like liquid Elmers glue. This prevents "chalking" also if it will be a while before you cover it. Easier to clean also when it is sealed.

    All sorts of stuff you can add to the mix, plasticizers, hot water, air, additives to slow the cure, or speed up the cure, fly ash, color, etc, etc. It sounds like you "guy" is knowledgable and the batch plant operators usually know their mix options for special applications.

    If the slab is to be covered I spend time on getting it as flat as possible, the finish look is not so critical. If you plan on gluing down, maybe a light broom finish would help. Again, check with your pro.

    When concrete is poured within 4 walls like a basement, "external restraint cracking" is possible. Basically as the slab is heated and tries to expand, it has no room to move and you get hairline cracks. Expansion edging helps, but its not so pretty. For a slab that will be covered I would not sweat these details. You basically don't want cracks and shifting like you see in sidewalks :)

    Concrete slab movement most often has to do with the subgrade prep and compaction, or lack of. If your current slab is stable, not cracking and shifting, you should be fine. The foam acts as a slip sheet, so cracking below will not telegraph through the over pour.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rossn
  • rossn
    rossn Member Posts: 76
    edited May 2023
    Thanks, Hot Rod!

    All very good information. This guy hasn't done this before, but did talk with the plant to get clarity. He did mention putting in some very shallow joints (over the top of the tubing) every 6-8', which seemed like a lot to me. the foundation I would call a B+ for cracking and movement. Some hairline cracks, but that is the extent of it. Pretty decent for 50+ years of age.

    In this case, flooring will be floating, so the smoother the better, but agreed looks don't add value.
  • hot_rod
    hot_rod Member Posts: 22,022
    Joints either sawed or troweled in really don't prevent cracking, they are intended to stop a crack from traveling across the entire slab.
    I would definitely not saw cut into a thin slab. I wonder that a trowel in expansion joint may encourage a crack, and make the final floor covering application more of a problem, with all those gaps to cross? Joints are usually on a 10' grid, what is the size of this room?

    If all this makes you nervous, a gypcrete pour would be another option. It is self leveling, doesn't crack, easy to add floor covering. It is not structural, basically sand, gypsum, portland and water. It's the stone in concrete that adds strength in a concrete mix.

    Gyp pours can really float tubing however, I'd fasten every 12" on thin gyp pours.

    Sounds like this overpour is more for a radiator than a custom finish floor? Keep the tube down, get it as flat as possible, be happy.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rossn
  • rossn
    rossn Member Posts: 76
    edited May 2023
    Yes, he was referring only to a short troweled in joint. I agree that the expansion joint may be a less known entity in this application and maybe not the best. I did look into gyp Crete, but after learning more about it here and elsewhere, don't feel it is the right product for my application and being below grade... just a lot of risks with moisture and stress loading.

    It's the lower level of the home, so 3BR, 2BA, great room, and 2 halls Most of that will be a thicker pour around 1-1/2 or 1-3/4". About 1/4 of it will be at the thinner 1-1/4" pour.

    Yes, the pour is strictly for insulating and heating purposes... not decorative. 12" fastening sounds good by me.

    Thank you.
  • Jamie Hall
    Jamie Hall Member Posts: 23,168
    My experience -- somewhat limited -- has been that if the glass fiber is well distributed in the mix, cutting expansion joints (they really are contraction joints, but never mind) is not needed. The cement in the concrete mix will microcrack -- that's inevitable -- but the fibers keep the cracks from ever getting to the visible point. You do, however, need some sort of expansion provision at the edges of a slab if, as in this case, it is thin and not integral with the surrounding concrete. Doesn't need much, and a thin strip of foam or somesuch will work if you don't need waterproofing.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    rossn
  • hot_rod
    hot_rod Member Posts: 22,022
    Here is how I detail an expansion joint, typically between a garage slab and concrete drive way.

    The "H" strip covers the foam edge. When concrete sets, remove the top portion, fill the 1/2" groove with a butyl caulk or polyurethane. In your basement you may not need to fill the gap, just floor over it.

    This plastic cap would be easier to float and trowel over compared to a raw foam edge. Glue it to the wall with foam adhesive. Shoot it in with a laser level and you have a nice screed board :)

    This would be used around the entire perimeter for the slab movement.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rossn
  • rossn
    rossn Member Posts: 76
    Thank you for sharing