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Concrete or Gypcrete?

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  • Concrete or Gypcrete?

    I would like to know if anyone has any comments on the advantages/disadvantages of concrete vs gypcrete for to encase radiant hydronic 'pex' tubes in a floor. I have no allegiance to either, but am just trying to figure out the best (and hopefully most cost efficient method) to build hydronic floors. What I have found so far is the following:

    Weight - Gypcrete weighs less than concrete, but if you design for it, that is not a problem. I'd be using Schluter's Bekotec anyway, which reduces the volume of concrete required per square foot of area drastically.

    Sealing - Generally, you should pour the screed (gypcrete or concrete) after you put up the drywall, to provide a good seal between the floor and the wall. They claim that gypcrete seals better than concrete but I don't think this is a problem if you use a concrete with better flow characteristics than normal concrete. But, generally, I would say that gypcrete would naturally seal and flow under the drywall with minimal encouraging.

    Strength - You can achieve much higher compressive strength with concrete mixes than gypcrete. Gypcrete, under ideal conditions, barely meets the recommended strength that the engineered wood floor guys require (3,000 psi or so) for glue down (but I hear this recommendation is being revised downward to 2,500 psi).

    Finishing - Trowelling is required with concrete and it is claimed not with gypcrete. But with many of the jobs I have seen, gycprete required patching to make it even. It simply does not pour entirely even, especially at the edges, and this requires patching later, which is not cheap. And, as for quality, I have yet to see a gypcrete floor ever have as flat and nice a surface as a trowelled concrete floor.

    Curing - Concrete takes longer to cure than gypcrete, but you can walk on it the next day. Gypcrete takes 90 minutes. I don't find this a big advantage. And I generally don't find that trades are standing around waiting for the gypcrete to cure (in 90 minutes) so that they can start work again. They just come back the next day anyway or you just pour at the end of the day. As for curing, concrete takes longer so you may have to wait a bit before laying down the floors, whether wood or tile.

    Sound properties & Fire rating - Gypcrete supposedly has better properties in these areas, but I am not using concrete for these attributes anyway.

    Cost - Concrete is about 1/2 the price of gypcrete (which means there is less of a savings if you are already using Bekotec which requires less screed) so this is an advantage. Also, there are few companies that supply gycrete (only 3 companies that supply and install gypcrete in Ontario, Canada) but there are hundreds that supply concrete.

    Thermal Properties - The 'gypcrete' guys (such as Maxxon, Hacker) claim that their products have better thermal properties. But Maxxon's comparative literature doesn't actually say that. An examination of the thermal coefficients for gypcrete and concrete shows that concrete has a higher coefficient and actually conducts heat better and holds heat better. The problem is, though, that if you turn the heat off the slab takes longer to cool down as concrete holds heat better than gypcrete. But this also should not be a problem unless you want to adjust the heat constantly. And, actually it can be an advantage as the better the slab holds heat the more efficient the system is.

    With Bekotec, you use less screed, so there is less mass to heat up. This makes the system heat up quicker.

    Shrinking - The gypcrete guys claim that concrete will shrink around the tubes because it cannot handle temperature fluctuations. This results in less efficient heat transfer from the tubes to the slab. Does anyone have any real-world information about this. I don't know how someone would know unless they did a lab test, because you have to know what is going on inside the slab. Also, I am wondering just what temperature fluctuation would have to happen for this to occur. Typically a slab, whether gypcrete or concrete, would not get any cooler than about 60F or hotter than 90F in a house anyway. Is this enough to cause the concrete to separate from the slab?

    Cracking - Yes concrete can crack a lot. But so can gypcrete. I just did a job where we've got cracks all over the place. The pour occurred 24 months ago and we have yet to cover the floor with wood or tile so we've been able to examine it over time. It's been through 2 winter cycles now and cracks appeared in the second cycle which were not there in the first.

    Also, since I now use Schlüter's product, Bekotec, on every job there is much less cracking than would occur in either gypcrete or concrete. In fact, there appears to be hardly any cracking at all.

    Fiber or shrinkage reduction additives can also be added to concrete to decrease the possibility of cracking.

    So I think it really comes down to getting the right concrete mix. And also knowing the answer to whether or not concrete shrinks around the tubes as it goes through heat and cool cycles.

    As for mix, in case you are wondering, I'd go with 1/2 minus crushed aggregate (crushed aggregate binds better than pea gravel) with double the fibre one would normally use, possibly a shrinkage reduction additive if you are worried about this, and a plasticizer additive could also be used to make the concrete more free-flowing.

    Anyhow, does anyone know the answer to this question about concrete separating from the tubes as it shrinks?

    Also, by the way, it seems like I am really promoting this Bekotec product. I have no vested interest in it, but it allows me to have a neater installation with all of the tubes exactly straight (as you just snap the pex in between the raised 'buttons'), it's easier for my own crews to use than having the hydronic contractor do staple down and I think it works much better than a thicker slab of concrete or gypcrete.

    Any comments?


  • #2
    Re: Concrete or Gypcrete?

    I just did a job where we've got cracks all over the place. The pour occurred 24 months ago and we have yet to cover the floor with wood or tile so we've been able to examine it over time. It's been through 2 winter cycles now and cracks appeared in the second cycle which were not there in the first.
    They haven't solved the cracking yet? I gave up on Gypcrete in the 70s because of all the cracking. I was involved in a lawsuit where the gypcrete was covered with vinyl asbestos tile, the owner removed the tile, fixalled the gypcrete and covered the gypcrete with carpet. The new owner pulled up the carpet for some reason, found the cracks, and sued the prior owner, and his contractor, for fraud, filling up the cracked gypcrete to hide the imperfections. He got a $300,000 second mortgage written off in the settlement.

    If you use it make the owner sign an informed consent letter that it will crack, and make him agree to disclose the informed consent letter if he ever sells the building.
    “President Obama doing well. His approval rating is at 51%.--- The other 49% are taxpayers.” Jay Leno


    • #3
      Re: Concrete or Gypcrete?

      I've been pondering the question of how best to do a radiant floor for several years. This is the best way to do it that I've found so far:

      That material gives you the best thermal conductivity (i.e., lowest water temperatures) and the least issues with installing various flooring materials (e.g., wood floors aren't much trouble).


      • #4
        Re: Concrete or Gypcrete?

        Well actually I am just trying to find the best method, rather than deal with consent letters. Bekotec's product appears to result in minimized cracking.


        • #5
          Re: Concrete or Gypcrete?

          What are you pouring the Gypcrete/concrete onto and what is the thickness of the pour?
          Andre T.


          • #6
            Re: Concrete or Gypcrete?

            I would be pouring it on to Schluter's Bekotec studded polystyrene screed panel. The thickness would vary because this is a panel that has 'buttons' or studs on it and the minimum screed thickness is 1-1/4" (32 mm) between the studs and 5/16" (8 mm) above the studs. See pics attached.
            Attached Files