Thin concrete slab

Harold

I had planned to use a 4" slab over a suspended floor with concrete being the finished surface. Based on the quality of the concrete work in the basement I am switching to a thin pour (1.5") with some carpet and some slate. I will probably go with concrete because of the cost of gypsum and the performance if it gets wet (e.g., kitchen, bath).

Almost everything I have been able to find places the concrete directly on the plywood subfloor (+ bond breaker) and puts any insulation undr the plywood, between the joists. A much cheaper installation would result from putting a sheet of foam between the subfloor and the concrete. It also has the major advantage of not having to finish the basement ceiling to fireproof the insulation to meet code.

Anyone have any experience with foam under thin slab?

Kevin_in_Denver

I can't think of any insulation with a high enough compressive strength to keep the 1.5" concrete from cracking under a large point load.

It sounds like you and I are at about the same point in construction of our very similar homes. I presume you've seen the website about another similar house, ourcoolhouse.com

Troy_3

Gypsum concrete

We have a local architect that specifies 3/4" polystyrene on the subfloor. We then staple out pex through the insulation and pour 1-1/2" gypsum concrete over the top. First job goes back 5 years. No problems yet. I personally would never pour portland concrete over a wood subfloor. Too much flex. The floor will crack all up over time. IMHO! But then I pour gypsum concrete. If you do this you better get oneof those exotic mixes. Don't use standard out of the truck sidewalk mix.

Kevin_in_Denver

concrete on wood

Tile guys do it all the time in bathrooms, no?

Troy_3

tile men

have installed cementitious tile underlayments for years. This is not your standard concrete mix. Here is a little history as I see it. Correct my misconceptions please. Few wood frame structures are designed to support the load of a concrete floor.That is why the Hotel/motel industry used 2" lightweight concrete pours for their wood frame structures. They installed 2" thick to give the floor some strength. Most of this industry has switched over to 3/4" gypsum concrete. Gypsum concrete will flex much more with much less cracking. When we install heating tube in gypsum concrete we add an additional 3/4" over the od of the tube to maintain the min. thickness. Adding heat to concrete creates a lot of stresses. There are an endless number of concrete mixes and additives. All I'm saying is - check your load design for your structure, check to make sure this mix and depth has been tested and approved for wood frame application. And that it will hold up to thermal stresses. Also make sure whom ever is designing your mix and min. depth understand that you are installing 5/8" tube or what ever size tube. It does have an impact on min. depth. all I'm saying is if you want to be a pioneer you must understand the liability.

Harold

Actually it is not all that hard to get strong foam. Some of the normal pink sheets have compression about the same as the assumed minimum of soil if an actual soil analysis was not done. This is used all the time under slab-on-grade. I also bought some high strength stuff and actually put it over the footers. It is used under things like bridges and runways. A point load on concrete will also, I beleive, be distributed by the concrete itself which lowers the actual point load effect.

I was thinking urethane (to get the R value in the thinnest layer) for the thin pour. As I mentioned in the oroginal post, I do not want to have to finish all of the basement ceiling. If I insulate the floor, as required for radiant, from below I would have to drywall the ceiling for fire code compliance. Putting the foam (about 3") on top of the floor and under the pour fixes that.

The floor is actually engineered for the 4" of concrete that was originally planned.

I do not view Gypcrete as a viable alternative. It probably provides the most level/flat results, but it is also susceptible to water damage and quite expensive. It is also relatively soft and I have seen complaints about long term surface damage where it is not really protected (e.g.,carpet).

I am looking for a good mix formula for the concrete layer. I do not think I am looking for a "lightweight" mix; just a strong one with small aggregate.

Any further thoughts please feel free to share. I also don't mind being told I am crazy if I can be shown why.

Kevin_in_Denver

Youker mix

This question was answered a few weeks ago...
search for "youker mix"

Kevin_in_Denver

Concrete as finished floor

Oftentimes if the finished concrete isn't pleasing, it can be ground for only about $1.50 per square foot. My architect calls it "poor man's terrazo" since some of the aggregate is exposed.

Just another little issue with this whole approach... The guy who did this on OurCoolHouse.com reported some trouble with the fibers in the mix. I believe he used polypropylene fibers and it was hard to keep them down. I have no idea how fiberglass would work.

Troy_3

concrete

What ever you use- please let us know how you make out and what you think as you progress through your install. also we are very interested in long term satisfaction. We can all learn.

S Davis

Concrete

Be very careful to give the concrete at least a 21 day cure and make sure your radiant heating installer raises the supply water temperature very slowly,over a period of a couple of days, this will minimize the cracking you will have.

S Davis

Apex Radiant Heating