Construction Design-Concrete-less Slab on Grade

Work on the concrete-less slab on grade continues.  At this point in the build we are dried in with mechanicals starting soon.  With this posting, I will go through constructing the unique floor system of the home.

In a recent posting about this project, I discussed the insulated concrete forms we used as the footings for the house.  The exterior walls were all built and the roof was installed before we could begin the floor system.  The reason being we did not want to have the completed floor system rained on.

As you can see from the photo above, the dirt floor remains after the shell of the home is complete.  The below grade plumbing rough-in has been completed and we are  final grading of the rock fines.  Rock fines are used in my area as the top layer of driveway material because of their ability to compact tightly.  This material worked well as the top coarse.  It took the leveling crew, who happen to be the concrete guys that installed the ICF foundation, about 6 hours to grade the rock fines for this 1700+ square foot home.

After grading, the next step was to install the first layer of foam.  The foam we selected was type IX expanded polystyrene (EPS), or the white bead foam.  Type IX EPS has a density similar to the extruded polystyrene foam (XPS), which is often used under a concrete slab.  At a little more than 2 inches thick, the foam’s insulation value is around R-4.8 per inch.  We installed nearly 4 1/2 inches of the product in two separate layers which gives us around R-20 below grade insulation.  I say around because this foams insulation performance is dependent on it’s temperature.  Listed R-values from the manufacturer were less at 70°F than at 50°F.  The photos below show the progress of the first layer.

After setting the first layer of foam, the next step was to install a vapor control layer.  The 6 mil reinforced polyethylene sheeting is placed between the two layers of foam to keep any ground moisture from entering the living space of the home.  We choose to place the poly between the sheets of foam to protect it from damage during installation.  The poly is sealed at the concrete footing to wood framing joint.

After the poly is installed, the poly was sealed with tape to all penetration, the below grade plumbing pipes along with the single interior footing and support column.  We also taped all the seams.  Next, the second layer of foam was installed.

Once all the foam is installed, the first layer of 3/4 inch Advantech subfloor is placed on top of the foam.  The subfloor is spaced to maintain a 1/4 inch gap around the perimeter to allow for any expansion.

Next step was to install a second layer of Advantech subfloor over the first layer.  This layer was rotated 90° and all seams were offset.  The two layers were glued and screwed together.

We ended up with a slight error in the areas where doors are to be installed.  We were hoping to have the Advantech flow through the rough openings, but ended up with a 1/4 inch height error so we cut back the top layer of subfloor, added a shim and installed 1/2 inch Zip sheeting instead.  The Zip added an additional layer of rot protection, an error that worked out in our favor.

Now that the floor is finished and we have had a chance to walk on it, we have noticed a couple things.  It is quiet and very comfortable to walk on.  Transitioning from the concrete garage floor to this floor is a big difference on my feet.  Overall, the installation went without any issues or unexpected problems.  I would definitely consider installing this floor again.

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