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.

11 Replies to “Construction Design-Concrete-less Slab on Grade”

  1. I just wondered about building permitts/engineer approvals for the “concreteless slab on grade flooring system using double layer EPS form, vapour barrier, double layered Advantek sheeting. I am trying to get more insights for a home addition project i am working in Ontario,Canada. What were the key obstacles in addressing building code/permitt apprivals. I have been advised to “hire a structural engineer” to get their stamp but want to get more background on this from others rather than starting from scratch. I have contacted a number of building permitt staff across Southern Ontario and no one is aware of this new floor system . Thanks Don

    1. Hi Don, The concreteless design we used is in a very rural setting. We were required to get a building permit but no one was concerned with how the house was going to be built. The county was only interested in the sewer system and setback from the lake and property lines. This is typical for rural builds throughout Minnesota, something I wish would change. I did discuss the system with an inspector in a nearby community. He only said that engineering would be required if it were being built in his area. Jake Bruton, who built a similar concreteless foundation, was in an area of Missouri that also did not require inspections. That build was designed by an architect (Steven Baczek) and I believe an engineer was involved. I wish I could be more help, I would start with a discussion with an engineer. Let me know if you have any specific questions on how we built our system. Good Luck! Randy

  2. Fifteen months later.. any issues with the concreteless floor? I’m liking what I see but would worry about settlement… Perhaps unnecessarily.

    1. Hi Scott,
      No issues. I’ve been back to the home several times. Homeowners really like it and we haven’t seen any settling. Thanks for the question!

  3. Hi Randy, I just discovered your blog and I wanted to let you know that I’m really enjoying reading it. Thanks for all of the great content!

    I’m planning a build and I really like the idea of a slab without concrete. I’m wondering, given your experience with this arrangement, if you were to add hydronic heat loops, where would you put them? My plan is to put them in the foam and then install foam around them, then I’ll fill the voids with sand before laying the OSB. If you have any advice it would be greatly appreciated.

    Thanks,
    Matthew

    1. Hi Matthew,
      Thanks for the comment! Your question is timely, I’ve just started working on a design using the concreteless concept with a heated floor, but I’m not very far along in the design. Look into Warmboard heated floors. It’s a subfloor system that has channels designed to accept the hot water heating tubes. I haven’t decided if the Warmboard panel should be installed as the bottom or top layer of the flooring system, the Warmboard would replace one layer of the Advantech flooring.

      If you have any other questions on the concreteless system, feel free to ask. The system does require more attention to detail over a standard foundation system. The homeowners that own the concreteless slab on grade home have nothing but praise. I’m very pleased with how the home turned out.

      Thanks for reading the blog and good luck on your build,
      Randy

  4. What about cost?
    You addressed the comfort walking on it. But 3500 sq f of advantech is not cheap.
    Can you tell us the final comparison cost?

    1. Hi William,
      In the case of the Concreteless project, the cost for the slab was slightly less than pouring concrete. This home is very rural, an hour away from the nearest concrete plant. Concrete placing and finishing in the area runs about $7 per square foot for a four inch slab. The cost figures the same amount of sub-slab foam insulation if either concrete or Advantech are used, so the cost difference is materials and labor to install subfloor. The cost savings was a few hundred dollars over concrete. It took the carpentry crew of two guys a couple days to install the Advantech. I’m sure cost will vary depending on location.
      Hope this helps, Randy

  5. Hi Randy,
    I saw on another website that you had used the master shower as a back up floor drain in the bathroom. How were you able to install the drain with no lip into the shower. I really like the idea of a concrete-less slab. Great job on the project. I hope to use this design in our new home build.

    1. Hi Eric,
      We made the master shower curbless by removing one of the layers of Advantech flooring in the shower enclosure and made a pan using dry pack concrete. We ended up with about a 3/4 inch pitch inside the shower. We used Ditra for the tile substrate for the bathroom floor which gave us a little extra floor height for the drain pitch. The shower floor was waterproofed using Kerdi. If we would have had a little more budget for the shower, I probably would have used a lineal drain at one of the walls. The Schluter System, both the shower system and the Ditra installed water tight works great in this application.
      Randy

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