Stage one of the Ecohome Demo house: slab-on-grade construction

This page follows the progress so far on our Demonstration House and detailing home orientation and slab construction techniques.

Slab-on-grade  for Ecohome's upcoming video building guide
Slab-on-grade for upcoming video building guide © Ecohome

The Ecohome Demonstration House is well underway, the following page chronicles the first stage of constuction, building the slab-on-grade. The house is oriented facing south to maximize passive solar heat gain in winter, but will be shaded in the summer by deciduous trees, wall mounted sun shades and a large overhang to prevent overheating.

Location and orientation of the house on the lot was chosen not only for passive solar heat gain, but to make best use of the terrain for drainage, to limit the cutting of mature trees as much as possible and to ensure a quality of life for occupants with a mix of privacy and usable outdoor space. 

Compacted slab on grade base
1. Compacted slab on grade base © Ecohome

1: After the land was cleared, compacted gravel was brought in to bring the site to a workable level. The house sits at about grade at the north side, but needed about 3.5 feet of fill at the south to level the building site.

Compacted gravel was tamped in stages to ensure a solid base. The machine we used can apparently tamp successfully to three feet deep, we did it every foot and a half for good measure. 

A retaining wall was built using rocks from the site to hold the compacted gravel and future slab in place.

Slab on grade forms
2. Slab on grade forms © Ecohome

2: It is more common to drive stakes in the ground then nail boards up as concrete forms, but we decided to pre-build forms, then raise and square them the way you would with walls. This was intended to make disassembly easier so we could reuse the wood. Hard to say if it was easier, but it went fine. 

Forms need to be well secured to avoid a blow out, including lots of braces to hold the weight of the concrete so forms don't bend. Three runs of 2x6's easily held concrete and insulation, and after forms were removed, that wood was used for framing. 

Plumbing infrastructure
3. Plumbing infrastructure © Ecohome

3: All plumbing infrastructure (water pipes, drains, radon stack, central vac, power conduits, etc.) was put in place next, below the coming insulation and concrete. We used insulated water pipes courtesy of Uponor for greater energy efficiency, which also brings us LEED points. 

You really need to be sure about your placement of plumbing at this point. Concrete is not so forgiving when it comes to making changes afterwards. 

Slab floor insulation
4. Slab floor insulation © Ecohome

4: After all the plumbing was in place, 8 inches of rigid insulation was installed on every surface. On the advice of Roxul engineers, we put high density EPS foam under the footing, as Roxul has not yet been tested against the weight of a footing and load bearing wall, but Roxul Comfortboard CIS (more dense than Comfortboard IS) was used below the main slab.

Comfortboard is great to work with, it cuts really easily and stays securely in place without slipping, allowing for tight clean joints. This is a nice added benefit when you are doing multi-layers as we did here. The joints of panels were overlapped as well to further reduce heat loss.
Slab on grade insulation
5. Slab-on-grade insulation © Ecohome

5: We placed 8 inches of Roxul Comfortboard vertically to protect the exterior of the slab, and we included a cement board outside the Roxul but inside the form. We later attached the cement board with plastic tie straps that passed through the insulation and vapour barrier into where the future footing will be; a 1.5" screw was put into the end of the tie strap to act as an anchor inside the concrete after it sets.

We really like this little detail, and a clear description will no doubt make it to our video guide; you can also get a look in the third big image below. 

The reasoning behind this step was to have a well-insulated slab with no thermal bridging, and a cement exterior ready for parging when the forms were removed. After disassembling the forms we were happy to see it worked exactly as planned.

Plastic ties were chosen over long nails or screws to prevent the heat loss that would come through metal, which our engineer assured us would be significant, far more than we would have expected before seeing software energy simulations. Metal fasteners would have effectively reduced the total R value by almost half, testament to the conductive powers of metal and why as a material it should be very selectively used in wall assemblies.

10 milSub-slab vapour and radon soil gas barrier
6. Perminator 10 mil Sub-slab vapour barrier © Ecohome

6: For added durability over the commonly used 6 mil polyethylene vapour barrier, we used the 10 mil 'Perminator' sub slab vapour barrier, courtesy of W.R. Meadows. The added thickness gives us more protection against accidental holes made during the construction phase, giving us greater confidence in it for vapour and radon gas protection. Any holes would also be easier to find with the green colour, as it tends to discolour and turn white where damaged. We found this out by trying to rip holes by jamming rocks through it, which we are happy to report is not an easy task.

Mesh supports over footing
7. Mesh supports over footing © Ecohome

7: To support the concrete mesh as it extends over the footing, we cut sections of mesh that sit on small foam feet. This also supports 4 rebars that will be imbedded in the concrete footing at exactly the position we want them. 

Despite that it may look somewhat labour intensive, this little detail took only a few hours to do and it ensured that our mesh was level, our rebars were well-positioned and our vapour mat was not punctured. 

Radiant floor tubing
8. Radiant floor tubing © Ecohome

8: Uponor also provided us with tubing for a radiant floor, complete with a floor plan layout for 10 individual zones, so the temperature in each part of the house can be controlled independently. 

Limiting the length of tube installed per zone is very important at this stage, too long a run and the water will no longer be hot towards the end of its return trip. There was no effort required on our part in regards to planning zones and zone lengths, it was all figured in the Uponor plan. So the tubing was installed by 3 workers in only a few hours.

Recycled concrete polished slab on grade floor
9. Recycled concrete polished slab © Ecohome

9: The final floor will be a polished concrete, which is completely non-toxic and very durable. The production of concrete is a heavy polluter, releasing one ton of GHGs (greenhouse gases) per ton of concrete. So we chose a concrete mix that included 50% recycled material, which significantly lowers the total emissions of our build, and brings in some LEED points as well.

There is an often overlooked advantage when you choose a slab over a basement - concrete is very expensive, so the reduced amount of concrete used in a slab along with not needing to install a subfloor and finished flooring product translates into a savings of many thousands of dollars.

That covers stage one,  check out our Demo House wall assembly page and see what's going on above grade. We will also be releasing a video building guide in the near future documenting each step of construction.

And if the very concept of slab-on-grade construction leaves you scratching your head or worrying about frost heave, we have multiple pages in our building guide to help warm you to the topic:

Slab on grade with Roxul mineral wool

Slab on grade with Roxul mineral wool insulation

slab on grade: reinforcement mesh supported over footing

Radiant tubes arriving in Mechanical room