After squaring and leveling our forms, we began installing the sub-slab insulation. We are using Roxul comfort board for many reasons (ComfortBoard 80 on the vertical sections and ComfortBoard 110 -which is more dense – on the horizontal and sloped sections). It has a much lower embodied energy than foam products, it is not affected by water, it retains it’s R-value through time, it is not attractive to bugs or rodents, and it is fireproof. It’s got a lot going for it. It is a bit itchy on exposed arms while working with it, though. We would start the day with long sleeves, but a major heat wave was passing through which meant the shirts were shed. By the end of the day the itchiness was definitely noticeable. But, given the other positive attributes to Roxul, we would use it again without a doubt.
Our design has EPS foam beneath the footer because Roxul does not have the compressive strength to support the footer section of our Monolithic slab. In that area we sourced a product called Atlas ThermalStar X-grade. Eps foam is friendlier on the environment since the blowing agents used in its production do not contribute to global warming -as do the ones used in producing XPS (Dow blueboard common to many lumberyards). Another benefit is that This ThermalStar product has been tested in it’s R-value retention over time where XPS has been known to lose R-value as the years progress. The thermalStar product is also produced with an integral termiticide just in case. We went with a 25 psi density to support the weight of the house.
Another benefit to Roxul is is cuts quite easily with an old handsaw. It holds an angle quite well.
We applied two layers of 2″ Roxul – offsetting all seams. This area will be reinforced concrete to support a load bearing internal post. (I should have ordered one extra sheet of ThermalStar!)
You can see the strings we referenced off of the tops of the forms to ensure we have a flat surface. This is very important for purposes of knowing how much concrete to order…
Our finished insulated pad (the recessed area with the pipes is to allow space for plumbing below the tub)
Next came installation of our vapor barrier. We chose Raven’s VaporBlock 15. It has quite a high puncture resistance and a very low vapor permeance. We taped and caulked all seams with their Vapor Bond tape and Tremco’s acoustical sealant.
Then came the re-bar. We ran 6 courses around the perimeter to strengthen the thickened edge and a grid of ½” rod 3′ oc in the slab. Just for good measure we added 6″ welded wire mesh atop this grid. It may be overkill, but we want to minimize any occurrence of cracks developing over time.
To hold the Roxul in place we found this cool detail on this site. Passing a heavy duty cable tie through a washer and then the exterior of the Roxul and the vapor barrier, we installed two screws through the cable tie which will anchor into the concrete. There is a nail beneath the tape just inside of the vapor barrier holding the cable tie in place. Though we haven’t removed the outer forms yet, a strong tug tells me that it worked! Also notice the foam feet we used to support sections of wire mesh that were cut to the correct size to hold the perimeter re-bar. The whole re-bar assembly was a little shaky until we added diagonal re-bar supports to it. After that, it was quite stout.
What to do when you need to carry twelve 20′ long sections of Re-bar on a compact car? Nail 8 16d nails (4 on each side) into a 12′ 2×6 and strap the re-bar to the board between the nails. Worked great!
And then the time came for concrete! None of us have a lot of experience with concrete and this was by far the largest pour Mike has overseen. But it went well!!! We are incredibly grateful to our friends and neighbors who came out to help, we could have never done it without you!!!
We rented a power screed that worked quite well. We set up wet pads 12′ apart, then would screed a line about a foot thick (on the left side of this photo). Then we would rotate the screed 90 degrees and flatten the middle of the section. This video shows it clearly. Our slump was about a 4 which was pretty stiff. If we were doing it over again we would go with something around a 5.5 or 6 to make it easier to spread and screed. The concrete trucks came so close together and we were so busy screening and spreading that we neglected to insert most of our anchor bolts into the wet concrete. Our plan at this point is to drill holes and insert bolts into anchoring epoxy.
The final surface is a little rough, but that is fine with us since it will be covered up by finish floor. Our plan at the moment is to have wood floors in the northern side of the house and a thin (approx 1″) layer of earth floor in the main living space. Earth floors have the benefit of being much softer underfoot than concrete as well as being a good thermal mass for storing solar gain in the colder months.
We are beyond elated to be finished with the concrete work and ready to get back to working with wood! At this time we would like to welcome friends from far and wide who have expressed an interest in helping with our build – we are ready for you!! Now comes the fun part : )