DC Rowhouse Renovation - Focusing on the inside: The new basement slab / by Gregory Upwall

Laying the under-slab insulation over the drainage gravel

Notice the white radiant tubing in-place above the steel bars

Now that we can turn our focus to the inside of the house we can start to think about the quality of the indoor environment that we aim to create. An important early consideration is in the design of the very floor that will separate our living space from the earth on which the house sits. As we prepared to install the new concrete slab in the basement there were important considerations in what went in place beneath the slab. After the old concrete floor was broken up and removed we had the contractor lay down a layer of gravel to cover the exposed soil. The gravel remained in place in the intervening months while we were focused on the rough framing and on getting the rest of house dried-in. But now with those important steps complete, we could finally focus on getting the new slab in place. 

 In years past concrete floors were poured directly on top of the soil, but today we have learned that this is not a good idea. Since concrete is a porous material and not a good insulator, both moisture and heat can pass through the slab, and need to be addressed in order to keep the house comfortable and dry. We were lucky in this location since the site is well drained and we did not have any issues with groundwater in this part of the city.

 As a side note, we cannot emphasize strongly enough the importance of addressing groundwater issues if they do exist, before installing any new interior concrete slabs or finishes. If you are in a location with high water-table or where signs of any prior flooding or water seepage have been witnessed, these need to be addressed and corrected, otherwise water and moisture can become a recurring problem that can lead to mold and mildew forming inside the house. Groundwater issues are generally best addressed by a civil engineer with experience in dealing with site drainage and groundwater issues. Commonly applied techniques for preventing water from entering basements and foundations include installing foundation drainage trenches and piping below the slab, and the installation of a properly sized sump pump. You should consult with a civil engineer if any ground water issues are present on a project.

View showing reinforcing steel bars (“re-bar”) and vapor barrier installed over the foam insulation

View showing foam insulation along edge of slab

 Since water was not a problem on this project we were able to install the new concrete without any foundation drains or sump pump. But even without active groundwater issues moisture is present in the soil and the temperature of the soil is considerably colder than our indoor temperature will be in winter. To address the moisture in the soil, our continuous layer of gravel will serve to percolate moisture away from the bottom of the slab. And to address the heat loss we installed a three inch layer of high-density foam insulation board (making sure that it was carefully fitted to create a tight and continuous layer) over the top of the gravel. Then we installed a layer of heavy mil plastic film over the top of the foam to help keep any moisture from passing through from the ground into the interior of the house. 

 We also had our plumber install the flexible tubing for radiant hydronic heating of the slab. It is important for this tubing to be laid in a uniform pattern to ensure uniform distribution of heat in the slab. With all of these measures in place the contractor could finally order the concrete and pour the slab. In this case they chose to mix the concrete directly on site since the access to the site was too narrow to bring a ready-mix truck in, and also because we developed our own custom mix for the concrete. For this project we plan to leave the concrete exposed as the final finished floor in the basement, and we wanted something a bit more distinctive than a common concrete slab. So we purchased special white colored Portland cement and a smaller washed gravel for our concrete mix. During the final finishing phases of the project we will have the surface of the slab ground and polished with a special floor sander and then sealed with a clear sealer. We have installed a similar floor on other projects and it has turned out nice. But since each mix and batch of concrete can vary we will not know that exact appearance that we’ll get until the end when we have the floor sander on site. We are excited to see how it will turn out and hopeful that we’ll be pleased with the result!