My last blog on ‘Sealing the Envelope‘ of our house included a quick description of the basement, outside walls, roof and air barrier for the house. In more detailed discussions with Murphy Cell-Tech, we came up with this set of laying for the basement.
The diagram to the right shows the various layers. The first thing that needs to be considered is the possibility that radon gases could build up under the basement of the house. High levels of radon are known to be carcinogenic, and rocks containing uranium are known to give off radon gas. At this point I believe all new construction (and renovations like ours) in New England need to plan for a way to remove radon gases if they exist.
For us the radon mitigation plan is to have a layer of perforated tubing under the insulation of the basement. That tube spirals around the ground of the basement and then joins at one point to a vertical stack that leads the gases up through the roof beyond any living space. After completely enclosing the house we will do a radon test. If the result is greater than 4ppm, then we can add a fan to the stack to actively draw the air from under the basement to the outside. If the radon is lower than 4ppm, then we won’t need to add the fan.
So the radon pipe goes directly on the dirt ground with crushed stone all around it to help level the space. Next are the two layers of foam board. The first one has a fire protection on it and gets taped together to act like one continuous layer. The second layer gets laid out to ensure a good overlap of joints. It gets taped as well to minimize the cracks.
Once we get that far, the next step is to have the walls sprayed with an insulation foam that seals right up to the foamboard on the floor. Finally a layer that is like a plastic sheet that acts as a vapor barrier over the entire surface.
Once that is done we will put down some plywood in the areas that people need to walk. For the most part our basement is not functional and we don’t expect to go down there much at all. Almost all our utilities will be in the utility room, located in the store/showroom.
I know the spray foam is used quite often in basements that have stone or masonry foundations since it doesn’t have to be a flat surface. We also talked about spraying the floor. Half of our basement is a small crawl space that we expect even less traffic and where it is very uneven. We are thinking that it might be more cost effective to spray that area rather than putting in all the effort to try to level it enough for foamboard.