Insulation below slab

We laid the first layer of 4” thick styrofoam insulation last week and Monday. The plumbers also started roughing in the plumbing through the foundation.But Monday night it rained - dripping into Tuesday. The insulation meant that the water did not just soak into the ground. So yesterday we mopped up the water, hoping enough would dry between raindrops that we could lay two layers of polyethylene film (to stop radon gas from migrating into the house). Regardless of the impact to construction, we really need that rain.

When moving the piles of more styrofoam closer to the house (and inside our fence) it appeared that some of the bundles were heavier than others. This ‘beadboard’ is made in several densities. We need the higher density in order to support the weight of the house. The lower density has less compressive strength. We then tested the foam that we had already laid and discovered that at least three sheets were not strong enough and three to six sheets in our pile were similarly too weak.

None of these sheets (actually blocks, since they are 4” thick) is marked in any way. Eli called the vendor and he will pick up the material that is below specification and deliver us new material. So we dug out the three sheets already installed that we identified as being inadequate - not a small task, since they are glued and taped in place.

Meanwhile the plumber did not return to finish the rough drain lines - no big problem because we were distracted by both rain and styrofoam.

And the heating engineer stopped by to see the progress. I discussed the thickness of insulation under the slab and he agreed that 8” total thickness (R-40) was probably excessive, given that the ground temperature under the insulation will probably never drop below 50°. I inserted a temperature probe at that point, so I can test that assumption. Regardless of insulating need, we now need the added height, so will install the next layer of 4" insulation when we are assured that it is strong enough.

Hopefully we will be able to place the concrete blocks today. We have the air distribution baffles already cut and drilled, so that should go quickly. [Photo was taken May 8. :)]>>> Next <<<