Post date: May 8, 2015 3:39:44 AM
For insulation under the floor slab, Eli the builder ordered two pound per cubic foot (2#) foam which is supposed to support 25 pounds per square inch. When it arrived, two of the bundles felt exceptionally light, so we set them aside. While installing, some of the foam also felt light and was easier to dent than others. We ripped that out.
We thought and thought and discussed, trying to find a way to measure the compressive strength — to no avail. [A carpenter’s intuition and thumb print is hardly objective evidence.) We even stacked two 50# concrete blocks on the foam and our heaviest person jumped on it to see if it would dent the foam. That was meant to model the concrete blocks, concrete floor slab, and human activity that we plan to load on top of this foam insulation. Yes, there was a slight dent.
Finally Eli was talking about one pound foam and pound and a half foam, and i realized that referred to the density. It dawned on me that we can easily measure the density of the foam. The receipt clearly said 2# foam. A 4’x8’x4” sheet is a little over 10 cubic feet. This foam block should weigh a little over 20 pounds if it met the sales specification of two pounds per cubic foot. I fetched the bathroom scale from the Common House. We put one sheet on the bathroom scale and it weighed 15#. We weighed a block that we had installed, then removed because it felt soft - and it weighed only 11 pounds!
I may have learned something in high school physics after all. The vendor will replace 15 of the 36 sheets, but we wasted a week.