A few weeks ago, I noticed an item in a foundation specification that I have been glancing over without any real thought. “Rigid, cellular thermal insulation with closed-cells and integral high density skin, complying with ASTM C 578.” What caught my eyes was integral high density skin. Are they trying to call out a specific for Extruded Polystyrene (XPS) Insulation because standard Expanded Polystyrene (EPS) would not naturally have a skin without adding it?
This triggered a technical conversation about skins on Rigid Insulation.
Does rigid insulation need a skin? What are the advantages or disadvantages of having a skin?
It is always helpful to go back and review the main function of insulation. Insulation’s main purpose is to insulate and provide R-value. Knowing the main purpose of insulation, do insulation skins provide more R-value? Skins can add R-value and skins can reduce R-value. A foil faced insulation could add R-value when it is used in the right circumstances such as a gap behind a masonry wall. Some skins like a clear poly laminated has no effect on R-value. Does an integral high density skin in XPS offer additional R-value? No, in fact according to newer research, it may have the opposite effect. How can this be? Why specified it as something important?
During a manufacturing process, XPS creates a type of skin during the extruding process and some call this an integral high density skin. This skin is maintained on the face and back of the board but the sides and the ends are trimmed during the manufacturing process. Many believe that trimming the sides actually expose and opens the board to moisture infiltration at a higher rate. The face and back have this skin but the ends and sides are open. Interesting, when you consider below grade insulation and how water moves down the foundation wall, water hits the top part of the insulation and water wants to enter in, however the top end does not have the integral high density skin. We know by testing, when XPS insulation is installed below grade, it takes in moisture and its R-value is reduced by 48%, from 5 R/inch to 2.6 R/inch. Some believe this phenomena is caused by water entering in easier at the edges and partially trap in by integral high density skin. So specifying this integral high density skin as a benefit, ultimately points out another reason not to use XPS below grade. Why pay a huge premium for XPS Insulation and get 1/2 of its R-value performance?
What about skins on EPS? Standard EPS does not naturally have a skin, however another advantage of EPS Insulation is that skins can be added. Many different types of skins with different properties can be added. A few examples of skins are ones that can be reflective, skins that allow permeability, skins that are vapor retarders and even skins that can provide high strength. In foundation insulation, where EPS is more effective, a permeable skin could be added to give even more strength. This skin could be even more effective against rough backfilling and fastener pull. This type of product would function great allowing any moisture that enters into to the insulation board to flow freely out and not trap the moisture in.
In this case, the specifier thought that XPS insulation with an integral high density skin would be a better product for below grade application. Unfortunately, it's not. With research showing that XPS insulation can loss 48% of it’s R-value below grade, Engineered EPS Insulation is a much better, safer choice.