News From Go Plymouth Foam

Most Applicative Strength for Below Grade Insulation

Over the last five years, Plymouth Foam has been doing research on below grade insulation and the effects on structures. One reoccurring finding has been the misguided use of insulation materials that have Compressive Resistance (CR) strengths that are “too high.”

We have been observing the approach of designers using the point load bearing calculation method and specifying insulation strong enough to carry that presumed load. Most of them used the triangular load path calculation. The thought process of the designers, were a conservative approach, the more PSI insulation strength the better - a type of “over engineering.”

Is this over engineering approach good? Does it have a detrimental effect on a structure? The Geofoam Industry (foam insulation beneath highways) learned the hard way and have adjusted their approach. They concluded that loads on slabs should not be look at as “Concentrated Triangular Point Load” but more in line with a slab that works more uniformly as a system as concrete slab distribute loads are more in an even fashion. The DOT and Geofoam Industry took a new approach to load issues. Use the least Compressive Resistant Geofoam Insulation that can handle the load.

How does this information translate to below grade insulation in residential or
commercial construction? This is where it gets interesting. Everything we learned and accepted, in regard to, below grade insulation, by the XPS Industry, has been misguided. We have really only looked at half of the equation and most of the time, we have only considered point loads instead of slab distribution loads. We have been concentrating on loads from the top down only. In most ways, we have been ignoring what the soil is really doing below - not just what the soil can bear. Why you ask? Maybe because it gets too complicated. Not making this a forum on soil engineering, let’s just simply say soils are not always consistent and are constantly moving.

So this brings up many questions including - how does soil engineering effect below slab insulation? What are the effects of expansive soils on insulation? Is over engineering insulation compressive strength on slabs bad or harmful?

Below Grade Compressive Resistence Less is Best

When we intersect structural engineering and geotechnical engineering, we find that in most cases - “Less is Best.” The lower the compressive strength, that still meet load requirements, is best. The insulation should act more as a stress cushion. Once the insulation has been in place for sometime, it should continue to act as a stress cushion.

The Theory of Plates on Elastic Foundations is a great way to calculate slab deflection and the resulting stress. The formula is (P/8)√(K/D). (Paper written by Timoshenko and Woinowsky-Krieger) We have been following these calculation in Geofoam for sometime now and have had less structural issues and have reduced Geofoam costs dramatically.

In regard to below grade insulation in residential construction, using products like XPS 250 with a 25 psi compressive resistance, as a standard, is not taking into account all of the factors in construction. This standard can be doing more harm than good. Because of the marriage of geotechnical and structural engineering, the industry has now began to understand this and revised its position on below grade insulation. It is time to move to these new standards.

Plymouth Foam is viewing this “Less is Best” change to run parallel with their research. We believe that this concept can reduce construction issues. Using products that are 10, 12 or 15 psi will have more advantages to the structure and in the end reduce cost. We call this the true definition of Value Engineering.


By John Calkins