Extruded Polystyrene the Best Roof Insulation

Extruded Polystyrene the Best Roof Insulation

I commend your staff for consistently delivering a very informative newsletter. However, a better understanding of durability and reusability should have been reached before making the recommendations in your article “Insulation Materials: Environmental Comparisons.” Being an independent sales representative for several roofing and waterproofing manufacturers (American Hydrotech, T. Clear, Neogard, Mirafi, and Steelox) with acquired experience working for a commercial roofing contractor (about 7 years) and residential builder (3 years), allows me to comment.

Insulation in roof/ceiling assemblies is typically designed for greater thermal resistance than wall systems. Therefore, roof insulation provides the greatest environmental impact.

Common low-slope roof insulations include: Isocyanurate, perlite, woodfiber, fiberglass, cellular glass, XPS, EPS, and composites. Your article stated that durability was important but devoted little time to this issue. Moisture resistance is the most important durability characteristic for roof insulation. Moisture-sensitive insulation should not be used in a dangerous environment (like a roof) because it risks compromising thermal resistance. Other important durability factors are resistance to:


•long-term thermal drift (real world, not laboratory)

•physical abuse (high compressive strength)

•dimensional change

Extruded polystyrenes (XPS) excel in all durability factors. Your generalization that rigid foam insulations produced with HCFC’s are prone to R-value drift is true for Isocyanurate but not for XPS.

The reusability of roof insulation is dependent on system design. The roof design should: prohibit insulation attachment with adhesive and promote drying of insulation. Recommendations to overlay insulation with sheathing make little environmental sense. This approach promotes isolation of the membrane contributing to premature membrane failure and allowing moisture to penetrate the insulation and sheathing.

A sustainable roof design makes environmental sense. The Protected Membrane Roof (PMR or IRMA), developed in the 1950s and underutilized has been offered by major roofing manufacturers. By sustaining the membrane life and insulating the structure, XPS insulation serves a dual role. The loose laid insulation and ballast cover provide protection from UV, freeze/thaw, and physical abuse by extending membrane life; the PMR design reduces maintenance costs. Think of the landfill space saved if the roof disposal frequency dropped 200-300% or more. Has EBN fallen into the same marketing trap that building owners have been lured into for years? As non-XPS manufacturers know, “we don’t promote it ’cause we can’t do it (PMR).”

My objections may have been avoided if the author had followed EBN’s own Perspective (

Vol. 3, No. 6), which reminds us “we are served best by keeping a broad perspective and looking at how each product fits into a larger system.” I hope EBN reconsiders its recommendation to avoid all HCFC insulations. There are simply too many benefits to XPS.

Stephen R. Shull

Rediger Associates, Inc.

Elgin, IL

Published March 1, 1995

(1995, March 1). Extruded Polystyrene the Best Roof Insulation. Retrieved from https://www.buildinggreen.com/op-ed/extruded-polystyrene-best-roof-insulation

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