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Perspectives
A Quarterly Information Source from Benchmark, Inc.
Volume 37 September 1999


Polyisocyanurate Roof Insulation

by Jeff Evans, RRC

This is the 3rd in a series about commercially available low slope roof insulations. Polyisocyanurate foam (also known as polyiso or iso) followed polyurethane foams into the roofing market in the early 1980s. Equal to polyurethane foams in insulating efficiency, iso foams have the advantage of better fire resistance properties, and so have completely displaced polyurethane foams in insulating boards.

Polyiso foams are manufactured in board form, typically in 4' x 4' and 4' x 8' sizes, and can be made in thicknesses from 1/2" to 4" thick. Tapered boards are also made, with the most common slopes being 1/8" and 1/4" per foot. The foam is manufactured through a chemical reaction of two liquids, polyisocyanate and polyol. The reaction causes the liquid to rise and harden into a cellular foam.

The insulating value of polyisocyanurate has been the subject of some debate over the years. Since the blowing agent used in making the foam is less dense than air, the R-value of the foam will drift or be reduced as it ages. Early in the manufacturing of polyiso foams, the R-value was listed as high as 8.33 per inch. Today there are two principal views on the in-place R-value of polyisocyanurate.

The Polyisocyanurate Insulation Manufacturers Association (PIMA) measures the aged R-value of the foam at 180 days, with a typical R-value of 6.0 for 1" thick material. The Midwest Roofing Contractors Association (MRCA) conducted a study that concluded a more conservative value of 5.56 should be used.

Uses
Polyisocyanurate foam has a number of properties that make it the most widely used roof insulation in the U.S. including:

  • Low cost
  • Thermally efficient
  • Fire resistant
  • Dimensionally stable
  • Can be used with hot asphalt
  • Can be used with contact adhesives
  • Can support normal roof traffic and loads

Polyiso has a wide range of applications. It can be mechanically fastened, mopped in hot asphalt, adhered with liquid adhesives, and also used in loose laid applications. It is widely available through a dozen or so major manufacturers in the U.S.

Limitations
As with any product, there are restrictions to using polyisocyanurate:
Blistering - Early in the development of polyiso foams, a problem with blistering of built-up and modified bitumen roofs applied directly over polyiso was noted. As a result, the NRCA recommends a coverboard of fiberglass, fiberboard or perlite insulation be used in hot applied built-up and modified bitumen systems. This seems to have alleviated the problem. Fully adhered single-ply membranes applied directly to polysio foam do not require this coverboard.


Delamination of the insulation facer.

 Facer delamination
We have experienced some delamination of the fiberglass facer from the foam when applying hot asphalt over the foam. This has been infrequent, and was explained by the manufacturer that the insulation had insufficiently cured.

Crushing
Polyiso foam has fairly good compressive strengths, typically 18-20 psi. This may not be sufficient on higher traffic roofs, and we have noted especially in single-ply projects that the foam can collapse from traffic loads in a fairly short time. 

Specify higher compressive strength board (polyiso can be manufactured in 25 psf board), or in high traffic areas consider using a layer of high density fiberboard or Dens-deck.

Collapsing Foam
We have also had a least one episode where tapered polyiso had collapsed (compressed from 2.5" to 1" thick). This occurred on a project where a layer of perlite was applied to the foam. The insulation collapsed two months after the foam had been applied. The manufacturer again explained that insufficient curing was the problem.

Outlook
The future for polyisocyanurate foam is that it will continue to claim over 50% of the flat roofing insulation market. It is a cost effective, versatile product that is well accepted by the roofing and specifying community.