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WSRCA TECHNICAL BULLETIN 2017-II-1: Knit‐line & Facer Irregularities in Rigid Polyisocyanurate Foam Roof Insulation

Posted By Chris Alberts, Western States Roofing Contractors Association, Wednesday, November 8, 2017
Updated: Wednesday, November 8, 2017



Greetings to Members of Western States Roofing Contractors Association:


Executive Summary

Changes in materials and methods used to produce rigid polyisocyanurate insulation boards for roof systems has been a concern for WSRCA members at different times over the years. As you may recall, WSRCA’s Low‐Slope Committee provided WSRCA Members with a Technical Bulletin in 2000 a follow‐up during 2004 (No. 2004‐1) about changes in blowing agents and the LTTR (i.e., published results of long‐term thermal performance testing) regarding R‐values per varying thicknesses of polyisocyanurate insulation. It appears that as the manufacturers continue to fine‐tune the polyisocyanurate insulation manufacturing process in order to comply with regulations, while still producing quality products with the necessary properties, issues are occurring and being observed in the field, of which our members should be made aware. The issues may require new action(s) by the industry. Reports from the field indicate that longitudinal depressions (e.g., also referred to as ruts or grooves) appear to be occurring with more regularity in rigid polyisocyanurate foam insulation boards delivered to job sites and some are being used in roofing systems. This issue that had previously only been seen infrequently, appears to be becoming common.


The ruts or grooves observed are associated with “knit‐lines,” which are lines formed during manufacturing when the flow of liquid‐state expanding foam from multiple mix heads meet as the material expands just down the manufacturing line from the liquid‐chemical outflow points. As two neighboring lines of expanding liquid foam material meet, if a slight amount of skinning‐over has already taken place, and the two masses of foam do not completely meld homogeneously, it leaves a visible line in the material. The two sides, under most ordinary manufacturing conditions, can bond together but there may be a readily‐visible line of compressed polyisocyanurate cells across the thickness of the board and these are called knit‐lines.


In the rigid roof insulation materials that are presenting issues, there are varying sizes of grooves or depressions in the facers of the foam boards along the knit‐lines, running the length of the board, as if there was not enough foam insulation material and/or foam expansion to fully fill in and contact the facer along where the depressions occur. As these affected boards are installed end‐to‐end on a roof, the grooves or ruts may align and extend across the roof. Depending on how the insulation boards are installed relative to the slope of the roof, the ruts may run perpendicular to the slope and can inhibit drainage. However, of greater concern is the possibility that the board’s facer could be unadhered and may also be bridging across these knit‐line depressions. In a fully‐adhered single‐ply roof  membrane system that is applied directly to the facer of polyisocyanurate insulation boards, there appears to be a potential for wind‐uplift issue(s) and/or damage as a result of the voids under the unbonded facer.


We note that this issue is being discussed at the National Roofing Contractors Association (NRCA) as well. An article written by Mark Graham, of NRCA’s Technical Section, regarding testing of rigid polyisocyanurate insulation board used in roofing, published recently in NRCA’s Professional Roofing Magazine, (Professional Roofing 12/02/2016, Another Round of Polyiso Tests, by Mark Graham.) touches on this subject. In the article, Mr. Graham states,


“…the issue of surface depressions associated with knit‐lines in faced, rigid board polyisocyanurate insulation is of particular concern. Although this problem was previously seen only in isolated instances, it now appears to be more pronounced and widespread with the current generation of polyisocyanurate insulation blowing agents and manufacturing processes. Polyisocyanurate insulation manufacturers need to improve the flatness of their roofing‐specific products, and appropriate evaluation criteria need to be developed and included in Faced Rigid Cellular Polyisocyanurate Thermal Insulation.”


Background Information:

As was stated in WSRCA Technical Bulletin No. 2014‐02 (“Updated Results of Long‐Term Thermal Resistance [LTTR] Testing, and R‐Value of Current Generation Polyisocyanurate Roofing Insulation”), over the past few decades the manufacturing of plastic foam roof insulations has undergone numerous changes, partly due to tightening regulations and governmental mandates to lower the amount of ozone‐depleting gases and VOC‐related blowing agents released into the atmosphere. Blowing agents evolved from the use of chlorofluorocarbon (e.g., CFC‐11) in the 1980s, to less volatile hydrochlorofluorocarbons (e.g., HCFC‐141b) with reduced ozone depletion potential and a lower global warming index in the 1990s, then changing again to pentane and cyclopentanes in the early 2000s.


Those earlier changes, and continued refinement of blowing agents, have caused manufacturers to invest significant assets in order to maintain producing successful polyisocyanurate roof insulations. Polyisocyanurate has been the most widely used rigid foam board in U.S. roofing assemblies for decades due to its high R‐value, fire resistance, and relatively low cost. Problems with dimensional stability had occurred over the years especially as manufacturer’s responded to regulations regarding potentially ozone layer‐harming chemicals used in blowing agents, and changed formulas. But over all, it has continued to be the most regularly specified rigid insulation board in low‐slope membrane roofing systems. Irregularities associated with the knit‐lines were reported occasionally in the past, but WSRCA Member reports are becoming more and more frequent and more concerning to WSRCA’s Low‐Slope and Industry Issues Committees. We have heard that some of the manufacturers have purportedly addressed this issue as merely an aesthetic one, but WSRCA, and other roofing industry organizations are concerned that this is a technical issue with potential for resultant roof problems.




Lack of Full Facer Adhesion:

When polyisocyanurate foam’s knit‐line grooves are deep enough and have left the facer unadhered along the length of the knit‐lines, the result is the same as facer separation, which can be a significant problem. Even though the knit‐lines result in the facer being unbonded at intermittently spaced portions of the board, across the surface of each insulation board, a relatively significant percentage of the surface area of the facer does not provide adequate wind‐uplift resistance for adhered systems. When we stop and think about this item or issue, it may be a problem over the life of any particular adhered roof system. Not all of our Member contractors, designers, and even some of our membrane manufacturer representatives may realize the potential magnitude of the emerging issue.



Variation in Board Thickness:

Judging by some of the photos being sent in by our WSRCA Members, and from what we have seen in the field on projects this year, polyisocyanurate roof insulation affected by visibly‐apparent knit‐lines can also result in a variation in board thickness, which can have another set of potential problems. One of these issues is continuous R‐value: Insulation R‐value averaging was once allowed by the Code. However, as you may be aware, most code jurisdictions no longer allow R‐value averaging, and now they require Total R‐value throughout the entire roof area. If the current polyisocyanurate insulation (i.e., which is knit‐line affected) varies in thickness, which some of the photos herein from WSRCA Members clearly show, then the Total R‐value of the roof system may be placed in question or jeopardy from a strict code violation perspective. And, if any of our Member contractors are having to guarantee the roof’s R‐value, which occasionally is required for sensitive projects, the Total R‐value over the full roof area may be affected by insulation that is compromised by knit‐lines with significant depressions along each of the insulation boards.





Potential for Condensation Issues:

In a mechanically‐fastened roof system with multiple layers of insulation board and potentially a cover board, where no vapor retarder is installed, the grooves in the boards create open pathways for air movement. In winter time heating climates, if there are any air leaks from the interior of the building, these pathways could help convey warm, moistureladen air through‐out a roof area and potentially form liquid water on the cold underside of the membrane from condensation, especially in our colder climate regions.


Problems at Membrane Lap Seam and Transitions at Boots:

Forcing a roof membrane to conform with an irregular substrate may be problematic at laps and seams in the membrane. The relatively deep grooves we are seeing in some insulation boards could increase the likelihood of voids and fishmouths at lap seams, boots, and patches, which could result in openings, the potential for moisture entry, and/or leaks.


Summary of Potential Issues: 

Unbonded Facer: Issues with the knit‐lines could be potentially problematic for wind‐uplift resistance where roof membranes are adhered directly to polyisocyanurate insulation boards if the facer is partially unbonded or has disbonded and delaminated from wind and/or thermal cycling movement along grooves or ruts at the knitlines.


Affected Adhesion: These irregularities may also interfere with proper adhesion between layers of insulation boards or coverboard in a roof assembly.


Effect on R‐value: Less of a concern, but still relevant is the possible effect on R‐value caused by changes in thickness along the knit‐lines. Especially if the roofing contractor is being required to guarantee R‐value of an installed roof system.


Air and Moisture Pathways: In a mechanically‐fastened system, grooves or ruts in both faces of rigid insulation board could provide pathways for air leakage from the building interior that may result in condensation within the roof assembly.


Drainage Interference: Grooves that run laterally across a roof that telegraph through the membrane can interfere with roof runoff and drainage.


Voids in Fully‐adhered Systems: Where roof membranes are to be fully adhered directly to facedpolyisocyanurate insulation boards, grooves in the insulation surface that intersect membrane laps can create difficulty in achieving a fully adhered seam, creating the potential for voids, fishmouths, and consequent moisture entry or leaks.



Purpose of Bulletin:

The irregularities that WSRCA is cautioning the western roofing‐industry about in this bulletin appear to be occurring with significant frequency throughout the Western States region. Because of the potential for these issues to become significant problems, which could result in wind‐related failures, reduced R‐value, standing water, debris accumulation, dark staining, increased membrane heat aging, and potential leaks, WSRCA desires all our members be aware of the issue. WSRCA encourages its members to be on the lookout and thereby try to prevent potential for problems on their projects before they occur. In addition, we want to encourage the manufacturing sector of our industry to take this issue seriously, and not as just an aesthetic problem, and to find ways to eliminate or address the problem of knit‐line irregularities and lack of facer adhesion.


In Conclusion:

As a result of continuing reported issues, WSRCA would like to update and restate our prior recommendations. In order to alleviate potential problems, WSRCA and its Industry Issues and Low‐Slope Committees continue to recommend the use of properly selected coverboard(s), appropriate for the climate and job’s conditions, and the type of roof system being installed, as a vital component in the successful design and construction of durable, long‐term, fire‐and hailresistant low‐slope roof assemblies. The use of a rigid coverboard over insulation that may have minor irregularities in the surface may help to minimize some of the potential problems those irregularities could cause.


1. Where possible and feasible, check the condition of polyisocyanurate insulation boards prior to accepting delivery and before installing them in a roofing project.


2. Educate supervising employees regarding the potential for compromised insulation boards and/or their unbonded facer(s).


3. Where possible on adhered roof assemblies, until all knit‐line affected insulation becomes a thing of the past, consider bead size and bead spacing of low‐rise foam and/or hot asphalt to attempt to more thoroughly adhere insulation and coverboards. Please note that WSRCA Low‐slope Committee intends to release a companion bulletin regarding lowrise foam insulation and coverboard adhesives in the near future.


4. Where possible and feasible, utilize coverboards rather than directly applying roof membranes to polyisocyanurate insulation boards.


5. Please continue to inform WSRCA regarding problems encountered in any of your roofing and waterproofing projects, and when possible provide photos as well.


Thank you for your support of Western States Roofing Contractors Association, and our active efforts to strengthen and advance technology and science in our industry, as well as to promote the art of roofing and waterproofing. We trust this information aids you to promote quality roof systems capable of long‐term successful performance.



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