The NRCA has recommended the use of cover boards over plastic foam roof insulation when incorporated into bituminous roof systems for years.

Say you’re a building owner or facility manager and you’ve solicited bids from three roofing contractors for a mechanically attached TPO system. You’ve heard this is the latest thing in roofing — TPO offers great reflectivity for energy savings, and it’s welded at the seams to provide a monolithic membrane once it’s in place.

The contractor you’ve done business with for years — good reputation and trustworthy — is thousands of dollars higher than the low bidder. So when you call him and tell him that, he wonders aloud what system configurations the other guys bid. You look through their bids and find the roof system descriptions: 2-inch polyisocyanurate roof insulation screwed to the steel roof deck and mechanically attached TPO. Your guy asks, “Did they include the insulation cover board?”

Your response: “Cover board?”

“Yeah, the National Roofing Contractors Association (NRCA) recommends installing a cover board over all plastic foam insulations,” your guy says.

You decide to question the other contractors. The response from one guy is that the membrane manufacturer doesn’t require a cover board for the warranty, while the other says that even the proposed insulation manufacturer doesn’t require one. So what’s the deal?

This is fast becoming a familiar scene in the roofing industry. The NRCA has recommended the use of cover boards over plastic foam roof insulation when incorporated into bituminous roof systems for years. The first bulletin regarding this practice was released as “Technical Development Bulletin #9” (July 1981) in a joint statement with RIC/TIMA (Roof Insulation Council of the Thermal Insulation Manufacturers Association) for cover boards over polyurethane foam roof insulation. The revised Bulletin #9 (September 1988) expands the bulletin to incorporate polyisocyanurate (isocyanurate) and phenolic foam insulation products.

Photo courtesy of Johns Manville Roofing Systems

What is a Cover Board?

The definition of cover board as described in the NRCA 5th Edition of the Roofing and Waterproofing Manual: “an insulation board used over closed cell plastic foam insulation (e.g., polyisocyanurate) to prevent blistering when used in conjunction with hot bituminous membranes. Suitable cover board insulation(s) are glass-faced siliconized gypsum board, glass-fiber board, perlite board, wood-fiber board or mineral-fiber board. Cover boards also are recommended between polyisocyanurate insulation and single-ply membranes to protect the polyisocyanurate.”

The NRCA expounded on this recommendation in the 5th edition of its manual with the following language, “NRCA also suggests that designers consider the benefits of using an appropriate cover board with roof systems other than those employing a hot-applied bituminous membrane.” The NRCA cites reasons for the suggestion as the reduction of possible incompatibility problems incurred using certain material combinations (i.e., polystyrene insulation and PVC roof membranes), reduction of thermal bridging or thermal drift of the roof insulation system, and where roof traffic may damage low-density primary insulations.

The Canadian Roofing Contractors’ Association (CRCA) also has recently issued a technical bulletin stating that the use of isocyanurate requires the contractor to develop a “proper understanding of its manufacture, storage and application to result in well constructed ... roofing systems.” To complement this technical bulletin issued to CRCA members, the Canadian standards for manufacture of isocyanurate insulation are changing. The new standards address, among other characteristics, compressive strength, permeability of the isocyanurate insulation and revised R-value testing.

Changes in the ASTM minimum standards for cellular plastic foam roof insulations have been proposed, with recommended changes in physical properties of the insulation boards forthcoming. These changes include: grading the isocyanurate by compressive strength (Grade 1 compressive strength is minimum 16 psi, Grade 2 is minimum 20 psi, and Grade 3 is minimum 25 psi); maximum dimensional stability limit of 2 percent (vs. the existing standard of 4 percent which would allow a 96-inch board to expand/contract 3 13/16inches); and redefining the published R-values of isocyanurate roof insulations with “long-term thermal resistance” (aged, in-place) R-values.

Why All the Commotion?

What are the reasons for the recommendation for a cover board in all cases where cellular plastic foam roof insulations are incorporated into a roof system? After all, haven’t polyurethane, polystyrene and polyisocyanurate foam roof insulations been used in most roof installations for years? It is estimated that over 65 percent of the low-slope roof systems in North America incorporate cellular plastic foam roof insulation, with 55.1 percent incorporating isocyanurate foam.

The failure rate of low-slope roof systems solely attributable to problems with isocyanurate, relative to the amount of isocyanurate used is likely to be low. However, when the cellular plastic foam roof insulation is problematic, the results can be catastrophic to the roof system. Most times, the roof system will require removal and replacement.

The following four conditions likely can be averted or compensated for by incorporating a cover board over a base layer of cellular plastic roof insulation:

1. Blistering of Bituminous Membranes. With the advent and eventual widespread use of cellular plastic foam roof insulations, the industry experienced problems with blister formation between the hot-applied bituminous membrane and the cellular plastic foam roof insulation. Blistering can occur when hot asphalt is applied to the facer of the isocyanurate insulation, and residual moisture in the polyisocyanurate insulation core is released by elevated temperature.

The release of the NRCA Technical Bulletin #9 and its subsequent adoption by most parties in the roofing industry virtually eliminated blistering caused by the application of hot-applied bituminous roof membranes directly over isocyanurate insulation. A desirable property for a cover board is low moisture content, therefore reducing the possibility of blister formation. When a cover board is installed over the base layer of polyisocyanurate foam insulation, blistering is unlikely because a buffer zone is created to prevent blister formation between the cellular plastic foam roof insulation and the bituminous membrane.

2. Dimensional Instability of Isocyanurate Boards. Shrinkage, cupping and warping of isocyanurate insulation boards can cause gaps between the insulation boards, resulting in the loss of thermal resistance of the roof system. Where shrinkage, cupping and/or warping of the isocyanurate boards occur in conjunction with a mechanically attached single ply (and when high humidity conditions exist on the interior of a facility), condensation of moisture can occur on the underside of the single-ply roof membrane.

When the roof deck is poured-in-place structural concrete and the polyisocyanurate roof insulation is mechanically attached beneath a single-ply roof system, the isocyanurate can be particularly affected by condensation. Resultant problems with dimensional stability of the isocyanurate are likely to develop because of the relatively high-equilibrium moisture content (approximately 30 percent by weight) inherent in concrete.

What can happen to the roof system as a result of these conditions?

  • Absorption of water by the isocyanurate boards, exacerbating the cupping/warping situation.

  • Deterioration of steel components like insulation/membrane fasteners and steel roof deck.

  • Spalling of concrete where penetrated by fasteners and exposed to repeated freeze-thaw cycles.

  • Drippage of water into the interior even when the roof membrane is watertight.

  • Cupping/warping of the isocyanurate boards also can cause problems in a fully adhered EPDM roof system when field seams of the EPDM occur over the warped/cupped edges of the isocyanurate boards. Because the roof membrane is adhered to the facer of the isocyanurate boards on either side of the joint, cupped/warped isocyanurate boards at joints stress the EPDM. The stress can be great enough to open the field seam.

The incorporation of a cover board with the joints staggered/offset from those in the base roof insulation layer may retard or prevent condensate from forming on the underside of the roof membrane by providing a relatively continuous insulation layer. It is generally accepted that minor warping, cupping and edge cavitation of isocyanurate boards incorporated into hot-applied bituminous roof systems has been “masked,” or covered over, by the inclusion of a cover board. It is likely that the use of a cover board in single-ply roof systems will provide the same type of “masking.”

3. Thermal Bridging at Fasteners. When single layers of roof insulation are mechanically attached to non-insulating structural roof decks (e.g., steel, plywood) the fastener heads/plates are a thermal conductor from the top of the roof system to the interior of the facility. When interior conditions are warm and moist and exterior conditions are cold, water can condense on fasteners penetrating to the interior of the facility resulting in condensate drips or “false leakage.”

Accelerated aging of single-ply membranes can occur over fastener plates because of thermal bridging, resulting in deterioration of the roof membrane at these locations.

Installation of the roof membrane system over a cover board adhered to the base layer of cellular plastic foam roof insulation provides a thermal break between the interior and the exterior. This greatly reduces the chance for condensation on fasteners on the interior of a facility and allowing the roof membrane to age at a more consistent rate at locations where fasteners exist.

4. Crushing of Cellular Plastic Foam Insulation. The current ASTM standard (C1289) for compressive strength of polyisocyanurate roof insulation is 16 psi for Type II isocyanurate. The standard has not been sufficient to produce isocyanurate resistant to crushing under point loads in many instances. Ballast operations for single-ply roof system installation, for example, can crush isocyanurate insulation, as can the repeated point loading beneath access ladders or equipment in single-layer isocyanurate configurations.

Crushing of cellular plastic foam insulation in mechanically fastened single-ply roof systems becomes extremely detrimental, as the level of fastener heads used to secure the insulation and roof membrane remains constant but the top surface of the insulation becomes lower. The crushed insulation creates a situation that allows the single-ply roof membrane to “tent” at fasteners, rendering the membrane easily damaged when roof traffic occurs on or near the fasteners.

One undesirable side effect of crushed cellular plastic foam roof insulation is the resultant loss of insulating value in the area where it is crushed. In some environments, this can result in condensation occurring on metal roof components.

An additional undesirable effect of repeated roof traffic or point load crushing of the isocyanurate is the separation of the facer from the isocyanurate foam core. This is of particular concern when the roof membrane is fully adhered to the insulation facer. Wind uplift resistance obviously is greatly reduced, not only in the immediate area, but adjacent the affected area also. Once wind forces begin to affect the weakest area in the roof system, the wind uplift forces will continue to peel the facer from the insulation core and the attachment of the roof membrane is compromised.

The use of high-density cover boards (e.g., wood fiber, moisture-resistant gypsum boards) disperses the point loads, typically over the entire cover board area, reducing the prospects of the compression damage and facer separation. Some cover boards, like moisture-resistant gypsum boards, have glass-fiber facers. Because of the encapsulation of facer fibers and the relatively high density of these boards, the facers are not likely to be separated by traffic/crushing.


Again, there are four essential reasons for the recommendation for incorporating cover boards in all low-slope roof systems over cellular plastic foam roof insulation: the cover board can retard blistering of a hot-applied bituminous membrane applied roof system; the cover board may mask any dimensional stability irregularities in the plastic foam insulation; the cover board will provide a thermal break for the roof system over steel fasteners (when adhered to the mechanically attached base insulation layer); and the cover board may reduce potential damage to the plastic foam insulation from traffic by dispersal of loads over a larger area.

The Wrench in the Machine

Of course, all the recent tech bulletins and standards revisions for isocyanurate may become moot if isocyanurate manufacturers are unable to formulate and implement blowing agent replacements that comply with the U.S. Environmental Protection Agency mandates to effectively ban the use of HCFC-141b (hydrochlorofluorocarbon) as a blowing agent. Then, it’s a whole different ballgame with more lessons forthcoming from the “School of Hard Knocks.”