Speaking up for venting base sheets

Editor's note: Following are the author's opinions about venting base sheets. Views expressed are not necessarily those of NRCA.

In the roofing industry, venting base sheets have received little media attention for a product group that delivers toughness combined with the need to prevent blistering. However, the application of venting base sheets over polyisocyanurate insulation has been debated within the roofing industry in recent years. Contractors have worried about the performance of polyisocyanurate insulation in such instances—concerns range from facer delamination to dimensional stability issues. In addition, because of concerns regarding blistering and the ability of foam plastic insulation to withstand foot traffic, NRCA recommends using cover boards over polyisocyanurate insulation.

Following is the history of these recommendations and arguments for using a venting base sheet over polyisocyanurate insulation.

Uses

Venting base sheets most commonly are installed over lightweight insulating concrete decks. Using venting base sheets is the primary option versus installing standard fiberglass base sheets because standard fiberglass base sheets can degrade if the lightweight insulating concrete deck's moisture content is too high. Alkalinity in a roof deck combined with moisture can attack the binder in a fiberglass mat and cause it to lose strength. Venting base sheets solve this problem because the granule surfacing on the sheet's bottom side keeps it from directly contacting the lightweight deck. In addition, venting base sheets can prevent accidental solid attachment of a roof system to a roof deck and the inherent moisture in the deck from blistering the roof system.

The second most common use for venting base sheets is as an alternative to a cover board. Cover boards are used in bituminous roof systems to separate a system from a substrate, such as an existing roof system, and they were the first material NRCA recommended to prevent blistering when installing a hot bituminous roof system over polyisocyanurate insulation.

Cover boards can prevent a concentrated pressure buildup directly under a roof system caused by the direct hot application of roofing materials to a substrate that may contain moisture. Cover boards also provide a good substrate for a roof membrane and, when installed over polyisocyanurate insulation, provide the benefits of a second layer of insulation. Such benefits include minimizing the effects of fastener backout and reducing thermal stresses on a membrane. Although cover boards are a common choice for installation over many substrates, problems, such as blistering caused by a cover board and difficulty working around penetrations, can cause a roofing contractor difficulty.

Venting base sheets help solve these problems; they don't contain moisture that can cause blistering and are not fully adhered, which prevents membrane blistering caused by other moisture sources. Venting base sheets also are easy to field cut and place around penetrations.

Venting base sheets serve two purposes: They separate a roof membrane from an underlying substrate, which prevents blistering, and they act as a suitable base for the same roof membrane installation. In built-up roofing (BUR) applications, a finished membrane is tough and rigid, and venting base sheets are a good base on which to build a BUR system. Unlike an insulation layer, venting base sheets combine with the plies in a BUR system to form the membrane, contributing to the overall toughness of the finished roof system. And because they are spot-attached or mechanically attached to a substrate, venting base sheets are the only bases that also provide "give" to roof membranes.

From solution to anxiety

In response to concern that blistering occurred when bituminous roof membranes were solidly adhered directly to polyurethane insulation, NRCA first recommended installing a cover board of insulation over foam plastic insulation in 1978. This recommendation was followed by a Technical Bulletin in 1979 that stated contractors should either install a cover board over insulation or a base sheet to allow for venting. In 1981, NRCA issued another Technical Bulletin that stated the recommendations regarding the use of either a cover board or venting base sheet had proved to be effective in minimizing potential blister formation and continued to recommend either using a cover board or venting base sheet over foam plastic insulation.

In 1988, NRCA recommended a cover board be installed over polyisocyanurate, polyurethane and phenolic foam insulation for BUR installations. And in March 2000, this position was expanded to recommend the use of cover boards over polyisocyanurate insulation in all low-slope roof membrane systems.

In 1988, the Midwest Roofing Contractors Association (MRCA) and NRCA issued a joint research report, "Results of Uplift and Crushing Resistance Tests of Polyisocyanurate and Phenolic Foam Roof Insulations When a Built-Up Roof Was Directly Adhered Under Simulated Field Conditions." The report was referenced as part of the rationale for NRCA and MRCA to recommend contractors discontinue using venting base sheets directly over polyisocyanurate insulation.

However, the report showed no crushing or damage occurred to polyisocyanurate insulation from mechanical equipment; phenolic foam insulation severely was damaged by mechanical equipment. Testing for facer delamination was based on the application of a solidly mopped bituminous roof system (as opposed to spot attachment of a venting base sheet) and poor results occurred only when asphalt was solidly mopped to insulation at elevated temperatures (significantly above the equiviscous temperature). In addition, the research clearly showed dimensional stability problems with materials used as cover boards.

The current recommendation for the use of a cover board found in NRCA's Technical Bulletin 2000-3 provides different reasons for the basis of the recommendation. Figure 1 lists these issues.

Figure 1: Some reasons why NRCA recommends using a cover board

The argument

Why would you use a venting base sheet directly over polyisocyanurate insulation? Review of Figure 1 shows a majority of reasons for using a cover board is an "ounce of prevention" paradigm with a "cure" that can have its own set of issues. Primarily, cover boards are used to cover polyisocyanurate insulation in the event there is a manufacturing defect in the insulation (the ounce of prevention), but cover boards (the cure) have their own performance issues.

The issues that should be considered are the percentage of roof systems that have cover boards directly installed over polyisocyanurate insulation that have exhibited blistering and percentage that have venting base sheets that have exhibited blistering. Blistering of roof membranes, particularly BUR membranes, provides a serious potential for roof system failure. And blistering can cause immediate roof leaks if there is any rooftop traffic, which can make the threat of damage from heavy traffic to the insulation relatively unimportant.

Additionally, fire resistance and the benefits of installing two layers of polyisocyanurate insulation can be found when using a venting base sheet and are not exclusive to using a cover board over polyisocyanurate insulation. Figure 2 lists issues that the use of a venting base sheet addresses and a quick summary of the benefits venting base sheets provide. The last few items in Figure 2 have been used as part of the basis for the recommendation for using a cover board; however, they actually support the use of a venting base sheet.

Figure 2: Some benefits of using venting base sheets

Contractors have worried about facer delamination of polyisocyanurate insulation, which can result in poor attachment of subsequent layers of a roof system.

If facer delamination can be caused by solidly attaching a ply sheet to the facer with hot asphalt as indicated in the 1988 joint research, the use of a venting base sheet that is not fully adhered should provide much less chance for delamination to occur than a cover board that is fully set in hot bitumen.

Furthermore, a venting base sheet that is not fully adhered provides a membrane that can "give" if there are minor dimensional changes in the insulation. If dimensional changes are large, there are serious problems for the thermal resistance of the system and system as a whole—regardless of what is placed over the insulation.

It should not be assumed that a venting base sheet is appropriate for installation over a clearly deficient polyisocyanurate board; a clearly deficient polyisocyanurate board is not suitable as a base for a roof system with a cover board, either.

A rational approach

Given the different parameters that each roofing project can present, the flowchart in Figure 3 can be used to determine options for a specific project. Once options are determined, a roofing contractor can determine which installation best meets a project's needs in terms of ease of installation, economics and building code requirements.

Figure 3: A flowchart to determine whether a venting base sheet or cover board should be used

Used for more than 25 years with a proven track record, venting base sheets have not been fully appreciated by the roofing industry. Not only do they provide a solid base for bituminous roof systems while preventing blistering, they are part of membranes; they contribute to the strength and toughness of finished membranes; and they are an alternative to the use of cover boards for many bituminous roofing applications.

Helene Hardy Pierce is director of contractor services for GAF Materials Corp., Wayne, N.J.

Venting base sheet basics

Venting base sheets first appeared in the roofing industry in the 1950s, and currently, there are different variations of these products available. Most are defined by ASTM D4897, "Standard Specification for Asphalt-Coated Glass-Fiber Venting Base Sheets Used in Roofing."

A venting base sheet is a glass mat coated on both sides with filled coating-grade asphalt. The coated glass mat then is surfaced on the bottom side with coarse mineral granules and may be "button-punched." It also may have an embossing pattern on the granule side where the granules are pressed into the filled coating with an embossing roll, leaving a pattern of channels for venting. The top side of a venting base sheet is surfaced with a fine mineral surfacing primarily to keep it from sticking in the roll.

Because of the mineral granules, venting base sheets typically weigh 0.75 pounds per square foot (3.6 kg/m²) to 0.95 pounds per square foot (4.6 kg/m²), and coverage per roll usually is 100 square feet (9 m²) or 150 square feet (13.5 m²).

Granule-surfaced venting base sheets are much more similar to a fiberglass-reinforced, mineral-surfaced cap sheet than to a standard fiberglass base sheet. In fact, there are some specifications where a manufacturer's cap sheet is inverted and used as the first ply in the same manner as a venting base sheet.

Literature from eight bituminous roof membrane manufacturers that offer venting base sheets shows several uses for these products.

Developed for application where solid adhesion to an underlying substrate may be less than desirable, these products either are mechanically attached or spot-adhered to a substrate and followed by multiple plies of a built-up roof or additional plies of a modified bitumen assembly.

The substrates to which these products often are applied include lightweight insulating concrete, noninsulated structural concrete, existing roof systems and polyisocyanurate insulation. These substrates cause blistering if a roof system is hot-mopped directly to the surface, and in the case of lightweight insulating concrete, the bottom-side granules also serve as a separator between the base sheet and lightweight concrete. Venting base sheets also may be used as a base ply when the first ply needs to be mechanically attached and higher wind-uplift resistance is needed and when recommended for use over gypsum boards.

Installing venting base sheets is done by mechanically attaching a base sheet to a structural deck or spot attachment to a substrate. Mechanically attaching base sheets in a common fastener pattern of 9 inches (229 mm) on center down the lap and two rows 18 inches (457 mm) on center in a roof's field results in about one fastener per square foot. Spot attachment is done either by rolling a sheet into spot moppings of hot bitumen; strip mopping a sheet into place (care must be taken to ensure hot bitumen strips do not flow together); or through the use of a "button-punched" base sheet. Perforated base sheets are installed dry and subsequent moppings of bitumen flow through the holes in the sheet and provide attachment for the roof assembly.

Whether a venting base sheet is installed by spot attachment with bitumen or mechanical attachment with a fastener typically depends on the substrate and type of roof system being installed. Wind resistance requirements also should be a factor when determining which attachment method to use.