Cover Boards: The Membrane and Insulation Protector

Continuing on our roof system component analysis—after discussion of the roof deck, substrate board, vapor retarders and insulation—we now have worked our way up to the cover board. For the purpose of this discussion, the cover board is defined as the board placed upon the insulation as the final substrate to which the roof cover will be placed.

The purpose of the cover board is multifaceted; it can include:

    Insulation Protection: Placed to protect the thermal layer from the often deleterious effects of repeated foot traffic, which can result in insulation crushing, loss of roof-cover adhesive, inability to resist wind uplift and mechanical- fastener puncture through the membrane.

    Asphaltic core boards are very flexible and will conform to irregular surfaces and offsets without fracture. Here crews work to install the cover board in bead-foam adhesive in preparation for the three-ply modified bitumen roof cover. PHOTO: Clark Roofing

    Asphaltic core boards are very flexible and will conform to irregular
    surfaces and offsets without fracture. Here crews work to install the cover board in bead-foam adhesive in preparation for the three-ply modified bitumen roof cover. PHOTO: Clark Roofing

    Enhanced Roof-cover Adhesion: Cover boards can enhance the bond between the roof cover to the substrate.

    Enhanced Resistance to Wind Uplift: Cover boards and their ability to enhance the bond of the roof cover to the underlying substrate can result in an increased wind-uplift rating above and beyond that which can be provided with organic-faced insulations. They reduce the possible effects of facer-sheet delamination.

    Enhanced Fire Resistance: Many cover boards will enhance the fire resistance of the assembly.

    Hail Protection: Numerous studies show the value of cover boards in enhancing a roof cover’s ability to resist damage by hail.

    Provides Separation: A cover board provides separation between a roof cover and insulation that may not be compatible or the attachment adhesive of the roof membrane is not compatible with the insulation.

    Reduces Thermal Shorts (Energy Loss): Thermal insulation is often attached to the roof deck with mechanical fasteners, which results in conductive heat loss, up to 7 percent according to the Rosemont, Ill.-based National Roofing Contractors Association. This is a large value when some roof covers, which utilize mechanical attachment, purport to provide energy savings. Furthermore, when only one layer of insulation is used (a cardinal sin in my opinion) an additional 7 to 8 percent energy loss can occur. Placing a cover board above mechanically attached insulation and/or a single layer of insulation will enhance the energy performance of the roof system.

    Enhanced Roof-system Performance: I firmly believe the use of a roof cover board in a roof system improves the overall performance of the roof system and increases the probability of the roof attaining a long-term service life, which is the essence of sustainability. NRCA agrees; the organization recommends the use of cover boards in all low-slope assemblies.

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Insulation Types, Application Methods and Physical Characteristics Must Be Reviewed, Understood and Selected to Ensure Roof System Performance

Designing and constructing roof systems (see my previous articles about roof decks, substrate boards and vapor barriers) continues with the thermal insulation layer. The governing building codes will dictate the minimum R-value required and, based on the R-value of the selected insulation, the thickness of required insulation can be determined. This plays into the design of the roof edge, which will be the subject of future articles. For now, let’s focus on insulation.

Photo 1: Polyisocyanurate (ISO) with organic facers

Photo 1: Polyisocyanurate
(ISO) with organic facers

Thermal insulation has multiple purposes, including to:

    ▪▪ Provide an appropriate surface on which the roof cover can be placed.
    ▪▪ Assist in providing interior user comfort.
    ▪▪ Assist in uplift performance of the roof system.
    ▪▪ Provide support for rooftop activities.
    ▪▪ Keep the cool air in during the summer and out during the winter, resulting in energy savings.

INSULATION OPTIONS

For the designer, there are numerous insulation material choices, each with its own positive and negative characteristics. Today’s insulation options are:

    ▪▪ Polyisocyanurate (ISO)

  • »» Varying densities
  • »» Organic facers (see photos 1 and 2)
  • »» Double-coated fiberglass facers (see photo 3)
  • ▪▪ Expanded polystyrene (XPS) (see photo 4)

  • »» Varying densities
  • ▪▪ Extruded polystyrene (EPS) (see photo 5)

  • »» Varying densities
  • ▪▪ Mineral wool (see photo 6)

  • »» Varying densities
  • ▪▪ Perlite
    ▪▪ High-density wood fiber

With today’s codes, the use of perlite and high-density wood fiber as primary roof insulation is very limited. The R-value per inch and overall cost is prohibitive.

Some attributes of the more commonly used insulation types are:
POLYISOCYANURATE

Photo 2: Polyisocyanurate (ISO) with organic facers

Photo 2: Polyisocyanurate
(ISO) with organic facers

    ▪▪ Predominate roof insulation in the market
    ▪▪ Organic and double-coated fiberglass facers (mold-resistant)
    ▪▪ Varying densities available: 18 to 25 psi, nominal and minimum, as well as 80 to 125 psi high-density cover boards
    ▪▪ Has an allowable dimensional change, per the ASTM standard, that needs to be understood and designed for
    ▪▪ Can be secured via mechanical fasteners or installed in hot asphalt and/or polyurethane foam adhesive: bead and full-coverage spray foam
    ▪▪ Has an R-value just under 6.0 per inch but has some downward drifting over time

EXPANDED POLYSTYRENE (EPS)

    ▪▪ Has good moisture resistance but can accumulate moisture
    ▪▪ Direct application to steel decks is often a concern with fire resistance
    ▪▪ Has varying densities: 1.0 to 3.0 pound per cubic foot
    ▪▪ Very difficult to install in hot asphalt; basically not appropriate
    ▪▪ Certain products can be secured with mechanical fasteners or lowrise foam adhesive
    ▪▪ Has stable R-values: 3.1 to 4.3 per inch based upon classification type

EXTRUDED POLYSTYRENE (XPS)

    ▪▪ Has good moisture resistance and is often used in protected roof membrane systems and plaza deck applications
    ▪▪ Direct application to steel decks is often a concern with fire resistance
    ▪▪ Has varying compressive strengths: 20 to 100 psi
    ▪▪ Not appropriate to be installed in hot asphalt
    ▪▪ Has stable R-values: 3.9 to 5 per inch based on classification type

MINERAL WOOL

    ▪▪ Outstanding fire resistance
    ▪▪ Stable thermal R-value: 4.0 per inch
    ▪▪ No dimensional change in thickness or width over time
    ▪▪ Available in differing densities
    ▪▪ May absorb and release moisture
    ▪▪ Can be installed in hot asphalt or mechanically attached

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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Substrate Boards

The third installment in my series on the roof system is about the substrate board. (To read my first two articles, “Roofs Are Systems” and “Roof Decks”, see the January/February issue, page 52, and the March/April issue, page 54, respectively.) For the purpose of this article, we will define the substrate board as the material that is placed upon the roof deck prior to the placement of thermal insulation. It often is used in part to support vapor retarders and air barriers (which will be discussed in my next article in the September/October issue).

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it.

The type of substrate board should be chosen based on the roof-deck type, interior building
use, installation time of year and the cover material to be placed upon it.

Substrate boards come in many differing material compositions:
• Gypsum Board
• Modified Fiber Reinforced Gypsum
• Plywood
• High-density Wood Fiber
• Mineral Fiber
• Perlite

Substrate boards come in varying thicknesses, as well: 1/4 inch, 1/2 inch, 5/8 inch and 1 inch. The thickness is often chosen based on the need for the board to provide integrity over the roof deck, such as at flute spans on steel roof decks.

TOUGHNESS

The type of substrate board should be chosen based on the roof-deck type, interior building use, installation time of year and the cover material to be placed upon it. For example, vapor retarder versus thermal insulation and the method of attachment. Vapor retarders can be adhered with asphalt, spray foam, bonding adhesive, etc. The substrate board must be compatible with these. You wouldn’t want to place a self-adhering vapor retarder on perlite or hardboard because the surface particulate is easily parted from the board. Meanwhile, hot asphalt would impregnate the board and tie the vapor-retarder felts in better. The substrate board must have structural integrity over the flutes when installed on steel roof decks. The modified gypsum boards at 1/2 inch can do this; fiberboards cannot. If the insulation is to be mechanically fastened, a substrate board may not be required.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

It should be more common to increase the number of fasteners to prevent deformation of the board, which will affect the roof system’s performance.

The substrate board should be able to withstand construction-generated moisture that may/can be driven into the board. Note: In northern climates, a dew-point analysis is required to determine the correct amount of insulation above the substrate board and vapor retarder, so condensation does not occur below the vapor retarder and in the substrate board.

Substrate boards are often placed on the roof deck and a vapor retarder installed upon them. This condition is often used to temporarily get the building “in the dry”. This temporary roof then is often used as a work platform for other trades, such as masonry, carpentry, glazers and ironworkers, to name a few. The temporary roof also is asked to support material storage. Consequently, the substrate board must be tough enough to resist these activities.

The most common use of a substrate board is on steel and wood decks. On steel roof decks, the substrate board provides a continuous smooth surface to place an air or vapor retarder onto. It also can provide a surface to which the insulation above can be adhered. Substrate boards on wood decks (plywood, OSB, planking) are used to increase fire resistance, prevent adhesive from dripping into the interior, provide a clean and acceptable surface onto which an air or vapor retarder can be adhered, or as a surface onto which the insulation can be adhered.

PHOTOS: HUTCHINSON DESIGN GROUP LTD.

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