PIMA Report: Effect of Roof Traffic and Moisture on Roof Insulations

The Polyisocyanurate Insulation Manufacturers Association (PIMA) released a research report suggesting that low-slope roofs using popular single-ply roof coverings may not be suitable for the use of mineral fiber (also known as mineral wool or rock wool) board insulation when subject to roof traffic and/or moisture accumulation.

The PIMA report titled “The Effect of Roof Traffic and Moisture on Roof Insulations,” was developed as a follow-up to previous research studies from Europe that evaluated the performance of mineral fiber subjected to a combination of simulated roof traffic and increased roof moisture content. The study suggests that moisture vapor may significantly reduce the compressive strength of mineral fiber insulation leading to a significant increase in overall roofing failures.

The research report concludes that:

  • After exposure to 95 percent humidity for 48 hours, single-ply roofing assemblies installed over two different types of rigid mineral fiber board insulation lost over 85 percent of their initial compressive strength when tested for only five cycles of a walkability test, recently developed in Europe to evaluate the effects of roof traffic on roofing systems.
  • Based on this observed loss of compressive strength, all of the roofing assemblies tested were rated as “Not Suitable” for roof traffic using a classification protocol developed in conjunction with the walkability test.
  • The reduction in walkability observed in this testing was slightly mitigated by increasing the thickness of the single-ply roof covering, but the benefit appeared to be minimal.

“It is well-known that moisture may collect inside roofing systems either from internal condensation or from external leaks,” says Jared Blum, president of PIMA. “As a consequence, the presence of water vapor inside roofing assemblies may be relatively commonplace. The data from this study, combined with prior work done in Europe, suggest that moisture vapor may significantly reduce the compressive strength of mineral fiber insulation. As a consequence, great care should be taken when using mineral fiber insulation if any significant level of roof traffic and/or internal moisture is anticipated.”

A copy of the research report, “The Effect of Roof Traffic and Moisture on Roof Insulations” is available for download at PIMA’s website and is also available from PIMA members.

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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