Effects of Roof Color on the Accumulation of Moisture in Roof Systems

Discusses the effects of roof color on condensation development in roof systems and offers suggestions for controlling moisture acccumulation. 

Introduction:

Moisture content within a roofing assembly may fluctuate significantly over the life of the roof depending on a variety of factors including, but not limited to: moisture in the existing roof assembly; interior and exterior temperatures; interior and exterior humidity conditions; deck type; under-deck ventilation; amount and location of insulation; and presence of vapor retarders / air barriers in the roof assembly.

The potential for condensation and buildup of moisture in a roof system from interior moisture sources has always been and should continue to be a design issue that must be accounted for in the roofing system design. Furthermore, the color, reflectivity, and emissivity of the roof surface can affect the potential for condensation and buildup of moisture in a roof system.

Unplanned moisture buildup in the roof assembly can result in deck deterioration including rotting wood decks and corrosion of metal decks, growth of mold and other bio-organic organisms, deterioration and reduction of the effectiveness of thermal insulation, premature failure and deterioration of the roof assembly membrane, and reemulsification of the water-based adhesive.

Effects of Roof Color:

The use of light color reflective roofing is increasing, driven in part by requirements to meet reflectivity mandates such as the California Energy Commission’s Title 24, Part 6 and regional code requirements across the U.S.

Reflective roof surfaces typically are cooler than non-reflective roof surfaces and as a consequence, reflective roof surfaces may conserve energy by reducing the cooling load on the building.

Changing the color of a roof membrane from a dark or non-reflective surface to a light color or reflective surface effectively reduces the amount of time and the degree to which the roof is in a “drying” mode. With a source of interior humidity, a light colored or reflective roof surface can allow moisture and liquid water to build up in the roof assembly without the opportunity to evaporate or dry.

Accumulation of moisture within roof systems can be exacerbated in buildings with elevated humidity or periods of excessive moisture generation and is often not addressed in the design of the building envelope. Some examples of moisture generators include:

  • Apartment / condo buildings (showers, cooking, air humidifiers, etc., produce high levels of interior moisture).
  • Swimming pools, food processing, paper mills, and foundries.
  • New construction with high interior construction moisture (i.e. from freshly poured concrete, space heaters, wet insulation installation, drywall installation, etc.).
  • A compact ceiling assembly where there is typically drywall, batt insulation, roof deck and membrane with little or no insulation above the deck, no vapor retarder or air barrier in the system, and little or no ventilation below the deck.
  • Reroof conditions where moisture may be present in the existing system.

Things to Consider:

In new construction projects the design professional must evaluate the anticipated interior and exterior conditions and design the proper water vapor control (this includes considerations for transfer of water vapor via diffusion and air flow) if conditions mandate. Their evaluation should include dew point calculations to ensure there will not be a condensation problem. They should determine the need for a vapor retarder, air barrier, or underside roof deck ventilation. Use of light colored or reflective roofing should be based on proper moisture vapor control and a full analysis of the potential for accumulations of moisture. If adequate water vapor control measures cannot be integrated in the design, use of light colored or reflective roofing may create condensation issues.

With regard to tear-off, recover, and coating applications, a roofing professional should evaluate the existing roof assembly for signs of water penetration and/or condensation issues (water stains, wet or deteriorated insulation, deck deterioration, mold). They should also confirm if there are interior vents (such as bathroom exhaust fans) and, if present, that they are all properly ducted to the outside and in good condition so they do not allow moisture to enter the roof system. A roof design professional or climate control specialist should be consulted to evaluate the existing conditions and to develop a plan to address moisture issues within the existing roof assembly.

Some things that can be done to help control moisture accumulation in the roofing assembly include, but are not limited to:

  • Remove wet areas within an existing roof system prior to recovering the system with a new assembly.
  • Provide insulation above the deck to shift the location of the dew point.
  • Use at least two layers of insulation with staggered joints to prevent moisture migration through the joints in the insulation.
  • Provide a vapor retarder and/or air barrier to the system at the proper location within the roof assembly.
  • Provide adequate ventilation below the deck to remove moisture before it enters the roofing system (always check with local codes to confirm below-deck venting requirements are met).
  • Use a fully adhered membrane system to minimize moisture migration within the roofing system.

Consult a roofing professional when questions and decisions are to be made on condensation and refer to ASHRAE for design guides and standards.

Modified Bitumen Roofing