I See a Cool Roof & I Want It Coated Black

No White Roofs Anymore, I Want Them to Turn Black

by Richard Norris, principal, Norris Consulting Services (with apologies to Keith Richards and Mick Jagger)

 

(Editor’s Note: Richard Norris is an engineering consultant in Fremont, California, specializing in building enclosure systems, especially moisture movement and condensation accumulation issues. He has a Master of Science Degree and Doctor of Engineering Degree from the University of California, Berkeley. Norris can be reached at Richard@RNorrisConsulting.com and www.rnorrisconsulting.com).

 

White, reflective roofs reduce the heat-island effect and lower air-conditioner energy usage during the summer. White roofs also lower the heat gain of those roofs during winter, increasing heating bills. Insulation above the roof deck, combined with air sealing, is much more effective at lowering air conditioner energy usage in summer and at decreasing heating bills in winter than a dark roof. Changing from a dark roof to a reflective roof, however, changes the building physics, affecting the way that the attic or roof-attic-ceiling assembly handles moisture.

It is crucial to understand this for roof replacements on residential buildings where there is no air conditioning, such as is common in the San Francisco Bay Area. These buildings are commonly reroofed with a reflective white roof, replacing dark roofs, without the addition of insulation above the roof deck. The California Energy Code (CEC) requires this in some cases and the roofing industry applies it, even when it is a very bad idea.

 

The Science

Heat gain from uninsulated dark roofs allowed the re-evaporation of the moisture that condensed on the underside of the roof deck on cool nights. When that moisture accumulates and remains in the roof assembly for long enough, it permits the growth of the decay fungus and mold.

White roofs replacing dark roofs without the addition of insulation change the building physics and can lead to the accumulation of condensed moisture in the roof deck and framing. This allows the growth of the decay fungus and mold. It is not certain that the substitution of a white roof for a darker roof will shift the building physics enough to allow decay and mold growth. It depends upon a number of factors, of which the reflectivity and emissivity of the roof covering system are only two, but they are the most important two in most cases.

We have a lot of white roofs installed now, thanks to the requirements of the CEC. Many of these roof assemblies have condensed moisture accumulating underneath them. But, many cool roofs were installed prior to the CEC requirement that there be insulation in most roof assemblies, and many have been installed since that requirement in areas where there is an exception for roofs over interior space that is not air conditioned. One must also consider whether there is a moisture-vapor retarder, which is rare in much of California. How much insulation is in the roof-attic-ceiling assembly, what type is it, and where is it?

If the pre-existing roof assembly was accumulating condensed moisture before roof replacement, changing to a white roof covering is likely to be the critical issue. It will reduce the amount of moisture that re-evaporates and is vented out of the attic or rafter bays or is drawn down into the warm interior.

 

What to Do?

First, don’t panic. In most cases this is resolvable, if caught in time. Is there insulation under the white roof? How much? Where is it? It doesn’t take much continuous insulation to prevent condensation. An air barrier is also important, and a vapor retarder can help, too. How do we know if moisture will condense and accumulate within the new roof assembly?

 

Identify the Issue

Determine if moisture will condense. A simple relative humidity test will give you a lot of the information you need. What is the interior temperature and moisture content? How does it vary? The critical time is usually just before dawn in the winter, when the roof is coldest and windows are closed. Next, draw the ceiling-attic-roof cross-section and find the dew point. If the dew point falls in an insulation layer, condensation will be minimal. If it falls below the insulation, moisture from the air will condense, turn to liquid water, and be absorbed by some materials. There may still be condensation at the rafters and at penetrations through the insulation, such as plumbing pipes and electrical conduit. It is safest to have continuous insulation on top of the roof deck, with no gaps or penetrations.

A decayed roof deck, under a light capsheet roof. Photo courtesy of Gustavo Guerrero of Ben’s Roofing.

You don’t have to worry about condensed moisture accumulating if the dew point is in or above the insulation layer and you don’t have penetrations through it. Be careful, though, because penetrations and any hole or gap through the ceiling insulation or through the air barrier at the ceiling level will allow moist air into the attic, bypassing the insulation layer. This is a common condition in residential, and sometimes commercial, construction, especially in California.

 

The Dew Point is Below the Insulation, Now What?

Two things can be done at this stage. The first is to use a more complicated hygrothermal model to determine if the moisture that condenses in the assembly will re-evaporate and dissipate during the day. The second is to coat the roof surface black, grey, blue, red, or any color that will absorb heat. The CEC may not permit these colors, so be sure to check with your city or county prior to making any changes.

If moisture does condense, not all is lost. If liquid water is re-evaporated during the daytime, when the sun hits the roof surface, and if it is dissipated by air movement in the attic space or through the ceiling into the building interior, the decay fungus and mold will not become established and grow. A knowledgeable individual in building enclosure systems design should evaluate the accumulation of condensed moisture and determine the likelihood of fungus and mold growth.

 

Coat it Black

Coating a white roof with a non-reflecting material can reduce the condensation accumulation so that the decay fungus and mold will not grow inside the assembly. The dark membrane absorbs more of the solar energy. This causes the attic air temperature to increase if there is no insulation above the roof deck. The increased attic air temperature allows re-evaporation of the moisture that condensed during the night. If you cannot rebuild the roof right away, you can probably stop, or at least slow, progress of the decay with a dark roof coating.

A cool roof provides benefits to society and to the building owners, under most circumstances. Replacing a dark roof with a cool one, however, can lead to the accumulation of condensed moisture and the growth of the decay fungus, wood rot, and mold. Understanding the basics of building physics can help roofing professionals avoid this pitfall and the building owner can avoid reconstruction expenses.

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