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For fabric roofs, R-values can be obtained in a number of ways, depending on light transmission needs.
By Bruce N. Wright, AIA
Like most wall systems in architecture, the R-value is a function of building up layers of materials that have different insulative values as well as water imperviousness. This holds true for fabric roofs, and higher R-values can be obtained in a number of ways, depending on light transmission needs. If light transmission through the fabric is not needed, then a sandwich of exterior structural membrane combined with an opaque thermal insulation material and liner can be employed.
If light transmission is desired, then an insulate composite (such as Birdair’s Tensotherm™ with Lumira™ aerogel technology), can help reach the desired performance. “If light transmittance is sought,” says David Campbell, P.E., principal and CEO of Geiger Engineers in Suffern, N.Y., “there is always a trade-off between the visual light transmittance and the thermal insulation of enclosure—not unique to tension membranes.”
With hot, arid climates, color and reflectivity of the top surface of a roof membrane can be key to performance. It stands to reason that the greater the reflectivity the better the shading, although some light transmission is desirable as daylighting to minimize electrical load. However, “a small amount of thermal insulation can be very beneficial,” Campbell says.
For cold climates, it is necessary that the dew point of interior air must be in or on the envelope surface. “This is no different than for other building envelope systems,” Campbell says. “If the envelope is not thermally insulated, it is likely that the interior surface will be at or below dew point, and some means of addressing condensation is needed.” This can be solved by making sure the inner surface performs as a vapor barrier with coatings designed to keep out moisture.
Environmental checklist
The big picture in building envelopes is about what to keep in and out, and what to let through the envelope. This checklist from engineer Campbell identifies top concerns by climate.
Hot, arid climates
- UV performance of membrane materials: Fluorocarbon and silicone materials, coatings and finishes stand up very well to UV exposure; PVC and other plastics do not perform as well. (UV protective coatings address this issue somewhat.)
- Color and reflectivity: The color and reflectivity of the top surface of the membrane are key, as well as its reflectivity as it ages and soils. The greater the reflectivity of the top surface, the better the shading offered to the covered space.
- Light transmission desired: Light transmission through the membrane is generally a desirable attribute as it allows beneficial daylighting. In hot, arid climates this necessarily results in additional cooling load. The key is to define the visible light transmission desired for daylighting and provide an assembly that achieves this with the least solar heat-gain coefficient (SHGC). The smaller the SHGC the better. (SHGC is a function of the solar transmission, the solar absorptance and the heat transfer of the materials or assembly.)
Cold climates
- The material chosen must be strong enough to carry snow and dimensionally stable so that long-term snow loads do not result in detrimental creep.
- Ideally, the design should encourage snow to slide free of the membrane, but only where this can be done without creating a problem with the sliding snow (where it lands could be problematic.) Ponding of snow on tension membranes is a significant issue that requires careful design consideration.
- Building enclosure: Consider that the dew point for the interior air needs to be in or on the envelope. This is no different than for other building envelope systems. If the envelope is not thermally insulated, it’s likely that the interior surface will be at or below dew point when it’s cold, and some means of addressing the resulting condensation is needed. If the enclosure is thermally insulated, the interior surface must be a ”vapor barrier” to prevent condensation from forming in the thermal insulation where the dew point temperature is reached.