Connecting the thermal dots

by Katie Daniel | March 4, 2015 10:59 am

FAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE
Despite recent code changes to improve the energy efficiency and overall thermal envelope performance of the building enclosure, parapets can continue to be an enigma in the design process. These somewhat-neglected building enclosure elements are often detailed without maintaining continuity of the thermal control layer (insulation) and air-vapor barrier (AVB) components incorporated in the surrounding wall and roof assemblies.

[1]

An uninsulated parapet can create a thermal bridge (a localized area of increased heat transfer between interior and exterior environments relative to the surrounding assemblies) in the building enclosure. In a positively pressurized building, air paths in the exterior wall or roof assembly may connect the conditioned interior with the uninsulated interstitial space within the parapet. This increases the potential for condensation and/or frost formation during colder winter conditions.

While a continuous perimeter air seal can be installed within the interstitial wall cavity at the roof slab level as a closure between the building’s conditioned interior and the parapet, this seal does not always address all air paths to the parapet. Consequently, solely depending on the continuity of such an air seal is questionable. Additionally, any exterior wall component bypassing the air seal and extending into the parapet that can serve as a conduit for air/moisture movement (e.g. an extruded mullion in a curtain wall assembly) must be properly detailed to eliminate air paths between the conditioned space and the parapet.

[2]

To minimize risk of condensation and/or frost formation during winter, the parapet can be thermally improved by installing a continuous thermal barrier around it, outboard of the AVB. This also allows the insulation to act as an isolator between the AVB and roof membrane, should compatibility concerns exist between products. Where parapets are insulated on the roof side, an acceptable sheathing substrate must be provided over the insulation before installing the roof membrane. In detailing the parapet, it is also important the integration of the roof AVB, wall AVB, and roof membrane occur at the parapet’s top horizontal surface to ensure proper sequencing, continuity, compatibility, and termination between these often dissimilar membranes.

As an example, the parapet shown here was constructed with a large, uninsulated cant that slopes from the parapet coping down to the adjacent roof deck; the insulation in the roof assembly was terminated at the toe of the cant, creating a thermal short in the building enclosure between the roof and wall assemblies (the dotted line indicates the edge of the roof’s thermal barrier). Condensation resulting from this thermal short was observed within the building interior on the underside of the metal roof deck directly below the uninsulated cant.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com[3].

David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com[4].

Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com[5].

Source URL: https://www.constructionspecifier.com/connecting-the-thermal-dots/

---