Building-integrated solar technologies must prove to be life-cycle cost-effective as the DoD will demand. Instead of being a bulky add-on, the integrated roof assembly results in a hardened building envelope with a high probability of withstanding today’s unpredictable weather patterns.
Importance of Sub-Purlins
In metal retrofit roof projects installed over an existing roof, specifically designed sub-purlins become the key component to make the retrofit application easy to install, economical and structurally correct.
Sub-purlins were first developed to provide a way to attach a new roof over an existing roof and meet all structural and wind-uplift requirements in the code. The design of sub-purlins must take into account the strength and wind-uplift requirements of any roof system that will be fastened to them. Design for wind uplift and loading capacity also focuses on how the roof attached to the sub-purlins will perform at corners and edges, where most roofing failures occur. Those structural requirements must meet local adopted codes. As energy efficiency of buildings became a more important design feature, these same sub-purlin products became a key component to achieving better performance from a retrofit roof application.
When considering ways in which to increase the energy efficiency of roofing, sub-purlin providers must accommodate energy-saving technologies—rigid and fiberglass insulation, above-sheathing ventilation, thermal conductivity, thermal breaks, thermal emissivity, solar reflectivity, solar-heat collection and solar-power generation— in the design of their systems while meeting code and structural criteria.
The sub-purlin system used in the demonstration project at Goodfellow AFB was made of 16-gauge hot-dip galvanized 50 ksi high-strength steel. The sub-purlins’ height of 2 1/2 inches allowed for the placement of rigid insulation in the cavity created by the sub-purlins and the roof panel ribs. Compression plates were then placed on the top layer of the rigid insulation. Panel clips were installed through the plates, the insulation and into the sub-purlin below. This type of installation created a 1-inch standoff layer between the insulation and the new metal roof panel, which creates a thermal break. That 1-inch cavity space is where the solarthermal system coils are placed to capture excess heat. The sub-purlins are designed to withstand additional dead loads for photovoltaic systems attached to the exterior roofing surface.
In addition, the installation of a subpurlin system has to be as intuitive and simple as possible, using standard tools and fasteners, which was the case on the Goodfellow AFB project.