Radiant Barrier Under Retrofit Metal Roof Creates Solar Air-Heating Roof
DATE: Spring 2012
American Solar recently installed radiant barrier insulation in a previously retrofitted metal roof to create a solar air heating roof.
The radiant barrier insulation system reduced the solar heating load on the old roof and building below and created air flow paths for solar heat recovery by air-to-water heating systems.
The Headquarters building of the Marine Corp Air Station in Beaufort, SC had been previously re-roofed with a metal roof retrofit system. The brown metal roof panels were installed on a steeply sloped truss frames above the old built-up asphalt roof, creating a metal roofed attic over the entire building. American Solar installed radiant barrier film insulation below the truss frames to create solar air flow paths in the attic that feed solar heated air to the existing roof exhaust fans. The design of the radiant barrier retrofit included air-to-water heat exchangers at the exhaust fans for solar heat recovery for building heating use.
Sunlight hitting the brown roof panels heats the panels to about 80 degrees F above outdoor air temperatures. The hot panels transfer some of their heat to the air in the attic by radiation from the underside of the hot roof panels. This radiation hits the old roof at the attic floor, heating it and raising the building’s cooling energy use. Warm air in contact with the bottom of the metal roof panels also heats up and moves up the panels to the top of the attic space.
The installation of the radiant barrier below the roof panels reflects the radiant heat back up to the panels. This decreases the heating of the attic floor below and increases the heating of the metal roof panels above the radiant barrier. The hotter panels give up more heat to the air in contact with the underside of the panels, increasing the temperature of the solar heated air rising to the top of the attic.
Tests at MCAS Beaufort showed solar air in the attic was 20F warmer above the barrier than below, with a measurable temperature difference, just inches on either side of the barrier. The solar air at the top of the attic was 40 degrees F warmer than the outside air reaching 110F in April. That solar heated air moving through the roof exhaust fans transfers over 100 million BTU per year to the atmosphere. Over 300 million BTU per year can be delivered to the geothermal heating system from solar heat recovered via air-to-water heat exchangers in the attic exhaust ducts.