Above-Sheathing Ventilation For Metal Roofs

Above-sheathing ventilation for metal roofs provides an air space above the sheathing of a roof deck that offers thermal benefits for stone-coated or standing seam metal roofs and yield energy savings in the summer and winter while also helping to remove unwanted moisture. 

Cool roofs have received much positive trade press where comfort cooling is the dominant building energy load.  In different climates with both heating and cooling loads, the wintertime effect reduces the energy benefit because the desirable roof heat gain in winter is diminished by the higher solar reflectance of the roof.  The Achilles heel of all cool roof systems continues to be the heating penalty that offsets the energy and cost savings associated with the cooling benefit of the reflective roof system.  A cool metal roof can save a home owner up to 25% in cooling energy costs compared to a dark-gray asphalt shingle.

A clear, couldless summer day was chosen to show the separate and combined effects of high solar reflectance and above-sheathing ventilation as compared to the asphalt shingle roof.  Venting the underside of the dark gray stone-coated metal shake caused significant reductions in the heat flow crossing the deck during solar noon.  The dark-gray stone-coated metal shake and the ashphalt shingle have almost identical reflectance and emittance characteristics, yet the heat flow crossing the roof deck of the dark-gray is just 70% of the heat flow crossing the roof deck of the asphalt control shingle.  The 30% reduction in heat flow is due to above-sheathing ventilation.  A  45% reduction in heat flow can occur from both above-sheathing ventilation and increased solar reflectance. 

Source:  www.metalarchitecture.com


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