Ventilating Cathedral Ceilings to Prevent Problematic Icings at Their Eaves

Abstract : Building heat from an unventilated steep-slope roof system can cause bottom melting of snow on that roof's surface. This often creates icicles, ice dams, leaks and structural damage at cold eaves. A prior study of attics showed that, to minimize such problems, attic ventilation systems should be sized to keep the underside of the roof below freezing when it is 22 F (-5.6C) outside. When it is colder than that, it is easier to ventilate with outside air, and when it is warmer than 22 F(-5.6 C) meltwater seldom refreezes at eaves. In this paper, mathematical expressions for sizing airways of cathedral ceilings of various slopes, lengths and insulating abilities are presented. Coldroom tests of 16 food- (4.9-m-) long airways, some undersized and some oversized, show that the mathematics produces airways that do indeed perform as expected. In some of these tests, airways were blocked by expanding fibrous glass insulation. Air barriers and rigid insulation boards are shown to offer solutions to this problem. Design guidelines in the form of graphs make the taste of sizing cathedral ceiling airways, as well as their inlet and exhaust openings, quick and easy.