Faced with maximizing the insulation of an existing cathedral ceiling, we were able to improve the usual insulation value by making our own vent chutes. As shown in the drawing, we used 1-in. foam-insulation board with foil on both sides. We cut the foam into strips that were equal in dimension to the distance between the rafters, plus 2 in. Then we scored the foam 1 in. from each edge. This sounds time-consuming, but the little scoring tool shown in the drawing made the job go quickly. The tool uses a utility-knife blade to cut a slice about 3/4 in. deep.
By using foil-faced isocyanurate foam board for the chutes, we were able to vent the roof and maintain a 7.2 R-value in 2 in. of space.
Bob Kelly, Kensington, NH
Edited and Illustrated by Charles Miller
From Fine Homebuilding #108
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What a waste of ti9me and foam board (not to mention the damage to our environment and planet! Densepack the cathedral ceiling with cellulose instead of partially filling with fiberglass batts or blown-in fiberglass and the ceiling does not need to be vented because no moisture-laden air will circulate and cause moisture damage. As a bonus, the total R-value will be much higher with dense-packed cellulose filling the full cathedral cavity, instead of a partial fill with fiberglass. Why prop up under-performing fiberglass with expensive foam? The old method of providing ventilation in the cathedral and slope ceilings was to prop up under-performing fiberglass which needs an open area to permit ventilation Dense-pack cellulose is packed full in the ceiling, just like in a wall, and air does not penetrate, nor does moisture. Work smart, not hard.
It's not necessarily the product that determines the outcome, as there are many time tested products readily available to use. Rather, an attention to a detailed installation combined with a chosen product + an awareness of the customers budget concerns leads to where we all want to be, a healthy and efficient place to live
(also, job well done with the foam vent chutes !)