The Macon Telegraph
Insulating Cathedral Ceilings
If there is one area of heavy timber construction that seems to be a mystery to owners and contractors alike, its how cathedral ceilings are built and insulated. A true cathedral ceiling consists of a heavy timber rafter that is covered above the top chord with a finished decking material such as V-jointed tongue & groove wood or drywall. This is in contrast to a vaulted ceiling where the structural rafters are hidden from view by the finished surface material nailed or screwed to the bottom chord of the rafter. Most stick frame builders are familiar with vaulted ceilings but few, if any, have ever built a cathedral ceiling.
All cathedral ceilings use timbers graded for not only strength but appearance in the home. After all, they will be the focal point of most great rooms, studies, and trophy rooms in the home. After the roof rafters, ridge beam, and collar ties have been erected, the first order of business is to rout the tops of the timbers where electrical is to be placed. Remember, a cathedral ceiling does not have a cavity between the interior finish and the exterior roof deck where wiring can be hidden, it must be in place before the decking is nailed down.
Since most cathedral ceilings have a wood finish, Ill explain that first. The 2" x 6" V-jointed, tongue & groove decking is nailed down directly on the rafters and allowed to overhang the gables approximately 2 feet while extending all the way to the rafter ends (or tails) at the eaves of the home. Next, 2x6s are installed on edge around the perimeter of the roof and attached securely to the decking before the felt paper, which acts as a vapor barrier, is nailed down on to the decking.
The insulation is normally a rigid urethane foam with a foil facing on each side to reflect heat and act as an additional vapor barrier. The insulating sheets are 5-1/2" thick and are the same size as a sheet of plywood. As the sheets are laid, the seams are sealed with a reflective foil tape similar to the tape used by heating and air contractors to make ducts airtight. To provide a sturdy base for attaching shingles or metal roofing, a plywood nailbase is laid over the foam insulation being careful to insure that the seams in the plywood are at least one foot from the taped insulation seams to reduce moisture drive. Plywood seams stagger over insulation seams to avoid moisture drive.
The plywood is nailed around the perimeter with common 8 penny nails, but is pattern nailed at 12 inches on center in the middle of the plywood with 70 penny pole barn nails. These longer nails have a ringed shank near the point that go through the plywood, through the rigid insulation, and bury 1 inch deep into the tongue & groove decking. This yields an aged insulation benefit of R=38 and will meet or exceed building code for insulation and structural integrity anywhere in the Southeast.
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