My recommendation for adding thermal insulation to the roof of a house without disturbing the interior.

If you have a house with a ceiling just under the roof, as is the case of a finished attic, garret room, "contemporary" house etc. with only one or two roof-planes, the following outline for the installation of thermal insulation might be right for you. Without mess or disruption to the interior space, you can increase the R-value of your roof. This "second roof" atop of your existing roof will house the new insulation; no invasion of the ceiling is necessary.

This work wouldn't be applicable to your antique house etc. if you want to preserve the originality, not allowing the alteration of the appearance of the facia area. The work would result in a 6-inch higher roof with corresponding 6-inch wider facia dimension.

R-value is the enumeration-scale reference to an insulator’s resistance to heat transfer.

Directions:

Remove the existing shingles and fascia board.

Install the rafters, constructed as a frame atop the old roof, using the same dimensions, therefore matching the existing roof-area. The rafters should probably be spaced at 16" intervals (16" centers) which will accommodate the standard 14 ½"-width-size fiberglass thermal insulation.

For installation of high R-value insulation, I recommend using boards of the 6-inch width for the rafters: "two by six"--which are actually 1½" x 5½". The additional cost compared to that of a smaller board- and insulation-size is negligible especially when considering the HVAC operational-cost savings. So, with the 6-inch sized rafters, the "6-inch" thickness insulation would be used to fill the spaces between the rafters.

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Then, the rafter framework should be covered by, at least, 5/8-inch CDX plywood to which the roofing felt and shingles will be applied. The designation: "CDX", means that the plies are stuck together with water-resistant glue.

Then install shingles or other roofing material. Lightly colored roofing should be used to further prevent solar heat transfer.

Then, install the appropriate, new-width fascia board. Coat the board with a durable, high-quality primer and paint to prevent rot.

Install vents to allow condensate evaporation. These would probably be located between each rafter-cavity through the fascia board—say, round and louvered approximately about 6 cm, diameter.

If you confer with a roofer or general contractor who is hesitant, or has little imagination about this or any construction matters, find another.

Before the plywood covering is installed, make sure that photos are taken of the new frame and insulation! This will certainly be of great value upon transference of the house to the next owner.

I guess that this improvement will reduce heating and cooling costs by approximately 30% or more.

Richard Stuart Otto, 13 July 2007