Insulating a Cathedral Ceiling

Dear NH,
I want to insulate the space between my den's cathedral ceiling and the roof. Can I just blow insulation in from the roof peak by removing the shingles and boring holes in the plywood? I read about this method in a magazine article. The article said this was once frowned on but seems to be the choice now. One local builder told me this once, but other roofers I talked to didn't believe me. Now I am really confused. Can you help?

Dear NN,
Blowing insulation between the roof and the drywall in a cathedral or vaulted ceiling is the easiest way to insulate this space. This can be done as you describe, but it must be done carefully so that there are no voids... areas without insulation.

Unfortunately, this installation can be potentially damaging to the roof unless there is some sort of ventilation under the roof deck. This is accomplished in renovations or new construction by installing special molded foam spacers that keep an air gap between the roof and the insulation. This allows a path for cooling air to move freely from a soffit vent to the ridge vent. Of course, many older homes did not have this sort of ventilation system built in, so if one wanted to insulate the ceiling, there was no realistic ventilation option except tearing down the finished ceiling... or ignoring the problem completely!

The two potential problems are 1) overheating of the roof deck, leading to premature aging of the shingles and 2) moisture from your home rising into the insulation, condensing and causing rot or mildew in the framing (along with lessening the efficiency of the blown insulation).

The results in your home are, in gambling jargon, a "crap shoot"! In any given home neither or both consequences may occur, depending on all sorts of factors, such as the total amount of air infiltration into the home (which would lessen moisture problems), the number of shade trees (which would lower roof deck temperatures) and the region of the country you live in.

The experience of roofers who know your area and have seen the problems should be helpful in determining the best way to do this sort of insulation retrofit, as well as consultation with insulation installers who use a variety of insulation products. Be wary of articles on any home repair situation that give you "the answer", especially when there are regional or local concerns. Realistically, all we writers can do is give you a variety of options... many possible "the answers". Only an experienced local professional can give you a more definitive local solution for your problem.


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