Reducing noise from above
A few years back, we remodeled our home and had a hard time finding a place on the ground for our air-conditioning units. The contractor ended up mounting them on 4×4 sleepers on top of a flat section of roof, which got them out of sight and away from any possible damage from coastal flooding. But the noise from the units is quite loud and annoying inside the house. In Keeping shop noise in the basement, you described several approaches to reducing noise from a basement shop. Any ideas that would work here?
Emmett C. Lee, Tampa, FL
Russell A. Bertrand, president of Bertrand’s Refrigeration in Wakefield, Rhode Island, replies: Because your air-conditioning units are installed on sleepers that are directly on top of the roof, the noise is being conducted directly to your house structure. Your whole house has actually become a sound chamber.
Fortunately, there is a fairly easy solution to your problem. You need to isolate the units from the roof with antivibration padding. One such antivibration pad, made by Bramec (800-843-9974), is a layer of cork sandwiched between two thinner layers of elastomeric material. Bramec’s pad is about 7/8 in. thick and comes in various sizes.
For your rooftop situation, I would cut the pad into 3-1/2-in. strips (the same width as the sleepers). I usually use a sabersaw or reciprocating saw to cut the padding. Put one layer of the pad directly on top of the roofing. The 4×4 goes down next, followed by another layer of padding. The air-conditioning unit is then placed on top of the whole assembly
Here in Rhode Island, the code allows us to place air-conditioning units directly on top of a structurally adequate roof without being bolted down. However, codes may vary in other parts of the country, requiring the units either to be bolted or strapped to the roof. I suggest checking with a building official in your area to be sure, especially if you are in a high-wind area.
By the way, we also use antivibration padding for noise reduction from air handlers installed in attic spaces. Air handlers are the fan units that distribute conditioned air. They are much lighter weight than air-conditioning units, so we often place them on a platform suspended from the rafters. In these cases, the antivibration pad is placed on top of the platform underneath the air handler.