Problem: My newish front loading washer shakes the house somewhat. As it speeds up during the final spin, there is a point at which it really shakes the house quite a bit; at full speed it is not too bad. I'd like to stop the "extra" shaking.
My diagnosis: I've checked that the washer is level. The washer is in a tight closet, with only 1/2" gap on one side, but it does not seem to be hitting the side wall. Since this only occurs at a narrow range of speeds, I'm assuming this is a resonant frequency of the floor system. Does this sound right?
Proposed solution: Stiffen the floor diaphragm to raise the resonant frequency above the washer's range. What is the best way to do that in this situation? The house is one story with crawl space, 26' between exterior foundation walls, with a girder at 14'. Floor joists are old 2x10s (2" x 9.5") #2 douglas fir, 26' long. Subfloor is old 1x6 (7/8" thick) T&G. The washer is basically in the middle of the 12' span.
Any suggestions appreciated. My only thoughts are to sister each of the two joists under the washer (if there is no plumbing interference) or to sheath the underside of those 2 joists (or 3 or 4?) with plywood (using screws to allow for future access).