I have an old cabin that we are fixing up. It’s 42ft long by 18 ft wide. The floor joists run across the width with one beam down the middle. They are 2×6’s on 24″ centers. I’m plan on putting in new 2×6’s and am wondering if it would be better to have them on 12″ centers or doubling up on 24″ centers. Thanks for the help.
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You would be fine with the joists on 16" centers. Think about what will go on top. Usually the less span between joists, the better to keep the subfloor from deflecting between the joists. Going 12" OC or doubling up at 24"OC takes the same number of joists, I'd go with 12" OC over doubling up for the reason stated above.
I did a thread on floor vibration a while back that will help explain what's happening somewhat:
http://forums.finehomebuilding.com/breaktime/general-discussion/floor-vibration
Putting joists closer together will do little to stiffen the floor. Only shorter spans or adding bearing will help significantly.
I followed ceramic tile forum
delete duplicate. original got lost in vapor for some time
Although I trust your statement in general, the folks at the ceramic tile form do suggest one other method of substantial stiffening a floor and that is to attaching (via glue and mechanical fastners) a 2 by to the bottom of a standard joist in the horizontal position. Essentially, you are creating 1 half of an I (eye) joist....actually its an inverted T.
It worked wonders under my kitchen under didn't require any additional bearling points in the basement. They have calcs over there that detail exactly the impact but I didn't delve into them.
Other than that, if you are replacing all the joists anyway are your really so limited that you can't go up to 2x8?
12" OC would be a PITA to insulate unless you were blowing in something.