Filling up a warped spot in subfloor

Q: I have older 2″ wide flooring on 1 x 6 sub flooring running diagonally on joists on a 2nd story. In one area, where a HVAC duct runs between joists, the sub floor has warped and dried out leaving a void in between the two. You can feel a soft spot when walking over it.


I have some access under the floor and can drill a hole at an angle to get a caulking cartridge tip in between the subfloor and hardwood. Would silicone work or what would you recommend? I see there are injectors but it’s more for concrete. Thanks for any input and sorry it’s so wordy.

A: You could use expanding foam.  I understand there is more than one type.  One which doesn’t expand or expands only marginally.  The other, original, has wide expansion.  It is very strong when it is cured.

Are you sure your sub floor is only 1X6?  How far apart are the floor joists?  I’m not surprised the floor is flexing if this is the case.  It isn’t thick enough.


Follow-up: Thanks for answering so quickly. 2 x 10 Joists on 16” centers. They’re spaced  about an 1/8” apart. The house was built in the mid 60’s. I thought about using the expanding foam. I have a can of it.


Some people online say it might break down from the weight of walking on it. But that was on concrete subfloor with engineered wood where the concrete wasn’t level. I think I’ll try a little foam. Thanks for your help. I do appreciate it.

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