Brown cardboard like underlayment

Q: I’m installing pre finished 3/4 thick by 4″ wide hickory Flooring. I am tearing out the 5/8″ particle board underlayment. Under the 5/8″ particle board is 1/2″ brown lightweight stuff that I am unfamiliar with. Looks like cardboard but it’s not. Under that is 5/8″ plywood. What is the stuff? I’m thinking maybe a sound barrier of sort. Should I take it out?

I plan on installing new 5/8″ plywood in place of the particle board. However to keep floor height even I need to use either 1/2″ plywood over the brown (sound board?) or remove it as well and put down 3/4 plywood. Your thoughts appreciated. This is a condo built in 1965.

A: Good you mentioned it is a condo. They do have rules with flooring which includes sound barrier so I assume that is what you are describing. Is there some flex in the material? I would lay your plywood over top of it. But I’d rather see minimum 5/8 ply over that. 1/2″ doesn’t give your fastener much to bite into. Once it penetrates through the tongue on the board, through the plywood it isn’t likely to go through the 1/2″ sound barrier into the 5/8 under that.

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