Is there any particular reason I cannot, or should not, use rosin paper as an underlayment for a floating, edge glued, engineered wood floor? I'm putting this over a crawl space I'm using as the heating/cooling system supply plenum. The intent is to have a semi-radiant heated floor so obviously I want the minimum possible thermal barrier under the flooring.

The best other choice I find is SCI (Sponge Cushion, Inc.) 1/8" rubber underlayment but it's a bit pricey. Rosin paper isn't listed in the flooring floating installation instructions.

You might need to give the flooring manufacturer a call and ask them if rosin paper underlayment will provide the desired function and if they will stand behind the warranty if RP is used..

From what I've read, most of the natural and synthetic underlayments add little in the way of R-value. What I can't find is if they a)hold up to the heat without breaking down and b)do they offgas with the heat? I understand that the heat that the floor "sees" is going to be only 85F or so.

BM