I loved the artical about Topaz in M.B. April 2012. Could you please tell me, when talking about lagging the boiler, what the "thin layer of insulating material" under the planks, is? I am new to steam and have never heard or seen this in any of the various books I have on the subject. As I am about to lag a boiler for my own boat this info. will prove very useful. Many thanks, Dave K.