I hope if you are doing it between floor joists that you are talking about a two story house where you are trying to heat both levels. If it has a crawl space or unused basement underneath, then you'll want to lay it over the floor sheathing between two crippled layers. Since you asked about a boiler, I assume you are attempting this yourself? Personally, I would have a pro do the job. Too complex a job for the do-it-yourselfer IMHO. I wouldn't even tackle the job myself, and I have 12 years construction experience. No matter how much money it seems like you would save by doing it yourself, you'd still be money ahead by hiring a pro. Trust me on this one.