Can this be done?? does it work for making low temp radiant floor solar heat without the expense of panels???
 do I need to protect the R 40 Sip osb panel surface under the metal roof( metal  mounted on fur strips on the Sips)..from condensation??..
  ...and if so, how best?? ZIP pnals on Sips?? Tar paper?? Ice edge roofing??...Any ideas if this is do-able?? Thanks guys..All new to this, but designing new TF home, 2000 sf and looking for real SIMPLE solar heat solutions...thanks...Lowell, Massachusetts area...

semasse64,

The SIP should be protected with a vapor barrier in a cold climate and a self adhering self sealing bituminous roll roofing underlayment in a warm/wet climate. Then I would use metal furring channels with grooves to snap the pex into. See image below. Here's a website to look at

http://www.libertynrg.com/purlin.html

The hollow purlins might be advantageous for letting the roof breath/dry better.

(Panelwright, Thank you for correcting me)

Thank you for putting your build location. It makes it much easier to provide sound advice based on building science that's applicable to your region.
Therefore, DO NOT put a self adhering underlayment directly on a roof SIP. This would violate one of the most common building science principles which is to allow an assembly to dry in at least one direction. If building in Massachusetts, the direction of drying should be to the outside. The majority of OSB SIP systems will detail a tape or similar vapor barrier at all inside panel joints. If you follow the manufacturers recommendation for sealing the system and then follow the advice of applying a vapor barrier to the outside, you may be setting yourself up for a problem in the future. If you decide to leave the tape off the inside and proceed with an exterior vapor barrier, you will need to ensure your joint sealing is 100% perfect (not likely). The one or more areas that warm and/or moist air finds it way to the exterior will result in a dew point and condensate accumulating in your roof assembly with no way to dry.

My solar advice; concentrate on a passive design that uses a complete SIP envelope with air tightness at <.8 ach @50. If you are successful in building an efficient envelope, you WILL be surprised at how little alternative energy is needed. Efficiency first!
Al

thanks for the advise Panelwright...i understand what you mean about efficiency first...given the low heat loads of this type of structure..what is best way to heat with solar???...sounds like pain in the A to install pex under roofing without worry about condensation...so, best to keep/use solar water panels mounted on roof surface???.....also, is cold or hot roof best with asphault shingles and/or metal roofing?? thanks again..steve