[Strawbale] Foams

[Andrew Webb]

I'd like to hear opinions on this problem I've been struggling with.  
There are two locally produced sandwich panels for roofing - one uses 
polyurethane and uses polystyrene sandwiched between two layers of 
corrugated metal.  The polyurethane one is 104mm thick for R4.8 (R27 US) 
and the polystyrene one is 140mm thick for R3.5 (R19.8 US).  R2.5 (R14 
US) is generally suitable for roof insulation in this climate but more 
is better.  The structural and cost advantages of the sandwich panels 
are impressive - they will span 6 to 8 metres and cantilever half of the 
span (so you may be able to do a house with only the walls as support).  
If you are happy with a raking metal ceiling then you don't need any  
further materials and they are easy to install.  They are prefinished in 
the factory.  Because of the metal ceiling they will have a thermal 
bridge from outside to inside, but this is a mild sub-tropical climate 
so it's not a major problem.

They need much less roof structure than conventional composite roof / 
ceiling systems of, say, rafters of engineered timber or steel for large 
spans and/or framing timber, more beams & supports for shorter spans, 
roof battens, metal roofing, insulation (polyester (up to R3.5), wool 
(up to R3.5) or plastic air-cell (up to about R2.5)), ceiling battens 
and ceiling lining of gyprock, masonite, plywood, or t&g timber, some of 
which need to be glued to the battens, ceiling paint/sealer and all the 
time and labour that goes into putting these components together; plus 
the various transport costs of the components (& labour), extra 
scaffolding, etc.

Not being a chemist, I would hazard a guess that the polyurethane and 
the polystyrene sandwich panels (in that order) have less negative 
environmental effects than the conventional composite system.  However, 
I still get a knot in my stomach when I think of specifying a 
polyurethane product that has twice the steel content of a conventional 
roof.  Is it just a matter of choosing the best of a bad bunch, is there 
an alternative I'm overlooking, or do you have any other thoughts on 
this?  I'm talking about fairly conventional solar / straw bale 
buildings built by conventional builders, not extreme alternative or 
owner-builder options.

Thanks,
Andrew