Thursday, July 29, 2010

SIPS for an R100 Roof

Well, the roof is not turning out to be easy.  the first option I've been working on is a 2x6 rafter system conventionally framed with sheathing and Grace Ice and Water Shield on the outside of it as an air and vapour barrier.  (PERSIST system).  Then 20" deep I-joists go on top of that assy, sitting on 2x4 purlins on the flat to minmimize thermal bridging.  Something is done at the top to form a 2.5" deep vent channel and then another layer of sheathing and then the roofing.  Long process, some big parts of it on scaffolding.  Costs are significant too.  Costing is complicated, but I have most of the numbers.  It is something like $19,000 of materials not including roofing.  Labour - who knows?

Then the SIPS solution.  the idea would be to frame the roof with the previously planned 2x6 rafters, then place the SIPS on top.  Air and vapour barriers are not needed, and neither is the vent channel.  It is solid insulation right through to the outside.  If code officials balk, then a conventional poly VB can be placed on the inside of hte inner frames.
Talked with Kent Truss in Barrie - they say the value of the SIPS materials for this application is about $30,000 delivered to site, (SIPS and their accessories only - no roofing).  The roof is a simple 8/12 pitch gable, total 2160SF on the outside.  Apparently this is about 2 truckloads of SIPS materials.
Installation is an additional $2/sf approx, but in our case, maybe double that.  Sips are max 12.25" thick - this is for manufacturing reasons, and also the 2x12 studs used to connect them.  since the thickest SIPs are R45.5, they would need to use 2 layers, the second being faced one-side only with OSB.  Special 14" long screws are used to secure the SIPS.  OSB facings are 7/16" standard thickness.

Also, largest SIPS panels available from Insulspan are 8'x24' (x previously mentioned 12.25"thk).


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