We are getting ready to build a new home and we plan to use SIP construction. However from the research i have done, even using SIP's for the roof isnt going to get me as high of an R Value as i would like.
  Anyhow i would like to know what everyone is doing to get the R50-R60 roofs i hear about. Thanks

ETA I should note that our attic space is to be finished out with knee walls and the rafters will act as the ceiling line. Thanks

What will finish the inside and outside of the roof sips?

Posted By jonr on 12 Jul 2011 08:25 AM
What will finish the inside and outside of the roof sips?

The roof sips will be placed on the rafters. I plan to finish the ceiling (bottom of rafters) with drywall. As far as the top (weather side) goes. As much as i would like to go with a steel roof i dont think its in the budget so it will probably be a more traditional asphalt type of shingle. But i am open to anything inbetween. I am thinking about closed cell spray foam in the rafter cavities before installing the drywall to help.

I've seen R55-R60 roofs done a number of ways.

On a retrofit I've been advising on recently they went with 6" of exterior rigid iso above the roof deck (~ R38 ish), with dense pack cellulose between 2x6 rafters (~ R20 center cavity, R18 whole-assembly with thermal bridging factored in) for a total of ~R55 ish. If go with deeper rafters you can do it more cheaply with more fiber/less foam, but the minimum foam/fiber center-cavity R-value ratios have to be designed for the local climate to prevent moisture accumulation in the roof deck. (On the local retrofit project it would have been fine to have only 40% of the total R in exterior foam, but there wasn't sufficient interior space to fatten up the cellulose so now it's ~65% foam-R. Using reclaimed foam from commercial building re-roofing/demolition takes the $ting out of it considerably.) This is the right approach for a kneewalled attic- don't try to air-seal & insulate the kneewalls, keep the rafter plane the insulation & pressure boundary of the building, and apply the foam budget to the exterior. Hunter, Atlas, and a dozen other iso manufacturers make panels with OSB nailer deck pre-applied (like a 4'x8' half-SIP) for exactly this type of situation, they come in both vented & unvented versions. But they're not cheap. A DIY with furring-mounted OSB or ply nailer deck can be less expensive, but it depends on local labor costs and the complexity of the roof lines.

A cheaper way to get there in attics without kneewalls is to use energy-heel/double-chord roof trusses designed to be able to handle ~20" of cellulose. It will usually take OSB or plywood on the underside to be able to support the weight of the cellulose- gypsum won't cut it for the long haul without sagging.

If you can't find a SIP that has the R value you need, then SIPs for the roof probably aren't the right choice. Perhaps use cross rafters to double the available space for cellulose and then add some rigid foam then OSB on the exterior.

Posted By jonr on 12 Jul 2011 12:41 PM
If you can't find a SIP that has the R value you need, then SIPs for the roof probably aren't the right choice. Perhaps use cross rafters to double the available space for cellulose and then add some rigid foam then OSB on the exterior.

Well wouldn't all this prove to be a bit expansive..... ?