Hello,

A client has hired me to come up with an insulation plan for the second floor of his old Maine farmhouse,  zone 6.  The plan is to create a conditioned living space.  It is currently framed out like a barn, with 6×6 rafters on 7 foot centers and purlins between them on 4 foot centers.  He has hired a designer who is calling for a structural ridge and rafters on 16 ” centers framed down to a  3 foot knee wall.  All of the new framing will happen inside of the old framing.  The existing metal roof and sheathing would remain.  

Here is what I am proposing for insulation for a vented roof assembly.

  First thing is to slap 2 inches of EPS on to the existing rafters, perhaps with some framing support behind as needed, to create vent channels in between the EPS and the sheathing.  The existing purlins would not interrupt the air flow between the ridge and soffit vents. 

Inside of the EPS there would be rafters, strapping, membrain and drywall. So, pretty straight forward.  Here are some small questions…

1. Will EPS dry to the outside and should it be air sealed?  We will tape the membrain as an airseal layer.
2.  In determining the r value of this assembly and the rafter depth, here is what I am figuring…. If we tape the EPS, I read somewhere on GBA that the air space between the rigid and the sheathing would be worth R-5.  2 inches of EPS rigid is  R-8.4.  2×12 rafters would give us 11 inches plus 1.5 inches of 2×4 strapping for an r value of 45 between rafters and strapping.  So, nominally the r value of the assembly would be 53.4 with an extra r-5 for the vented space. 
3.  This seems like a great assembly for dense pack cellulose and I have figured the r values based on that.  Is 2 inches of EPS on 3.5 foot centers strong enough to resists the pressure of dense packing?
4. Finally, a large flat roofed dormer is planned for the south side.  The client wants to do an unvented roof assembly here, with 51% of the rigid up against the sheathing.  Is there any problem integrating an unvented dormer roof assembly with a vented roof?

Thanks,

Stephen 

Thanks