[Greenbuilding] insulating on sheathed roof

[Speireag Alden]

On 2007, Dec 16, at 11:12, George Tremblay wrote:

> Thanks for your comments, Ben.  I've been gradually re-siding my way
> around the house, installing foam on the exterior all the way up to  
> the
> roof sheathing (strapping over the foam to leave a rain screen  
> under fiber
> cement siding), so I believe the envelope would, in fact, be pretty  
> well
> sealed with the proposed plan.  I can see that removing the roof  
> sheathing
> and spraying foam in the rafter bays could produce a tighter seal, but
> does it make sense to trash all of the fiberglass batts and existing
> sheathing?

     I missed the early stages, so I'm sorry if this is not relevant.

     If you're going to put foam on top of the existing roof deck,  
why not make that roof deck your barrier?  There's no law that says  
that the barrier must be at the exact inside of the insulation.  It  
could be within the insulation, as long as it's not too close to the  
outside.

     A layer of hot-mopped tar, or bituthene membrane, and foam on  
top of that, with the roofing of your choice on top of that, and you  
could leave the existing insulation, under the existing roof deck,  
right where it is and not worry about sealing it.

     But I'm probably missing something... like the joint where the  
rafters meet the wall.  Hm...

-Speireag.