Spray foam
I have a weirdly designed house.
The attic in the lean-to addition has 2×6 joists (actual 2×6 – old wood). The joists sit directly on top of the wall framing for the ouside wall, and extend outward from there. They’re notched to sit flat on the outside wall header. – there’s less room for insulation there, as teh joists are no longer the full six inches.
There was never any room for both insulation and ventilation, so we have a ‘hot roof’. It was created using three sheets of XPS foam sandwhiched together, and sealed with copious amounts of canned sprayfoam.
In the winter, we saw striping on the roof when the first snowfall happened, so we were thinking of getting a DIY sprayfoam kit and adding more insulation – and encapsulating the joists.
The issue is where the roof joists meet the outside wall – there’s no way to add insulation to those without lowering the ceiling on the entire roof.
How big of a thermal bridging issue will it create if I sprayfoam the joists on the entire roof, except at the bottom?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Anders,
Q. "How big of a thermal bridging issue will it create if I spray foam the joists on the entire roof, except at the bottom?"
A. The thermal losses will be relatively small from the perspective of their effect on your energy bills, but could cause problems with ice damming. I would go ahead with your plan, and monitor the ice dam situation.
If you don't want to lower your ceiling, but you still want to improve the thermal performance of the roof, another option is to install rigid foam above the existing roof sheathing. Of course, if you do that, you will also need to install new roofing.
I was worried about condensation as well - Is there a chance of condensation on the underside of the foam (or worse, between the roof and the foam), if not enough insulation is applied?
Anders,
The method you used to insulate your rafter bays is called the "cut-and-cobble" method. Here is a link to an article with more information on the method: Cut-and-Cobble Insulation.
Q. "Is there a chance of condensation on the underside of the foam (or worse, between the roof and the foam), if not enough insulation is applied?"
A. Yes, which is one reason why many experts advise against using the cut-and-cobble method for unvented cathedral ceilings. The article I linked to includes a few anecdotes of failures.
To the extent that you are successful at reducing air leaks and cracks with your two-component spray foam kit, you will be lowering the risk of your cut-and-cobble assembly.
Dear Anders,
While spray polyurethane foam is a great solution for air sealing and insulation, I would strongly encourage you to find a qualified contractor to apply the product. You can find some additional guidance about the safe use of these products here: http://polyurethane.americanchemistry.com/Resources-and-Document-Library/CPI-Statement-on-the-Safe-Use-of-Two-Component-Low-Pressure-Spray-Polyurethane-Foam.pdf