The insulation at eaves level will leave your loft area comfortable for use and free from condensation. By installing the insulation at rafter level, it keeps the loft void warmer and prevents condensation build-up, which can otherwise occur when increasing insulation at ceiling level. The warmer, drier, cleaner roof space eliminates the risk of pipes and tanks freezing. http://m.youtube.com/embed/ggLAUsiuI_o
After researching this, Im curious about a method called poor mans sprayfoam where foamboard is cut to size between studs and sprayfoamed around the perimeter. Aside from the labour doing the job yourself, the cost seems comparable, is one better than the other? What about the quality of the canned sprayfoam around the perimeter? When i see sprayfoam done, it is never a full depth of the studs, so you would get 2.5-3 inches in a 3.5" (2x4) wall. Any thoughts? 
A. You can use closed-cell spray foam that way if you want, although you should realize that the spray foam will prevent the roof sheathing from drying inward. This approach should only be used if the roof sheathing is bone dry, because it will never dry when sandwiched in this manner. Most building scientists would prefer to see fiberglass rather than closed-cell spray foam under the roof sheathing in this type of assembly.

Roof Coating Seal

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