Theodo

If you're wanting to convert that attic into living space, then consider replacing that fiberglass insulation with 2 1/2 or 3 inch thick slabs of extruded polystyrene foam.

There are two kinds of polystyrene foam; expanded and extruded. Expanded polystyrene foam is the stuff that looks like it's made of "beads":



Extruded polystyrene foam is most commonly blue or pink in colour and is sold under the trade names of Dow "Roofmate" or Owen's Corning "Pink" polystyrene insulation:



Not only does extruded polystyrene foam have an R value of 5 per inch, compared to about 3.5 for fiberglas and 4.5 for expanded polystyrene insulation, but since extruded polystyrene foam is impermeable to air, humidity can't get into it and condense into water or freeze into frost. So, extruded polystyrene foam is it's own vapour barrier and doesn't require that polyethylene vapour barrier sheeting be installed over it.

Both Dow and Owens Corning make all kinds of metal channels that nail or even staple onto the sides of the rafters to hold the foam panels in place. You caulk around each panel with expanding foam (like that Great Stuff stuff) and then trim off the excess expanding foam before installing your drywall. In this way, you establish an air gap between the insulation and the roof decking from the eve to the ridge.

Extruded polystyrene foam is more expensive than expanded or fiberglas, but it will pay for itself in energy savings in both summer and winter.