I recently purchased a 75 yr. old story and-a-half, just outside of Edmonton, Alberta. As the home is not very energy efficient, I'd like to start with the roof. The existing ceiling joists are 2X4 construction - with nothing there for moisture barrier or insulation, at all. Behind the knee walls is the exterior of the home. The attic is very small - from ceiling to roof peak is only about 24 to 30 inches, at best. There are two schools of thought here. First, insulate in between each ceiling joist, using blown-in insulation, then the attic (using same). Second, is to remove the existing shingles and roof deck - so that the roof trusses are fully exposed Then, add 2X4's on top of the old, to create 2X8's. Then add a 6ml. poly vapour-barrier, followed by about 5 1/2 inches of fiberglass insulation, then a vent (for air flow). Then, of course, re-apply the roof decking and re-shingle (whew!). I hope you can understand all that. Any advice you give would be greatly appreciated! I would likely plan the work to be done sometime in September. PS. I would also add either roof vents or a ridge vent, whichever way I decide to go.
Answer:
Let's try to make life a bit more simple shall we? I like your idea of being willing to remove the roof decking so let's go with that. Now fill in between the rafters with R-14 bats and then cross strap the rafters with 2X4s on edge and again, fill in between the straps with R-14 bats. This makes the newly installed 2X4 straps rather flimsy so now let's introduce 2X2s going the same direction as the original rafters before we apply the roof decking. We now have R-28 insulation and a 1-1/2" cavity for air to travel from vented soffits to a baffled ridge vent that goes gable to gable along the top. We do not however, have a vapour barrier. It may be getting a bit late in the year to work outside anyway so lets work inside. Line the whole of the newly worked-on section of the ceiling with 6mm poly and you have a vapour barrier. Over the poly, we install 1/2" or perhaps 5/8" drywall (the thicker the better for temperature retention - winter and summer) and we have the best of all worlds. We have a nice new ceiling to prime and paint, we have a good VB, R-28 insulation, an air movement chase above the insulation where it belongs, and as good a ventilating system as you'll find for any attic new or old.
