I'm at the stage now of insulating the loft. Because of adding the HVAC it's now going to be considered as living space and it has to meet the insulation codes. This requires me to have a minimum of R-15 in the walls, R-19 in the floor, R-30 in the 'cathedral ceiling' areas (the sloped sections under the roof rafters), and R-38 in the horizontal ceiling area.
Though I would have preferred closed cell foam, I didn't want to break the bank. Fiberglass turned out to be the cheapest option, but it's exceedingly time consuming to install it to a high standard. One big problem was the shallow depth of my roof rafters: 2x8. This gave me only 7-1/4 inches to achieve R-30. With foam that would have been easy, but going with the fiberglass meant that I would need to have roof vent baffles under the insulation, further reducing the available depth.
I came up with a scheme that would do it but it was a ton of work. First, I made baffles from 1/2 polyisocyanurate foam board. I had to have a 1" clear vent channel depth, so I used spacer strips cut from the foam board. Then foam board panels had to be cut to size to fit snugly for each rafter bay. This reduced the available depth for insulation by 1-1/2 inch leaving only 5-1/2 inches.
One benefit of the polyiso board is that it has a high R- value of R-3.6 in only 1/2" thickness. Another benefit is that this is foil faced, so it provides a great radiant barrer when applied with the foil side facing the ventillation gap. The R-3.6 gets to be deducted from the amount of fiberglass I would need, leaving me with about R27 to do in fiberglass.
The code also required me to have a solid backing behind the insulation in my knee walls; the bats can't be left open faced. So I used the foam board behind the knee walls too.