I have a 2x6 stick framed attic that I need to insulate. I need at least R-30 to meet code. I will need to increase the depth of the rafters to around 10 inches in order to fit batt insulation. Conventional wisdom says to fur on an additional 2x6 to gain the required depth. Any ideas, special products, etc..?

You might consider using isocyanurate rigid insulation...a 4 9/16 thick panel will give you R-25 within the 2x6's (leaving an inch above for ventilation), and then you could add another inch on the rafter faces and glue the gypsum board to that with construction adhesive. (It must by code be covered with gypsum board)

There are other code issues with finishing an attic...make sure you research them all.

You said attic. Are you trying to insulate the floor or convert it to a vaulted area? The floor is easy just pile up the insulation making sure you keep a ventilation space at the rafters using a foam or cardboard spacer. In a vaulted area the foam idea can work, although not a favorite of mine. If you go that way make sure you spray foam any gaps between the rafters and the foam board. This foam must be covered with sheetrock to make it fire safe. I'd rather fur out some and use high-density fiberglass using foam or cardboard ventilation channels all the way from suffit to ridge to provide air space for venting. .

To create the ventilation channels, we chose not to use those flimsy foam or cardboard baffles. Instead, we routered in 1/4" grooves one inch from the top edge of both vertical faces of all rafters (before we put them up), then we cut hardboard to slip into the grooves at its edges, creating a 1" ventilation baffle. It's nice and rigid. We also, like you, have 2x6 rafters, and so now that leaves us only 4" for insulation. We're going to spray in urethane foam, for about R-24, which in Sacramento is fine. If the inspector demands more, we'll just sheet over with rigid polyurethane sheets for an added R-6. Then sheetrock. You have to call out a specialty contractor to do the spraying, but you can do the extra sheet yourself, as they're available at places like Home Depot.

I hope that hardboard was perforated, otherwise you have effectively BLOCKED ventilation of the insulation.

You route the rafters?? I can’t believe anyone would do such a thing. What a total waste of time and effort not counting weakening an already undersized rafter. Plus if the roof sheeting is already on it’s almost impossible to do. If you have to it that way pieces of ½”x 3/4” parting stop nailed to the each side of the rafter is a lot easier not to say a lot faster. I also don’t understand why you are using an undersized 2x6 rafter in the first place. You talk about a foam insulation baffle as being flimsy and you use a 2x6 rafter???

Richard.
In california ventilation isn't used to control moisture issues the way it is in cold climates. It doesn't really get cold enough to cause the underside of the roof to act as a condensation plane. And even then, thing how much inward roof drive they have during the day.

So rafter vents are required to vent interior humidity in cold temperatures.

Instead, they are used to maintain shingle warranties, and allow inward drying of rain.

Routing rafters is probably not the smartest thing ever, but sometimes you gotta do what you gotta do what they code guys let you get away with. Why not just use a furring strip running along the rafter as a nailing base? Nailed to the rafter, or the roof deck. Either way would work.

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