G'day: I am winterizing our cottage and wish to preserve the interior appearance of the ceiling. Since our cedar shake roof is in need of replacement we are considering applying the insulation to the exterior prior to installing the new roofing material. I assume I will have to toenail new 2x4's or 2x6's to the existing rafters to begin. Can anyone give advice on the exact procedure - I checked on buildingscience.com with no success - thanks in advance - Jim

WOW Jim that's a loaded question where to start?
First will your existing roof be able to accept and carry the weight of a secondary framed roof over it? What type of existing roof framing is there? Stick framed with 2 x 6, 8, Or trussed?

This question has been asked for many years and the answer to the Best insulating hands down is Spray foam. Its about 3 times more then fiberglass insul but it has more R value then any other insul inch per inch.

That said I'm with Jmc1324 on the issues of scabbing a new roof over the old and the weight factor problem that may be here.
Adding a 2x4 or 6by is not going to do much for adding space for any R value. Don't' know where this cottage is in Geo but 9" (R) 30+ is the norm for cold areas. Maybe rethink this on what you are going to gain out of the amount of work ratio. I use 2x12s on all vaulted ceilings when I stick build here to get the (R) value and the air space needed to vent the roof. Less roofs in dim width is shot with spray foam.

Of all the roof info available there is a reason why you can't find how to scab on a roof.

As for CODE: if built before the IRC code and only doing for personal energy savings any amount of insulation is better than none I do a lot of remodeling even though I would like to see the most insulation installed if the existing ceiling or roof is left intact no code official can enforce new codes...

If a dormer is added to the house / cottage that new roof must meet current codes not the existing.