[Greenbuilding] SIPs vs. fix/improve the roof/ceiling

[Justin Close]

I have this odd little idea (or is it a dream?) about my current home.  
I live in a 1955 stick built house with no insulation in the walls 
(2x4).  The ceiling (2x6 rafters) is sort of insulated: once by some 
grayish blowin material and then an overcoat of pink fiberglass.  It 
isn't very well sealed at all or terribly continuous.  The roof itself 
sags in a few spots (visible from the backyard) and has some problems 
with flashing around the chimney (centrally located in the house - but 
the exposed outer brick is decaying and needs to be rebuilt, according 
to one tradesman). 

My little idea is to remove the roof and replace it with structural 
insulated panels (SIPs).  This would solidify and straighten out the 
roof and provide needed insulation and sealing.  It would also allow for 
less concern when installing lights in the existing ceiling (won't have 
to penetrate the insulation layer).  Is this possible, practical, 
sensible? 

Let the discussion ensue...

Justin