[Greenbuilding] upgrading a 1959 1800 sf building in Montreal

[Stewart Abbey]

I recently purchased a 30'x60' building built in 1959. A 20x20 garage with a 
room above exposed on three sides.  Crawl space with 2 floors and a flat roof.  
Exterior is brick in good shape.  Tar and gravel roof was redone in 2008.  Some 
windows have been changed, about 2/3rds are the the 1960,s screw-on aluminum 
guillotine type that leak like a sieve!!   Oil fired hot water heat (with 
original cast iron boiler probably 60% efficient).  One heating zone for each 
floor. Main floor has 3 apartments,  2nd floor has 2  41/2room apartments.  
Heating loops all pass though the crawl space.  Walls are 2x4 construction and 
would have the standard of the day 3" of rockwool or fiberglass.
Oil consumption was 5700 liters in 2010, and 5000 liters in 2011=$4500

    Last fall I had the crawlspace walls and rim joists blown with 21/2" of 
foam.  Installed plastic on the crawlspace floor.
    The 2 second floor apartments will be empty next month.   I'm thinking of 
blowing densepack cellulose into the 2x10 flat roof cavity from the inside 
through 3" holes in the gyproc. The current ceiling insulation is 3" rockwool or 
fiberglass.  COST $5000+ plastering.  The cost seems a lot but I know it is a 
slow job to do right.  The contractor says 1 1/2 days per unit.  

    We are probably getting gas on the street this summer and I am considering 
going to a 98% efficient
gas condensing boiler. That alone should cut the heating bill by 50%. Gas is 93 
cents per cubic meter.
At the same time we would add zones so each unit would have it's own zone and 
thermostat.
I am not planning on doing major renovations as the building is sound and well 
built.

Anyone have advice on where to get the most bang for the buck?
Stewart Abbey
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