These folk are recomending a technique here
http://www.spray-insulation.co.uk/ca...insulation.htm but of
course they are selling the stuff.

I also have an MT4 IR temperature sensor
http://www.raytek-northamerica.com/c...ekNorthAmerica
to look for gaps (I got it on ebay for $35 but it works)

Apparently a 12oz can of great stuff = 13 litres so a wall 12ft wide by
8ft high and 1" cavity should take 16 cans according to my calculations
(lets say 20 to be safe) and they are about $10 each in home depot so
about $200 to insulate a wall. and I'd have 12 to 15 small holes to
fill. I have the equivalent of 6 sections of wall like that to fill.

Alternative, rip off the drywall, leaving the nice cove at top and the
baseboard... spray behind nice cove and skirting board = 5 tins spray =
$50, 3 sheets of 1" foam board = $50, 3 sheets drywall about $50 =
about $150.

Ok now the big question is so how much energy have I saved by spending
my $1200..... basically going from r4 to r8 walls.

PS. gas bills were $350 a month but that was before I insulated the
basement and roof and put in the 94% furnace, old furnace was 60%
efficient at most.

I found this http://ergosphere.blogspot.com/2006/...back-time.html
which seems to say that using 1" of foam boards you get your money back
fairly quickly (1 to 3 yrs) as it would halve the heat loss through the
walls, and then diminishing return as you get thicker board.

Now if I saved 50-60% by doing the roof and basement and furnace
already, my annual gas bill should be $1000 or so instead of $2000 - or
maybe it will end up at $2000 with rising/falling gas prices, and if I
hadn't insulated I would be looking at $3500 dollar bills this year

So if doing the walls saves 20% a year... equals 6 year payback at last
years gas price, or 3 year payback if the gas price is high. And
hopefully nice and warm and no wall condensation!