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The R-value of a wall assembly takes into account a multiitude of things.
Thickness
Type of studs
Spacing of studs
Type of insulation
Type/thickness of exterior sheathing
Exterior veneer, or finish
Number, size and type of penetrations
Etc.
It's not as simple as the R-value of a certain thickness of insulation.
You don't give us any details, so who knows what your contractor should use. The numbers sound low, unless you have a bunch of windows, etc.
The actual calculations for tonnage needed are pretty involved and there's a whole bunch of factors to consider. It's more complex than it sounds and maybe your guy is being conservative. On the other hand, I have a total of 12 tons (3 units) in a two story house slightly smaller than yours, on a heavily wooded lot, in a slightly milder climate. 6 tons sounds a little short, to me.
JR
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