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Remember heat moves up. By air sealing the bottom plate area in the crawl space or basement then sealing the top plate along the attic, you in effect have stopped air flow moving through the walls. If the air is not moving the existing insulation works much better.
An example of this that I have talked about before is a sweater on a cold day. You put it on go outside. Feels real warm. A breeze blows then all of a sudden you feel the chill going through you. Same thing happens with insulation. It keeps you warm, until air blows through it then it allows for the heat to be drawn away. Stop the air, you stop the cold.
This is why foam works so well. It stops the air from moving. There are other insulation products that also as part of their system that are sprayed on the new wall to air seal then their green insulation product gets put in. With foam its one stop system. You get the benefit of the higher R value plus the air sealing all in one package.
Foam the top sills, both along the outside walls, and along the tops of the inside walls. Even if you do not see any openings, The space between the wall board and framing is enough to draw out lots of air from the house. Do the same in the basement., Inside walls and outside walls if the basement is not heated or cooled. If the equipment is located in this area, then just do the outside wall area where the framing meets the foundation wall. You do not need a lot, just seal the area with one or two inches then add fiberglass if you like.
Lots of folks think that the vapor barrier stops the air in the wall, Not true. The vapor barrier is just that, a vapor barrier. It stops moisture from getting into the insulation to a point in which it can cool down and condense into a liquid causing mold and other issues. Where within the insulation does this occur? This would depend upon outside temps and how much air is moving through the wall. I have seen condensation form on the back of the vapor barrier and in some cases against the outside plywood. We see this a lot when people install fiberglass insulation in an attic against the roof sheathing. Moisture gets trapped and with cold winter roof the plywood gets wet and rots the roof from the backside out. That is why with fiberglass insulation or even blown in cellulose you need to ventilate to remove moisture. Not heat.
As far as doing the walls to get to your answer, Foam is injected ideally from the outside of the house. As it expands it crushes the old insulation that was installed when the walls were put up. A long pipe is pushed down and in some cases up to it cannot move any more, then as the foam comes out the hose is pulled out until it fills the cavity.