Well zander... your on the right track,but...
As you said, "new homes" have a material like tyvek or tar paper on the outside . The real purpose of this stuff is to repel water in rain storms, the air barrier is a farce, marketing at its best!!It may slow down the air but it still comes on through.
Now we get to the old home part where things DO leak as I said earlier, It needs to be done correctly in an older home to keep out water, Otherwise it defeats the purpose of insulating if you rot the house away.The exterior clading is the most important part of this senario.
Now about foam insulation, this stuff is a sponge when it is wet. No kidding I have installed it on 3 jobs. (20 years in remodeling historic homes from 1648 on up)
The last job we had with foam had a leak from a chimney flashing area and we found it 2 years later when the chimney sweep stepped through the roof.The insulation was saturated!! The roof had decayed so bad we had to replace the missing rafter area surrounding the chimney.The kicker was the no rain in 2 months.
I am a big fan of foam in new houses because you control how the inside stays dry. I am not a fan in remodel projects as you can see.
Also with the blown in it does dry out after time if it gets wet.
Again check the web site I mentioned earlier and any links. There is a lot of misinformation out ther and this may help sort some of it out for everybody.As a remodeler and a home inspector I get to see the failures of the trades. I would hate to see some of my earlier attempts at remodeling myself.