It depends on the kind of insulation that was installed. If it was fiberglass insulation, then definitely yes, there should be a vapour barrier over the insulation to prevent moisture from condensing inside it during the winter.
Tyvek is NOT a vapour barrier. It's like a masonary paint in that it will allow individual H2O molecules to pass through it, but not liquid water. I can't say whether Tyvek does this the same way, but masonary paints do this by using particular kinds of plastic resins for the binder.
This is where I fly off on a tangent...
Quality paint resins are made out of the same plastic that Plexiglas is made of, called "poly methyl methacrylate". Imagine a long polymethyl methacrylate molecule as a long piece of wire, and a masonary paint resin as that long wire scrunched up into a ball. The spaces between the segments of wire in each ball are larger than the diameter of a single H2O molecule, but smaller than the distance between H2O molecules in liquid water. So, a single H2O molecule can pass through the masonary paint, but not liquid water. In this way, liquid water can evaporate out of the wall through the masonary paint, but rain water can't get in through the masonary paint. That helps to keep the wall interior dry. And, if I had to bet, I'd bet Tyvek works the same way. Whenever a solid material will allow moisture vapour to pass through it, it's said to be "breathable".
And this is where I come back to the subject at hand.
If the insulation in the wall is extruded polystyrene foam (like Roofmate), then it's impermeable to air infiltration anyway, and doesn't need any vapour barrier.
Expanded polystrene insulation (the foam bead stuff) is typically blown with blowing gas, and depending on the amount of blowing gas used, the foam beads can interconnect with each other, and that creates permeability through the insulation. How much permeability their is is something you really don't know, so in the case of expanded polystyrene insulation, you'd be better off with a vapour barrier over it just to err on the safe side.
Last edited by Nestor_Kelebay; 04-18-2009 at 10:05 PM.