Insulation, Radiant and Vapor Barriers - EPS over blown-in

I have plaster and lath walls with blown in insulation. To increase the insulation of the walls, I'm considering removing the plaster, putting the pink styrofoam (EPS?) boards over the laths, then putting a vapor barrier and sheetrock over that. Would it cause a problem to have the EPS and the blown-in installation combined like this? Thanks.

I doubt if you have real vapor barrier in your present walls except the natural barrier provided by the plaster and paint. If that is the case, you could use a vapor barrier.

Some may argue that you do not need a barrier over the combination of plaster, paint and foam.

Dick

I was thinking I'd remove the plaster because I don't mind doing a little demo, but I was thinking I'd leave the laths there to keep the blown-in insulation in place to save me the trouble of having to replace the entire system. I was initially thinking of adding the foam to the outside when I replace the siding, but I think there would be an issue with having EPS outside and a vapor barrier inside wouldn't there?

If your existing walls are working without a vapor barrier, why would you put one in?

Also, you more than likely have enough insulation. At which point your real source of heat loss is actually air leakage. So when you have the walls open, put up an air barrier. Like tyvek. Not a vapor barrier.

I agree with Doug that if your insulation levels in the walls are adequate, you'd do better to concentrate on air leaks rather than more insulation.

But even so, EPS foam would work better on the outside as an air barrier than just housewrap alone, and EPS foam on the outside provides a thermal break between the environment and the studs whereby a lot of heat energy is lost through through the wood itslef by conduction.

The proper place for EPS is on the exterior, not interior.

And Using foam on the exterior eliminates the need for a vapor barrier on the inside...

I didn't think that EPS was a moisture barrier though and that even if you did use it on the outside, you'd still need a moisture barrier on the inside?