I'm working on the addition of my house, roughly 29'x25', 1 story, with a 4:12 pitch roof.

It has a suspended concrete slab floor, with in-floor heat tubing, over a basement. (The basement slab also has in-floor heat tubing.)

The walls of my addition are roughly lined up with the outside of my suspended slab. Each wall consists of two 2x4 walls, with 1/2" of space in between them (so I have a thermal break). The outer 2x4 wall has 1/2" plywood sheathing on it. We are planning on using a certain amount of spray foam insulation on the inside of the 'outer' 2x4 wall (to seal the wall), and then, depending on finances, filling the rest of the wall cavity with some less expensive insulation.

We'll be putting metal siding on the outside of the house.

I need to put a certain amount of insulation on the outside of the suspended slab, to keep the heat loss down to a reasonable amount. However it turns out that my wife doesn't want the addition to be 'fatter' where the slab/foundation walls are (because of the insulation) than the wood wall that is above it.

I'm wondering what someone, that knows what they are doing, would do in my situation? The trade off is more money spent now (on insulation, and possibly a curtain wall) versus less money spent later on heating the addition.

If R12 on the outside of the slab is acceptable, then we could just strap the wood wall with 2x4s on the flat. That would give us 1.5" (thickness of the 2x4) and ~1" (thickness of the metal). So we could go with 2" of foam insulation (~R12) and then parge over it.

If > R12 on the outside of the slab will pay for itself in the long run, then we'll have to go with a curtain wall. If we go with a curtain wall then we are considering putting John Manville polyiso insulation on the outside of the outer wall, to get more insulation into the wall where the studs/headers/etc are.

I'm wondering, if we put the foam board insulation on the outside of the wood wall, and spray foam on the inside of the wall, what I should do w.r.t. a 'drainage plain' to make sure that any moisture that gets into the wood of the wall, has a place to go. (IE. in case my roof leaks, which it shouldn't, but...) Would Typar on the outside of the wood wall, with the foam board on top of that, help? What do people do, who are adding foam insulation to the outside of an older home?

> put the foam board insulation on the outside of the wood wall, and spray foam on the inside

I know lots of people have built such "moisture trapping" walls without problems. For example, poly in the inside and rigid foam on the outside is common around here.

Only other response I can think of is to put a ventilation plane though it - a layer that allows a tiny amount of end-to-end horizontal air flow through the entire wall. For example, studs with a few holes in them and some tiny vents somewhere. You want enough air flow to eventually dry the cavity but not enough to affect heat loss.

I have used this method and was very satisfied .However I was not told about small vents and looks like a good advice

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