Your house wrap should go UNDER your insulation.

While you're happy about new windows, it does present a small problem.
Fitment.

The windows were spaced into comply with the previous wall structure.
If you add inches of foam/Rockwool, what does that do to the fitment around the window?

This will incur you some additional cost (probably metal flashing/sill pan to mate the window to the new wall structure).

As for "breathability". You do NOT want your insulation layer(s) acting as the lungs of your home. Because such setups inevitably accumulate so much detritus that you may as well be smoking Camel Unfiltered. A pack at a time.

But what you're really asking is, how do you want to control moisture flow in your home.

You can rely on uncontrolled flow by leaving big holes in your wall/insulation, leaving a window open, etc.

Or you can looking at your climate control (HVAC) systems and see what can be done to manage moisture in the home.


Your house wrap should be vapor-open. So efforts SHOULD be made to air-seal. NO, you shouldn't make such assemblies "airtight", as in your home it'd promote condensation and mold growth in the wall assembly. Then lay the insulation over it.
If you decide to go EPS instead, don't do it as a single thick layer. Do it as two layers. So there's a breathing space between the layers, and you can tape the outer layer.

Basically there's minimal difference (insulation-wise) between EPS and XPS. XPS starts out a bit better-rated. But as it loses blowing agent from the cells, the rated R-value decreases until you're left at EPS levels. Translation: WASTE OF MONEY.

If you need an R10. that's essentially 3 inches (3.85/inch...roughly). Would put it on as a 1" layer and a 2" outer layer.
And yes, most EPS board is strong enough to take being firred out. Most of it is rated for a compression-resistance between 10 and 60 PSI, though places like Dow and OC will GLADLY sell you stuff up to 100PSI.
Just as an aside, 20+ PSI is usually more than strong enough to use under the foundations/slabs of a house.
The only reason your foam coffee cup seems weak is that you're dealing with just 0.07 inches of foam thickness.

So your rain screen plan is a good one.

The siding, flush or firred, provides minimal R-value. Most home-grade wood siding/framing is about R-1/inch. Your siding is likely not even going to be an inch thick.

That's okay though. Between your existing insulation (guessing R11-R13-ish) and slapping R10 on the outside, you should be fine.

Hi there, thank you very much for the detailed reply and the information!

As for the windows, my hope is that being as I'm removing all of the existing siding, which is currently old 'bottle dash' stucco, covered with backer and then vinyl, removal of all this will gain an inch or two of 'depth' to help accommodate the added thickness of insulation however you may be correct in the fact that some difference in depth may still need to be addressed with flashing, etc. I haven't begun tearing down the old siding so may have to pull down a small section to see what I'm dealing with.

Just making sure I follow you on "breathability" I was referring to moisture/vapour permanence, not necessarily straight air flow. I can certainly see not wanting air to be able to flow freely through insulation to the interior of the home. I'd assume though that the interior vapour barrier would prevent most of this air flow? And I may be misinformed with respects to 'house wrap' does a vapour permeable house wrap still not act as an air barrier for wind, etc?

I had debated one layer EPS vs. two layers, I had thought that this was mostly to prevent continuous seams, but based on your comments, does two layers promote better vapour movement away from the sheathing? By a "breathing space" do you mean an actual space or simply the marginal space created when two panels are installed one over the other?

Thank you for the info regarding EPS vs XPS, and compression resistance, very helpful, glad to hear I'm on the right track. The reason I asked is that manufacturers are always happy to sell a more expensive product, promoting it as 'better' even though in many cases it's overkill and a wast of money.

I suppose the next step is determining what sort of wall thickness I'll end up with in relation to the windows so I can plan accordingly.

Yeah. You want to remain vapor open to the exterior. You don't want gaping holes in the wall system

Essentially, a single layer of taped insulation is a vapor barrier. You already have one on the interior face of the wall. You don't want to trap the moisture that way.
By going with a layered approach (and yes, staggered seams) you cut thermal bridging at the foam seams.

Yes, the house wrap serves as an air barrier. As does layered foam. Now, count up all the fasteners being drive through all of this. Wrap, foam1, foam2, furring, etc...
Ever heard the term "belt AND suspenders"? That's my thinking behind this method.

And yes, you're going to have a certain amount of vapor movement past the foam seams. So you'll see some condensation in between the two foam layers. Not terrible. The foam surfaces will act like a drainage plane. And, if done right, won't be significant enough to worry about icing up inside the wall.

Thanks again for the info, very helpful.

You mention taping outer insulation seams in your initial reply, is this to prevent exterior liquid moisture from penetrating the insulation?

Also, the manufacturer I am looking at going with for the EPS specs Type I compression at 10psi. Just to confirm, should this be sufficient to support furring strips? Type II would come in at 16psi but offer significantly reduced vapour perms and only very marginally better R value. I'd prefer to go with Type I if the compression won't be an issue. Any recommendations here?

Taping the outer seams is to prevent weather (both wind and water) from outside. It's always possible that high winds can get up under your siding and pull insulation panels (and your siding) out.
The bottom of the wall should be detailed with metal bug-guard/screening that allows the escape of water from within the wall structure.

10PSI should still plenty strong. You aren't resting the whole weight of a house on it. You're simply nailing through it.

If you're worried, talk with an engineer about it.

Drew,

Here's a Youtube video dealing with insulation detailing. Some of it addresses the bottom-of-wall detailing.

Note: This guy's down in Texas, so he doesn't have your condensation concerns.

And pardon his doofy, dudebro manner. The guy knows his stuff, he's just trying too hard to be affable.

https://youtu.be/IDFSnjuNl5U

Posted By Dilettante on 29 Apr 2020 05:47 AM
Drew,

Here's a Youtube video dealing with insulation detailing. Some of it addresses the bottom-of-wall detailing.

Note: This guy's down in Texas, so he doesn't have your condensation concerns.

And pardon his doofy, dudebro manner. The guy knows his stuff, he's just trying too hard to be affable.

https://youtu.be/IDFSnjuNl5U

Thanks for the link much appreciated! This is very helpful.