Hi,

This is my first post on FH.  I am a DIYer not a contractor but more of the type to read Taunton and buildingscience.com and such.

I recently chopped into my building envelope for the first time to install a larger replacement window (most of my DIY work was on a different house).  I knew it would be a bit interesting to see what I found but I’m a bit baffled.

Here’s what I found, from inside to out:

drywall

6mm polyE

2×6 w fb batts

a mix (in this particular wall) of soltex sheathing and 1/2 plywood

old cupped 1/2″ vertical pine siding

a ~2″ air gap with various and haphazard lathing

1″ XPS

3/8″ stucco layer with wire lath directly in the XPS.

No tar paper, no felt paper, no building paper, no tyvek.

Now, I’ve done enough reading to know that an air gap behind stucco isn’t crazy but it’s not clear to me what it’s purpose is behind 1″ of XPS. I know XPS is not completely impermerable but it seems like vapor diffusion and air movement will not transport water in or out of a closed space that is sandwiched between polyE and XPS.  The bottom of the stucco has screed in some places and not in others. Needs it in others because the mice like the air gap.

There isn’t any strong evidence of bulk water intrusion (no serious mold or anything).

Anyways, I ended up digging into this because we upgraded a 60 30 window to 70 40 with the corresponding enlargement of the exterior opening, new header/trimmer, etc.  In rebuilding the sill, I was left wondering what to do in terms of drain plane and air gap.  In the end, I eliminated the air gap in the window area sill down by adding 2″ of XPS behind the existing 1″.  I’m not sure I love that decision but I’m making a custom concrete slab sill so I don’t think much moisture is getting in that space.  The bigger question is what to do next time and to the walls in general?

To add to the history of the house, I knew to expect the stucco on pine siding. That mod was done in 1987.  What I don’t know is how reasonable this system is?  Many other things done by the homeowner/contractor during the 1987 remodel are pretty bad. Lots of mystery nut cap electrical unions in the walls and ceilings, holes in drywall behind caps left behind (now used by mice), very very bad shower membrance work …  I’m planning to do many more windows so I need to figure out the best wall system for the vicinity of my retrofits and the degree to which I should scope creep additional stucco replacement in walls that are getting enlarged openings.

I should mention I’m in Colorado, situated in a reasonably sunny location.  Technically Zone 5b but it seems like designing for Zone 6 summers isn’t a bad idea (Santa Fe got moved into 6, so will Boulder in 10 or 20 years?).  The house heats efficiently but gets too hot in peak summer (lacks central ac).

To summarize my questions:

– Do I need an airgap behind 3/8″ stucco (not much reservoir there)?

– Shouldn’t the XPS be interior to the air gap?

– If I decide to redo entire walls, does anyone have a self-furring drain mat product they like that might let me eliminate the large interior air gap (and thus insulate more)?

– What do you think of the old existing walls system?

 

Thanks so much for reading