Insulating exterior walls and moisture
Hi all,
I have a 1960s single family home located in climate zone 3, coastal California (95062). I’m looking to insulate our exterior 2×4 walls, which are currently uninsulated. Inside out, we have drywall – 2×4 – building paper – stucco. We’ll be insulating from the interior.
The site drains fine but the soil is essentially level with the top of the stem wall. The stucco extends perhaps 2″ down the stem wall, making it buried in the dirt. I don’t believe there is any weep screed. There are some areas with slight disoloration in the stucco on the north side of the home near the soil. Overall not bad, however.
My fear is that by insulating the home, I could cause a moisture problem. Is there anything I should do to be on the cautious side? Completely re-stuccoing and insulating from the exterior is not in the budget.
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Replies
To deal with the extreme peak moisture drives of stucco requires an air gap between your building paper and the insulation layer. A half-inch is usually good enough.
Any fiber insulation needs an air-barrier on the exterior side facing that gap too. My preferred method is to install 1" wide strips of 1/2" foil faced polyiso as spacers on both sides of the studs
and one mid-way between the studs as spacers using a thin bead of foam board construction adhesive to make it stick to your building paper. Use cut'n'cobbled 1/2" foil faced polyiso as the air barrier, with a foil facer facing the air gap, which adds another ~R1 to the performance, and blocks extreme moisture drives when the sun is driving the moisture out of the stucco from reaching the cooler interior side.
Cut the polyiso air-dams about a half-inch narrower/shorter than the cavity space. Tack it in place with dabs of foam board construction adhesive, tape any seams with a decent temperature rated foil HVAC tape, then seal the quarter inch perimeter gap between the foam & framing with can foam.
For milled 3.5" 2x4s that would eat up 1" of cavity depth, leaving 2.5" for fiber insulation, for full-dimension 2x4s it would leave 3". The foam layer is worth R3, the air gap with the foil facer another R1, and compressing a "contractor roll" R13 into 2.5" would perform at about R10, compressing it to 3" would be about R11.5. Blowing it full of cellulose would deliver R9.25/R11 for 2.5"/3" respectively.
If it's a full-gut rehab (trim included) and you can afford to lose an inch of interior space, making stud-width edge strips out of 1" polyiso (or half-inch polyiso for full-dimension 2x4) would allow use of full-depth high high density R15 fiberglass or R15 rock wool, bringing the total cavity R to something close to the current IRC code minimum R20. But even if it's only R19-ish at center cavity, the 1" polyiso over the framing fraction would make most walls pretty much meet IRC code minimum on a U-factor basis.
More:
See: https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing
If the roof overhangs are shallow and the stucco is often wet, it may be worth venting your gap spaces with up to a 1" hole at both the top and bottom of each stud bay (whatever diameter fits whatever purpose-made bug-screened vent plugs are available near you) to allow the space to convection-vent whenever the stucco temp is higher (or lower) than the outdoor air temp: eg"
https://ventmastersstore.com/products/1-round-open-screen-vent-tab-style-mill?variant=5129870596¤cy=USD&utm_campaign=gs-2019-11-13&utm_source=google&utm_medium=smart_campaign&gclid=EAIaIQobChMInNLbqPSS5wIVC5yzCh1d4Qx4EAQYBSABEgLikfD_BwE
With 2' overhangs and no splash-back to speak of that may not be necessary. Most new stucco gets by with a vented weep screed at the bottom-only with no venting at the top that would allow full convective venting. Venting only the bottom of the stud bays would likely be enough, and that's where it does the most good, since the bottom is where any incidental bulk-moisture that gets in ends up.
Dana - thank you very much for the detailed response. I had read the article about insulating an old house with no sheathing and this reconfirmed my thoughts.
Follow-up - We are yet to begin demo. If we find that sheathing is installed, can batt insulation simply be put into the cavity? I'm still cautious about the buried stucco and lack of weep screed.
Additionally - if possible, I'd love to avoid the use of foam (embodied energy, potential off-gassing, added cost). Does the use of mineral wool batts and my relatively mild climate affect your response at all, for both the sheathing scenario and lack of scenario?
My guess is the mineral wool may help slightly over fiberglass being hydrophobic but it still is not an air barrier. I guess I'm trying to figure out how terrible it would be to forgo the polyiso in either scenario. Am I off in thinking that forgoing the polyiso if there is no sheathing is a terrible idea, and forgoing it if there is sheathing is a dice-roll?
Thank you again.