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Discussion Starter #1
1949 Ranch-style home in a hot, humid area in Texas. The exterior is stucco and it is in good condition. There's no insulation in the wall cavity. The original stucco is applied to lathe on building paper, but there's no wall sheathing, just the diagonal runners you can see here.

Mold is growing on the studs, and it was all over the back of the drywall.

I was thinking of adding a 1/2" polyiso sheet in each wall cavity attached to the runners, and then spray foaming over the polyiso with Soythane to seal and insulate. Thoughts?
 

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GC Builder/Remodeler
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First, kill all the mold. If that isn't done, nothing else you do will matter in the long term. Chlorox is not the chemical tool for this task. Consult with a firm that does mold remediation to learn what you're up against and maybe they will even recommend or sell you a product suitable for homeowner use.

Next, don't waste your time with plastic sheet moisture barriers. That's just extra work. Use a good quality, CLOSED CELL foam insulation - I believe there is a closed cell version of Soythane - which will act as a moisture barrier and insulation as well as sealing infiltration areas around windows, doors, and other exterior wall penetrations. Based on your description of your location, the recommended placement of the moisture barrier is on the outside of the walls for the conditioned space. Spraying the closed cell product on the inside of your exterior walls will accomplish that placement.

Good luck with the mold. If you don't eradicate it, your house could become unmarketable or, worse, uninhabitable even by you.
 

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As pointed out, you MAY need a VB on the outside. If you do, then closed cell foam would be a good choice, I think. (1) Get experts in, as mentioned, to KILL the mold. (2) Check w/ local engineers, etc, on insulation/VB needs for your area. If the VB goes on the outside, I would think it would be IMPERATIVE to ensure that your drywall can breathe. If not, you have a double VB and that is disaster. Google the ADA (airtight drywall approach), which would likely mean tearing out all your existing drywall and doing it right. Sorry about that, but all that mold means you have a very serious problem which may mandate a very serious solution. Good luck. j
 

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Discussion Starter #4
Thanks for the help. The plan is to rip out all the drywall to upgrade the electrical and plumbing to code, and to install some quality insulation. I will be bringing in a specialist to kill all the mold in the walls and the attic.

How should I design an insulation system that will
  • Allow the stucco to dry after a rain
  • Form an air-tight barrier to the inside of my house
  • Minimize the chance for re-growth of mold after the remodel

The polyiso sheet with outward facing foil was to help with the long hot summers, and to maintain the air pocket on the inside of the stucco. Then I would use a closed cell foam to seal and insulate on the inside.

Is there anything I can do to cut down on the mold growing on the studs & runners? Maybe it won't be an issue if the interior of the house is sealed off? Anti-mold, water-proof paint? Am I over-thinking this?
 

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greenbuildingadvisor.com has lots of info on walls, and stucco just came up recently. they describe the ADA approach if you search for it. whatever you do, DO NOT have a double vapor barrier (one insides and one outside) or you WILL get more mold. also search for joe lstiburek and see buildingscience.com
 

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