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Adding insulation to existing walls. Blow-In or Spray Foam

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I have never used spray foam to add to existing walls. Makes sense that it would be a better way to go. My biggest hold up is expansion. Easy enough to bow a door or window with foam. I'm sure controlling fill is the key, but hard to keep up with through a small hole in the drywall.
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I wouldn't use fillable foam as I am still hearing issues from others with it filling fully & not shrinking later - hire a pro to spray it or look into dense pack with cellulose

Use the proper type around windows & doors (low-expansion) you wont have issues
Think about blown in rock wool as an option. Better R value than cellulose, it's moisture resistant (unlike cellulose or open cell foam), mildew resistant, and fireproof.
I don't think I have ever heard a recommendation for blow in rock over cellulose!

A dense pack of cellulose will net you a pretty good airseal, which will not allow air to carry moisture through the product(moisture resistance), and it is treated to be mold, mildew, and fire resistant. It may achieve a higher r-value in the general sense, but you are not comparing apples to apples when you factor in the air leakage rock wool is going to allow.
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