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My home is in central Indiana -- climate zone 5. It has lasted 100 years with no insulation at all. I've removed all of the super thick and heavy 3/4" cement sheet rock on the exterior walls and found that the exterior cladding is all 1" pine boards, with a few patches and holes.

The house doesn't have any exterior wrap -- and right now it has vinyl siding over that 1" pine. I'm planning on leaving the vinyl siding on until I save enough $$ for hardi-plank, and at that time I'll add the house wrap and do a bunch of exterior sealing.

I've tallied up the exterior walls square footage and I've got ~1300 SQFT to insulate. I'm DIY.

I'm considering -

  1. Spray 2" thick closed cell foam i.e froth-Paks, I would require about 2600 board feet of coverage. This would cost about $3000 for 4 froth pak 620s. This would give me ~R13, and 3.2 water vapor permeance.

  2. Spray 1" thick closed cell insulation, then pack 3.5" fiberglass batts into the leftover 2.5". 1" spray foam is R6. The bats are 3.5" R11 packed to 2.5" is R9, which gives me total of R15. R11 is .22 a foot, which brings my cost down to $1700. This would give me 6.4 water vapor permeance.

  3. glue 1" thick R4 EPS foam board insulation, leaving a 1/4" margin around edges, seal those edges with spray foam to create a vapor barrier. then pack R15. the EPS, R15 batts and several cans of spray foam would cost around $1500, total R Value would be 15.

My question is this -- I'm leaning towards option #2, but are there other options that might be more cost effective and still eliminate condensation on cold exterior wall, provide a comparable R value and water vapor permeance? I'm currently investigating using a sprayed or rolled on liquid waterproofing material, which would let me use full thickness R15 bats, but the products I've found so far would be more expensive than froth-paks. I'm also considering 6-10 mil plastic sheeting glued or taped to back of stud-bay, but I haven't figured out how to air seal the plastic to the sides of the stud bays.

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  • Have you considered using a vapor permeable continuous insulation and simply leaving the wall vapor open? (You'll still want an air barrier somewhere in the wall assembly) Commented Dec 25, 2020 at 15:48
  • Side note - insulation makes it harder to run cables. So if you have any wall open, then run any electrical/ethernet cables you can while the chance presents itself. The bare minimum is to run a pull-cable from your ceiling and/or under floor space to anywhere that you might want jacks in the future, and note that.
    – Criggie
    Commented Dec 26, 2020 at 0:16
  • Note that if you don't want to rip out and redo all the wall surfaces, blown-in insulation is a perfectly viable alternative. Also note that if the house is old enough to have balloon construction, then if you are going to open walls this is a good opportunity to improve fire blocking (though the insulation will also help reduce the chimney effect).
    – keshlam
    Commented Feb 6 at 15:01

3 Answers 3

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I'd go with rock wool batt insulation in the stud bays.

Rock wool has several advantages over fiberglass. Better sound deadening ability. Better fire resistance. R-value unaffected by moisture. Low air permanence ( slows air down = slows heat loss ). Higher recycled content.

Tape and acoustic seal poly on the interior stud surface for your vapor/air barrier (climate zone 5 is vapor barrier on the warm side of the insulation). Once you've got your drywall up if you want better air sealing get aerobarrier to come in.

Spray foam insulation over time will crack so the air sealing ability is over-rated. The propellants for spray foam are not typically very environmentally friendly. While the foams are treated with a chemical to prevent fire spread they will burn when they burn they emit toxic fumes. The fire retardant chemicals are not non-toxic - avoid putting chemicals in your house if you can. Spray foam if improperly installed can be stuck in a non-cured state and continuously release volatile organic chemicals (VOCs).

I still use spray foam but I try to minimize where I use it.

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Vapor barrier goes on the WARM side of the insulation. So plastic up against the boards and insulation on top is a bad idea.

If you've got the walls down to studs and sheathing, furring out the studs to give you more depth will make the most effect on insulation, at the cost of a little interior space. If you want to combine fiber batts and rigid foam, packing the cavities with fiberglass/rockwool (or cellulose, ) and sheathing over the studs with styrofoam (so there's no thermal break in the foam layer from the studs) followed by drywall will give you a significant upgrade.

Cellulose tends to be the most "bang for the buck" insulation-wise, but is not quite as good as some on a R-per-inch basis - however, it's very good with moisture management and reducing airflow.

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Contrary to comments I've read here, cellulose is equal, or better, than other blown/batt choices in R-value, plus it air-seals each wall cavity it is packed into. It's far superior to ill-fitting batting and has a higher R-value than the foam at your proposed depths. Foam is dangerous, expensive, and a drain on our environment. A 100-year-old house is especially leaky. Cellulose will find every leak and plug it in. A 100-year-old house has full 4-inch 2x4 construction (or deeper), so it has the depth capacity to hold more cellulose, providing an even greater R-value. The fire-retardants in cellulose are also used as wood preservatives. It's all positive. Your home will be a much different environment - remarkably draft-free and easy to condition in summer and winter.

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    "Cellulose will find every leak and plug it" and spray foam won't? Also, I have cellulose blown into the walls of my 1890s house (in central IN). It's OK, but it ain't all that great. There were still plenty of air leaks.
    – FreeMan
    Commented Feb 6 at 15:07
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    "it air-seals each wall cavity it is packed into" No it doesn't. I'm not saying cellulose it bad at all and I'll even say that it is great for old home retrofits, but it absolutely is air permeable. Commented Feb 6 at 22:40

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