I have a 90-year old house in Seattle. Exterior walls are constructed as follows: lath and plaster, uninsulated 2x4 stud bays, plank sheathing, cedar clapboard, 1" foil-faced styrofoam, vinyl siding. The house leaks like a sieve, and costs a mint to heat. I can't seem to find consensus on how to approach updating and weatherizing a house like this, though. What's the best way to seal the drafts, add insulation, and manage any moisture moving through the wall? Ideally, I'd like to keep the existing lath and plaster as most of it is in decent shape. I would like to ditch the vinyl and re-side with clapboard.

4 Answers 4


Provided you don't have knob-and-tube wiring, this one is easy. Remove the vinyl siding, cut holes through the EPS foam and the sheathing at the top of each stud bay, and inject dense-packed cellulose into the empty stud bays. Should be pretty cheap and help a ton. I wouldn't use retrofit-style non-expanding foam. It'll be more expensive, highly flammable, and some people have allergic reactions to it once it's installed. Then add as much additional EPS foam over the existing stuff as you want. XPS foam, Polyiso foam, or rigid mineral wool will also do fine. Then install 3/4" thick vertical furring strips (usually 1x3s) over that exterior insulation to form a drainage channel (important since you're in a rainy climate), and then nail your new siding of choice to the furring strips.

If you do have knob-and-tube wiring, you need to keep the stud bays empty until you have it entirely replaced with modern cable.

  • I presume the advice is similar for cedar shingle siding, except that you'd remove single shingles for each bay (or one top and one bottom; I've seen both suggested), cut a hole behind them, install the blown-in through that hole, plug the hole and reinstall the shingle...? Or is there another preferred approach? (Late-18th-century construction.)
    – keshlam
    Dec 1, 2014 at 21:53
  • Yeah, basically. The beauty of injecting dense-pack cellulose is that it's pretty non-destructive. Cheap and effective too.
    – iLikeDirt
    Dec 1, 2014 at 22:23
  • Thanks. Need to do that or have it done, I think -- not completely certain because I'm not sure what's in the walls now, if anything.
    – keshlam
    Dec 2, 2014 at 1:59
  • Thanks, @iLikeDirt. What about the vapor barrier (faced EPS or polyiso) being on the outside of the house rather than the inside? Is there an issue with moist air from the house interior moving through the wall, condensing on the inside surface of the rigid, and leading to rot or mold? This is where I get confused with what I read online (air and vapor barriers). I assume your above advice includes taping all panel seams? Does this effectively inhibit moist air traveling through the walls? Dec 2, 2014 at 4:22
  • Condensation won't happen since the insulation on the outside will keep whatever is inside it warm enough to be above the dew point. To do this, code says you need R-2.5 for a 2x4 wall and R-3.75 for a 2x6 wall, but the more the better and that's why I suggested adding even more. Taping the seams improves your air barrier and prevents air from either side from getting through, which is really more about heat loss than moisture flow.
    – iLikeDirt
    Dec 2, 2014 at 14:52

If you're going to rip off the siding, then insulate from the outside.

If it were my house, I would remove the clapboard and sheathing, then spray in expanding foam. Follow the recommendation of the foam insulation manufacturer for how to finish the outside before you install your vinyl siding.

Another good alternative is to blow in loose insulation. Since you're removing the siding, it will be easy to do. Then you can wrap the outside in a vapor barrier before you install new siding.


The best solution would be to fill the 2x4 cavities with foam. This could be done either by filling each cavity from the top or by removing the siding. You have to be careful to use the correct foam (non-expanding) if you are going to fill the wall cavity without removing the siding. There are many companies that sell non-expanding foam kits and here's a link as an example of such a product:


As long as you have well insulated windows/doors, you will realize super low heating/cooling bills and be blown-away by the superior performance/comfort of this type of insulating system.

Don't let the seemingly low R value of the spray foam affect your decision. The overall performance is not only in R-value but also in the incredible bonding of the product which eliminates air infiltration.


I live in Canada in a cold climate,I was always told that if you're going to install vapor barrier in your walls,it is best to install it on the inside portion of the walls and allow the outside cavity to breathe as you have to allow moisture and condensation to disperse from the walls.I am currently renovating my home after having major mold mildew and condensation issues. I have worked for various construction companies and we never installed vapor barrier on the outside walls.We always installed plywood or OSB and a rigid insulation,such as Styrofoam or reflective fiberglass panels, on the exterior and fiberglass batts and vapor barrier on the interior to eliminate condensation issues.You may have to take your walls down to bare studs and address all problems,such as knob and tube wiring,lack of insulation or any other issues found.You won't know until the demolition is done.All depends on your budget and timeline as well. Good Luck.

  • You almost never want a dedicated vapor barrier such as sheet polyethylene in your walls. These things cause more problems then they solve.
    – iLikeDirt
    Jan 27, 2015 at 22:59

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