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I live in western Washington, where we see plenty of rain, but only a month or two of freezing temperatures each year. My exterior walls are drywall with horizontal wood siding. I feel crazy as I write this, but I don't think there's any sheathing. The cavities are empty aside from any wiring.

I'm considering cutting holes high in the interior walls, blowing in insulation, patching, and painting.

Is this sort of wall suitable for insulation, and is this a project I can do myself?

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This sounds like a good candidate for the procedure you outline, done with loose fill cellulose insulation, with two caveats.

First of all, while this could conceivably be a DIY project, you can't rent a typical cellulose blower from Home Depot or the like because it doesn't have the power to really densely pack the stuff in there; as a result, the material will settle and you'll wind up with an uninsulated band on top. You need one of the more powerful professional machines that can do wet-blowing and dense-packing.

Secondly, you live in a rainy climate, and it's likely that the empty wall cavities have been permitting the boards of your wood lap siding to dry readily when they get wet due to convection within the wall and heat leaking out from the interior. These traits are bad from the perspective thermal comfort and utility bills, but fixing them will in all likelihood reduce your walls' drying potential, which could lead to your siding rotting and the paint peeling. If your house has very wide roof overhangs or a wraparound porch or something such that the walls rarely get very wet, this may be no big deal. But if not, the outlines project is much riskier and it may be worth it to expand the project and remove the siding so you can install modern sheathing with a weather-resistant barrier like double-layer tar paper or Tyvek. And if you're going to do that, you might as well add even more insulation in the form of several inches of rigid foam or mineral wool boards over the sheathing--or even in lieu of wood sheathing, in the case of foam!

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    +1. Advice I've seen from the pros is that the rental blowers are fine if you just want to pile cellulose fill into the attic, not great if you want to fill walls due to the packing issue. The other point... arggh. I think I want more insulation in my walls, but I do not want to deal with ripping off and replacing the cedar shingles while they're in good shape if I can possibly avoid it. And I presume I couldn't nail shingles into insulation outside the sheathing in any case, so the wall-wrap might be the right answer there. – keshlam Dec 8 '14 at 20:29
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You will need a specialty insulation blower and not the one you find at home depot or lowes. You would drill holes at the top of the cavity and blow in your insulation being careful not to overfill and blow the dry wall out. The process is slow and time consuming but can save you a lot of money especially if your walls are not insulated.

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