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Old 01-11-09, 08:55 AM   #10
Daox
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I'm sure you could convert a conventional house to a passive house, but it would include basically tearing off every outer wall in the place to add more insulation. Say an average 2x4 wall is filled with fiberglass. That is R-11 per ColoradoENERGY.org - R-Value Table. Not even remotely close to what it should be with a passive house (R40+). Even with 2x6 construction it doesn't get that much better as it would only be about R16.5. You'd literally need to thicken your walls, or use a better R-value insulating material. Fiberglass is about R3.5 per inch. Polyurethane which is what I believe is the blown in foam is R6.5 per inch (and seals the house air tight). Even using that good stuff, you'd need 2x6 construction and fill it completely full to get close to R40.

What they did on my house which is old real 2x4 construction is add a layer of polystyrene to the outside of the wall between it and the house wrap and siding. I'm not sure how thick it is unfortunately, but thats another R2 to R5. If I ripped out the lathe and plaster and redid the walls ever, I'd think about throwing up 1 to 2 inches of polyiso insulation on the outside wall before putting up drywall. This would add another R7.2 to R14.5 to the wall and cover up some of the woods thermal bridging, but it would also make the room a bit smaller. In my case, I'd be willing to do that. If I did all that, my walls would still fall quite short of R40 unfortunately. It would probably put me somewhere around R26-33, but a heck of a lot better than my current ~R16.
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