Remodeling 1910 balloon framed home with no insulation in walls
I am looking for thoughts on how to most efficiently insulate and air seal my old home..without breaking the bank. i have rough 2 x 4 balloon framed walls, 16″ oc. My current favorite idea is to insulate those bays with r-15 roxul comfort batt, then to frame a 2nd 2 x 4 wall staggered from the first and put a 2nd layer of r-15 roxul. I am also wondering about how much dead air space, if any, i should leave between the two walls. thank you for any and all input
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If you are double-studding, why not consider dense packed cellulose? If you go w/ two layers of Rox, the air space will lead to convective loops and denigrate your R value of the wall, I believe. Too, an air space like that is not a great insulator. If you want a space between the walls, stack Rox batts together like hay bales, or dense pack the whole thing.
There are lots of ways to beef up the R-value of your walls. Your suggested way (demolition of interior finishes and installation of a second 2x4 wall) is neither the cheapest nor the one that achieves the best performance.
Among the options to consider: exterior foam over the existing sheathing, exterior Roxul mineral wool over the existing sheathing, and interior foam over the existing plaster or drywall. You could also install blown-in cellulose in the existing 2x4 walls, either in conjunction with an added layer of foam or simply as a stand-alone insulation measure.
Whatever you do, don't plan an insulation system with a deliberate "dead air space." Insulation is always better than dead air.
There are a a couple of things I think about when is comes to using batts as a thermal barrier.
1st is that 40% of the stud cavities are not the standard 16 or 24 inch on center. Then there are all the wires, etc that further complicate the install. It boils down to do I want the performance based on a basically perfect install.
2nd I heard Joe L say in talking about retro fits that dense cellulose come close to an air barrier. So much so that it negates the worries about a lack of an air barrier in situations where is is not practical to install an air barrier. In this case he has the option of creatinga good air barrier but what it illustrates is that you will get a good insulation job void of air leaks that you may get with batts.
Cellulose can be combined with other options such as interior or exterior foam. He would still have the option of a double wall or furring out perpindicular to give the thermal bridging break.