Requesting comments on a retrofit double-wall design
I’m in the design stage of a whole-house remodel of a 1964 ranch in Evergreen, Colorado (high and dry, Zone 5B, but right on the edge of Zone 7) and would appreciate any comments on a proposed wall design. The existing home’s walls are 2x4s with 1/2″ fiberboard and plywood siding on the exterior and 1/2″ gyp on the interior. Cavity insulation is old rockwool batts. What I’d like to do is remove the drywall and rockwool, fill the existing cavities with blown-in cellulose, sheath the interior of the existing walls with 1″ XPS, frame a new 2×3 wall, 1″ off the interior surface of the XPS and insulate that wall (and the space between the new 2x3s and XPS) with mineral wool batts. I’ve attached (I hope) a sketch of what I’m proposing. I’d like to leave the existing fiberboard and siding in place. The XPS would be the air barrier, taped at all seams, floor and ceiling air barrier and to the new window bucks/gussets.
After reading Martin’s Sept. 17, 2010 post “Are Dew Point Calculations Really Necessary,” I used the suggested simple method to calculate the temperature of the inside surface of the XPS, as I would think this is the critical condensing surface. The materials outboard of the XPS have significantly higher vapor permeability. Using the average of Dec., Jan., and Feb. mean temps as the exterior temp (about 28 degrees, in our case) and 70 degrees and 35% as the interior temp and relative humidity, I calculated that the interior surface of the XPS is about 54 degrees. The dew point for 70 degree air at 35% humidity is about 40 degrees, so my wall is above the dew point. No problem, right? Just to be conservative, I did the same calcs with the average low temps for Dec., Jan., and Feb., (about 13 degrees F) and got a temperature on the inside of the XPS of 47.6. Still above the dew point. Then to be really conservative, I ran the calcs at an exterior temp of 1 degree F (the design temp for Heat Loss Calculations at my location). Temp of inside face of XPS at 1 degree outside: 43. Still above the dew point. So, this is good, right?
What I like about the wall design is the ease of building it from in inside of the house and that it leaves me with a 2×3 (really 2×4 because the studs are stood off from the air barrier) chase for MEP items, while protecting the air barrier. Any drawbacks that I need to consider?
I’m basically teaching myself this stuff and figuring it out as I go along, but thought this would be a great forum to get comments. The remodel won’t be going for Passive House certification, but the design is heavily informed by those principles.
Thanks for all the great information on this site. I look forward to your replies. Cheers.
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