Rigid insulation on the interior face of the wall
I am considering putting 2″ of rigid insulation on the INSIDE of my perimeter wall in a home I am preparing to construct. This will serve as a vapor retarder and add r10 to the wall assembly that will include 5.5″ of dense cellulose (r20 + r10 is not a bad wall). Here is the breakdown of the wall I am proposing: hardi Plank on 1×4 furring strips, Tyvek or similar air barrier, plywood sheathing, 2×6 framing at 16″oc, 2″ rigid (xps) with taped joints, GWB.
By using plywood sheathing instead of OSB and including a 3/4″ drainage gap I expect the wall will be able to dry to the outside if water gets inside the wall and should avoid rot. Thoughts?
The real question in my mind is the impact of the interior rigid. I have never seen this done and am trying to determine why. The pros of this are clear: limits thermal bridging, issues of fastener sagging are eliminated (see explanation below), interior GWB meets code requirement for fire protection, insulation work can be done from the interior standing on the floor, stockpiling insulation inside the house is easy fast and cheep. Cons are casing extensions for Windows, electric outlets needs to be well sealed or located elsewhere. What else? The material cost won’t change BUT the labor is cut considerably (especially since I could do it myself before the GWB sub arrives. Installing it myself on the exterior is too much for a average skilled diy guy like myself.). So what am I missing here? What is the downfall of the assembly I am proposing?
I have gotten great advice from this forum in the past. Please share your thoughts again if you don’t mind.
Here is some background on the project that might help:
I am an architect preparing to build a house for my family south of Boston. I’ve posted questions about vapor retarders and insulation on perimeter walls previously but the mystery of this topic continues for me. The dialogue and advice has been terrific. I released the drawings for bid and have spent the last couple weeks analyzing the numbers and talking it through with the contractors. The guys I am talking with are folks I’ve worked with multiple times and the repore is good so I am continuing to learn more and more about these current thoughts on building science and sustainability (that’s why they call it practice). The conclusion that we’ve reached is that to add rigid insulation on the exterior is an expensive proposition both material wise (no surprise) and labor wise (bigger surprise than I expected). Working from staging, lifting material two stories, temporary fastening, concerns about screws sagging / furring shifting under the weight of cement board planking (if the rigid is more than the 1.5″ min thickness (I was proposing 3″) the moment on the screw can be significant and if spaced far apart it is likely they will cut into the rigid and sag a bit in a few years), and eastern MA labor costs are making this assembly unachievable on my budget.
I’ve designed many houses over the years and I consider myself to be a thoughtful advisor to my clients. Designing a house for myself over an extended period of time has given me the opportunity to consider every decision much more carefully than is typical in practice and the cost is much more of a personal issue. I’ve been an environmentalist since I was a cub scout in the early 70’s (yes, I am in my mid fifties and have practiced for 30 years) and want to do the right thing here but I have to admit I am realizing the financial cost of some of my sustainability decisions is very high and a challenge for my financial abilities. $13000 (material cost) worth of rigid insulation buys a lot of electricity and the electricity doesn’t need to be financed through a mortgage. Banks only lend 30% of your gross income so additional big ticket materials like this insulation cost are challenging to fit into a budget if excess cash is not available. These are more difficult choices than they seem and help explain why the average home is vastly less sustainable than I would like it to be.
I want to insulate this house really well (my original design called for a 12″ double stud wall which was totally out of the question price wise) but I need to design a way to limit the cost of that insulation as much as possible to make it financially achievable. Is the assembly I propose above that solution or is their an inherent flaw in it that I am missing?
Thanks for your thoughts and advice.
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