Insulation for Exterior Brick Wall
Climate zone 2 – all brick, one-story 2400+ sq house.
We are to choose between 2×6 studs with full fill open cell spray foam insulation on the inside part of the wall VERSUS a combo of 2×4 studs with 3.5″ nominal fill open cell spray foam PLUS 1″ of some sort of exterior rigid foam insulation (trying to figure out if we want the premium Rockwool Comfortboard 80 or just the Dow Board/Owens Corning pink Foamular XPS board — we have a stiff budget, gulp).
I would be happy with choosing the XPS rigid foam combined with the nominal 3.5″ spray foam on the inside of the exterior wall BUT knowing that XPS foam can have a thermal shift due to shrinkage over time plus the attraction ants and termites have to it makes me want to rethink and choose Rockwool instead, which is more expensive. Unfortunately, I am still working on finding out the price comparison on these products mentioned. If I go with Rockwool it’ll probably be the thinnest they have (1.25′) as we are in hot and humid Houston. My builder thinks that the open cell spray foam on the inside of the exterior wall is more than enough for our needs and that it’s overkill to try to add to that but with my research it seems like air leakage is a big deal to have to manage besides bulk water management. Good thing is, we as the homeowners have the last say on what ends up in our future home.
Please help out with the best cost-effective strategy for our build. (CZ 2). Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Before settling on a solution you may want to factor in availability and lead time. Currently Rockwool is on 4 month+ delivery in our area, necessitating a shift from RW external insulation to double stud dense pack on a current project.
please elaborate what a double stud dense pack is...i hope more people will chime in on their recommendations for my dilemma...yes, rockwool is at a 200-day lead time last time I checked unfortunately.
Since you asked about them, here’s an article for you:
A Case for Double-Stud Walls. Double-stud walls filled with dense-packed blown-in cellulose insulation is a favored wall assembly among high-performance home builders in cold climates.
You want a solution that air seals, eliminates thermal bridging, has as high R value as possible, can actually be implemented within your time frame, fits the budget, is environmentally sustainable and does not introduce other issues (insects/termites).
Your 2x6 stud option has the issue of thermal bridging, but meets most of the other goals.
Your 2x4 + ext foam option has the environmental and insect issue, and lower R value.
Suggest 2x6 top and bottom plates, with 2x4 studs at 12" oc, offset every other one (one flush to inside, next flush to outside), except at window and door jambs, with open cell foam for air sealing, followed by damp sprayed cellulose to fill the balance of the wall. That seems to hit all the goals.
- The offset studs reduce thermal bridging
- The 2x4 @12" oc studs are probably less expensive than 2x6 @ 16" or 24" oc.
- The open cell foam addresses air sealing (although there are more environmentally friendly ways to do this, it is difficult to impossible to obtain the required quality unless you do it yourself, so open cell spray foam is a slight compromise).
- The damp applied cellulose fills the balance of the wall with insulation and is more environmentally friendly and far less expensive than filling the wall with spray foam.
I'm not sure it is worth while chasing high wall R value in your mild climate. Most of your energy savings are probably had at an R5 wall assembly, if your walls are double brick, you are pretty close to that already.
If this is new construction and you need to meet code, there is not much point going above code min. There is no ROI at all for a 2x6 SPF wall in your climate. Keep it simple and build a standard brick veneer + stud wall with batts.
Either way, your effort is best spent on air sealing the structure, if this is an older home, 1" to 2" closed cell spray foam does a pretty good job of sealing up the walls. You definitely don't need more than that.