Help with wall system design for residential project, ideas and critiques welcome!
Preparing to begin a “pop the top” renovation. Existing house will be reduced to exterior brick and block walls and existing subfloor system over existing basement.
New interior first floor wall framing against existing brick/block.
New second floor construction to be clad in brick.
New 1,000sf rear addition to be clad in hardi plank.
Please see attached picture depicting my plan for each wall system.
Questions:
For existing 1st floor block/brick wall, should I incorporate any type of vapor barrier, or allow the wall to breathe toward the inside?
Do I need to specifically manage the transition of the existing 1st floor wall structure to the new second floor wall system with regard to water transmission?
Will dense blown cellulose be an acceptable insulation choice for the stud cavity?
For 2nd story wall, how much room do I need to plan for between the polyiso foam board and the new brick cladding? Any other important considerations that I need to be aware of?
Any other thoughts or recommendations welcome!
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Replies
You probably want an air gap between the brick and the polyiso. MortarNet has a lot of products for brick specifically. Also, you didn't specify what climate you are in. Why only 1" of polyiso? Now is the time to maximize that insulation layer, I would advocate for a lot thicker foam layer. You can use brick ties that keep the brick wall connected to the sheathing even through many inches of foam.
Thanks for the reply. I am in zone 4, Virginia. I will look into more exterior insulation. I had considered only 1 inch to eliminate thermal bridging without adding too much additional complexity or cost. That said, you make a good point that I only get this one chance to do it right.
In zone 4 the exterior rigid you show is enough for condensation control for a 2x6 wall. The cheap R value upgrade is to go from 2x4 to 2x6. Cost delta is pretty minor and exactly the same build complexity.
Most of your walls is ok except the original one. What you have there is structural masonry wall. Unfortunately that is not easy to insulate from the inside, the one that is guaranteed to work is spray foam. You can read more about it here:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-105-avoiding-mass-failures
I've tried to insulate that type of wall with interior rigid and ended up a moldy mess. Had to pull the finished wall off and spray foam.
The other option is to treat it like a veneer wall and build with a gapped stud wall same as your other walls but you have to get ventilation and drainage through the exterior to the gap.
Thank you for the 2x6 recommendation. I will price out that change, but I expect that you are right.
Regarding the insulation of the existing structural masonry wall, I really appreciate the article that you linked. As I am in zone 4, it looks like I may be ok using "wall approach five", but will need to include a fluid applied water control layer to the interior face of the masonry wall. I expect that this would be something like a Prosecco R guard?
That should also work. The interior coating needs to be vapor open, make sure to check the data sheet. You want to use higher density batts with this setup and very important to get a decent warm side air barrier. You want to minizine the amount of moist air from the house that can get to the cold brick in the winter time. If you are using a vapor retarder membrane, make sure it is well sealed the perimeter framing and best use air tight (sometimes called vapor tight) device boxes.