To spray, or not to spray (Zone 8a SE, 140+yr. Historic 3 wythe) – GreenBuildingAdvisor

Long time listener, first time caller here. Love the community and appreciate all of the contributions and support.

We have a renovation project underway in a ~140 year old 3 wythe MMU two story historic building located in South GA. It is a mixed use space, with the first floor commercial and second floor residential, and approximately 1800 sq.ft. per floor. 

We are evaluating insulation options for 2 of the exterior walls on the second level and are restricted to interior only solutions. The plan so far is to frame a 2″x4″ wall 1 1/2″ away from the brick/plaster, insulate and cover with a smart vapor retarder, and then install perpendicular strapping to allow for electrical and drywall installation.

At this stage, the recommendations for insulation are all over the place, primary with foam type and where (sills/ceiling vs full wall cavity, etc.) and we’re struggling a bit, even with researching here, on how best to move forward with it. I’ve used mineral wool in other projects and areas of this building, love it, and plan to use it here as well. It’s just coming down to that same old question of open or closed cell, and where. I’d like to just use it to control air infiltration, but then it’s back and forth on water/vapor considerations. Were hoping the community can share some wisdom and offer some sound guidance and recommendations.

Some additional information on the building –

The first level walls will remain either exposed brick and original plaster, and the attic space is well vented and will remain so. Both levels will be conditioned with modern HVAC and the ceiling/floor separating the levels will be insulated with mineral wool and a smart vapor retarder along with new substrate and finished floor.

We will be applying siloxane to the exposed exterior walls to help with reducing water driving inward, as well as dealing with any tuck/point needs. 

We realize humidity is a huge factor in our zone, and freeze/thaw cycles not so much. Air infiltration is another major factor to contend with, along with the variables of mixed use and overall age of the building.

I’m sure I left out a pile of other helpful details and will do my best to answer any questions. I really appreciate any feedback y’all have and look forward to hearing your thoughts.