Interior wall assembly for older CMU wall structure
I have a unique project. WWII era building with 1st floor CMU construction. It has been used as workshop and the “office” many years. The space will be heated in the cold months and intermittently cooled during a fairly short period during the summer when it gets hot and humid several weeks every summer
Its slab on grade and there is no insulation below grade. Exterior is primed and painted with exterior latex. Despite never having any water intrusion, inside of walls were treated with a Sherwin Williams drylock type product to brighten the space up several years ago.
I’m currently finishing the interior walls to insulate here in Zone 5. Insulation will need to be on the interior due to the architecture and design of the building. I am using 2″ of Owens Corning Foamular 250, glued and mechanically fgastened to the inside of interior CMU. All seams and where the foam meets the concrete floor will be caulked and taped. The Foamular is intended as both insulation as a moisture resistant barrier for the assembly.
The 2×4 framed walls are offset about 7 inches from the CMU wall due to the support columns they utilized. So there will be approximately 2″ of air in the wall assembly between the foamular and stud framing.
I am more concerned with mitigating possible moisture issues but want to get as much R value as possible. My questions are:
1. Can I add un-faced batt fiberglass insulation in the stud walls so that the wall assembly would be latex paint, drywall, unfaced batt insulation, 2″ air gap, 2″ foam and then CMU wall?
and
2. Should I seal the portion of the concrete floor behind the wall assembly to ensure that I don’t have moisture migrating up through 6″ of 80 year old uninsulated concrete floor?
Thank you.
Replies
Here are a few thoughts. Sounds similar to a basement.
rock wool might be a better choice in the stud cavity.
Do the math to make sure the interior surface of the foam stays above the dew point in local worst case temperatures.
I would say treat the floor in that gap just like the wall, 2 inch foam board. Seal it well.
the gap will need fire blocking.
I was approaching it sort of like finishing a basement except that its dry and all above exterior grade. I have a copy of the Builder's Guide to Cold Climates by Joe Stiburek I reference pretty often but since this is retrofitting an older structure that only got me so far. Good idea on the rock wool. Thank you. I've used in the past but totally forgot to consider it. I may be a bit in the dark on dew point calculation since this is a larger space and since we are not sure what the temp and RH will be maintained at during winter....Its been a long while since we have seen any sustained temps in the single digits or lower. Winter up here is now normally in the teens or above.
I'm installing fire blocking....specific areas with screws so that I can pull the fireproofing and blocking off periodically to visually inspect down the 2 inch air gap from the unfinished but insulated floor space above to make sure all is well.
Keep in mind fire blocking needs to separate the concealed space every 10 feet or less in the horizontal direction, as well as at the top and bottom.