Insulating top of basement slab – Is moisture still a problem with a barrier?
Is insulating the top of a non-walkout basement slab floor in climate zone 6 really valuable if the slab has a moisture/vapor barrier beneath it? Goal is carpet on top for most to the space. What happens in the event of water flooding from a toilet or clogged drain?
My reading on GBA and Building Science have lead me to believe my greatest enemy is the dew point and possibility of mildew smell and maybe mold. National home builder of the neighborhood does not insulate the slab on either side when they install carpet. Floor temps in homes they build feel fine to feet in socks in the dead of winter.
I am struggling with the effort & cost to insulate the on top of the slab. I would do 3/4″ or 1″ XPS with 5/8″ OSB Tongue and Groove attached to the slab. On the effort side of things, framing is complete so additional blocking (double the sill plate in essence) will be needed to have a functional sill plate for drywall to rest on due to the new floor height (exterior walls will need spray foam trimmed to accommodate blocking), door openings would need some modification, and my strategically place air returns (just high enough to not necessitate notching baseboards) would need moving.
Climate zone 6 (Twin Cities, MN) Home built in 2014. Two story with a basement (not walkout. Roughly 6′ below ground). No ERV/HRV. Constant running fan in upstairs and a fresh air intake in the basement utility room. 2.2 air change per hour (door blower test by energy consultant). Basement walls and rim joist are sealed up with spray foam. Basement slab is uninsulated and has thick plastic barrier between it and it’s rock base.
In the summer I run a 70 pint portable dehumidifier in the utility room set to target 50-55 RH. With the dehumidifier running in the past with the uninsulated basement, the basement has felt dry and of course cool. AC is used heavily during the Summer. We typically target a 74* temp on the main floor which equates to roughly 78* upstairs (central return on main floor). Summer basement temps are unknown as I have not monitored them in past summers. As part of the basement finishing I have added multiple air returns throughout the basement which are situated a few inches off the floor to help circulate air.
Resources I have read through:
Dew point and slab temperature
https://buildingscience.com/documents/insights/bsi-003-concrete-floor-problems (Moldy carpet, latex floor adhesives. Both slab on grade in warm climates.)
https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/98936/adding-insulation-floor-vs-adding-insulation- (comment 7)
https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/what-s-best-basement-flooring-system (comment 2)