Mr_brighttt OP t1_ja6i1vi wrote on February 27, 2023 at 5:01 AM

Reply to comment by ajtrns in Waterproofing and insulating interior basement by Mr_brighttt

Fair enough. I just can’t afford to excavate around my entire house and do it again right. This won’t be a forever home just a “for a while home” and want to make it more thermally efficient and comfortable place. I don’t have a ton of extra head room so I was likely going to not insulate the floor. If I do end up doing it at some point, I’d make sure I’d seal the floor to wall transition. Since I’m not insulating the floor, is there benefit to placing a sill gasket or other type of thermal break below the rigid wall insulation?

If the dimple mat wouldn’t be ventilated, I’d probably just forego it then as an unnecessary cost increase

ajtrns t1_ja6kg8y wrote on February 27, 2023 at 5:25 AM

when considering these systems, there's a hierarchy of priority:

bulk water

water vapor

thermal comfort

your proposal is starting with thermal comfort and is not dealing with bulk water or vapor very well. so just know that, if it's not already clear.

rigid rotproof insulation against the interior walls can be sealed at the bottom to prevent water that condensates behind the foam from dripping out from beneath it. the foam itself is the "thermal break".

a big issue with your proposed system is that, if you seal everything at the top of a typical concrete basement wall, that will shove the moisture in the wall upwards into the wood floor framing. the moisture, which now cannot escape to the interior through the foam, has to go somewhere. and if your wooden sill isnt itself sealed (not common in older houses) it will see the moisture.

mold will grow behind the foam. and moisture will rot the sill and rim joists and first floor joist ends of the house. over time.

to repeat, there is no common known passive interior basement insulating method that does not generate mold and moisture in unwanted places.

Mr_brighttt OP t1_ja6ld3i wrote on February 27, 2023 at 5:35 AM

So should I only seal the rim joists and accept it as enough to help with air leakage and call it enough? I bought the house from family and there’s never been any bulk water issues for >10 years. As far as vapor goes, is there anything I can do to allow the concrete block walls to still dry to the interior? Mineral wool bats mechanically fastened to the walls? Since air penetration through the walls is minimal.

I can’t envision any significant open air gap behind the rigid insulation allowing the insulation to be effective thermally

ajtrns t1_ja6m9ym wrote on February 27, 2023 at 5:45 AM

there's no easy way to retrofit a vapor barrier between the top of a concrete wall and wood sills / rim joists.

i can't say how the mineral wool would perform. it would likely become a mold substrate, and whatever interior wall finish surface is covering the mineral wool, the foundation side of that would see the condensation.

i couldnt give any more useful guidance on this. as a handyman and carpenter i'm often asked to retrofit spaces. i have torn out more than 10 basement interior insulation assemblies, perhaps average age of 20-30 years old. until active ventilation products mature, or new insulation technologies are invented, there's just no way i could guarantee my work under such conditions. i only water/vaporproof and insulate from the exterior. i'm wrapping the exterior of a house and its slab in 2" of foam this week.

to be clear, mold and moisture get trapped either way -- interior air or exterior ground contact. the difference is that the exterior assembly does not foul interior air quality, and can exist for decades in a moldy wet subsurface environment and perform its job well. not so for interior assemblies.

Mr_brighttt OP t1_ja6mmyk wrote on February 27, 2023 at 5:49 AM

I look forward to building a home some day with perfected water management and continuous insulation exteriorly too. Cheers