Submitted by Mr_brighttt t3_11d24d6 in DIY
ajtrns t1_ja6grhk wrote
there is no real solution. so whatever you do, plan to possibly have it removed in 10-20 years or less. if you are in a humid/continental climate.
if you have to insulate from the inside, your idea is sensible. you can and should waterproof the concrete wall first but such interior coats of paint will usually fail over time. make sure the rigid foam and whatever you cover it with are themselves waterproof / rotproof (some foams are open cell or have absorbent paper covers). it's just not worth dealing with wood or fiberglass in these conditions.
the foam or the mat can be the vapor barrier if they are detailed/taped correctly.
to make your idea really work, the dimple mat airgap needs to be actively ventilated. i don't know of a retail product for this. it doesnt have to be constant. but periodic air changes.
you aren't going to insulate the floor?
you can help this system out by shielding the exterior of the basement wall from moisture. french drains, umbrella sheeting in the ground to send water 5-10' away from the foundation.
the right way to insulate a basement in a wet climate (really any climate) is from the outside. with closed cell foam on the exterior of the house.
Mr_brighttt OP t1_ja6i1vi wrote
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
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
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
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
I look forward to building a home some day with perfected water management and continuous insulation exteriorly too. Cheers
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