Waterproofing and insulating interior basement

Submitted by Mr_brighttt t3_11d24d6 in DIY

Help with an old house?

Climate zone 5. Suspecting no exterior waterproofing on the exterior of the concrete block foundation since it was built in 1908. Or if it’s there, probably not amazing condition.

I am wanting to apply rigid insulation to the interior. I have no bulk water problem. Should I just go the safest route and put a dimple mat on the walls first? Seal the gap at the top of the insulation but then do I keep the bottom open in case water does get in? Or is simply having the mat there and sealing the bottom of the rigid insulation make it enough of an air gap to dry again back through the foundation? Wouldn’t leaving it unsealed at the bottom lose my air barrier? I suppose probably not much air getting through the foundation if any. How to pick what dimple mat and what depth?

Or should I just apply a water proofing liquid membrane before the rigid insulation? No dimple mat if I do that I would assume since that’s the whole point of the membrane. Which liquid membrane should I pick?

Should I put a sill gasket on the foundation floor for the rigid insulation to sit on? How to pick one?

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SteeleRain01 t1_ja6ceju wrote

Don't put any liquid membrane on the inside, I don't think that's ever a good idea and the potential pressure build up will eventually cause it to fail. I'm a huge fan of the dimple matt approach. One side of my basement, the floor is at ground level. I ran dimple matt from that side on the floor, and up the wall on the other side covering the wall that touches the dirt. On the opposite wall where I started the dimple mat at the floor, I just did rigid foam because I didn't need possible water barrier on that wall. For the wall with the dimple mat, frame a 24" OC wall over the dimple mat and fill with rockwool.

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Mr_brighttt OP t1_ja6efxy wrote

I don’t necessarily want to frame a wall in over the top. I was hoping be able to do just rigid insulation on top of a dimple mat if helpful and then an FRP (fiberglass reinforced plastic) panel or something similar on top of that. Maybe plywood. It’s not ever gonna be a completely finished space. I just want it to be insulated

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SteeleRain01 t1_ja6hv4b wrote

Tracking. How would you keep a wall sandwich like that together? If you use something like furring strips, you'll end up drilling so many holes through the dimple mat into the wall it seems like it defeats the purpose. I did the wall more to hold my dimple mat in place AND give space for an insulation R-value cheaper than using foam.

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Mr_brighttt OP t1_ja6ia3p wrote

Hoping there’s an adhesive I could bond the rigid insulation to the plastic panels. If it’s plywood, you make a good point. Maybe I should use 1x2 furring strips with concrete anchors, then the rigid foam, then the plywood. That would allow for secure ways to attach shelving for myself and future owners without having to go down the concrete anchor route again.

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the_real_abraham t1_ja6wgii wrote

Everything I found online said don't do anything you just said you did. You'll have to fix it very soon.

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Mr_brighttt OP t1_ja8aitf wrote

Well it would be helpful if you posted the right way to do things then. Or at least discuss specifics on my plan.

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the_real_abraham t1_ja8ulkh wrote

I'm just talking about my google experience. I started my search with "best way to attach foam board to concrete." Every article I read started with, "This is not recommended but if you're set on it..." I ended up on this ad which was very informative and similar to my situation. I know this is a DIY sub but sometimes it's just not worth it and reputable companies provide warranties. A good warranty is worth it's weight in gold.

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SteeleRain01 t1_ja7q81g wrote

There are so many different approaches for sure. Using dimple mat on the basement floor with plywood subfloor is a widely accepted practice. I could find no logical reason why taped and sealed dimple mat on the walls wouldn't work as well. I do know since I've done it, my basement is 100 times dryer and more comfortable. This article informed a lot of my decisions. Ihttps://www.greenbuildingadvisor.com/article/carmic-house-taming-a-basement-from-hell

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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.

https://youtu.be/kwn0Vjw_ji0

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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

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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.

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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

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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.

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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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doskey123 t1_ja8s08e wrote

From what I understand and have read it is not a good idea to seal walls of from the inside because you will just trap the water in the walls. What they do here in my county (Germany) if you don't have the means to excavate the whole building is to push a liquified compound through holes drilled from the inside which then saturates the walls, stopping water from coming through and providing thus an outside seal.

With some technical skills its possible to do it yourself. E.g. see this description of the barrier in EN. The link describes a seal-off for humidity coming from the ground, but another option (vertical barrier) exists. Same process, just more holes.

https://www.isotec.de/en/workmanship/sealing/horizontal-barrier.html

Also, why don't you get a company to judge the situation and make an offer? And then use their knowledge for your own gains. You don't have to pay them for the full job, they will just take some money to have an assessment which they will give you in writing or directly. But sound interested. And in the end, you just happen to find somebody else or change your plans ;-) .

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hoeding t1_ja7utq1 wrote

Run poly from the floor to dirt level on the concrete wall, seal it to the floor, build a 2x4 stick wall, insulate with fibreglass then run poly from floor to ceiling on the interior. Drywall unnecessary but now you are ready for it.

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Mr_brighttt OP t1_ja7zjbk wrote

I always thought double vapor/water barrier was bad

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ajtrns t1_ja8bbe8 wrote

this method you're describing is now considered bad and likely to fail in 10-20 years. if you don't mind moldy rotting walls, this is the method to choose!

alternatives are not readily available, so there is a tendency to revert to your idea for lack of other good options. OP's idea of using all-rotproof materials is an improvement over putting wood, fiberglass, and drywall in a basement.

https://youtu.be/kwn0Vjw_ji0

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hoeding t1_ja8tm8l wrote

I'll take tips on better options, but this is how builders build new around here in climate zone 7/8 circa 6 years ago. I've got one of these walls open right now and the only evidence of water is ice on the exterior concrete wall above the ground level which is expected around here. The wood isn't going to rot because it can still breathe to the exterior through the top.

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ajtrns t1_ja8vxer wrote

your climate in 7/8 has almost nothing to do with OP in 5. especially if you're in an arid part of 7/8.

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