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OT: Walkout basement concrete wall insulation

ShaunM

Aluminum
Joined
Apr 13, 2015
This is off topic and the wrong forum, but with the breadth of skilled trades that participate on this site I figure I can get some good info.

We are adding a bedroom to our mostly unfinished basement. Climate zone 5, bordering on 6 in Iowa. The house is a 2018 modular built into a hill at a roughly 45 degree angle so the basement is walkout with 1 wall entirely exposed and an adjacent wall that is about 50% exposed. The other two are fully backfilled. The basement is entirely poured concrete with 9 foot walls so the exposed basement walls are bare concrete.

Previous owner glued 2” Foamular 150 XPS to all walls and framed 2x4 studwalls inside of that. Both were done subpar, but not bad enough I want to rip it all out and start over. They did not bother to tape/seal any joints of the XPS. That thickness should offer a nominal R10 which I believe is inadequate for our climate zone.

We have not experienced a winter in this house yet so we can’t say how warm the basement will be, but we want to take any necessary steps to keep the bedroom cozy. It will have one wall on the fully exposed side, one on a fully backfilled side, and the other two walls will be interior partition walls. That said, we would like to add more insulation to the two exterior walls and I would like some input to avoid any moldy mistakes that might happen.

Spray foam over the XPS? Fiberglass batt? Mineral wool batt? Cut and cobble more rigid foam in between the stud bays? Fiberglass is most susceptible to mold, mineral wool is better, but both are a breeze to install. I don’t mind cutting and cobbling more foam if that’s the least risky – more XPS or use EPS instead? Regardless of technique I will attempt to seal the existing rigid foam joints and any other air leakage spots I can find (including the joist bays on the sill plate).

The basement is dry and cost is not a concern. Thanks for your input.
 
The basement walls in my place had been framed and drywalled in the 60's, previous owner ripped most of it out. I found out why when I ripped a section out, visibly it looked fine, but inside the wall it was filled with nasty made me gag mold. I think the proper way to do it is to have some vents in the drywall so moisture can escape. Note, I'm in the desert.
 
In Maine I used a plastic vapor barrier on the ground and then gray 3/4" styrofoam on the plastic. Then I poured the concrete floor. The slab absorbed the heat in the garage and was a delight to have the tractor and car inside the garage. I kept it heated all Winter using a vented propane heater. It was very economical.

I built the house next and wrapped the foundation in the same Styrofoam. The cellar was not heated never dropped below 55 degrees.

So I would recommend not allowing the cement foundation to come in contact with the outside temperatures.

Remove the backfill, seal the concrete from moisture, do a killer drainage system to keep water away from the foundation. Insulate the concrete and backfill.
 
I would think somehow you'd want to create a 3/4"ish airflow gap between the concrete and the interior foam. Supply air at the bottom and give it somewhere to go at the top. Probably apply some waterproofing sealer to the concrete as well.

I mean, if the outside isn't very well drained and sealed, you're going to have moisture coming inside. Probably best to figure out a system to get the moisture out from between the concrete and insulation.
 
I would think somehow you'd want to create a 3/4"ish airflow gap between the concrete and the interior foam. Supply air at the bottom and give it somewhere to go at the top. Probably apply some waterproofing sealer to the concrete as well.

I mean, if the outside isn't very well drained and sealed, you're going to have moisture coming inside. Probably best to figure out a system to get the moisture out from between the concrete and insulation.
Nope
Access to air is access to moisture. The moisture does not come from the concrete, it comes from the air. Sealed up is the way to go.
 
greenbuildingadvisor.com for specific details.
Seal the seams
rockwool or whatever in walls.
2018 build should have an insulated foundation, but whatever

Insulate the floor. Above website for strategies for your floor finish of choice.
This whole building a wall has always seemed a waste of space to me, but its there, use it.
IF you had 2 inches on the floor with vapor barrier adhesive, plywood then finish floor, the 2 inches sealed on the wall, with r13 in the stud bays, build the walkout wall to R20+ best windows you can afford and you will not have a problem with heat.
Airtightness.
Before you start closing in, airtightness.
 
Tear it all out (or maybe you can get the foam out and leave the studwall) and build a studwall about 2" inside the concrete wall. Have the concrete sprayed with closed cell foam 3 - 4" thick. The wood is well away from any moisture source, the wood is captured for stability, and you have a continuous air, moisture, and temperature barrier on the walls. Install wall covering of your choice.
 
To be clear, moisture ingress is not an issue. The basement is dry. My concern is warm, humid interior air condensing on the cold concrete walls. The two walls in question were drywalled by the previous owners. We are tearing it off to add insulation. The drywall we have removed looks as good as new - no molding on the inside. Which maybe answers my initial question.

There are guides to determine the ratio of foam-to-batt insulation that will avoid condensation, but none of the models I found were for exposed concrete walls, only backfilled walls. Ideally, someone could chime in from experience whether the 2" of XPS in place is sufficient enough to add mineral wool. Quick and easy.

It's also entirely possible I'm over-analyzing the situation. Thanks for everyone's thoughts thus far.
 
Condensation on the interior can happen when warm moist air contacts the cold concrete. If the entire surface of the concrete is sealed, no moisture will condense there. That's why I suggest spraying closed cell foam. Nothing will get to the cold concrete from inside because it is completely sealed. I doubt you can 100% caulk the foam that is there now.

I get wanting it quick and easy, but can't say that's the right answer. Go ahead and put up the batts and find out.
 
eleven responses only three locations so where is the OP located? is he in the desert like I assume since most responders seemed to be there.
Bill D
 
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Nope
Access to air is access to moisture. The moisture does not come from the concrete, it comes from the air. Sealed up is the way to go.
If the backfilled walls were not sealed properly, that is where the moisture gets into the concrete and it will bleed into the interior wood/insulation/drywall. You can add sealers to exterior walls before backfilling, but water will find a way in. I'm a firm believer that one side of the wall needs to breathe, interior or exterior, your choice.
 
Putins' s. 1.4 Billion dollar mansion is reported to have had the interior gutted and rebuilt twice because of mold. Cast concrete building with stud walls inside, marble and gold did not prevent mold from the damp walls.
Bill D
 
Very little moisture typically migrates through poured concrete foundation walls, but water or moisture often enters at cracks, form ties, or penetrations. The foam insulation that you have is a decent barrier, if well sealed. If cost is not a concern, I would spray foam between the studs. That will seal the joints and give you plenty of insulation.
 
Most of the ratios foam/fluff have to do with the condensation point. You want it to be inside the foam where it cannot happen. I don't think you will have that problem with concrete, as the concrete moderates temperature swings. And IIRC the risk is sheathing rot, which is not an issue here.
Air infiltration is the enemy for several reasons. Ledger boards, top of the foundation wall, wire and plumbing penetrations.
Tape the foam and fiberglass or rock wool[supposedly better with any moisture] in the cavities and you will be fine.
 
For closure's sake, went with closed cell spray foam. Wasn't my first choice just because I'd prefer to not use foam, but it seemed like the best option considering the circumstances. Took 4 vendors and 5 weeks to finally get someone to come out and work a job this small, but it's done and I'm mostly happy with the results.
2" on the wall over the 2" XPS and 3" in the rim joist bays over the 2" XPS. $1700 for ~350 total sq. ft. Moving on to hanging drywall. Thanks for the suggestions.
 
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