How to insulate direct plastered basement walls
Hello!
Im an avid DIYer and fabricator and I like to think I understand how buildings work, but I’m stumped by this one, looking for someone who has some direct experience with this issue:
Im looking to finish my basement here is beautiful Denver CO. It is under my 50’s ranch, and is poured concrete that has been plastered directly over. There is mostly no water issues except for one area where it’s pretty clear to me that a glaring grading issue and a small vertical crack are to blame. Im planning to epoxy inject the crack and repair the grading issue and don’t expect that to be a problem (have successfully done this in another area)
but my question is weather or not, and how to insulate the foundation walls where the concrete has been plastered. While I mentioned above my plan to mitigate this one water issue, my foundation is coated with a thin layer of bitumen, as was standard in the 50’s, and expect it isn’t performing 100% anymore, so I do expect there would be some passive moisture vapor from the soil In the foundation walls, if not right now, then likely in the future. Or any future flooding issue on the exterior may or may not make it inside ( broken sprinkler line, overflowing gutter, etc) Bottom line, I don’t trust the foundation walls to be 100% dry, though I don’t expect to have major water intrusion issues, either, once I repair the above mentioned area.
I am a bit afraid of placing XPS along the Inside of the foundation walls, as seems to be the standard practice, because I’m afraid it would trap any moisture in the plaster on the concrete wall, causing it to crumble and/or mold. In some areas, the plaster has separated from the wall a bit, but in most areas it is very well adhered, so it would be a nightmare to remove it all. It would basically have to all be chipped or ground off 🙁
I’m Colorado, where the weather is really fairly mild, save for a few weeks of extremes each year. Any applicable code requirements for a remodel aside, now I’m wondering if I would be better off leaving the plaster as is on the foundation walls. As they are, any vapor or from the exterior can just dry to the interior, no moisture trapped anywhere, and no problem, right? If I were to insulate the first floor joist bay ends along the perimeter and insulate the slab with an XPS subfloor panel, How much money an I really going to save by insulating the basement walls… Is 100 bucks a year worth not launching into weeks of nasty grinding plaster off the walls, thousands in insulation, or digging up and waterproofing the entire foundation exterior???
anyone have any experience with plaster right on a basement concrete wall??
thank you for any insights!
cjx1
Replies
You are right to concern yourself with water intrusion. No point in doing anything if it will all get wet. I would leave the plaster on (assuming you are referencing true plaster and not some drywall type product). If the plaster is loose and coming off easily, I would remove it. If you have to forcefully remove it, you’ll be taking some grout out of the joints etc with it, not good.
You can waterproof from the inside, a method I’ve had to do quite often on older homes where exterior work isn’t feasible or cost effective. You will need to install an interior drain and sump pump. To do this you will have to remove a small section of concrete slab all around the perimeter. You’ll need to install a strip type of drain pipe tight against the wall, connect that to a sump pump. Then you need to install the same dimple board that is used as exterior water proofing against the interior wall. It’s important to make this as water tight as possible and seal the top where it meets the mud sill. At the base have it go beyond the strip drain, so water will drain between wall and dimple board into the strip. Then pour concrete back in the trench you made against the dimple board. Can glue your foam insulation against that and install framing as usual. It works well, but do not skimp on sealing the dimple board.
In Colorado, I doubt it's even possible to trap enough water in a concrete foundation wall to create a problem. I wouldn't hesitate to insulate with EPS, which does allow some moisture transmission. Heck, I'd do it in New England, where we get as much moisture in a month as parts of CO get in a year!
I would add that it's very important to fix the existing bulk water issue first.