Why do you shim up basement walls?

t b
Updated: March 3, 2021

I'm looking for instructions on how to properly repair and finish a basement wall. I don't just mean drywall, insulation and vapour barrier, I also need to know what to do about the masonry behind it. I need the correct instructions, so it doesn't cause more damage. There are 'solutions' that actually make things worse, such as painting a spalling brick wall. That makes it worse eventually. If you hire the wrong contractor, or if you do it yourself based on incorrect advice, you'll damage your house. There is some very helpful information on your site but I could use some additional details. For example, On your WHY ARE BASEMENTS MOLDY? HOW TO FIX OR PREVENT A MOLDY BASEMENT page.

  • Shim up bottom plates to allow for water to pass under in cases of mild flooding.

What do you mean by that? Are you saying don't put the studs all the way down to the floor? Because water needs to be able to get behind the wall? Wouldn't that defeat much of the purpose of finishing the wall? I don't want water getting behind the wall and I don't want mice or bugs coming in through the gap. There has been flooding here, but if it ever happens again, I don't want it getting behind the wall, which would probably mean I'd have to tear the finished wall down and do it over. If flooding gets behind the wall once, maybe I could leave it. But twice or three times, I'd have to tear it down.

Do you just leave the bottom edge unfinished? It seems to me this would defeat the purpose of keeping rodents, bugs, cold air, and water out, which is what the wall is meant to accomplish. The 'finished' parts of my basement are like that now, and it's a mess. I'd like to finish those walls completely, so nothing can get in or out, and the area can be kept clean. I can't keep it clean now because it's impossible. The masonry wall behind the existing drywall and lath is spalling, falling out, and spilling onto the floor.

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Responses (4)

t b 3 years ago

I guess what I want to do here isn't 'finishing', I just want an air barrier. I want to keep mice, bugs, water, and cold air out. I don't really care about having a nice framed drywall wall in the basement, if that doesn't do anything. I thought finishing the wall according to the specifications found on this website would incorporate an air barrier. So what I need are instructions on how to construct *something* that keeps all the unwanted stuff from getting in.

Emmanuel B. Cosgrove 3 years ago

As an air barrier and for moisture protection, you’re best to have a 6 mil poly barrier directly against the concrete wall. It won’t however do much for heat loss. And in all but the most cracked and leaky foundations, there isn’t typically a ton of air leakage through walls that would be stopped by the addition of an air barrier. I also can’t see it doing much about mice, bugs perhaps if you do a superbly excellent job, but they also can usually find the few chinks in your armor.

Another thing that won’t really be helped by a poly membrane is bulk water infiltration. If you have water that comes in regularly, you are better to deal with that either with exterior grading away from the house, or an interior trench dug around the perimeter where you could install a weeping tile that is directed to a sump pump. 

I know that last point seems a little unusual, but cracking a concrete floor is not actually that difficult, it would only be 3 or 4 inches thick and able to be smashed with a sledge hammer and removed.

I’m not trying to give you a ton of new projects here to do, but I just don’t see a poly barrier solving some of the issues you mentioned.  

t b 3 years ago