I have been studying the latest engineering on how to insulate and frame basements after tearing out a partially finished basement that had wood and insulation in direct contact with the concrete.

Bonus picture:

4000lbs of material moved

From my research, it seems that the best option is to install rigid foam insulation glued and screwed to the blocks and taped at the seams to create an effective air, moisture, and vapor retarder and then frame the walls in front of that.

However, as you can see from the picture above and the picture below, there is a large cast iron sewage pipe that covers approximately 3/5 of the walls.

Sewer pipe and irregular wall profile

I'm thinking that my options here are:

  1. Where the gap is big enough, follow the contour of the wall with 1/2" rigid foam insulation. Where I can't get the 1/4" or the 1/2" (e.g. around the collars), I'll leave an opening and fill with spray foam. Put the framing in front of the pipe and fill the entire remaining gap with insulation (rock wool). The problem is that this will require a lot of insulation along these walls. Probably 9-10" of insulation to fill the cavity.
  2. Where I have the sewage pipe, install the framing in front of the pipe and install 2" rigid foam board glued onto the back of the framing. I'd go thick in this case because I can and to prevent a temperature gradient causing moisture to form on the inside of the right foam. In this case, there will be a 5-6" air gap from the backside of the wall assembly to the concrete. I figure as long as I can make the assembly relatively air-tight, it will prevent moisture from moving from the conditioned space to the wall. And in any case, the walls will be bare concrete. In general, I was thinking of doing this for the whole wall since the sloped transition ledge runs along the entire perimeter and makes it difficult and time consuming to install tight foam board pieces.
  3. Like (1), install rigid foam board and use closed cell spray foam where necessary. Then, frame a half wall below the pipe and a half wall above the pipe and frame a ledge around the pipe. Install insulation in wall assembly. The problem with this approach would seem to be that the wall won't be very strong structurally with a break in between unless I attach the upper half wall against the concrete. And plus, it probably won't pass inspection.

I should note that the original wall in this area was framed in front of the pipe with paper-faced insulation installed into the wall assembly not touching the concrete and this was the only wall in the basement where the insulation was not discolored and the back side of the wall did not show any signs of mold so in a way, I am inclined to go with #2. I'm inclined to build the whole wall this way to avoid excessive seams and cuts to accommodate the sloped transition ledge.

A bonus challenge is what to do around the mechanicals. There is not enough space behind the water heater to put rigid foam and build a wall assembly in front of it. Should I move it? Should I frame around the room and put foam board directly on the backside of the framing?

I'd really appreciate some input on the best way to manage insulating and framing this to avoid moisture issues in the future.

2 Answers 2


Skip option number 3, you don't want to have two half walls with a framed in gap between. It's not stable so you'll have cracking and likely other issues.

Option 1 implies that you need to fill all the open space with insulation, that's not the case. You need insulation between the outside and inside, but once you have enough insulation installed (for your desired R value) and a continuous envelope around the home, there's no need to keep adding more to fill a void.

So your options are either 2 or one unlisted option. Your option 4 is to insulate against the exterior wall and behind/around the pipe as best you can, and then install your wall in front of the pipe. The main difference between option 2 and option 4 is whether you insulation directly against the concrete wall or inside your framed wall. Inside the framed wall will be easier and likely use less insulation material. Against the concrete wall will allow you to more easily fish wiring inside the framed wall.

  • With "4", I was thinking of using 2" XPS directly glued to the "exterior" side of the steel stud framing instead of against the concrete wall itself. In this case, I could leave the wall cavity uninsulated. There is a lot of mixed info on having an air gap behind the wall assembly. Any thoughts on this? Oct 27, 2015 at 14:38

I know this is old but when renovating the first thing you should do is move this over to PVC. Second bolt some pvc collars to the side walls so you don't have the metal guides hanging down. Third build a 3/4 box around it and sandwich roxul against wall and around pipe. The roxul will insulate and it will also keep much more sound out than foam.

  • The interesting thing is that I've had at least two contractors and one plumber come through and neither have recommended replacing the cast iron pipe with PVC; I actually explicitly asked the plumber if he would recommend it and he said no. His take was that the cast iron pipe would probably be around longer than the house. Jan 27, 2016 at 13:53

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