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Old 12-26-2017, 04:00 PM   #1
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Insulating basement walls - dealing with waste pipes


Cape Cod home.
Built in 1955.

I'm not the original owner.

Basement is "full" (7' from concrete slab to the "bottom" of the floor joist which are 2x10). Cinder block exterior walls.

The issue (I'll try to post pictures if that will help) is that on 1.5 (half the "long" wall and all the rest of the one short wall) is the waste line (PVC) that eventually ties into the sewer P-Trap. House was apparently originally on septic and converted to sewer at some point. The sewer P trap is about 18" above the concrete floor. Right before the P-Trap is a lovely fresh air inlet that seems to like to be bitter cold (when we get around 0 or lower (Farenheit) and of course, the the inlet cover faces the direction the wind likes to blow. There are times I can see some frost on the PVC pipe on those cold days.

There is evidence of efflorescence on the very bottom row of the cinder blocks, but I've not seen any moisture since I bought in 1998, but then again I also had gutters installed on the house a year later and directed the downspouts 8' from the walls.

That being said:
I'd like to finish the basement (well most of it, there's always the utility room).

My original idea was to put 1" poly iso (although after I saw the price I may rethink that) rigid foamboard against cinder block. But I can't get it behind the PVC waste line that runs against those two wall portions. So do I just kinda butt it as close as it'll go?

If so, do I "seal" around the PVC pipes and rigid foamboard somehow?

I would then like to frame, but I kinda "need" every inch possible, so I'd like to avoid just using 2x4 up against the PVC pipe (well leave like 1/4" for the expansion/contraction of the PVC so that it doesn't make noises when hot water/cold water goes down. Although that's a possibility to that if necessary and then use Roxul for the stud bays, but I'm not sure about that fresh air inlet (apparently it's still required by the UPC? There's 3 other vent stacks that go to the roof, but I thought I read somewhere that you must have a vent stack right before the P trap on the "house" side). Anyway, I didn't do the plumbing, it was there already.

Other "ideal" option would be to either notch the 2x4 or use 2x6 and notch them so that I could put the framing against the 1" poly iso and then Roxul the bays?

I'd seal the rim joist areas with either spray foam kits or I suppose I could cut some foamboard, but whatever. Fiberglass/etc. doesn't seem to do much for sealing out air infiltration and these old houses ....

I'm not worried about the waste/sewer line freezing (I think it's 4" PVC) with that fresh air inlet, it's just a combination of the fact it gets fairly cold, and heat loss, but also don't know if compromising the foamboard will results in moisture problems either?

Seems like a dumb question, but figured better to see what's around (remarkably Google was not very helpful on this matter. You can find how to frame stuff, but very little on insulating around basement waste pipes--seems to all be insulating around water supply pipes to keep them from freezing).

Oh, live in Albany, NY, BTW.

Yes, would get everything permitted, etc.
Just plotting/planning my spring time project. haha
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Old 12-26-2017, 06:18 PM   #2
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Re: Insulating basement walls - dealing with waste pipes


Pics would help but it sounds like a tricky situation. I guess it would not be possible to reroute the line right?

I had to deal with that with my water line, but I had some play in it so I was able to bring it out and then vapour barrier behind it. You want the pipe on the warm side.

For a waste pipe though, you might be able to get away with having it on the cold side, so you could box it in and insulate/vapour barrier around it, I would think. But let's see what others say, as I'm not positive on that. If there is any P trap involved then you don't want that on the cold side.
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Old 12-26-2017, 06:53 PM   #3
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Re: Insulating basement walls - dealing with waste pipes


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Pics would help but it sounds like a tricky situation. I guess it would not be possible to reroute the line right?

I had to deal with that with my water line, but I had some play in it so I was able to bring it out and then vapour barrier behind it. You want the pipe on the warm side.

For a waste pipe though, you might be able to get away with having it on the cold side, so you could box it in and insulate/vapour barrier around it, I would think. But let's see what others say, as I'm not positive on that. If there is any P trap involved then you don't want that on the cold side.
So here's some old pictures, I'm not sure if it'll help explain things, LOL!
The PVC portion of the pipe is attached to the cinder block with anchor bolts and metal strapping. I might be able to losing the one side a smidgen to get "something" behind it. Or at least the parts where it's not the fittings/couplings. I don't think the P-Trap part would ever freeze, but I just didn't know what was good/bad about insulating waste pipes.

Should they be on the "cold" side, or the "warm" side. I would think (for the most part) on the "warm" side, but that darn fresh air inlet is the one that may be the problem. Granted, how much heat loss would I get? I don't know.

(the "first" picture has the fresh air inlet and it goes down to the P-Trap)

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Old 12-26-2017, 09:18 PM   #4
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Re: Insulating basement walls - dealing with waste pipes


That's weird, why is that first one going outside? Where does it go to outside? That other one that goes along the entire wall, that's going to be kind of a pain. Maybe see what others say, might be a better way to deal with this than I can think of.

I wonder if this is actually for something else. Are you sure it's waste water and not some kind of vent?
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Old 12-26-2017, 09:29 PM   #5
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Re: Insulating basement walls - dealing with waste pipes


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That's weird, why is that first one going outside? Where does it go to outside? That other one that goes along the entire wall, that's going to be kind of a pain. Maybe see what others say, might be a better way to deal with this than I can think of.

I wonder if this is actually for something else. Are you sure it's waste water and not some kind of vent?
The vertical pipe that you see is the fresh air inlet vent. It goes right through the cinder block wall and has one of those old metal plates with holes on it.

It connects to the PVC and then the PVC goes right into the cast iron P trap for the sewer and then the sewer pipe goes through the wall outside (horizontally mind you).

Kinda like this, but my sewer pipe isn't underground:

http://chestofbooks.com/home-improve...-Air-Inlet.jpg

Yeah, the horizontal one is going to be a "fun" one.

I mean, I *think* I understand the principle of sealing the "envelope"/exterior walls with the foamboard (moisture barrier for the insulation/etc.), so if I can't have a continuous seal, that's what I'm mostly concerned with.

But if the folks agree that it's not that big of a deal (just run the foamboard up to the PVC on the top and bottom and that's "good enough"?
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Old 12-26-2017, 11:43 PM   #6
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Re: Insulating basement walls - dealing with waste pipes


The vent one I would not worry about it being on the warm side as it won't see any water, so that one should be easy enough, just box it in and insulate around.

The horizontal one I would consider rerouting. so that it ends up being on the outside of the wall. Probably have to do that first, put some temporary supports, then build the wall behind it and then support it to the wall once it's drywalled.
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Old 12-27-2017, 08:53 AM   #7
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Re: Insulating basement walls - dealing with waste pipes


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The vent one I would not worry about it being on the warm side as it won't see any water, so that one should be easy enough, just box it in and insulate around.

The horizontal one I would consider rerouting. so that it ends up being on the outside of the wall. Probably have to do that first, put some temporary supports, then build the wall behind it and then support it to the wall once it's drywalled.
Ah, OK, so kinda like doing a false half-wall or something? At least if I wanted to cover it up so that the PVC waste line isn't visible.

the other, (probably easier) option would be, if I could support the pipe temporarily and lose a strap to have it come from the wall an inch or so and put the rigid foamboard behind it, so that I would get a continuous barrier?

I'll probably have to get some plumber estimates if it really needs a re-route, as you'd and up having to put a pretty sharp elbow to get it to stick that far out and go into the cast iron sewer P-trap.

I did take a gander and I can actually fit a 1" piece of foamboard between the P-trap and the cinder block wall, so it might be semi-doable.

I just wasn't sure of any implications of keeping the waste side on the "hot" or "cold" side of a wall, as well as implications of not having a continuous barrier along the cinder block.
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