Piping in exterior basement walls
For our basement reno we would prefer to keep our washer/dryer along the exterior wall where the previous owners positioned them, and add shower controls/head along the same side. We’re insulating on the interior, and the new assembly where piping is present will be 8″ concrete foundation wall – continuous 2″ XPS (R10) – 2×6 wood stud wall 16″ OC filled with Rockwool (R22) – 1/2″ drywall. Supply piping would be run roughly in the middle of the rockwool. Concerned about the risk of frozen pipes, I inquired with the city. They advised me:
The City’s requirement for waterlines in exterior walls is that they are located on the warm side of the insulation. So, concrete wall à insulation à waterlines à vapor barrier à drywall.
My question for the community is: How would you interpret the “warm side of the insulation” in our proposed assembly? Would you expect the XPS and ~2inches of Rockwool to be enough to prevent freezing? Should we consider going with thicker foam to be safe[r]? Or, is it obvious that we should avoid the exterior wall altogether? I wish it were easy to do so but we’re quite limited on square footage and layout options.
Climate zone 7A (Calgary AB).
Thanks,
E.
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Replies
You're below grade here, so right from the start things are a little less risky. I would just position your pipes in the "inner half" of the 2x6 wall, so maybe 2" or so back from the interior facing face of the studs, then you end up with about R24 worth of insulation between the pipe and the foundation wall, and around R8 between the pipe and the interior. That's probably sufficient to prevent any issues. If you're really concerned, just compress or notch the batts around the pipes, so that there is no insulation between the front of the pipe and the interior side drywall.
Bill
If you're doing a reno there's no point in half measures. Just make the wall a little bit thicker. Make the wall thick enough for the required insulation plus a space for the plumbing.
Note that the plumbing shouldn't go within 1-1/2" of the face of the stud where it passes through studs unless there is a steel plate to protect it.
I'd do 1" of continuous foam and then a 2x6 wall. You need R13 plus continuous, so that's 3-1/2" of fiberglass, less if you use foam board. So that leaves 2" for the plumbing. A 1/2" pipe will have to be recessed 1/8" into the insulation, a drain pipe will need a steel plate.
E,
In climate zone 7, if you have R-10 of continuous foam, and use a 2"x4" wall with 3 1/2" batt insulation, you don't need to worry where the pipes are in the stud wall. They won't freeze, the interior face of the foam won't collect moisture, and you will have more than enough insulation for a basement wall from an efficiency perspective.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
All, thanks for your replies. I was unaware of the importance of the foam/fluffy insulation ratio. Our plumber had requested the 2x6 studs for more room to run the plumbing. Based on the linked table, do I understand correctly that the main options are:
1) 2x6 insulated to full depth - Increase foam from from R10 to R15 (e.g. 3" instead of 2" XPS)
2) 2x6 insulated with 3 1/2" batt - Stick with 2" foam; plumber runs piping on the inner face of the batt
3) 2x4 insulated to full depth - Stick with 2" foam; plumber makes it work
I should have also pointed out that it is unlikely that the rim joist will get insulated because of our cast-in joists. With that in mind, are we better off running the piping horizontally across this wall assembly so that it stays similarly insulated (albeit with more exposure along the exterior wall), or drop it down vertically at the nearest joist where a short 90 degree turn will expose the piping to the uninsulated rim joist area? In theory we will be allowing interior heat to keep the inner rim joist "warm". In the latter scenario the best I can think of is to insulate the pipes themselves.
Thanks.
You'll have to check what your local jurisdiction requires, but if they have adopted the most recent version of the IECC there are three options for Zone 7 for a basement wall:
*R19 between studs, this is usually done with batt insulation on a 2x6 wall.
*R15 of continuous insulation. This is usually done as 3" of XPS foam sheets.
* R5 continuous insulation and R13 between studs. This is usually done as 1" of XPS against the wall and 2x4 studs filled with fiberglass insulation.
For a variety of reasons I wouldn't recommend the R19 option, even though code allows it.
I would either do 1" of foam with a 2x6 wall with 2x4 insulation in it and the plumbing in the space, or 3" of foam with an uninsulated 2x4 wall and the plumbing in the space. Either way your wall is 6.5" thick.
DC,
The OP is in Calgary, Alberta.
E,
1. This option yields a total of R-35, which is an awful lot for a below grade basement wall.
2 & 3. Either works. Fiberglass batts suffer from convective currents when there is an airspace around them so #3 is more efficient.
Will you be drywalling the ceiling? If so I'd avoid running the pipes close to the uninsulated rim-joist area.
Hi Malcolm, would you have the same reservations with ceiling tile?
E,
I have a friend in Calgary who several years ago sent me pictures of frost build up on the inside of his un-insulated rim-joists. I doubt that translates into the possibility of the pipes freezing as they will be a good 10" inside those. My concerns about either drywall or tile are probably misplaced.
If you box around the pipes and the rim joist, that would stop a lot of air movement, and then you might have issues. The further up into the Frozen North you go, the more of an issue that can be. If the pipes are in open air and there is no air barrier between the interior air space of the home and the pipe/rim joist area, then I wouldn't be too concerned as long as there is a decent space between the pipe and the rim joist.
For the OP, is there any way you could 'foam in' some EPS blocks tight against the rim joist to insulate it? EPS would get you some R value, but also allow a bit of inward drying, especially if you only did 1" or so. If you have no vapor barrier on the exterior, you might be able to add a bit of R value here without adding too much risk to your joist ends. I do understand your concern here though.
Bill
I would argue that the r-value, including air layers, of a masonry wall with no insulation is similar to that of a layer of drywall, and that a pipe between them is not really in the heated part of the house.
DC,
As I understand it the whole 8" concrete wall will be insulated with whatever option the OP chooses from the suggestions we made. Because the joists are embedded in the concrete they don't intend to insulate the rim-joists in case that causes moisture problems.
The question is whether running waterlines in the joist bays, then down into the new insulated stud wall would risk them freezing because there would be no insulation between them and the rim-joist about 10" away. The pipes w0uld be open to the joist bays, but closed off from the basement by a drywall or tile ceiling.
Right, that's basically as I understand it. Although I don't thin there's a rim joist, the joists are set into the concrete.
So on one side of the pipe you have 8" of concrete and then the Alberta winter. On the other side you have 1/2" of drywall and then a heated interior.
Concrete is about R-0.1 per inch, so R0.8 for the concrete side. Drywall is R1 per inch so R0.5 for the concrete side.
There is so little insulation value to concrete that the drywall provides meaningful insulation by comparison. So the pipes aren't really inside the conditioned part of the house.
DC,
But there is no drywall between the pipe and the heated interior. The drywall or tile is on the ceiling. The pipes run along the joist bays. The floor structure is surrounded by and part of the conditioned space above and below. The question is how far the cold from the embedded joists extends back into those heated joist bays?
Malcolm--
You say "heated joist bays." What's heating them? I see them as unheated spaces.
DC,
You've lost me a bit here. Are you saying that in a house with more than one storey, the floor separating say the first and sec0nd floors is not part of the conditioned area of the house? Isn't that like saying the interior walls in a house aren't part of the conditioned area because they have drywall on both sides?
The floor system shares an area equal to the size of the floor plan on both the top and bottom from which it gains heat. It loses some to a very small area at the perimeter.
I see now.
The floor/ceiling area is much greater than the rim area. So the temperature is going to be close to the room temperature. For some reason I was thinking the drywall area was the same as the rim area but that's not right.
DC,
So to return to the original question: How close to the un-insulated rim-joist can those pipes in the joist bays be before there is a risk of freezing?
I was assuming the joists were only partially embedded in the concrete, so the pipes would be turning down into the new stud walls about 10" away from the rim-joist. If the concrete continues up to the floor level, those pipes might be quite close to it, and might get quite cold.
[Reply to #18]
Malcolm --
OK, here is my calculation. Let's assume the house is 24' wide, the joists are 2x12 which for simplicity we'll assume are 12" high. In one linear foot of wall you have a square foot of rim area, which is connected in the joist bay to 12 square feet of floor and 12 square feet of ceiling. Let's further assume that the concrete of the rim area has an r-value of 0.8, the floor and ceiling are both 0.5, and every surface is coated with a layer of air that is also R0.5 (the insulation levels are so low in this scenario that the air layers count.) Further assume that it's 70F indoors and 10F outdoors.
Putting that all into a spreadsheet, I get that the air inside the joist bay is at 68.2F. The inner surface of the concrete is at 52.1F, the outer surface is at 26.1F. The inner surface of the drywall is at 68.9F and the interior-facing surface is at 69.3F.
I'm assuming there is no temperature gradient within the bay, I don't see a reason to assume otherwise.
So the inside of the bay, even against the concrete, there is no danger of freezing. There is no danger of condensation along the ceiling or floor, which is something I was concerned about. There is a danger of condensation along the concrete wall, which is going to be at about the dewpoint of interior air.
The heat flow through the rim area is 32 BTU/hr/sf, which is equivalent to an assembly with an R-value of 1.86, which you might see in a mediocre window or door.
All this assumes no air infiltration. If the rim area is leaky all bets are off.
DC,
"There is a danger of condensation along the concrete wall, which is going to be at about the dew-point of interior air."
Which explains why my friend's place had the frost on the rim-joists in a similar situation
My joists are *mostly* embedded -- usually only 1-2" air gap at the top: https://www.greenbuildingadvisor.com/question/insulating-basement-with-partially-cast-in-concrete-joists
This means the pipes would turn down ~3-4" off the wall if I go with a 2x4 stud wall, so pretty close. Over the last few years of having the basement gutted, I have not observed frost build up on the wood blocking that sit against the concrete at the end of the joist bays (DC, I believe you're technically correct that I don't actually have a rim joist), but that could be meaningless once the new assembly is in place.
I'm gathering from the discussion that running the pipes horizontally below grade in a fully insulated 2x4 wall might be our lowest risk option here. Mercifully, our plumber confirmed that a 2x6 wall is a nice-to-have, not a need-to-have.
Bill, to your comment about EPS in the joist bays (#11), see the link above for my thought process around insulating (or not) the embedded joists. I wish I had the risk tolerance to install 1" EPS, but not sure I can bring myself to it. Your suggestion is compelling because the permeance of the EPS would be higher than the XPS. On the exterior is plywood sheathing with 1 or 2 layers of felt paper, metal lath, and stucco transitioning down to parging on foundation. Not sure if that affects your idea?
Why the resistance to a 2x6 wall? It's the easy way out.
Square footage mainly.
E,
See DC's post #21. He confirmed you are safe running the pipes any way you want.
My experience with houses in southern Alberta is that because of the dry climate you can get away with things like embedded joists that elsewhere would have rotted out in a few years. Like you I am pretty risk adverse, but adding an inch of EPS as Bill suggests isn't anywhere as worrying where you are as say here in the PNW.