Tub and shower trap in concrete slab sitting on bedrock (ledge)

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Ed Dooley

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I’m building a bathroom addition on my house in Vermont. It’s 10′ x 18′, with the 18′ wall parallel to the house. Ten feet out the ledge is right at grade, at the house it’s only 9-12″ down from grade to ledge. I’ll be pinning a poured concrete wall to the ledge, at grade that wall height will be about a foot up from grade, at the house the wall will be approximately 2′ high. Inside the wall will be tamped gravel, 4″ of rigid insulation, topped by a 4″ concrete slab. My main question is how to keep the 2 traps from freezing. With enough soil below the slab I wouldn't worry about it freezing so much, but with bedrock almost at the surface it won't be very warm below the slab and insulation. I thought maybe I’d make an insulation box to go under and on all 4 sides of the traps, but I’m wondering if there’s another standard practice for this. I can’t find anything on-line.
 

John Gayewski

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I don't see how this could freeze being inside of the new footprint of the building. Insulate around the exterior footing and the heat shouldn't escape fast enough to freeze. Rather than thinking of keeping cold out think about how your going to keep heat in.
 

Ed Dooley

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Thanks, but remember I said I'm right on bedrock. And there is no footing, as I said, I'm pinning the new concrete walls to the bedrock, which is the biggest strongest footing on Earth. :) The insulation under the slab and on the inside of the short stem walls will keep the room warm, but the shower and tub drains will be fairly close to the exterior wall, and the bedrock transfers outside cold very easily underneath the rigid insulation. I'm in Vermont, with a lot of experience of cold weather and bedrock. Twenty below zero means the bedrock transfers that cold pretty far horizontally. The usual practice of putting horizontal foam just under grade outside the slab doesn't work with bedrock on the surface the way it does with soil.
 

John Gayewski

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The wall is your footing which gets insulation around the perimeter. The reason the ground gets cold is because there's no heat at the surface you have heat at the surface and insulated sides.
 

Jeff H Young

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turn tub around put drain on other side? shower trap isnt that close to outside wall is it?
 

Ed Dooley

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Thanks. I don't think any of this addresses the main problem, that there is bedrock at the surface, with insulation around the exterior walls and over the gravel on the ground, which will keep the room warm, but not prevent cold from getting under that insulation. There is no protection from the cold/freezing bedrock projecting its cold far under the slab, below the slab insulation, which in our climate travels horizontally many feet under the slab. My concern is how to keep traps, which without additional protection will be below the slab insulation, from freezing. I was wondering if there was something other than my plan of making an insulated box around and below each trap. If you don't live in a very cold climate and have bedrock (we call it ledge here) then you may not know just how far that 20 below zero temperature travels horizontally. I did a test with my shop/barn, also sitting on ledge, a few years ago, wondering how much to insulate under the floor. I put down 4" of XPS in an 8' x 8' square under the barn, with a thermometer underneath the insulation at the center. At 20 below zero the thermometer still registered below zero. With deep soil, when you lay down insulation outside a building, the insulation allows the natural heat of the soil below to keep the ground above freezing. That isn't true with surface bedrock.
 

wwhitney

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The only good option is to arrange your foam insulation so that the traps are within the thermal envelope of the building.

Basically your five sided foam box idea, where the edges of the box should tie into whatever horizontal insulation you have under the floor, which should tie into the building wall insulation. And no insulation above the trap.

Cheers, Wayne
 

Ed Dooley

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The only good option is to arrange your foam insulation so that the traps are within the thermal envelope of the building.

Basically your five sided foam box idea, where the edges of the box should tie into whatever horizontal insulation you have under the floor, which should tie into the building wall insulation. And no insulation above the trap.

Cheers, Wayne
Thanks Wayne, It's looking like that's the way to go! I thought maybe there were enough examples of a successful option with slab, surface bedrock, cold climate, etc., but I've yet to find anyone anywhere who's said "that's my exact situation, and I did it by _______________________ " (fill in the blank)
 

John Gayewski

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I obviously can't explain this will enough but the cold can't overcome the heat from your building, provided your exterior is insulated. It just can't. The earth is 50 degrees or so from below. Your house is 70. The trap is maybe a for from the 70 degrees. Your insulated from the only cold side.

Do whatever keeps you sleeping well, but if the walls are insulated the only heat loss that can occur is through conduction from below which has to overcome the 70 from above. If you had another building that registering below freezing then you had to be right next to an exterior wall with no insulation.
 

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I can’t seem to get this part across, the Earth will *not* be 50 degrees below this addition. It’s sitting on solid rock, which comes to the surface, not soil. The extreme cold, as I’ve said at least once, makes the bedrock as cold as the outside air, often 20 below zero in the winter, and that cold travels right under the insulation under the building, where the traps will be, as I’ve said has been tested in my other building. And the water will sit in those 2 traps for 8-10 hours at a time, and will freeze. I will insulate around and below the traps. Thanks.
 

John Gayewski

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I can’t seem to get this part across, the Earth will *not* be 50 degrees below this addition. It’s sitting on solid rock, which comes to the surface, not soil. The extreme cold, as I’ve said at least once, makes the bedrock as cold as the outside air, often 20 below zero in the winter, and that cold travels right under the insulation under the building, where the traps will be, as I’ve said has been tested in my other building. And the water will sit in those 2 traps for 8-10 hours at a time, and will freeze. I will insulate around and below the traps. Thanks.
No I understand what your saying. I'm saying it's definitely a 50 degree rock when you get to 4 or 6 feet.

The rock is sitting on 50 degrees unless it's literally floating in the air. Unless the best footing on earth isn't on the earth then it's 50 degrees. And your trap is sitting on the 70 degree side of a sandwich that has 50 on one side and 70 on the other.
 

wwhitney

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No I understand what your saying. I'm saying it's definitely a 50 degree rock when you get to 4 or 6 feet.
I think what the OP is validly concerned about is somewhat crudely represented in the drawing below. The blue line is meant to represent the possible frost line (and whose shape is only a guess on my part. The thick red line is the thermal envelope of the house, with a break in it above the p-trap. The house sits on a frost protected shallow foundation, the red line on the right is the wing of insulation intended to keep the soil under the house from freezing.

Where the house sits on the bedrock, no such insulation wing is possible, and the bedrock is directly exposed to frigid air temperatures. That will let the frost line extend under the house as shown. The bedrock is also a better thermal conductor than typical soils, so the frost line depth in the bedrock is deeper below the surface than it is on the right under the soil. Which means the frost line will extend farther under the house than it would at an unprotected corner with soil underneath.

Now how far the frost line will extend under the house on the bedrock side, and how high it will be, and whether it could actually reach the p-trap is hard to know, so depending on the details of the OP's situation, it may never freeze. But extending the thermal envelope to enclose the p-trap is easy insurance.

Cheers, Wayne


CrudeFrostLine.png
 

John Gayewski

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I think what the OP is validly concerned about is somewhat crudely represented in the drawing below. The blue line is meant to represent the possible frost line (and whose shape is only a guess on my part. The thick red line is the thermal envelope of the house, with a break in it above the p-trap. The house sits on a frost protected shallow foundation, the red line on the right is the wing of insulation intended to keep the soil under the house from freezing.

Where the house sits on the bedrock, no such insulation wing is possible, and the bedrock is directly exposed to frigid air temperatures. That will let the frost line extend under the house as shown. The bedrock is also a better thermal conductor than typical soils, so the frost line depth in the bedrock is deeper below the surface than it is on the right under the soil. Which means the frost line will extend farther under the house than it would at an unprotected corner with soil underneath.

Now how far the frost line will extend under the house on the bedrock side, and how high it will be, and whether it could actually reach the p-trap is hard to know, so depending on the details of the OP's situation, it may never freeze. But extending the thermal envelope to enclose the p-trap is easy insurance.

Cheers, Wayne


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Wayne there's no need to go out of your way to explain something I have understood from the beginning. Thank you, but the heat loss doesn't work out here. As I said above he should do whatever makes him the most comfortable.

Your drawing doesn't show the wall he's building (that needs insulated). It's actually going out of its way to prove something that in all liklehood can't happen.
 

wwhitney

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It's actually going out of its way to prove something that in all liklehood can't happen.
If the p-trap is outside the thermal envelope, It's going to depend a lot on some of the distances that haven't been specified.

If the shortest path from the cold air to the p-trap that avoids all the insulation is only, say, down 12" (because then you hit ledge, and so your exterior vertical insulation can't go any deeper), over 24" (because the p-trap is near the exterior wall), and up 6" (because that's where the p-trap elevation ends up), that's only 42" of soil/ledge separating the p-trap from the outside air. If the frost depth in Vermont is 4' or more, freezing of the p-trap seems like a definite concern.

Now if the shortest distance ends up 10', I agree it seems unlikely, unless that distance is all through ledge, and ledge is like twice as thermally conductive as soil. In that case it would be harder to say.

But the easy solution is to make it so there's no path from the outside air to the p-trap that bypasses all the insulation, i.e. to put the p-trap within the thermal envelope.

Cheers, Wayne
 

Ed Dooley

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Thanks again Wayne. I drew a very rough sketch to show that the issue is even more severe than what you've drawn, or that John seems to understand. 4" horizontal rigid insulation under slab, 3'4" vertical inside short concrete stem walls. As you can see the bedrock is very close to grade, even up against the house at its deepest point. I'm going with the insulatio around the traps. Thanks to all for their help and input!
 

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wwhitney

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I drew a very rough sketch
Nice sketch. Is freezing of the drain lines themselves an issue, or just the p-trap? Are the drain lines going to be in the 4" insulation layer, or down in the gravel?

Either way, the solution is the same: maximize insulation between the item and the bedrock/outside air; minimize the insulation between the item and the interior conditioned space.

BTW, I don't think there's any reason the insulation has to be just below the slab, or even level. So you could put down some minimal fill to flatten the top of the bedrock, put down the 4" of insulation, and then add gravel on top of the insulation to create a level substrate for the slab. There might be some details I'm unfamiliar with that would be necessary to protect the insulation from sharp gravel on top.

Your sketch doesn't show slab edge insulation, but you don't want the interior concrete to be continuous with the exterior concrete exposed to cold air. The slab edge insulation should be continuous with the underslab insulation and the in-wall insulation. I.e. an unbroken thermal envelope.

Cheers, Wayne
 

Ed Dooley

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Nice sketch. Is freezing of the drain lines themselves an issue, or just the p-trap? Are the drain lines going to be in the 4" insulation layer, or down in the gravel?

Either way, the solution is the same: maximize insulation between the item and the bedrock/outside air; minimize the insulation between the item and the interior conditioned space.

BTW, I don't think there's any reason the insulation has to be just below the slab, or even level. So you could put down some minimal fill to flatten the top of the bedrock, put down the 4" of insulation, and then add gravel on top of the insulation to create a level substrate for the slab. There might be some details I'm unfamiliar with that would be necessary to protect the insulation from sharp gravel on top.

Your sketch doesn't show slab edge insulation, but you don't want the interior concrete to be continuous with the exterior concrete exposed to cold air. The slab edge insulation should be continuous with the underslab insulation and the in-wall insulation. I.e. an unbroken thermal envelope.

Cheers, Wayne
No drain line freezing issue. The existing drain line exits the house and sits on bedrock with only 1" foam on top of that, covered by 2" of soil. It's been like that for decades. It follows the outside of the house like that for 24+ feet. I've done many slabs, and always gravel 1st, insulation right underneath concrete (no sense having to heat the gravel!), a tried and true method, especially if you install radiant heating as we may. I mentioned the slab edge insulation in the post with the sketch. It's common here to use interior vertical insulation, which comes up to the top of the slab for a thermal break.
 

wwhitney

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(no sense having to heat the gravel!)
Thanks for the further info, it all sounds good except for the above. The gravel inside the thermal envelope would just add thermal mass, like the concrete slab does. It wouldn't increase or decrease heat loss.

Anyway, just an option to reduce the foam origami you will need.

common here to use interior vertical insulation, which comes up to the top of the slab for a thermal break.
Just curious, do you ever use interior continuous insulation? That would let the wall envelope be continuous with the slab edge insulation. But I imagine it's common just to have a discontinuity in the thermal envelope at the mudsill.

Cheers, Wayne
 
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