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Will plumbing P-traps under rigid foam of a monolithic concrete pour risk freeze damage?

kjginma | Posted in Energy Efficiency and Durability on

We are soon pouring a frost-protected shallow foundation here in MA. When the plumbing was installed, the ptraps for some of the fixtures (floor and shower drain) are below the place where the 6+ inches of EPS will sit Effectively, these traps will not be in the insulated envelope.

I am concerned about possible freeze issues in this scenario and need to decide whether I put a layer of foam on top of these traps (this would be to protect the house from the cold) OR leave off the foam over the ptraps (so that the warmth of the house will keep the ptraps warm enough in Winter).

I hope that GBA has a consensus opinion about what is the proper action for me. It is too late to put any foam underneath these ptraps so the only question is whether to put EPS foam on top or not. If no GBA consensus, I will endeavor to put a thinner layer of foam over the ptraps to hedge my bets.

Thanks, everyone.

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  1. Robert Opaluch | | #1

    I'm not an expert in this topic, but think you have little to worry about, realistically. The ground temperature about 5' underground is out of phase with the current season (warmest in winter). So the temperature underneath the slab, at least 5' from the edge of the slab, will not be affected much by the outdoor temperature, more by average ground temperature. The average ground temperature where you are (where in MA??) is about 50 degrees or so. No worry IMHO. Besides, you are sending warm water down that pipe to warm the pipe and ground surrounding the pipe, or any bit of ice that you believe might form. If you have these traps near the edge of the slab, you could put wing insulation out from the edge of the foundation a foot or two, making the path to outdoor temperatures even longer. Wing insulation, especially at corners, is required in climates colder than anywhere in MA. So not required for you. You could also use some pipe insulation near the slab edge. But if you really thought you'd encounter freezing drain pipe, it would occur away from the house not under it, with a frost protected slab.

  2. kjginma | | #2

    Hello, Robert. We are in Central MA (Air Freezing Index = 1500). The foundation slab is essentially sitting ON the surface, with no frost walls, footings and no basement -- the final grade soil will only be about 8 inches deep, sloping away.. This is an FPSF The ptraps will be approximately 1.5 feet below grade, no closer than 3 feet to any exterior slab wall. The challenges will be the floor drains, one of which will hardly ever be used except for relief valve and similar overflow. I will try to attach a cross-sectional diagram of the slab. BTW, this kind of foundation is not well kmown here ... I have had to point to the residential building code section R403,3 repeatedly. Judging from your reasoned response, I gather you are of the opinion that it is not the heat FROM the house that will be the savior of my ptraps, but instead it will be the normal heat of the earth protected by the well-insulated house above it. If so, then it should be BETTER for the ptraps if some EPS insulation is placed above them, in the space immediately under the slab.

  3. kjginma | | #3

    Here is the cross-sectional diagram of the slab.

  4. Jon_R | | #4

    The ground underneath a FPSF doesn't freeze - that's the whole point of it.

  5. ethan_TFGStudio | | #5

    Kenneth, I didn't see the cross section... perhaps it isn't attached?

  6. GBA Editor
    Martin Holladay | | #6

    Robert gave you the right answer.

    I wrote an article on this type of foundation. Here is the link: Frost-Protected Shallow Foundations.

    In that article, I wrote, "Fortunately, these shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth." The proof: The main manual for FPSFs has always included details for unheated buildings like garages. This type of foundation will keep the soil under the building from freezing, even if the building above is unheated.

  7. Robert Opaluch | | #7


    Hopefully Martin’s article and links address your concerns.

    Yes, the stable ground temperatures as well as some heat from the slab and through the pipe itself would counter the outdoor air freezing temperature. The R-value of plastic drain pipe is about R=0.8/inch. R-value of cast iron pipe would be low. So heat would flow from the house through the pipe, as well as be heated from water that is drained from the bathroom shower. Modifying the depth of insulation around your p-traps seems an unnecessary complication.

    Soil does have some R-value and a lot of thermal mass, which stabilizes its temperature below the surface. The deeper you go in soil, the less seasonal temperature variation. Remember, geothermal systems work because unlike surface soil temperatures, the underground temperature remains more stable throughout the year, warmer than winter temperatures, and cooler than summer temperatures. Uninsulated basements are typically cooler in summer and warmer in winter than the outdoors, closer to the underground temperature, since concrete has little insulation value. Most of the basement outer surface is in contact with below grade soil.

    You can look up the average annual temperature for your area in central MA, which is well above freezing. The average yearly temperature in Worcester, MA is 47.8F (8.8C), well above the 32F freezing temperature. That’s an approximation to the temperature of the earth, say five or more feet below grade.

    You also might find out the temperature of the water from a nearby city water supply as another approximation to your ground temperature.

    Frost depth in MA is assumed to be 48” (less nearer the coast, could be higher at high elevations NW MA). So you could have an uninsulated concrete foundation if the footer is 48” below grade, even though concrete has very little R-value (but lots of thermal mass). Your p-traps seem to be 48” or more from the nearest outdoor air surface at its closest point. If you are still concerned, you could add wing insulation just outside your foundation edge nearest the p-traps, or use pipe insulation. Or you could use a temperature probe or pour hot water down your floor drain if you get worried mid-winter. (The water is the floor drain p-trap might evaporate over time if no water ever drains through it?)

  8. kjginma | | #8

    Thanks, again, Robert, for your detailed and thoughtful response.

    The seldom-used utility room floor drain is something that would benefit from the cheap insurance of horizontal foam wing so I'll save some scraps for that. Because the foundation is being built atop a 4 foot deep plateaued mound of frost-non-susceptible rock my horizontal wings will be near surface and will need some masonry protection from incidental gardening damage.

    I am hopeful that there is only negligible cooling effect of subslab via the passive radon venting in the winter -- the FPSF papers I have read do not quantify such an effect. Mindful of the chilly 'polar vortex' cold snaps of recent years, I moved the radon stack far from the ptraps in question.

    The slab diagram upload did not seem to work for me, so I'll summarize for posterity. I have a lot of EPS immediately under the 4+ inch slab (~10 inches = R40) and 11 inch grade beams (3.5 inches = R14). I lucked out to find a commercial roof renovation and ended up paying $10 per sheet, *delivered* for decent R14 4x8s. I had not planned on any horizontal foam until this thread, because 'frost heave' is a nonissue with the deep base of heaped rock everything sits on.

    Yay for GBA and the great guidance folks provide me as I build my own house.

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