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Community and Q&A

Insulated slab details for zone 7 walkout

kenorakq | Posted in Energy Efficiency and Durability on

My build is in Kenora Onrtario Canada (zone 7). The basement is a walkout; I need to incorporate a sewage lift station under the floor to pump (shower/toilet/sink and washing machine) water up to my septic
I need to know how to detail the insulation and poly layer under the concrete.
Do I insulate the sides and bottom of the hole the tank (110 gallon) goes into; if so do the joints have to be taped like building an foam box; I can’t have the sewage freezing in the tank! Is that even a concern?
I will have a separate sump pit (40 gallon) that will pump collected perimeter water to the exterior. Should I insulate that hole in the same fashion as the sewage tank?

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  1. GBA Editor
    Martin Holladay | | #1

    1. This is a question for an engineer. The value of an engineer's services are worth the cost.

    2. In theory, it's certainly possible to create details for a big insulated sump that is large enough to hold this tank. I understand the desire to create an insulated sump, and it's possible. However, if your engineer advises a solution that isn't insulated, I don't think you have to worry about your sewage freezing in this location.

  2. kenorakq | | #2

    I appreciate the reply but don't have an engineer involved in this build. I thought someone out there has run across this before (maybe not!) I am hoping someone will chime in...

    The issue with the sump pit is less complex, I think, since there won't be any run off to deal with at freezing temps an uninsulated hole seems to make sense, in that case I would tape or use acoustical sealant around the top of the pit..

    I'm wondering if there is even an issue or need to insulate the sewage lift station if its completely under the insulated floor...thoughts....

  3. user-4524083 | | #3

    Tim - I'm no engineer, but your proposed solution looks too complicated to me. The in- ground tank will not freeze if the basement does not freeze. The soil below the slab will be at a temperature roughly equal to the year-round average temperature of your location. I would try to make some type of insulated cover that was reasonably well insulated and tight, but could be removed. A good application for EPS or polyiso, maybe have the cover in 2 pieces with neoprene boots over the pipes.While this may be challenging, it seems less cumbersome than trying to insulate the whole pit. I respectfully disagree with the need for an engineer for this, but also understand why our host cannot tell you what he would do if it were his house, as it is not his domain.(Neither is it mine!) The cost of the engineer consult would likely exceed the cost of the remedy. Assuming the rest of your house is "pretty good",the heat loss from the basement to the pits will be less than a good window in the wall. Good luck finding a solution.

  4. Expert Member

    Tim, there may be something particular to your situation I don't know about, but in every case where I've been faced with pumping effluent to a septic tank, I've opted to put the pump chamber outside the house rather than use a sump pit. When things go wrong, which they invariably do over time with pumped systems, the problem is in your yard not your basement.

  5. rocket190 | | #5

    To the OP, this is just a standard ejection pit (lift pump) required because your tank inlet is higher than the lowest invert of your basement plumbing? If so, there typically isn't a need for that much storage capacity, although I have seen capacities that large for grinder pump setups. Liken it to a sump pump that is designed to handle effluent. Your upstairs plumbing should exit the house without the need to be pumped. If so, there is no risk of the tank freezing even without insulation unless you are planning to leave your house ingested for an extended period. If the ground was ever cold enough to freeze your tank the rest of the plumbing would be frozen in your house and you'd have greater issues.

  6. user-4524083 | | #6

    I like Malcolm's idea, if that is possible,but it sounds like all this plumbing will be in the basement. In that case,limiting the tank to ONLY the basement plumbing, as Rick suggests, is the way to go.

  7. kenorakq | | #7

    Only the basement plumbing will be going into the lift station... The tank size was recommended by POLYWEST the local manufacturer... They related that a 110 us gallon tank with a pump on the bottom really only has a range of about 50 gallons since it never gets filled to capacity before being pumped out.. And there is always about 10-15 gallons in the tank even when pumped clean. They added it should have enough usable capacity to handle the washer draining while someone showers and the sink or shower drains!

  8. Expert Member

    Tim, as others have posted you don't need to insulate the tanks. A larger issue with a tank that size and shape might be hydrostatic pressure forcing it upward. I would make sure the area around it had good drainage.

  9. kmer7777 | | #9


    I think ten inches of EPS under the slab is a great way to go - one I see as an investment. Trouble is, there are some side effects that can work against you if you are not prepared for them. Perhaps you considered this already, but since heatloss to the footing will be substantially reduced, you will likely need to compensate with horizontal insulation on the exterior to prevent heave, especially at the walkout. I've been called to this situation before, after the fact. It can easily be prevented with proper design and consideration of the unique site.

    I don't recommend experimenting with the investments like yours, so as for an answer to the question you asked, I do agree with Martin that a professional's input is worth further investment on this one - but then I'm slightly biased because I am an engineer. Yes, we do charge a fee for our services but make no apology for that.

    If you can't find someone locally that is equipped to deal with this you could track me down online. I'm driving through Kenora in a few days and could provide you with a quote for your very interesting project.

    All the best.

  10. kenorakq | | #10

    Hi, thanks for the replies, I learn something every time I read a post. The soil is well drained and sloped towards the lake, I don't thing lifting will be too uch concern. For context I can add that I have a house in Winnipeg (very poorly drained clay) I added a sump pit last year and noted the water level was at the bottom of the basement slab, even with the drain tiles. After a hole was scooped in the swamp under the house a pit was put in place and filled with water to make in neutrally buoyant. This was kept flooded till the concrete set around the pit, after a week we turned on the pump and enjoyed it pumping every ten minutes for about a month. It now has about 18 inches of water (its lowest level and hasn't erupted through the floor. FWIW the Red River gumbo around Winnipeg is famous for its horrible drainage. With the much better drainage in Kenora I don't anticipate an issue.

    Kelly, Yes I'm interested. However I'm a little lost when it comes to what an engineer can offer me and at what cost....

    .I will be in Kenora Monday March 16th to Thursday the 19th..I can PM you my contact email if you would like to talk.

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