Construction Design-Code Minimum House-Foundation

The design at the Code Minimum House calls for a frost protected shallow foundation.  I have built several homes using this type of foundation and found it works well for a budget friendly build.  Most of my experience is using XPS foam for the insulation.  On this current build, there is no XPS, a new to me concrete form and a different sub-slab insulation are being used.  This blog post outlines the products and processes we used.

The photo above shows a frost protected shallow foundation for a cabin I built in 2011.  I used this foundation design for several different past builds.  XPS insulation is used for the slab edge and horizontal frost wing.  The hand built forms and foam installation are labor intensive.

Fast forward to the code minimum build of 2020.  This build uses a manufactured form by Mono Slab.

These two cross sections are the sample I was given for the EZ Form.  Made from EPS instead of XPS foam, the product is a little better for the environment.  It measures 16″ x 16″ and the notch (in the upper picture) is to receive a 2″ x 8″ form board.  The lower picture shows what the form will look like after the concrete pour.  A portion of the form (a triangle) is designed to be removed, leaving some insulation at the slab edge and the lower area covered by soil.

One issue with the form sample I was given is the insulation value and frost protection “wing” does not meet the code requirements for climate zone 7 and the air freezing index of 3,000 (the values for my area of Minnesota).  The frost protection requirements for all climates is listed in articles R403.3-R403.4.1 in the 2018 IRC, the current code in force for Minnesota.  I ended up designing the detail below to meet the requirements.

As you can see, an addition “horizontal wing” of insulation would be needed when using the 16″ x 16″ form.  We later learned that Mono Slab has a form designed for cold climates call the Arctic Form, which measures 18″ x 30″.  This is the product we ended up using and was able to eliminate the additional horizontal frost protection wing.

Once the deep utilities were installed, the water line which is eight feet deep and the main sewage trunk line, which is 3 feet deep, and site compaction and leveling were complete, the concrete crew could begin.  The crew had almost all the forms set in less than a day.  The plumber then installed all the below grade waste lines along with a few water lines.

The next step was to add the sub-slab insulation inside the forms.  It is best to thermally break the concrete slab from the ground in all climates.  We are installing an in-floor heating system which makes this thermal break all the more important.  Typically we would be adding some sheet foam which would be cut and fit inside the EZ Forms.  This is a labor intensive process.  The concrete contractor suggested we have a closed cell spray foam (CCSF) contractor spray the thermal break.

Not only did the R-10 of CCSF provide a thermal break, it is also a vapor barrier and it did a great job sealing around all the plumbing pipe penetrations.  The spray foam crew had the entire 1500 square foot slab plus 750 square foot attached garage foamed in less than four hours with two guys.  I would estimate a crew of four guys would have taken eight hours to cut and place sheet foam.

Once the sub-slab foam insulation had been installed, the heating contractor installed the in-floor heat piping.

This photo shows the ends of the plastic heat tubing that will eventually be tied to a boiler manifold.  This home has well over 2,000 lineal feet of 1/2 inch heat tubes run through the concrete slab.  Heating contractors in my area typically figure one lineal foot of tubing per square foot of home.  1500 lineal feet was used in the home with the remainder in used in the attached, heated garage.  If you look closely at this picture, you can also see the fiberglass rebar that was used to strengthen the concrete.  First time I have seen the fiberglass rebar used in my area.

The first concrete pour.  The concrete contractor hired a pump truck for this job because of the limited access to two of the sides of the slab.  You can see they fill the thickened footing area of the foundation first and then move to the four inch slab.

The last picture shows the finished slab with the 2 x 8 wood forms and insulation triangle removed.  The area has been backfilled and we are ready to start framing.  Next post, the wall and roof framing.

4 Replies to “Construction Design-Code Minimum House-Foundation”

  1. Excellent post about the process of using the ez mono slab forms. As a fellow contractor in Minnesota, my only question is about the utilities. The water line, the main sewer, and then the branch drain lines for first floor bathroom and kitchen. Does using slab on grade techniques require any type of special insulation or heating method for water and sewer? Specifically p traps located under the slab or water lines like you specify? Thank you in advance.

    1. Good question. Slab on grade foundations requires careful planning and layout for the mechanical systems, especially the plumbing. The ground directly below the slab will stay above freezing, even with sub-slab insulation present, so there is little concern with things freezing. I have seen sewer lines freeze after they leave the space under the slab, this is usually due to settling of the soil between the home and septic tank or main sewage line at the street causing a dip in the line. That’s an installation error, not a problem with a frost protected slab. The main waterline feeding the structure will usually enter the home from deep underground. The waterline for the code minimum house, which has a well, was six feet down until it got to the edge of the structure, then it started sloping upwards under the slab until it reached the location of the mechanical room, which was about 6 feet inside the slab’s edge, again, we are under the slab at that point, the soils will stay above freezing. Because is a deep excavation directly under the thickened edge of the foundation, we needed to be sure proper soil compaction was completed before forming and pouring the slab.

      Something I have noticed recently is the pipes and other penetrations through the slab not sealed correctly to the sub-slab vapor barrier. A proper slab on grade installation will include the rock for the radon mitigation system. During blower door testing of the home, I’ve seen air moving up from under the slab, I believe I’m pulling air down the radon pipe, into the rock bed where it finds a path around a poorly sealed penetration in the slab and enters the home. It’s usually not a lot of air, but I have been able to detect it. If air can find its way through the concrete slab, so can radon. We need to do a good job sealing the pipes around the sub-slab vapor barrier.

      One last thing I try to plan for with slab on grade construction is to eliminate as many penetrations as possible that would normally move through an exterior wall. The electrical service conductors, cable and internet cabling, and if possible, any gas lines I try to have sleaved under the slab and up into the mechanical room or other location where they would terminate. The easiest way to tighten a home and improve durability (water leaks in the wrb) is to eliminate as many holes in the air/water control layers as possible.

      Thanks for the question, John!

  2. Hi Randy
    This is very helpful. I am planning to pour my slab this fall in MN and build my shed this winter. My question is if I’m pouring a slab with all the same insulation practices you described here, but do not initially heat the space and we have a typical MN winter, do you see a high risk of frost heaving? Or would it stay out since it is enveloped in insulation? I also have 2’ of gravel and geo textile fabric under slab that has been prepped and sitting for over a year so base is solid. Any thoughts on this is appreciated!

    1. Hi Brandon,
      I’ve been in a few discussions about this recently. My thoughts are, as long as the soil is well draining and not expansive, and there is at least R-10 sub-slab insulation under the entire slab, the slab should be fine. If there is any risk of the soils moving, you could always install a layer of concrete blankets or straw over the top of the slab. I would include a few feet of temporary insulation to cover the insulation wing that is buried outside the slab as well.

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