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Retrofitting Hydronic Heating into an Uninsulated Basement Slab

August 10, 2016
Read time: 2 mins

Direct the heat toward the feet

Retrofitting in-floor heat is a great way to warm up a chilly basement bathroom. But unless you have insulation under the heating system, your customers are going to spend fat stacks of Benjies heating the earth every month.


Step by step:

  • To keep the heat where you want it, cut out the slab before installing the heating system.
  • Dig deep enough to allow for gravel, insulation, and a slab on top.
  • Cover the earth with compacted gravel to support the slab and keep water from wicking in.
  • Reinforced plastic over the gravel stops water vapor.
  • Rigid insulation comes next surrounding the new slab on all five sides.
  • You will need some sort of steel reinforcement, either wire mesh or rebar and then install the hydronic tubing.
  • Pour the slab over the tubing, let it cure, and cover the whole assembly with flooring—and be confident that your customers Benjies will stay in the bank.


2012: IECC:
2015 IECC:
  • Section 402: Building Thermal Envelope
  • R502.1.1.1: Building Thermal Envelope (existing building)
  • R 502.1.1.2 Heating and Cooling Systems (existing building)


—From the fat-stack detail-pack of Otogawa-Anschel Design+Build, in Minneapolis, Minn. Thanks to Dr. Joe Lstiburek for the technical assistance and talking down to us so patiently. 


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