Radiant Floor Heat in a Shop or Garage Slab in Spokane: R-10 Insulation, Tubing Depth, Joints and Permits (2026)

Thinking about radiant floor heat in a new shop, garage or barndominium slab around Spokane? It's one of the few things you can't add later without tearing out concrete, so the insulation, tubing depth, joint layout and permits all have to be settled before the pour. Here's what Washington's energy code and the radiant and concrete industries actually say, and how it changes the way a heated slab gets built.

Washington requires R-10 under the whole heated slab

This is the part that surprises people. Both of Washington's 2021 energy codes treat a heated slab differently from a regular one:

• Homes and barndominiums (residential code): WSEC-R Section R402.2.9.1 says the entire area of a heated slab-on-grade floor has to be thermally isolated from the soil with at least R-10 insulation, and that R-10 is required on every compliance path. It's on top of the perimeter insulation the code already requires for slabs
• Unheated residential slabs, for comparison: Spokane is in Climate Zone 5, where the 2021 table calls for R-10 at the perimeter, extending 4 feet (up from 2 feet in the 2018 code, per WSU Energy Program)
• Shops and commercial buildings (commercial code): WSEC-C Table C402.1.3 lists heated slabs at R-10 at the perimeter and under the entire slab, versus R-10 for 24 inches for an unheated slab
• Soil gas systems: if a radon or soil-gas system below the slab increases air movement under it, the residential code says the heated slab has to be isolated from the sub-slab gravel layer too

Which code applies depends on the building and its use, so we confirm that with your building department during the permit. Either way, the heated-slab insulation number is the same.

How deep the tubing goes, and why slab thickness changes

Hydronic radiant heat runs warm water through PEX tubing cast into the slab. Where it sits matters:

• Cover over the tubing: REHAU's installation guide calls for 2 inches of concrete over the pipe in a structural slab, and allows 3/4 inch only in a non-structural topping
• ACI's guidance: the ACI 302.1R floor guide, as summarized by CalculatingConcrete, describes about 2 inches of concrete under the pipe (not less than 1 inch) and 2 to 3 inches over it
• What that means for thickness: that stack-up is why heated slabs are often thicker than a plain 4-inch floor. Our shop slab thickness guide covers what the slab needs for lifts, trucks and equipment, and the engineer or slab design sets the final number
• Engineering check: REHAU notes the pipe usually doesn't require re-engineering the slab, but a structural engineer or architect has to verify that

Control joints and tubing have to be planned together

Every slab needs control joints, and a heated slab still does. The problem is the saw blade:

• Saw-cut depth: joints are cut to at least 1/4 of the slab depth, and not less than 1 inch, per NRMCA guidance cited by CalculatingConcrete. In a 5-inch slab that's a 1-1/4 inch cut
• Keep loops out of joint lines: REHAU says circuits should be laid out to cross saw-cut joints as few times as possible, and any pipe that does cross has to sit low enough to stay below the blade
• Sleeves at joints: REHAU recommends protecting the pipe 15 inches on both sides of a joint it passes through
• Spacing doesn't change: For Construction Pros notes joint spacing in a radiant floor is the same as any slab, 24 to 30 times the slab thickness

So the joint layout gets drawn first, then the loops. More on spacing in our control joint guide.

Plan the no-drill zones before the pour

Once tubing is in the concrete, you can't drill anywhere you like. Before we pour, mark out:

• Car lift and equipment anchors, racking, and any bolted machinery
• Interior wall plates for offices, bathrooms or a barndominium's living space
• Floor drains and plumbing stubs, which go in first so the tubing routes around them. See our shop floor drain guide
• Zones and the manifold: a shop bay, a garage bay and living space may each want their own loop and thermostat, with the manifold on a wall you can reach

Take photos of the tubing with a tape measure in the shot before the pour. They're the only map you'll have later.

Vapor barrier and base under a heated slab

The insulation board sits on a compacted base, with the vapor retarder and the tubing laid out on top according to the system's design. REHAU's guide says the subgrade should be compacted, flat and smooth so it doesn't damage the pipe or insulation, and that wire mesh, if the design calls for it, has to sit flat with sharp ends pointing down. Our vapor barrier guide covers where the barrier goes and why it matters for shops and barndominiums.

Permits for a heated slab in Washington

• Building permit: the slab is part of the building permit for the new shop, garage or home, and the energy code insulation gets checked there
• Electric radiant: Washington L&I says electrical work requires a permit and inspection, with a few exceptions listed in WAC 296-46B-901. One narrow exception covers listed heat mats with factory lead-ins embedded in tile grout, which doesn't describe cable cast into a structural slab. Some cities run their own electrical inspections, so the permit comes from the city there and from L&I everywhere else
• Hydronic: the boiler or water heater and its piping are typically reviewed under the mechanical and plumbing codes, so ask your building department what they need

Heated slab or not? Questions to answer first

• Will the building be heated at all? If not, a plain slab with the code's perimeter insulation is simpler
• Do you want the option later? Some owners insulate under the slab and cast in tubing now, then connect a heat source years later. It's the only way to get it without new concrete
• What will be anchored to the floor? Lifts and heavy equipment shape the tubing layout and the slab thickness
• Is it a metal building? The slab is only part of the heat loss. Our metal building insulation guide covers the walls and roof

What it costs

Heated slabs, under-slab insulation and radiant systems are quoted per project, since slab size, thickness, insulation, the heating system and who installs the tubing all change the price. If the slab is going under a new building, Raven's current installed rate for PEMB and pole buildings is about $60 to $75 per sq ft, and electrical, plumbing and utility hookups are extra.

Get a heated slab quote

Raven Buildings pours shop slabs, garage floors and foundations across Spokane and Eastern Washington, and we put up the steel buildings and pole buildings that sit on them. Call 509-350-2016 or request a free quote with your building size, how you plan to heat it and your address.

Sources

• Building pricing: Raven Buildings' current installed rate for PEMB and pole buildings
• 2021 Washington State Energy Code, Residential, R402.2.9.1 Heated Slab-on-Grade Floors (UpCodes)
• 2021 WSEC-R, 2nd Edition (Table R402.1.3, SBCC)
• WSU Energy Program, 2018-2021 Slabs and Below Grade Walls FAQs (PDF)
• WAC 51-11C-402121, Table C402.1.3 Commercial Envelope R-Values
• REHAU Radiant Heating Systems Installation Guide (PDF)
• CalculatingConcrete, Heated Concrete Floors: Tube Depth, Joints and Cost
• For Construction Pros, Contraction Joints for Radiant Floors: Best Practices
• Washington L&I, Electrical Permits, Fees and Inspections
• WAC 296-46B-901, Electrical Work Permits and Exemptions