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Thermal Breaks

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Concrete carries heat out of the house. A thermal break cuts that path, and it has to be continuous.

I am a:

Tap your level and the boxes below open to the advice for you. Remembered on this device.

What it isA strip or layer of insulation that cuts the heat path through concrete, mainly at slab edges.
Your jobRun the foam continuously along the edge and joints, hold it in place through the pour, and protect it above grade.
The hard ruleNo gaps. A cold bridge shows up only in the first winter.
The checkThickness and continuity against the plans and the energy requirements, before the pour.

By the end you can

  • Explain why a slab edge is a cold bridge.
  • Place and hold edge foam so it stays continuous through a pour.
  • Choose foam the maker rates for the use.
  • Know when a thermal break is an engineer's job.
Stop and read

Rigid foam burns, and the smoke is toxic. Keep torches, heat guns and welding sparks away from it. Solvent-based adhesives and sprays can melt foam, so use only products the maker approves. Wear a dust mask and eye protection when cutting, and ventilate when using a hot-wire cutter. Foam inside a building normally needs a fire-rated covering, so check your code.

Foam under a slab or beside forms is slippery when wet or frosty. Keep rebar chairs and tie wire out of walkways, and cap rebar ends where people walk.

The big idea

Concrete conducts heat very well. A slab edge or footing that runs from the warm inside to the cold outside carries heat away, and that cold surface collects condensation and frost. A thermal break is a layer of insulation that cuts that path.

A US Department of Energy lab handbook says the slab perimeter is where most slab heat loss happens, and that vertical insulation outside the wall or grade beam is the only method that cuts edge heat loss for an integral grade beam and slab. It also says that whenever insulation is placed inside the foundation wall or under the slab, the joint between slab and wall must be insulated, or a thermal bridge causes significant heat loss.

Where a thermal break goes

  • The slab edge: the most common cold bridge. The US handbook says edge insulation at the slab-to-wall joint is generally limited to about 1 in. (25 mm), because that is the room there is.
  • Between the foundation wall and the slab, where the design calls for it.
  • Any other place concrete runs from inside to outside: exposed slab edges, and pads or steps that touch the slab or wall. Your design and the energy requirements say which.
  • Embedded joist ends: Natural Resources Canada says not to put more than 25 mm (1 in.) of foam board where joists are fully embedded in the concrete, because the cold concrete can make the floor above uncomfortable and prone to damage.

The thickness and R-value come from your plans and the energy requirements, and they vary by province and tier. See Slab Insulation for examples.

Materials

  • XPS or below-grade-rated EPS for edges and anything below grade. The US handbook says below-grade foam must be approved for that use, and that it can take on moisture that cuts its effective R-value by about 35 to 44%.
  • Higher-strength foam where the edge carries load. Ask the maker for the compressive strength, and the designer for what is needed.
  • Structural thermal breaks (for example at balcony connections) carry load and come from an engineer's design. Do not improvise them.
  • Check that the foam type is rated for the use by the maker. For polyiso, foil-faced board or any board you are unsure about, ask the maker before using it below grade. NRCan says foil-faced board may deteriorate in contact with cement.

Installing and holding it

  • Place foam along the whole slab edge, against the foundation wall or form, so it is continuous.
  • Make sure it does not get in the way of the formwork, and that it stays put when the concrete is placed.
  • Get full contact between the foam and the concrete, and seal the joints so air and water cannot get behind it.
  • Protect any exposed foam above grade with parging, a panel or a flashing. NRCan says to cover exposed exterior foam down to at least 100 mm (4 in.) below settled ground level.
  • Protect the foam during backfill and compaction, and check the thickness against the design.
  • Frost-protected shallow foundations use the foam as frost protection, so they follow the design exactly (see Frost Protection).

Example (numbers made up). A slab 100 mm (4 in) thick with a 36 m (118 ft 1 in) perimeter needs an edge strip 36 × 0.1 = 3.6 m² (39 ft²). One 1.2 m × 2.4 m (4 ft × 8 ft) board cuts into 12 strips 100 mm wide and 2.4 m long, and the perimeter needs 36 ÷ 2.4 = 15 strips, so 2 boards. Add extra for corners and cuts.

Common mistakes

  • Slab edges left uninsulated
  • Foam the maker does not rate for below grade
  • Exposed foam with no protection
  • Foam that blocks the drainage layer
  • A gap where the foam shifted during the pour
  • Wrong thickness for the climate
Pro note

Cold bridges are silent. You do not see them until the first winter, when the slab edge sweats or the floor is cold.

Weather note

Freeze-thaw cycles stress joints at the slab edge, and rain behind foam traps water. Heat expands foam and can open its joints. Install on a dry, stable day.

What the inspector looks at

Foam at slab edges and joints against the plans and the energy requirements, before the pour. Confirm what your building department checks.

✅ Check yourselfTap a question
What is a thermal break?
A layer of insulation that cuts the path heat takes through concrete, such as at the edge of a slab, where the slab meets the cold outside.
Insulation goes inside the foundation wall and under the slab. Does the slab-to-wall joint still need insulating?
Yes. A US Department of Energy lab handbook says that whenever insulation is placed inside the foundation wall or under the slab, the joint between slab and wall must be insulated, or a thermal bridge causes significant heat loss.
A slab is 100 mm (4 in) thick and the perimeter is 36 m (118 ft). How much edge foam area, and how many 1.2 m × 2.4 m boards if you cut 100 mm strips?
36 × 0.1 = 3.6 m² (39 ft²). One board cuts into 12 strips 2.4 m long, and 36 ÷ 2.4 = 15 strips, so 2 boards. Add extra for corners and cuts. The sizes are made up.
Why is foam rated for ground contact needed at an edge that goes below grade?
Below-grade foam can take on moisture, and a US lab study found this can cut its effective R-value by about 35 to 44%. Use a type the maker rates for below grade, and keep water off it.
Can you build a balcony thermal-break connection yourself from foam strips?
No. Structural thermal breaks carry load and come from an engineer's design.
The foam shifts while the concrete is placed. What do you do?
Stop and reset it. A gap at an edge becomes a cold bridge you will not see until the first winter, so hold the foam in place through the pour.
🧰 Gear for this stepMust-haveRent itNice to have
  • Must-haveLayout and cutting:
  • Must-haveFoam: XPS or below-grade-rated EPS strips for slab edges, and higher-strength foam where loads are heavy
  • Must-haveFixing and sealing:
  • Must-haveProtection:
  • Must-haveChecking: a tape measure, to check thickness against the drawings
  • Must-havePPE:
  • Must-haveDust and fumes: a dust mask or respirator (CSA Z94.4) when cutting foam, and ventilation when using a hot-wire cutter
👷 Who can do thisTap your level
Home DIY
A capable owner can place edge foam on a garage or shed slab before a pour. Do not guess on structural thermal breaks such as balcony connections, and do not skip the energy check with your building department. Foam is slippery when wet or frosty, so keep the work area tidy.
Laborer or helper
Cut foam strips to length and hold them in place while forms go up. Do not try to build or alter a structural thermal break. Tell the lead if foam shifts, tears or is left out at an edge.
1st-year apprentice
Cut and fit edge foam and keep it in place during forming, under supervision. Learn where cold bridges form and which foam the maker rates for ground contact.
Apprentice, 2nd to 4th year
Fit and fasten edge foam to the forms, then check it has not moved before the pour. Ask before cutting any foam at a corner or joint.
Journeyperson
You own the continuity of the thermal break at slab edges and joints, and how it is held through the pour and protected above grade. Stop-work triggers: a gap at an edge, a foam type the maker does not rate for the use, or a structural thermal break with no engineer's design. Send structural breaks to the engineer.
Lead hand or foreman
Check that the break is continuous at every edge and joint before the pour, and that it is held in place through the pour. Protect the exposed edge after stripping. Send any structural thermal break to the engineer.
Contractor or business owner
Price edge foam and sealing to the energy requirements for the job. Document thickness and continuity with photos before the pour. Cold floors and sweating slab edges show up as callbacks in the first winter, so check before the pour.
Starting a new business
Confirm the energy requirements with the building department before you price edge foam. Photograph thickness and continuity before every pour. Agree in writing with the owner what is included, because cold edges show up in winter.
By Province:

The technique is the same across Canada. What changes is the energy requirement and the R-value, set by Part 9, Section 9.36 as each province adopts it, and by the energy tier or step your province or municipality has chosen. I dropped the old list of provincial adoption dates because I could not verify it. Ask your building department which requirement your permit is under. The Slab Insulation page shows real examples from Alberta and BC.

See Canada by Province for the full comparison.

Sources:
  • Foundation Handbook, Location of Insulation (slabs) (US Department of Energy lab, undated: slab edge, thermal bridging, below-grade foam)
  • Natural Resources Canada, Keeping the Heat In, Section 6 (issued 2014, modified December 2024: retrofit advice, not a code)
  • Section 9.36 energy tables and the provincial R-values were not read here. Structural thermal-break products were not researched, and are an engineer's decision. The foam-safety advice is general practice and follows the makers' data sheets. Strip counts are calculated and re-checked.

Last reviewed October 2026. General information, not engineering or legal advice.

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