Slab Insulation

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

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A slab only performs as well as the layers under it. Walk every layer before the pour.

I am a:

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

What it isThe layers under a slab on ground: base, moisture barrier, foam and the slab itself.

Your jobLay each layer clean, flat and sealed, in the order the plan shows, and walk them before the pour.

The hard ruleNever pour over wet or frozen ground, standing water under the poly, or a covered radon pipe.

The checkBase, barrier, foam, radon pipe and edge insulation against the plans, before the pour.

By the end you can

Name the layers under a slab and why the order matters.
Say what Part 9 says about slab thickness and drainage under it.
Count foam boards and keep the layers clean for the pour.
Know whose number sets the insulation thickness.

Stop and read

Rigid foam burns, and the smoke is toxic. Keep torches, heat guns and welding sparks away from it, and use only adhesives the maker approves. Wear a dust mask and eye protection when cutting.

Foam sheets under rebar are a trip hazard, and wet or frosty foam is slippery. Keep rebar chairs and tie wire clear of footpaths, and cap protruding rebar ends where people walk.

The big idea

Slab insulation cuts heat loss through the floor, protects the slab edge from frost and makes the floor warmer. It sits in a layered system: compacted base, then a barrier against ground moisture, then foam, then the slab. The slab only performs as well as the layers beneath it, so walk them before the pour and fix what you see.

What the code says

From Part 9, as printed in the Vancouver Building Bylaw 2014 (a municipal reprint, possibly older than your province's edition):

A drainage layer is called for under a floor-on-ground (Article 9.16.2.1). Garage, carport and accessory-building slabs are exempt from that rule. The text I read gives no thickness (an older 100 mm figure is struck out).
A floor-on-ground slab shall be at least 75 mm (3 in) thick (Article 9.16.4.3).
Water must be kept from getting under the floor by grading or drainage (Article 9.16.3.1).
The radon rough-in (Article 9.13.4) describes a 100 mm granular layer and a 100 mm vent pipe, with Radon Area 2 exempt. Check your edition and province.

The insulation requirement comes from the energy rules (Section 9.36), which I did not read. It varies by province, tier and whether the slab is heated.

What the rules look like in practice

Two real examples show how much this varies, and why you must check your own:

Source	What it says	Notes
Town of Gibsons, BC, building department handout (cites BC Building Code 2018)	Unheated floor-on-ground above the frost line: R-12 insulation running 1.2 m (4 ft) from the slab edge, horizontally or vertically, and the edge may be R-6. Heated floor: R-12 under the whole slab, and the edge must not be reduced.	One municipal handout, undated, from the 2018 code. It may be out of date under newer BC codes. Read your own.
Alberta Municipal Affairs, STANDATA 19-BCV-005 (December 2019)	A variance for hydronic (radiant) floors: where the floor is fully below the frost line, at least RSI 0.88 (R-5) under the slab is compliant; where it is above the frost line, at least RSI 1.32 (R-7.5). It describes this as an alternative to the 100 mm under-slab insulation that Section 9.36 would otherwise require, and it does not remove the insulation requirement.	Written for the 2019 Alberta edition. The edge requirement refers to CSA B214 (at least RSI 0.88 below a heated slab and at its edges).
Natural Resources Canada, Keeping the Heat In, Section 6	Basement slabs are seldom insulated, because most heat is lost through the upper foundation walls. On frost-susceptible soil, slab-on-grade houses get impermeable insulation placed horizontally in the soil, sloped 1:5 for at least 1 m (3 ft).	Retrofit advice, not a code. It gives no R-values for slabs.

Neither number is "the" number. Your drawings, your energy tier and your building department decide.

The layers

Foam under a slab

Use XPS, or an EPS type the maker rates for ground contact and for the slab load. A US lab handbook says below-grade foam can take on moisture that cuts its effective R-value by about 35 to 44%.
Choose compressive strength for the load: ask the maker for the product's figure and the designer for what is needed. This matters most for garages, shops and slabs that carry machinery. I dropped the old kPa ranges because I could not verify them.
Ask the maker before using polyiso or any board you are unsure about below the slab. NRCan lists polyiso boards for exterior walls, and says foil-faced board may deteriorate in contact with cement. Ground-contact rating is product-specific.

Subgrade and base

Compact the subgrade and fix soft spots. Do not let the foam sit on loose or wet fill.
Place the granular base or drainage layer your design calls for. Its thickness comes from your design and code edition.
Make the base flat and uniform, so the foam does not rock or bridge.

Moisture control

Place polyethylene where the code or design requires it, with the laps and tape the design specifies. Where it sits relative to the foam comes from the design. I found no code sentence on it.
Seal around pipes and penetrations, and repair any tears before the pour.
NRCan's simple dampness check: tape a plastic sheet about 1 m (40 in.) square to a slab. Condensation on the top means high interior moisture. Condensation on the underside means moisture is moving up from the ground, and may indicate radon.
Radon: where a rough-in is required, set the piping and the gas-permeable layer before the foam goes down.

Placing the foam

Stagger the joints, butt the panels tight, and tape or seal them as the design says.
Run the foam to the slab edge, and add edge foam as a thermal break (see Thermal Breaks).
Keep the foam clean and in place while the reinforcing and concrete go on.

Example (numbers made up). A 6 m × 8 m (19 ft 8 in × 26 ft 3 in) slab with foam over the whole area is 48 m² (517 ft²). A 1.2 m × 2.4 m (4 ft × 8 ft) board is 2.88 m² (31 ft²), so 48 ÷ 2.88 = 16.7, which is 17 boards, and about 19 with 10% extra for cuts and waste.

Common mistakes

Foam the maker does not rate for ground contact
Edges left uninsulated
Gaps between panels
Damaged poly
Foam too weak for the load
A soft or uneven base
Water trapped under the poly

Pro note

The slab only performs as well as the layers beneath it. Walk the layers before the pour and fix what you see.

Weather note

Cold stiffens foam and weakens tape. Rain and frost on the poly and foam must be dealt with before the pour, because water trapped under the slab does not leave.

What the inspector looks at

The base, the barrier, the foam and the edge insulation against the plans, before the pour. Confirm what your building department checks.

✅ Check yourselfTap a question

What is the layer order under a slab, from the ground up?

Compacted subgrade, a granular base or drainage layer, the barrier or foam as your design shows, then the slab. Each layer has to be right for the others to work. Follow your plan for where the poly sits relative to the foam.

What does the Part 9 text I read say about slab thickness and what is under it?

A floor-on-ground slab shall be at least 75 mm (3 in) thick (Article 9.16.4.3), and a drainage layer is called for under it (Article 9.16.2.1), with garage, carport and accessory-building slabs exempt from that. The text I read gives no thickness for the drainage layer. Check your edition.

A 6 m × 8 m (19 ft 8 in × 26 ft 3 in) slab gets foam over its whole area. How many 1.2 m × 2.4 m boards with 10% extra?

Area 6 × 8 = 48 m² (517 ft²). Each board is 2.88 m² (31 ft²), so 48 ÷ 2.88 = 16.7, which is 17 boards, and about 19 with 10% extra. The sizes are made up.

Water is trapped between the poly and the foam on the morning of the pour. What do you do?

Stop and deal with it. Water trapped under a slab does not leave. Remove it and fix the layers before the pour.

Who decides the foam's compressive strength for a loaded slab?

The designer or engineer. Ask the maker for the product's compressive strength, and do not guess for garages, shops or slabs that carry machinery.

A radon rough-in is required. When does the pipe go in?

Before the foam and slab go down. Part 9 describes a 100 mm granular layer and a 100 mm vent pipe, with some low-radon areas exempt (Article 9.13.4). Check your edition and your province.

🧰 Gear for this stepMust-haveRent itNice to have

Must-haveLayout and levels:
Must-haveBase (hand tools):
Rent itBase (machine): a plate compactor
Must-haveFoam and poly:
Must-haveCutting and fitting:
Must-haveRadon rough-in: pipe, fittings and a labelled cap where the code requires it, set before the foam goes down
Must-haveRebar support:
Must-havePPE:
Must-haveDust and fumes: a dust mask or respirator (CSA Z94.4) when cutting, and ventilation when using a hot-wire cutter

👷 Who can do thisTap your level

Home DIY

A capable owner can lay a base, foam and poly under a small slab such as a shed or garage when the plan is clear. Do not skip a required radon rough-in, lay foam over wet or frozen ground, or change foam strength on a loaded slab. Hire a pro or an engineer for heated or structural slabs.

Laborer or helper

Carry and lay foam sheets and keep the layers clean as the crew directs. Do not lay foam over wet or frozen ground, and do not step on or bend the radon pipe. Tell the lead if foam is damaged or a layer is out of order.

1st-year apprentice

Cut and lay foam, tape poly and keep the layers clean, under supervision. Learn the layer order, how rebar chairs sit on foam, and why trapped water stays.

Apprentice, 2nd to 4th year

Lay foam and tape poly in the order the plan shows, and keep rebar chairs from punching through. Keep the radon pipe clear. Report puddles under the poly before the pour.

Journeyperson

You own the layers under the slab: base, radon pipe, foam, poly and edge. Walk them before the pour. Stop-work triggers: wet or frozen ground, a covered radon pipe, or foam strength in doubt. Take heated or structural slab loads to the engineer.

Lead hand or foreman

Walk every layer before the pour and make the call to stop if the ground is wet or frozen, the radon pipe is covered or the foam strength is in doubt. Take heated or structural slab loads to the engineer.

Contractor or business owner

Price the base, foam, poly and radon rough-in to the code and energy requirements for the job. Keep dated photos of each layer before the pour, plus the inspection record and product data. Cold or wet slabs lead to claims.

Starting a new business

Price the whole assembly (base, foam, poly, radon rough-in) to what your building department confirms. Take dated photos of each layer. Ask your insurer about slab moisture claims, and keep the inspection record.

By Province:

The national baseline is Part 9 (Section 9.16 for floors on ground, 9.13.4 for radon, 9.36 for energy). Each province adopts its own edition and sets its own energy tier, and some municipalities add rules, so under-slab and edge insulation requirements differ by province, by municipality and by date. The table above shows two real examples. I dropped the old per-province list of editions, tiers and radon details because I could not verify it. Ask your building department which requirements your permit is under.

See Canada by Province for the full comparison.

Sources:

Vancouver Building Bylaw 2014, Part 9, Section 9.16 (municipal reprint of NBC Part 9, possibly older than your province's edition: Articles 9.16.2.1, 9.16.3.1, 9.16.4.3)
Vancouver Building Bylaw 2014, Part 9, Section 9.13 (Article 9.13.4, radon rough-in)
Town of Gibsons, Floor on Ground Insulation Guidelines (municipal handout citing BC Building Code 2018, undated)
Alberta Municipal Affairs, STANDATA 19-BCV-005, Insulation Below a Heated Floor (December 2019)
Natural Resources Canada, Keeping the Heat In, Section 6 (retrofit advice, modified December 2024)
Foundation Handbook, Location of Insulation (slabs) (US Department of Energy lab, undated)
Section 9.36 energy tables, provincial radon details and foam compressive-strength figures were not verified. Board counts are calculated and re-checked.

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