Built Right Detail 04: Slab Edges & Under-Slab Insulation

A builder’s net zero envelope course by Xerox Builders · October 2026

The slab is part of the envelope, and everything under it is a one-shot job. This Detail builds the layers under a basement or slab-on-grade floor in the right order: the capillary break, under-slab insulation, the sealed poly, the slab-edge thermal break and the radon rough-in now required across BC.

The slab is part of the envelope

Heat leaks down into the ground and out through the slab edge. Ground moisture wicks up through concrete. Soil gas, including radon, enters through cracks, joints and pipe penetrations. Once the concrete is poured none of this can be fixed, so every layer under the slab is a one-shot job.

The layers, bottom to top

1. Undisturbed soil, with under-slab plumbing and the radon pipe roughed in.
2. Clean, coarse granular layer. It breaks capillary rise and gives soil gas a path to the radon pipe.
3. Rigid insulation, rated for the load it carries.
4. Polyethylene air and vapour barrier, lapped, sealed, run up the foundation wall and sealed around every pipe.
5. Concrete slab, with an insulation strip separating the slab edge from the foundation wall.

Some designs place the poly below the insulation instead of above it; both are used. Follow the drawings and keep the barrier continuous and sealed either way.

Capillary break and vapour control

Concrete and fine soil wick water upward. A layer of clean, coarse stone breaks the wick because the gaps between stones are too large for water to climb. Under the BC Building Code 2024, the prescriptive gas-permeable layer is at least 100 mm of clean, coarse granular material with no more than 10% passing a 4 mm sieve, so one layer does both jobs. The poly above it stops vapour and soil gas. Walking on poly tears it and opens laps, so keep traffic off it and repair damage before the pour.

Under-slab insulation choices

The engineer confirms the compressive strength needed under footings, posts and other concentrated loads.

How much insulation

A heated slab, with heating pipes or cables in the concrete, needs insulation under the whole slab and around its edge, and the edge insulation is not reduced. For unheated slabs above the frost line, the prescriptive path described in local climate zone 4 handouts allows a perimeter band 1.2 m wide, or insulation run 1.2 m down the wall. In practice, most homes built to the BC Energy Step Code in Metro Vancouver insulate the whole slab, and the energy model sets the thickness. Requirements change between code editions, so confirm the current values with your city and energy advisor.

Breaking the slab-edge bridge

Place a strip of insulation between the slab edge and the foundation wall before the pour, joined to the under-slab insulation and to the wall insulation above, so the insulation line is unbroken. Alternatively, insulate the foundation on the outside, with the insulation flashed and protected above grade, so the whole wall and slab edge stay warm. Detail 05 continues this line up into the wall.

Radon: required across BC

The BC Building Code 2024 removed the old area exemptions: new Part 9 homes across the province need a radon rough-in, for building permits applied for on or after March 8, 2024 (unless the house is designed for radon under Parts 5 and 6). The rough-in is the gas-permeable layer, a continuous sealed air barrier above it, and a 100 mm pipe from below the slab to the outdoors, sealed where it passes through conditioned space, insulated where it runs through unconditioned space, and labelled every 1.8 m and at every change of direction. The outlet must not create a hazard: the provincial bulletin describes keeping it well away from windows, air intakes and property lines. A fan is not required, but the bulletin suggests leaving a clear space around the pipe (about 1.2 m high and 500 mm across) where one could go, with an electrical box nearby. Cities run their own inspection stages: Burnaby, for example, books the first inspection when the gas-permeable layer is in and before the air barrier is installed, while Richmond inspects the pipe layout and then the granular layer and air barrier. After move-in, the owner should test. Health Canada advises action above 200 Bq/m³, based on a long-term test of 3 to 12 months, with at least 91 days in the heating season.

Sequencing with the plumber

Under-slab plumbing and the radon pipe go in first, with block-outs where pipes pass through footings. Then the granular layer, levelled and inspected. Then insulation, cut tight around every pipe, with edge strips in place. Then the poly, lapped, sealed up the walls and taped or booted at every pipe. Protect it until the pour and photograph every area. Talk to the plumber before the job starts so pipes are grouped where possible: fewer, tidier penetrations are easier to seal.

Site checklist

• Under-slab plumbing and radon pipe roughed in; footing block-outs in place
• Granular layer at full depth over the whole area and inspected
• Insulation grade matches the drawings; joints tight
• Edge insulation along every exterior wall, joined to under-slab insulation
• Poly lapped and sealed, run up the walls and sealed there
• Every pipe through the poly taped or booted
• No tears or footprints in the poly before the pour
• Radon pipe protected during the pour, labelled every 1.8 m and at each change of direction
• Inspections passed and photos taken of every area

Test yourself

Try to answer each question, then click it to check.

Next: Detail 05, Foundation-to-Wall Transition. See also our net zero homes page.