Key takeaways
- The slab edge is a thermal bridge, so perimeter insulation matters most where the floor meets the outside air rather than at the centre of the slab.
- Whether slab edge insulation is required depends on the climate zone and the slab type the energy provisions address, and the builder should confirm that for each job rather than assume it.
- The insulation has to be protected from sunlight and from damage during the works, and it has to be continuous with the wall detailing at the junction.
- The junctions where the slab meets the wall, the doorways, the services and the external slabs are where most of the performance is lost.
- Slab edge insulation is far cheaper to get right before the pour than to fix afterwards, so the detail should be checked before the concrete arrives.
The slab is usually the most carefully designed part of a house, and its edge is often the least thought about. The centre of a slab sits on the ground and is shielded from the weather, but the perimeter is exposed on its outer face and sits close to the air the building is trying to keep out. That exposed edge is where heat is lost in winter and where the floor gets cold, and it is where condensation and mould can start if the detail is poor.
This article covers the slab edge and its junctions only: the perimeter insulation, the way it meets the walls, the doors and the services, and the checks to make before the concrete is poured. Wall-frame insulation, air sealing and subfloor ventilation are separate subjects. The focus here is the seam between the slab and the rest of the envelope, because that seam is where the performance of the floor is won or lost.
The cost of getting the detail wrong is paid in two places. The first is comfort and running costs over the years the house is lived in, which the owner notices and mentions. The second is the cost of fixing it, which is high once the slab is down. Insulation placed before the pour is cheap. Insulation added afterwards means breaking out the edge, which is rarely done well.
The detail also matters for the way the work is assessed. Inspectors look for continuity at the junctions and for evidence that the documented layout was built, and the boards are visible at only one stage of the job. That makes the pre-pour check the main opportunity to show the detail is right, and it is the stage most often skipped when the programme is tight.
Why the slab edge loses heat
Heat moves through a building by conduction, and the edge of a slab offers a short, direct route between the warm inside and the cold outside. Concrete conducts heat far better than the insulation in the wall next to it, so an uninsulated edge becomes a thermal bridge: a line along the base of the building where the envelope is weaker than the surfaces either side. In winter that line is cold to the touch, and the floor near the perimeter feels it first.
The slab near the edge is also a problem for condensation. Warm, moist indoor air meets the cold concrete and can condense on it, especially in rooms where furniture sits against the external wall and the air does not move. Insulation keeps the slab edge closer to the indoor temperature, which reduces the risk of condensation and the mould that follows it. The benefit is greatest where the floor has a hard finish that draws heat away from the feet, and it is the difference between a floor that feels reasonable and one that is always cold at the edges.
The benefit also shows up in the energy the house uses. A floor that loses heat at the perimeter makes the heating work harder to keep the living areas comfortable, and the effect is strongest in the rooms with the most external wall. The slab edge is a small part of the envelope by area, but it runs the full length of the building, which is why a continuous detail matters more than the size of the surface would suggest.
When the energy provisions ask for it
The energy efficiency provisions of the National Construction Code set the minimum performance a new home has to meet, and they are the first place to look when deciding whether slab edge insulation is required. Whether it is required depends on the climate zone the house sits in and on the type of slab. Some combinations need the perimeter insulated and others do not, and a builder who assumes the answer is always yes, or always no, will be wrong some of the time. Read the provisions for the specific job, and confirm the answer in the building’s energy documentation before the slab is designed.
Where the energy documentation requires perimeter insulation, the detail is part of the approval. It is not a matter of the builder’s choice of product, and a substitution that lowers the performance below the documented level is a change to the approval. The Your Home guidance on insulation from the Australian Government is useful background on why the insulation matters. It is written for owners as well as builders, which makes it a good thing to show a client who asks why the edge detail is costing money.
Keep the energy documentation with the job file, and make sure the slab detail on the drawings matches it. When a question comes up at inspection, the decision is far easier to defend if it can be traced from the requirement to the drawing and from the drawing to the boards on site.
Vertical or horizontal: choosing the board layout
Rigid insulation boards come in two main layouts at the slab edge. Vertical boards sit against the outside face of the edge beam or footing, running down from the top of the slab, and they are the most common arrangement for a slab on ground. Horizontal boards run out from the edge beneath the ground surface, which lengthens the path heat has to travel and suits some site conditions better. Some designs combine both. The engineer’s drawings and the energy documentation will say which layout is intended, and the builder’s job is to build that layout rather than the one that is easiest to fix on the day.
The board has to be the type and thickness the documentation specifies, and it has to be fixed so it stays where it was put. The boards are usually held in place by the formwork or by fixings cast in during the pour, and they need checking before the concrete goes in. A board that moves during the pour leaves a gap at exactly the point where the detail was supposed to work, and nobody sees it until the floor is cold.
Detailing the slab edge from the trench to the pour
- 1Confirm the requirementCheck the energy documentation for the climate zone and the slab type before the slab is designed.
- 2Agree the layoutSettle whether the board runs vertically, horizontally or both, and show it in section on the drawings.
- 3Fix the boardsHold each board against the formwork or cast-in fixings so it cannot move during placement.
- 4Protect during the worksCover or shield the exposed board, and sequence the backfill so the board is not knocked out of position.
- 5Detail the junctionsMake the insulation continuous at the wall, the doors and the services before the concrete goes in.
Protecting the board from light and impact
Most rigid insulation boards are damaged by sunlight and by impact. Exposed expanded polystyrene breaks down under ultraviolet light, and a board left uncovered during a long construction programme can be worn back before the house is finished. Protection is usually a render, a cover, a plaster finish or another permanent barrier, and the detail should show what is intended. A board that is exposed for the life of the house is no longer doing the job it was installed to do.
Impact damage comes from the trades. Excavators, barrows, the forming crew and the backfill all pass the edge of the slab, and a board that is knocked out of position or broken during backfill leaves a gap that nobody sees. Protect the boards during the works, and make the backfill sequence part of the detail rather than something that happens after the slab is finished.
Where the slab is exposed for a long time, because of a delay in the frame or a change in the programme, the protection should be checked again rather than assumed to be intact. A board that looked sound at the pour can have lost its surface by the time the walls go up, and a quick look before the cladding is fixed is cheap insurance.
The wall junction is where the detail is won or lost
The junction between the slab edge and the external wall is the most important point in the whole detail. The edge insulation has to be continuous with the wall insulation above it, so there is no gap where heat can pass around the end of the board. In a brick veneer house that junction also sits close to the damp-proof course, and the insulation must not bridge it in a way that traps moisture against the wall or the slab. The designer and the builder need to agree how the layers meet, and the drawing should show that agreement in section rather than in a note.
Look closely at the point where the wall framing lands on the slab. A stud or a plate sitting directly on the slab edge can bridge the insulation, and in steel-framed construction that bridge can be serious. The guidance in the article on thermal bridging and steel framing fixes applies the same logic to any member that crosses the insulation layer, and it is worth reading alongside this detail.
Doors, services and external slabs
Doorways at the slab edge are a weak point because they are where the insulation is most likely to be interrupted. A sliding door threshold, a garage door opening or a step down to a patio all break the line of the edge insulation, and each break needs its own detail. Where an external slab is not connected to the house, the insulation at the junction must still be continuous, and the builder should not assume the slab in the garden will take care of itself.
Services are the other source of breaks. Pipes, conduits and drains that pass through the slab edge leave gaps around them, and those gaps are often filled by whoever is nearest, with whatever is handy. Sleeve the services, seal the penetrations with a material that suits the job, and check that the insulation is still continuous around each one. A penetration that is never sealed is a path for air and heat, and it is the sort of small defect that adds up across a whole perimeter.
Coordinate the slab detail with the waterproofing and the drainage at the same time. The point where the slab edge meets the external ground is where water is most likely to sit, and insulation that traps water against the concrete does its job poorly. The fall of the ground away from the house matters as much as the board does.
Checking the detail before the pour
The time to check the slab edge is before the concrete arrives, when the insulation, the fixings and the services are all still visible. Walk the perimeter with the drawings and confirm that the board is in the position shown, that the type and thickness match the documentation, that the fixings hold the board where it was placed, that the junctions are continuous and that the penetrations are sleeved and sealed. Record the check with the date and photographs, and keep the record with the slab inspection file. The list below is the working version of that walk.
Junction checks before the pour
- Board in the position the drawings show, on every side of the building
- Board type and thickness matching the energy documentation
- Fixings holding the boards in place through the pour
- Continuous insulation at the wall junction, with no gap at the base
- Thresholds and door openings detailed, not left to the trades
- Services sleeved and every penetration sealed
- A record of the check, with photographs, filed before the pour
Worked example: an edge detail caught before the pour in western Sydney
A builder in western Sydney was building a single-storey brick veneer house on a slab with a perimeter edge. The energy documentation required vertical insulation boards on the outside face of the edge. The formwork crew fixed the boards on the day, but they stopped them short of the line where the damp-proof course met the wall, leaving a strip of exposed concrete at the base of the wall on the north side. The strip was not obvious from inside, and it would not have been visible once the bricks were laid above it.
The supervisor walked the perimeter with the drawings before the pour, as the checklist required. He saw the gap, held the pour, and had the boards extended to the correct line, fixed with a timber batten so they could not shift during placement. The inspection was arranged for the same afternoon, and the corrected detail was in place when the inspector arrived.
The repair took a morning and a few boards. Left until after the slab was down, the same gap would have meant breaking out the edge to reach the base of the wall, with the cost of that work, the delay to the frame and a cold, damp corner in the house for its whole life. The habit of walking the perimeter before the concrete arrived was the difference, and it cost nothing but the time it took.
The crew was briefed on the change, and the supervisor added the perimeter walk to the pre-pour checklist for the rest of the company’s jobs. The owner was given a plain-language note of the edge detail in the handover pack, so the floor’s performance was explained rather than left as a mystery when a cold corner was noticed in winter.
Lead in. The mistake that costs the most is a gap at the wall junction, because it is invisible once the wall is built and expensive to reach once the slab is down.
The energy detail sits within the wider performance of the house, and the building envelope and air sealing article covers the joins that the slab edge has to meet from the other side.
For how a detail like this fits into the wider CPD picture for NSW builders, the PointCPD guide to NSW builder CPD sets out how the scheme works, and the courses on offer are listed in the catalogue.
Questions builders ask
Is slab edge insulation always required in a new house?
No. Whether it is required depends on the climate zone the house sits in and the type of slab being built, and the energy provisions of the National Construction Code set out the combinations. Confirm the requirement for the specific job in the energy documentation rather than assuming the answer.
Should the insulation go on the outside of the edge beam or underneath the slab?
The layout is set by the drawings and the energy documentation, not by the builder’s preference. Vertical boards on the outside face are the most common arrangement, while horizontal boards suit some sites, and some designs use both. Build the layout that is shown.
Can I add slab edge insulation after the slab is poured?
Only with difficulty. Adding it afterwards usually means excavating the edge, which is slow, expensive and rarely done to the standard of a detail built in from the start. It is far better to get the detail right before the concrete arrives.
Does the insulation need protecting from sunlight?
Yes, in most cases. Many rigid insulation boards break down under ultraviolet light, so exposed board needs a render, a cover or another permanent barrier. The detail should show how the protection is provided, and the protection should be in place before the house is finished.
What is the most common mistake at the slab edge?
A gap at the wall junction, where the boards stop short of the line where the wall meets the slab, or where the insulation is not continuous with the wall insulation above. These gaps are invisible once the wall is built, which is why they should be checked on the slab before the pour.
How do I handle pipes and drains passing through the slab edge?
Sleeve the services so the insulation can stay continuous around them, and seal each penetration with a material suited to the job. An unsealed penetration is a path for air and heat, and across a whole perimeter these small gaps add up.
Sources
- National Construction Code: energy efficiency provisions · Australian Building Codes Board
- Insulation · Your Home (Australian Government)
Published 24 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
