Who this guide is for
- Self-builders reading two ground floor build-ups that differ only in layer order
- Owners whose heating designer and floor designer have proposed different arrangements
- Anyone choosing a floor covering before the base beneath it is specified
- Owners planning embedded heating in a new ground floor
- Anyone trying to understand why a screed thickness became a structural question
One position, and three things that move with it
The ordering question runs through the whole floor. Placing insulation under the slab puts the concrete on the warm side, so the slab stores heat and the barrier normally sits lower in the stack. Placing it over the slab puts the concrete on the cold side, moves the barrier's likely position, and means the layer above has to carry loading across something compressible.
Those are not three separate decisions that happen to be related. They are one decision expressed in three places, which is why the entries treat the layer order as the defining property of the assembly rather than as a detail of it.
- Whether the slab is inside or outside the insulated envelope
- Where the moisture barrier is likely to sit in the stack
- What the layer above the insulation has to do structurally
- What the perimeter has to carry past the junction with the wall
Below the slab: the concrete is inside the envelope
With the thermal layer beneath, the slab sits inside the insulated envelope. The entry puts the consequence in terms of behaviour rather than numbers: the slab stores heat, and the floor responds to heat input differently from one where the concrete is on the cold side. Whether that is wanted depends on the heating strategy, and that is a designer's judgement rather than a product choice.
The barrier normally sits lower in the stack in this arrangement. Its continuity is then settled at the places nobody sees again — laps buried under the pour, the turn-up at the perimeter, and the collars around pipes and ducts fixed before any concrete arrives.
Above the slab: the screed stops levelling and starts carrying
This is the half of the decision that surprises people. A slab finished flat may take a covering directly; one that is not needs a levelling layer. But where the insulation sits above the slab, that layer stops levelling and becomes an unbonded screed carrying loading across something compressible.
The floating screed entry states the relationship plainly: a screed floating over something compressible is spanning, so the softer and the more variable that layer is, the more the screed has to do. Change the insulation and the screed changes with it, which is the reverse of how the two are usually discussed, where the screed is chosen and the insulation treated as fill.
The barrier is not a fixed item in the stack
Because the barrier's position follows the insulation, an arrangement change is also a moisture-strategy change. The library is explicit that no single layer in this floor is damp-proof on its own; moisture control is a property of the completed and correctly detailed assembly, and a sheet lapped short, punctured or stopped at the slab edge simply relocates the moisture path rather than interrupting it.
A further consequence sits with drying. A floating build-up over a barrier can generally only dry upward, so what is underneath sets how long the layer above takes to dry and what may be laid over it.
The perimeter is where both arrangements are decided
Edge isolation and thermal continuity are the same detail seen from different sides. The compressible perimeter strip exists so the slab can move without loading the wall; the upstand insulation exists so the thermal layer does not stop where the floor meets the structure. Where either is omitted or squeezed out, the floor gains a hard connection and a cold path in one move.
The slab edge entry describes that junction as a line rather than a detail: it runs continuously around every floor of the building, and it is where the floor's insulation and the wall's insulation have to meet, alongside the damp course and, at upper levels, fire separation at the facade.
What the finish inherits
Once heating is embedded at this level, the covering choice is bound to the thermal layer position beneath it. Where the build-up carries heating, the floor's response is governed by what sits under the pipes as much as by the pipes themselves, and whether heat goes upward or into the structure is decided by the layer beneath.
This is why the entries keep returning to the same sequencing point: the floor covering is best chosen early enough for the base beneath it to be specified around it, rather than selected once the base has already been built.
Why the entity page will not tell you which
The slab entry names the position of the thermal layer as a design decision rather than a product decision, and then stops. That is the correct boundary for a reference entry describing one construction. The judgement depends on the ground, the heating, the finish and the perimeter details, none of which an entry about a slab can see.
Settling the insulation position
- 1Ask where the thermal layer is intended to sit relative to the slab, and what reasons were given.
- 2Ask what that position settles about the barrier's position in the stack.
- 3If the insulation sits above, ask whether the layer over it is a levelling layer or a spanning one.
- 4Ask who is designing that layer, and against which compressible layer beneath it.
- 5Establish how the perimeter isolation and the edge upstand are both provided at the same junction.
- 6Ask which drawing shows the floor barrier lapping into the damp course in the wall.
- 7Establish whether the build-up can dry upward only, and what that means for the programme.
- 8Confirm whether embedded heating is intended, and when that decision has to be final.
- 9Ask whether the intended floor covering has been chosen early enough to specify the base around it.
- 10List the services penetrating the floor and confirm their positions are fixed before any pour.
- 11Ask how the compressible layer and the perimeter strip will be protected between laying and covering.
- 12Ask what record of the build-up will be handed over for whoever alters the floor later.
Common mistakes to avoid
- Treating the insulation position as a thermal question alone, when it reorganises the barrier, the perimeter and the layer above.
- Reading a screed over insulation as levelling, when over a compressible layer it is acting as a spanning element.
- Choosing the insulation and leaving the screed unchanged, when the two are designed against one another.
- Squeezing out the perimeter strip or the edge upstand, which costs isolation and thermal continuity in one move.
- Assuming a build-up can dry in both directions, when what sits beneath commonly leaves only one.
- Fixing the floor covering after the base is built, when the base is normally specified around the covering.
- Adding embedded heating to a build-up that was designed without it, which changes depth, drying and joint positions together.
When to involve a professional
- Where the thermal layer sits relative to the slab is a design decision made by a qualified professional rather than a product selection.
- Screed make-up and depth over a compressible layer depend on the layer beneath and the loading above, and are design matters.
- Whether a board can be loaded beneath a slab, and how it behaves against ground moisture, is determined by the manufacturer's documentation and the designer.
- Moisture control is a property of the completed and correctly detailed assembly, and no single layer carries it alone.
- Nothing here states a thickness, a thermal value, a drying period or a loading, and none can be inferred from it.
Frequently asked questions
Questions readers ask about this topic
Why does moving the insulation change the screed?
Because a screed laid over a compressible layer is spanning rather than levelling. It carries loading across something soft to the base below, so its make-up is decided with the layer beneath it rather than chosen independently of it.
Does insulation above the slab make the floor warm up faster?
The entries describe the mechanism rather than a result: with insulation below, the slab sits inside the insulated envelope and stores heat; with insulation above, the slab is outside it. How a particular floor responds is a design matter for the heating and floor designers together.
Where does the damp-proof membrane go?
Its likely position follows the insulation rather than being fixed. What the entries do state is that continuity decides its behaviour, and that a sheet stopping at the slab edge relocates the moisture path into the wall junction instead of interrupting it.
Can the perimeter strip be left out if the floor is not acoustic?
The strip is doing two jobs at that line. It keeps the slab or screed from bearing hard against the wall, and it is where the insulation line is carried past the point the floor stops. A hard contact there costs both at once.
Why will the reference entry not recommend an arrangement?
Because it describes one construction and cannot see the ground, the heating strategy, the finish or the perimeter details that decide the question. It names the misunderstanding, states what each position changes, and leaves the determination with the designer.
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