Short answer: if you are laying a new concrete floor in a heated room, yes, and building control will expect to see it. The longer answer depends on whether the floor is new, replaced or simply being topped up, and on how much depth you have to work with. Here is how it works for homes in and around Manchester.
Building regulations treat a ground floor as a thermal element. A new one, or a substantial replacement, has to meet a set level of heat loss, measured as a U-value. Lower is better.
For a new floor in an extension or new build, the target is 0.18 W/m²K. For an existing floor that is being dug out and replaced, it is 0.25 W/m²K. Whichever council covers your address, whether Manchester, Salford, Trafford, Stockport or Bolton, building control or an approved inspector will want to see the insulation in place before it is covered.
There is no single figure, because the calculation depends on the shape of the floor. A small room with a lot of external wall loses more heat at the edges than a large, square one, so it needs more insulation to reach the same U-value.
As a working guide, rigid PIR board of around 100mm to 150mm will normally achieve 0.18, and 75mm to 100mm will normally achieve 0.25. EPS (expanded polystyrene) is cheaper per board but needs to be roughly a third to a half thicker to do the same job. Your designer or insulation supplier can run the exact calculation from the floor dimensions.
Do not forget the edges. A 25mm strip of insulation stood upright around the perimeter stops heat escaping sideways into the walls, and it doubles as the expansion gap for the screed.
A lot of the housing stock here is Victorian and Edwardian terraces and semis, in places like Chorlton, Levenshulme, Didsbury, Prestwich and Sale. Many have solid floors that were laid straight onto the ground with no damp proof membrane and no insulation, or timber floors that have since been filled in with concrete.
Digging these out to fit 100mm of insulation plus a slab and screed is not always possible. Foundations on older terraces can be shallow, and you should never excavate below the bottom of the footings without structural advice. Much of the area also sits on heavy clay with a fairly high water table, so a deeper dig can mean dealing with wet ground.
The regulations allow for this. If the full thickness is not technically feasible, you are expected to fit the best you reasonably can. Thinner high performance boards, or a liquid screed that needs less depth than sand and cement, can recover 20mm to 30mm, which is often the difference between a threshold that works and one that does not.
If you are fitting underfloor heating, insulation stops being a box to tick and becomes the thing that makes the system work. Without it, a large share of the heat goes down into the slab and the ground instead of up into the room. Warm up times get longer and running costs rise.
The usual build-up is slab, then insulation, then a separating membrane, then the pipes, then screed. Over heating pipes, a sand and cement screed is typically 65mm to 75mm thick, while a liquid screed can be around 50mm. That thinner layer also responds faster to the thermostat.
Insulation is one of the cheaper parts of a floor. As a rough guide, 100mm PIR board tends to cost in the region of £15 to £25 per square metre for materials, depending on brand and how much you are buying. The bigger costs are usually excavation, muck away and making good, which depend on access, how deep you need to go and what is found once the old floor is up.
It is far cheaper to fit insulation now than to break out a finished floor later, so if the floor is coming up anyway, this is the moment to do it properly.
Yes, as long as the slab is sound and dry and you can afford the lost height. Check door thresholds, the bottom stair riser and ceiling height first, because even 75mm of insulation plus screed raises the floor by well over 100mm.
Either can work. Below the slab is common in new extensions, while above the slab and under the screed is the better choice with underfloor heating because the floor warms up faster.
Replacing a ground floor is normally notifiable work, so you should contact your local council building control team or an approved inspector before starting. They will want to inspect the damp proof membrane and insulation before the screed or concrete goes down.