How to insulate a concrete floor

Inspired by a helpful YouTube guide. This walkthrough is based on the UK video "How to Cut and Install Rigid Insulation Board Kingspan Ecotherm Celotex PIR", which covers the practicalities of handling and cutting PIR insulation boards neatly. The section on cutting technique alone is worth a watch - it is easy to crack or crumble boards if you rush it.

1. Check the floor and head height

Concrete floors in older properties sometimes have active damp or rising moisture. Check for white tide-marks on the walls near the floor, soft plaster, or a musty smell. If there is active rising damp, that needs treating before you lay insulation - trapping it underneath makes things worse.

Measure the floor-to-ceiling height and work out how much floor level rise you can accept. A standard approach - DPM, 50 mm or 75 mm PIR board, 18 mm tongue-and-groove chipboard - adds around 70–95 mm. In rooms with low ceilings, that matters.

2. Clear the room and prepare the slab

Strip out any existing floor coverings. Sweep the slab thoroughly and fill any significant cracks or hollows with a floor-repair mortar. Large voids under the slab can cause the insulation boards to flex and eventually crack, so do not skip this step in older properties.

If there is an existing DPM (damp-proof membrane) under the slab - often a polythene sheet from when the house was built - you may not need to add another one. Check with a moisture meter if you are unsure. When in doubt, add one anyway.

3. Lay the damp-proof membrane

Roll out a minimum 1200-gauge polythene DPM over the whole floor. Overlap joins by at least 300 mm and seal with DPM tape. Turn the sheet up the walls by 50–75 mm all round and secure it temporarily with tape. This upstand gets hidden behind the skirting board later.

Take care at doorways and pipe penetrations - these are the spots where moisture finds its way through. Use a mastic sealant to bed the membrane tightly at pipe exits.

4. Cut and lay the insulation boards

PIR boards - Celotex, Kingspan, or similar - cut cleanly with a sharp long-bladed knife and a straight edge. Score the foil face deeply, then snap. Keep the dust down and wear a dust mask when scoring - the foam particles are not something you want to breathe in.

Lay the boards tight against each other with joints staggered - no cross-joints that run the full width of the room. Butt the boards firmly together and tape every joint with foil tape. This stops warm air from the room finding its way down between boards and condensing on the cold concrete.

5. Tape the insulation joints

Go over every joint with proper foil tape - the self-adhesive kind sold specifically for insulation board. Press it down firmly. Skimping on this step is common, but unbroken joints make a real difference to the thermal performance of the finished floor.

At the perimeter, leave a small gap between the insulation board and the wall - about 10 mm. This is the expansion gap for the chipboard or screed that goes on top.

6. Lay the deck or screed

For a floating floor finish, tongue-and-groove P5 moisture-resistant chipboard goes on top of the insulation boards. 18 mm boards, glued joints, with edges staggered. It is relatively quick to lay and can take most floor finishes: tiles, LVT, carpet, or wood.

If you prefer a screed finish - because you want underfloor heating or a very flat tile base - use a minimum 65 mm bonded screed directly over the PIR. In that case, check the PIR boards are rated for compression under screed; standard flat-roof boards are not suitable.

7. Refit doors and skirtings

The raised floor level will almost certainly mean trimming doors. Mark the door bottom on the new finished floor level, take the door off, and trim with a circular saw or jigsaw. Seal the cut end with a primer or wood hardener to stop moisture entry.

Refit or replace skirting boards over the perimeter expansion gap. The new skirting hides the upturned DPM edge neatly.

When to call a handyman

Call Richard if you find active rising damp, structural cracks in the slab, or the existing DPM is compromised. Floor insulation in properties with complex layouts - lots of doorways, multiple rooms at different levels - also benefits from someone who has done a few. Trimming doors so they swing freely on an insulated floor is one of those jobs that sounds straightforward but can catch you out.