Domakihub How to Fit Floor Insulation Boards: A Step-by-Step Guide
Cut energy bills and boost comfort by laying rigid PIR insulation boards on any concrete or timber subfloor.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
Fitting rigid insulation boards under your floor is one of the most cost-effective upgrades you can make to a home, holiday rental, or hotel room — particularly in Greece where uninsulated concrete ground floors can feel ice-cold in winter and radiate summer heat upward. The video above walks through a real installation of 100 mm PIR boards on a concrete subfloor, while this guide breaks the process into clear, ordered steps with material choices and practical tips drawn from current best practice. No building permit is required for insulating an existing ground floor in most cases, but any work that touches the structural slab, damp-proof membrane, or underfloor heating pipework should involve a qualified contractor.
Choosing the Right Insulation Board
Rigid foam boards fall into three main families: EPS (expanded polystyrene, the cheapest and most widely available), XPS (extruded polystyrene, better moisture resistance, suitable under screed), and PIR/PUR (polyisocyanurate, the highest thermal performance per millimetre, foil-faced). For a Greek ground floor on concrete, PIR at 80–100 mm typically achieves the target U-value of 0.25 W/m²K or better without losing too much floor height. XPS at 100–120 mm is a good second choice where any residual moisture is a concern.
Always check that the board carries a CE mark and an appropriate compressive strength rating — 150 kPa (CS(10)150) for domestic foot traffic under a floating floor, 300 kPa or more if you plan to pour a screed on top. In Greece, procurement from builders' merchants in Athens, Thessaloniki, or major island depots (Heraklion, Rhodes) is straightforward; PIR boards are sold in 1200 × 2400 mm sheets of varying thickness.
When to Call a Professional
Fitting insulation boards is a DIY-friendly task on a clean, dry concrete slab, but three situations require licensed tradespeople. First, if the existing slab shows rising damp or efflorescence (white salt deposits), a damp-proofing specialist must treat the substrate and install a continuous damp-proof membrane before any insulation goes down — skipping this traps moisture under the boards and leads to mould. Second, if underfloor heating pipes or electric mats are being laid on top of the insulation boards, a certified plumber (for wet systems) or electrician (for electric systems) must connect and commission the system. Third, any structural alteration to the slab itself — coring for drainage, lowering the floor level — needs a civil or structural engineer.
For a straightforward floating-floor installation on a sound, dry slab, a competent self-builder or general handyman can complete the work. Allow roughly 1–2 days for a typical 40–60 m² room.
Tools and Materials Checklist
You will need: rigid insulation boards (PIR or XPS at specified thickness); a heavy-duty utility knife or handsaw for cutting; a long straightedge and tape measure; a damp-proof membrane (minimum 1000-gauge polyethylene sheet) if not already in place; vapour-check tape to seal membrane laps; timber battens or chipboard tongue-and-groove flooring panels (if building a floating floor deck); and a foam gun with expanding PU foam or foil tape to seal board joints. Avoid using adhesive directly on PIR foil if the floor will later be screeded — follow the screed manufacturer's guidance on isolation layers.
How to Fit Floor Insulation Boards — 5 Steps
- 1
Prepare and dry the subfloor
Sweep the concrete slab clean and check for moisture using a polythene sheet taped flat to the floor for 48 hours — condensation on the underside means active damp that must be treated first. Fill any cracks or hollows with a fast-setting floor-levelling compound and allow it to cure fully. A level subfloor prevents boards rocking, which causes squeaking and, in time, cracked joints.
- 2
Lay the damp-proof membrane
Roll out a continuous polyethylene DPM (1000 gauge minimum, or 250 micron) across the entire floor, running it up the walls by at least 150 mm. Overlap sheet joins by a minimum of 300 mm and seal every lap with vapour-check tape rated for DPMs — untaped overlaps allow moist air to bypass the membrane. Trim the upstand neatly once the floor finish is installed; it will be hidden by the skirting board.
- 3
Cut and lay the insulation boards
Starting in one corner, lay the first board foil-face up. Score PIR with a sharp utility knife and straightedge — a single firm pass, then snap the board over the edge of a batten; a second pass severs the foil backing. Butt boards tightly together and stagger the joints by at least 300 mm in a brick-bond pattern so no continuous joint runs the full length of the room. Leave a 10 mm expansion gap around all walls. Tape every board joint with foil tape rated for PIR to prevent warm air bypassing the insulation through gaps.
- 4
Install the floor deck or screed
For a floating timber floor, lay 22 mm tongue-and-groove chipboard (or the equivalent in plywood) directly over the insulation boards, gluing the T&G joints and staggering the panel joints from the board joints below. Do not screw down into the insulation — the deck floats on the boards. For a wet screed over XPS, first confirm the board compressive strength is sufficient (CS(10)300 or higher), then pour and level the screed to the specified depth (typically 65–75 mm for a unbonded screed). Walk on the boards as little as possible before screeding to avoid displacement.
- 5
Seal edges and finish
Once the deck or screed is down, trim the protruding DPM upstand with a knife or scissors at finished floor level. Fit a foam acoustic/expansion strip between the floor edge and the wall — most skirting board profiles will cover this neatly. Check the overall floor height against door thresholds and adjust door bottoms if needed. Finally, reinstall or fit new skirting boards to conceal the perimeter expansion gap and the DPM upstand.
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FAQ
- How thick should floor insulation boards be on a Greek ground floor?
- For a concrete ground-floor slab in Greece, 80–100 mm of PIR or 100–120 mm of XPS is typically sufficient to meet a U-value of around 0.25 W/m²K. Older buildings with single-pane glazing and uninsulated walls benefit from pushing to 120 mm PIR if floor height allows, since heat loss through the floor is proportionally larger when other elements are also poorly insulated.
- Can I lay floor insulation boards myself or do I need a contractor?
- Laying rigid insulation boards on a sound, dry concrete slab is a task most competent self-builders can handle without a contractor. However, if the slab has moisture problems, if you are integrating an underfloor heating system, or if the building is listed or subject to local planning restrictions, you must involve a licensed professional. In Greece, any changes touching the structural slab also require a civil engineer's sign-off.
- Do I need a building permit to insulate my floor in Greece?
- In most cases, insulating an existing floor from the interior — adding boards and a floating deck without altering the slab — does not require a building permit (oikodoniki adeia) in Greece, as it is classified as a maintenance or energy-improvement intervention. However, if the work is part of a broader renovation that increases the building's total energy footprint or changes floor levels, a building permit or at minimum an energy performance certificate (PEA) update may be required. Always confirm with your local urban planning office (poleodomia) before starting work.