Concrete floors are known for their strength and durability, but without proper insulation, they can also be a major source of heat loss. Whether you're working on a new extension, a full renovation, or simply looking to improve comfort at home, insulating a concrete floor is one of the most effective ways to boost energy efficiency and reduce heating costs.
In this guide, we’ll walk you through how to insulate a concrete floor the right way. From understanding when and why to insulate, to choosing the right materials and installation method, whether you’re working above or below the slab, you’ll find practical advice to help you get a high-performance finish that lasts.
Why Insulate a Concrete Floor (and When You Should)
Concrete is solid and long-lasting, but it’s also a poor insulator. Left untreated, it acts as a thermal bridge, drawing heat away from your space and making floors feel cold, particularly in winter. That means more energy is required to maintain comfortable rooms, resulting in higher heating bills in the long run.
Insulating a concrete floor helps reduce heat loss, improves comfort underfoot, and contributes to a more energy-efficient building. It’s crucial for ground floors, where contact with the cold sub-base or earth below leads to the greatest temperature drop.
When You Should Insulate
- New builds: Building Regulations require solid floor insulation that meets minimum U-value standards. Insulation is typically laid below the concrete slab.
- Extensions and refurbishments: Where accessible, floors should be upgraded to improve thermal performance, particularly when replacing floor finishes or installing underfloor heating.
- Conversions and retrofits: Garages, basements or older properties with exposed concrete floors are prime candidates for above-slab insulation to improve warmth and usability.
The Benefits at a Glance
- Reduces cold spots and draughts
- Helps lower energy bills over time
- Supports compliance with Building Regulations
- Enhances comfort and usability of living spaces
- Ideal foundation for underfloor heating systems
Types of Insulation and Where to Place It
There’s more than one way to insulate a concrete floor, and the right method depends on whether you’re working on a new build, a renovation, or retrofitting an existing floor. The key difference lies in where the insulation sits: above or below the concrete slab.
Insulation Below the Slab
This method places rigid insulation on top of the compacted sub-base, underneath the concrete slab itself. It’s commonly used in new builds and extensions, where floor levels can be planned from the ground up.
Advantages:
- Excellent thermal performance
- Protects insulation from foot traffic and wear
- Keeps internal floor levels low
Considerations:
- Needs to be planned before pouring the slab
- Requires a damp-proof membrane (DPM) either above or below insulation
Insulation Above the Slab (Retrofit)
If the concrete floor is already in place, the simplest option is to lay insulation boards on top of it, followed by a layer of screed or chipboard. This is ideal for refurbishments, garage conversions, and any job where lifting the floor isn’t practical.
Advantages:
- No need to remove or replace the concrete slab
- Suitable for most renovation projects
- Easier to combine with underfloor heating
Considerations:
- Raises the finished floor level – check door clearances, thresholds and skirting
- May require adjustments to existing services or fixtures
Common Insulation Materials
- PIR (Polyisocyanurate) Boards – Rigid, high-performance insulation with foil facings. Ideal for both under and over-slab applications.
- XPS (Extruded Polystyrene) – Moisture-resistant and strong; often used in below-slab builds or where compressive strength is important.
- Phenolic Boards – Higher thermal performance in thinner layers. Good when the floor height is limited.
- Expanded Polystyrene (EPS) – Cost-effective and widely available, though not as thermally efficient as PIR.
Step-by-Step Installation Guide
How you insulate a concrete floor will depend on construction — solid floors and suspended floors each require different approaches.
Solid Floors
Solid floors, like concrete or stone, are dense and typically better at retaining heat than suspended floors, but they still allow heat loss, especially in older homes. Since you can’t insulate beneath a solid floor without major excavation, the insulation must be added on top of the existing surface.
-
Clean and prepare the surface: Sweep the floor, remove debris, and check for any moisture issues.
- Install a damp-proof membrane (DPM): Lay a continuous membrane across the floor to prevent moisture from reaching the insulation. Overlap and tape joints securely.
- Lay rigid insulation boards: Use PIR or phenolic boards. Lay them tightly together with staggered joints and foil tape where needed.
- Add optional acoustic or underfloor layers: To reduce airborne noise in echo-prone spaces (e.g. converted churches or older homes), consider layering an acoustic barrier mat or sheeting to improve sound installation.
- Install your floor finish: Apply screed, chipboard, or your final floor covering (e.g. laminate or engineered timber).
Note: Be mindful of the added floor height. You may need to adjust door thresholds, trim doors, or raise sockets and skirtings to accommodate the new build-up.
Suspended Floors
Suspended timber floors rest on joists above a void; this void can cause significant heat loss if left uninsulated. The insulation must fill the space between joists, either from above or below.
If you have cellar or basement access:
-
Measure and cut insulation boards or slabs: Rigid boards (PIR or mineral wool slabs) can be friction-fit between joists for a tight seal.
- Support with insulation netting: Roll out insulation netting beneath the joists and fix it to each joist using staples or battens. This holds the insulation firmly in place.
- Check airbricks: Do not block airbricks. Ensure at least 2 inches of ventilation below the insulation to avoid damp or timber decay.
If you do not have access from below:
-
Lift floorboards: Carefully remove floor finishes and boards to expose joists.
- Install rigid insulation between joists: Cut to size for a snug fit. Avoid gaps to prevent draughts and heat loss.
- Alternatively, use insulation rolls: Natural insulation (like sheep’s wool) can be suspended in netting below the joists. Ensure it extends fully across each cavity.
- Replace floorboards and finish: Once insulation is securely in place, reinstate the boards and lay your chosen floor finish.
Insulate Floors Properly with the Right Products and Advice
Insulating your floors is one of the most effective ways to boost comfort and reduce energy loss in any property. With the right materials, proper installation, and attention to detail, you’ll not only meet building regulations but also create warmer, more efficient spaces for years to come.
Visit your local branch for expert advice, or sign up for a Trade Account to benefit from competitive pricing, priority service, and simplified ordering.