Underfloor heating (UFH) provides a gentle, even warmth that feels luxurious underfoot — but its efficiency depends heavily on one thing: insulation.
Without proper insulation, much of the heat your system produces escapes downwards into the subfloor instead of rising into the room. The result? Higher running costs, slower heat-up times, and reduced comfort.
This guide explains why insulation is so critical for UFH, how much you actually need, what types work best, and how to ensure your system performs at its most efficient.
Why Insulation Matters So Much in Underfloor Heating
Underfloor heating works by radiating warmth evenly across the floor surface. Unlike radiators, which heat the air directly, UFH relies on large surface areas and lower operating temperatures to maintain comfort.
That makes insulation beneath the pipes or cables absolutely vital. Without it, the system will waste energy heating the subfloor instead of your living space.
Key benefits of good insulation:
- Prevents downward heat loss – keeps warmth where you want it.
- Improves efficiency – systems heat up faster and use less energy.
- Reduces running costs – lower energy consumption over time.
- Protects floors and structure – prevents overheating of materials below the system.
Insulation is essentially the difference between a high-performance, low-cost UFH system and one that’s expensive to run and slow to respond.
How Heat Moves — And Why Insulation Stops It
Heat travels in three ways: conduction, convection, and radiation. In UFH, most heat is transferred via radiation and conduction — the warmth rises upward through the floor covering into the room.
However, without an insulating layer, a significant portion of that heat conducts downward into the screed, subfloor, or ground. Studies show that up to 50% of output can be lost downward if insulation is inadequate.
This not only wastes energy but also makes the system take longer to reach temperature, which can double the time before your home feels warm.
Building Regulations and Insulation Requirements
In the UK, Part L of the Building Regulations governs energy efficiency for floors. It requires that all heated floors have sufficient insulation to meet target U-values — a measure of how well a building element resists heat loss.
Recommended U-values for floors:
- New builds: typically 0.13 W/m²K or better.
- Retrofits and renovations: up to 0.18 W/m²K (depending on feasibility).
The exact insulation thickness needed depends on the material used and the construction type (solid concrete, suspended timber, etc.).
While regulations set the minimum standard, UFH performs best when insulation exceeds the minimum — especially in older properties.
Typical Insulation Thickness for Underfloor Heating
Here’s a practical guide based on floor type and insulation material:
| Floor Type | Insulation Type | Typical Thickness | Notes |
|---|---|---|---|
| Solid concrete (new build) | Rigid PIR (polyisocyanurate) board | 75–100 mm | Excellent thermal resistance; easy to install under screed. |
| Retrofit concrete or screed floors | High-performance PIR or phenolic boards | 50–70 mm | Thinner boards can achieve low U-values with high efficiency. |
| Suspended timber floors | Mineral wool or PIR between joists | 100–150 mm | Leave airflow below insulation for ventilation. |
| Overlay or low-profile systems | Thin EPS or XPS insulation boards | 10–25 mm | Designed for minimal floor height increase in renovations. |
As a rule of thumb:
- New builds: aim for at least 75 mm PIR beneath the heating layer.
- Retrofits: use as much insulation as floor height allows, ideally 25–50 mm high-performance board.
If space is tight, advanced materials such as vacuum-insulated panels or phenolic boards offer excellent thermal resistance in minimal thickness.
The Role of Edge Insulation
It’s not just what’s under the UFH that matters — what’s around it does too.
Edge insulation strips (typically 8–10 mm thick) should be fitted around the perimeter walls before screeding. These prevent heat loss into adjoining walls and allow the screed to expand safely as it warms.
Without edge insulation, heat can “bleed” into walls, and expansion can cause cracks in the screed or floor covering.
What Happens If Insulation Is Inadequate?
If insulation is missing or insufficient, several problems appear almost immediately:
- Slow response times — it takes much longer to warm the room.
- Uneven heat — parts of the floor may stay cooler.
- Higher running costs — energy use rises significantly.
- Inconsistent temperatures — thermostats may overshoot as the system struggles to reach the setpoint.
- Reduced comfort — the floor never feels as warm as expected.
Over time, you may notice higher energy bills and increased wear on your heating source, especially boilers or heat pumps that must work harder to compensate.
Insulation Options Explained
1. PIR (Polyisocyanurate) Boards
The most popular choice for underfloor heating in the UK. They offer excellent thermal performance (λ ≈ 0.022 W/mK) and are available in thicknesses from 25 mm to 150 mm. Common brands include Celotex, Kingspan, and Recticel.
Best for: New builds, solid floors, and retrofits where space allows.
2. XPS (Extruded Polystyrene) Boards
A durable, water-resistant option with good compressive strength (ideal under screeds or tiles). Slightly lower thermal efficiency than PIR (λ ≈ 0.030 W/mK) but excellent in high-moisture areas such as bathrooms.
Best for: Bathrooms, kitchens, or wet rooms; compatible with electric UFH mats.
3. EPS (Expanded Polystyrene) Boards
Cheaper and widely used in construction. Provides decent insulation but is less efficient than PIR, so thicker layers are required for the same performance.
Best for: Budget installations or when thickness isn’t an issue.
4. Phenolic Foam Boards
A high-performance material with outstanding thermal resistance (λ ≈ 0.018 W/mK). Excellent for retrofits where every millimetre counts.
Best for: Older properties with limited floor height.
5. Mineral Wool or Fibreglass
Used mainly between timber joists in suspended floors. Provides thermal and acoustic insulation but needs to be supported by netting or boards underneath.
Best for: Traditional timber floors and upper storeys.
Insulation and Heat Pumps — A Perfect Partnership
If your underfloor heating is powered by a heat pump, insulation becomes even more critical.
Heat pumps operate at lower flow temperatures (typically 35–45°C), so minimising heat loss ensures they work efficiently. Insufficient insulation forces the heat pump to run longer, increasing electricity use and reducing the system’s Coefficient of Performance (COP).
A well-insulated floor helps maintain stable temperatures and keeps the heat pump running at its optimal efficiency.
Retrofitting Insulation in Existing Homes
Adding insulation during a retrofit can be more challenging, but it’s not impossible. Here are the main approaches:
1. Low-Profile Overlay Systems
These combine thin heating pipes or mats with integrated insulation panels, typically 18–25 mm thick. Perfect for existing homes where raising floor height slightly is acceptable.
2. Insulation Boards Above Existing Floors
For electric UFH, thin XPS boards (6–10 mm) can be laid directly on existing concrete or tile floors before heating mats are installed. They improve responsiveness and reduce downward heat loss.
3. Suspended Floor Retrofits
If you have timber floors, insulation can be fitted between joists from below (e.g. from a basement or crawl space) using mineral wool or rigid boards supported by battens.
Balancing Efficiency and Floor Height
One of the biggest challenges in retrofits is managing floor height. Adding insulation, pipework, and a screed layer can easily add 50–100 mm to the finished floor level.
Solutions include:
- Low-profile wet systems (10–20 mm pipes within pre-routed boards).
- High-performance thin insulation (phenolic foam, vacuum panels).
- Integrated overlay boards that combine insulation and UFH channels in one layer.
These allow for energy efficiency without major structural changes or door-height issues.
Best Practices for Installing Insulation Under UFH
- Lay insulation boards tightly together to prevent heat bridges.
- Seal gaps and joints with foil tape to maintain a continuous thermal layer.
- Use edge insulation around walls.
- Ensure boards are level before laying pipes or mats — uneven insulation leads to air pockets and inconsistent heat.
- Pressure-test and photograph the layout before screeding, so future works avoid pipe damage.
- Install a vapour barrier if required to prevent moisture ingress from below.
A few hours of preparation at this stage can save hundreds of pounds in running costs over the system’s lifetime.
Example: Energy Savings with Proper Insulation
Consider two identical 25 m² rooms with water-based UFH:
- Room A has 75 mm PIR insulation (U-value 0.13 W/m²K).
- Room B has only 25 mm insulation (U-value 0.25 W/m²K).
Room A reaches target temperature 40% faster and uses up to 30% less energy to maintain comfort. Over a year, that could save £150–£250 in heating costs, depending on energy prices.
It’s easy to see how adequate insulation pays for itself quickly.
How to Check Your Existing Insulation
If you’ve bought a property with UFH already installed and aren’t sure how well it’s insulated:
- Look at build documents or floor plans for insulation specs.
- Use an infrared camera to check for heat loss (cooler spots often indicate poor insulation).
- Ask an engineer to measure floor U-values or thermal performance.
Upgrading insulation in key rooms can make a noticeable difference in comfort and cost.
Common Mistakes to Avoid
- Skipping insulation to save floor height – it always costs more later in wasted energy.
- Using the wrong type of insulation – compressible materials can deform under screed, causing cracks.
- Leaving gaps between boards – even small gaps create cold bridges.
- Neglecting edge strips – heat escapes sideways and screeds may crack.
- Failing to insulate suspended timber floors properly – air gaps beneath can pull heat away rapidly.
Each of these mistakes can reduce system efficiency by 10–30%.
Final Thoughts
The secret to efficient underfloor heating isn’t just advanced thermostats or smart controls — it’s insulation. A properly insulated floor allows your UFH system to heat up quickly, retain warmth for longer, and operate at lower energy cost.
In most UK homes, at least 50–100 mm of high-quality PIR insulation beneath the system is ideal. Where space is limited, thin high-performance boards or overlay systems can still provide excellent results.
The investment in good insulation pays off every day, with faster warm-up times, lower bills, and a more comfortable, energy-efficient home.
Before you install underfloor heating — or if you’re improving an existing setup — make insulation your first priority. It’s the foundation of true efficiency and long-term performance.