In 2022, the UK building regulations were updated, setting new benchmarks for the thermal performance of buildings. These updates aim to improve energy efficiency and reduce carbon emissions, a crucial step towards a more sustainable future. The revised Part L documents specify minimum U values for new constructions and renovations. The focus is on three key components: roofs, walls, and floors. Achieving these U values is essential for compliance, but it raises the question: how much insulation is needed to meet these standards?

To address this, we’ll explore the required thickness of three popular insulation materials: Expanded Polystyrene (EPS), Rockwool, and Kingspan K5. Each material has its unique properties and efficiency ratings, which influence the thickness needed to reach the desired U values.

Current regulations

New Constructions

For newly built structures, the updated regulations have set the bar higher for energy efficiency, with stricter U values as follows:

• Roofs: 0.11 W/m²K
• Walls: 0.18 W/m²K
• Floors: 0.13 W/m²K

These values are designed to ensure that new buildings are constructed with optimal insulation, minimising heat loss and reducing energy consumption.

Renovations

Renovations present an opportunity to bring existing buildings up to modern standards of energy efficiency. For new elements introduced in existing dwellings, the regulations specify:

• Roofs: 0.15 W/m²K
• Walls: 0.18 W/m²K
• Floors: 0.18 W/m²K

These requirements are slightly less stringent than those for new constructions, recognising the challenges and limitations that can come with upgrading existing structures.

Existing Elements

For existing elements within existing dwellings, the regulations provide limiting U values, setting a baseline for the minimum acceptable level of insulation:

• Roofs: 0.16 W/m²K
• Walls: 0.30 W/m²K
• Floors: 0.25 W/m²K

These values are more lenient, acknowledging the practical difficulties in retrofitting older buildings but still pushing for improvements in energy efficiency.

How much insulation will you require?

EPS (Expanded Polystyrene)

EPS is a lightweight and cost-effective insulation material, known for its thermal resistance and ease of installation. To achieve the specified U values, the required thickness of EPS will vary based on its density and specific thermal conductivity, which is typically around 0.032 – 0.038 W/mK for general-purpose EPS.

• Roofs (New Builds): To achieve a U value of 0.11 W/m²K, approximately 220-250 mm of EPS might be needed.
• Walls (New Builds): For a U value of 0.18 W/m²K, around 140-160 mm of EPS could be required.
• Floors (New Builds): To reach a U value of 0.13 W/m²K, you’re looking at needing about 180-210 mm of EPS.
• Roofs (Renovations/New Elements in Existing Dwellings): To achieve a U value of 0.15 W/m²K, approximately 250-280 mm of EPS is likely required.
• Walls (Renovations/New Elements in Existing Dwellings): For a U value of 0.18 W/m²K, the thickness remains around 140-160 mm, similar to new constructions.
• Floors (Renovations/New Elements in Existing Dwellings): To reach a U value of 0.18 W/m²K, about 200-230 mm of EPS might be needed.
• Roofs (Existing Elements): To meet a U value of 0.16 W/m²K, approximately 100-120 mm of EPS could be necessary.
• Walls (Existing Elements): Given the less stringent requirement, aiming for a U value of 0.30 W/m²K might require about 90 mm of EPS.
• Floors (Existing Elements): For a U value of 0.25 W/m²K, around 100 mm of EPS is estimated to be required.

Rockwool

Rockwool, a mineral wool insulation, offers excellent thermal and acoustic insulation, along with fire resistance. Its typical thermal conductivity is around 0.034 – 0.040 W/mK.

• Roofs (New Builds): For a U value of 0.11 W/m²K, roughly 230-260 mm of Rockwool would be needed.
• Walls (New Builds): A U value of 0.18 W/m²K might require about 150-170 mm of Rockwool.
• Floors (New Builds): To achieve a U value of 0.13 W/m²K, approximately 190-220 mm of Rockwool could be necessary.
• Roofs (Renovations/New Elements in Existing Dwellings): For a U value of 0.15 W/m²K, roughly 260-290 mm of Rockwool would be needed.
• Walls and Floors (Renovations/New Elements in Existing Dwellings): The requirements would be similar to those for new constructions due to the similar U values specified for walls and increased thickness for floors to meet the 0.18 W/m²K U value.
• Roofs (Existing Elements): Achieving a U value of 0.16 W/m²K might require around 120-130 mm of Rockwool.
• Walls (Existing Elements): For a U value of 0.30 W/m²K, approximately 110 mm could be sufficient.
• Floors (Existing Elements): To meet a U value of 0.25 W/m²K, about 110-120 mm of Rockwool is likely needed.

Kingspan K5

Kingspan K5 is a high-performance insulation board. It has a lower thermal conductivity compared to EPS and Rockwool, typically around 0.020 – 0.023 W/mK, making it more efficient in thinner layers.

• Roofs (New Builds): To meet a U value of 0.11 W/m²K, around 130-150 mm of Kingspan K5 might suffice.
• Walls (New Builds): For a U value of 0.18 W/m²K, approximately 100-120 mm of Kingspan K5 would be required.
• Floors (New Builds): To achieve a U value of 0.13 W/m²K, you would need about 120-140 mm of Kingspan K5
• Roofs (Renovations/New Elements in Existing Dwellings): A thickness of around 150-170 mm might be enough to achieve a U value of 0.15 W/m²K.
• Walls (Renovations/New Elements in Existing Dwellings): The requirement stays at about 100-120 mm to meet the U value of 0.18 W/m²K.
• Floors (Renovations/New Elements in Existing Dwellings): For a U value of 0.18 W/m²K, approximately 160-180 mm of Kingspan K5 would be required.
• Roofs (Existing Elements): To reach a U value of 0.16 W/m²K, about 80-100 mm of Kingspan K5 could suffice.
• Walls (Existing Elements): For the less stringent U value of 0.30 W/m²K, around 60 mm might be adequate.
• Floors (Existing Elements): Achieving a U value of 0.25 W/m²K might require around 80 mm of Kingspan K5.

Considering renovations and existing dwellings

When introducing new elements or renovating existing elements in dwellings, the required thickness of insulation to meet or exceed the specified U values may differ. It’s important to conduct detailed calculations based on the specific conditions of the building. This includes its existing thermal performance and the interaction between different building elements. Professional advice should be sought to ensure compliance with building regulations while optimising the energy efficiency of the property.

Achieving the updated U values is not just about complying with regulations; it’s about moving towards more energy-efficient and sustainable buildings. Carefully selecting and installing the right amount of insulation is key to this process. You must ensure that buildings are comfortable, cost-effective to heat and cool, and have a lower environmental impact.

Thankfully, there are government grants available, including the Great British Insulation Scheme, eligibility for which you can check on our partner’s website, courtesy of their impartial funding eligibility calculator (Insulation Grant Funding).