When insulating a home or commercial building in the UK, you are required to meet legal standards, not just trying to keep it warm or reduce energy bills. The main set of rules comes from Part L of the UK Building Regulations, which focuses on conserving fuel and power in both new builds and renovation projects. These regulations help ensure buildings are energy-efficient and environmentally responsible.
Whether you're building from scratch, converting a loft, or upgrading insulation in an existing property, understanding these requirements will help you choose the right insulation and avoid costly mistakes.
Part L: The Core Insulation Regulation
Part L sets out minimum thermal performance requirements for new builds, extensions and loft conversions, renovations and retrofits, and tells you how energy-efficient your building’s envelope (walls, roof, floor, etc.) needs to be. To meet these standards insulation must help achieve target U-values (a measure of how much heat escapes through a particular building element).
For full details and the latest official guidance, you can refer to the Approved Document L on the UK Government website.
What Is Regulation 40?
Regulation 40 is one of the rules within the UK building regulations that focuses on saving energy in buildings. It states that:
- Buildings should be built in a way that uses as little fuel and power as possible.
- Care should be taken to reduce heat loss in winter and heat gain in summer through walls, roofs, floors, and windows.
This rule helps make sure buildings are energy-efficient, which saves money on bills and is better for the environment. It also ties into Part L of the regulations, which sets specific standards for insulation and energy use. With energy prices rising and the government pushing for lower carbon emissions, following Regulation 40 is more important than ever.
U-Values and Compliance
A U-value measures the rate of heat transfer through a building element — the lower the number, the better the insulation.
Part L sets out maximum U-values that must not be exceeded. Here's a breakdown of typical targets:
| Element | New Buildings – Max U-Value (W/m²·K) | Older Homes – Max U-Value (W/m²·K) |
|---|---|---|
| Roof (pitched or flat) | 0.11 – 0.15 | 0.16 – 0.18 |
| Walls | 0.18 | 0.26 |
| Floors | 0.13 | 0.22 |
| Windows / Doors | 1.2 | 1.6 |
How Thick Should Insulation Be?
There’s no one-size-fits-all rule for insulation thickness. However, the general trend is:
- Floor Insulation: 100mm to 150mm
- Wall Insulation: 50mm and 100mm
- Roof Insulation (lofts): 250 - 300mm
- Flat Roof Insulation: 120 - 150mm
Is 200mm Enough?
For many applications (like mineral wool in a loft), 200mm–270mm is typically enough to meet or exceed U-value requirements. However, if you’re using materials with lower thermal efficiency, you might need to install thicker layers to achieve the same performance. Just keep in mind that available space can limit how much insulation you can fit.
What Happens If You Over-Insulate?
Over-insulating can reduce ventilation and cause condensation issues, increase installation complexity, cost and offer little return after a certain point (diminishing R-value returns).
Always balance material type, application space, and target U-value for cost-effective, code-compliant insulation.
What’s a Good R-Value?
R-value measures thermal resistance — the higher, the better the insulation resists heat flow. A good R-value depends on:
- Material type (e.g., PIR > Mineral Wool)
- Required U-value
- Space limitations
To dive deeper into how R-values and insulation thickness relate, see our full guide: Insulation Thickness and R-Value: What You Need to Know.
When choosing insulation to meet building regulations, it’s important to consider not only thermal and fire performance but also the environmental impact of different materials. Understanding which insulation options are more sustainable can help you make informed choices that benefit both your project and the planet.
For a deeper dive into the sustainability of insulation materials, check out our detailed guide on The Environmental Impact of Insulation Materials. This resource will help you balance compliance with green building goals.