As energy efficiency standards continue to tighten, insulated doors are becoming a critical part of modern building design. Once viewed as a secondary consideration, thermal performance is now a core requirement for external doors across commercial, industrial, and public-sector projects.
In this guide, we explain what insulated doors are, how they reduce heat loss, and why thermal performance – measured by U-values – matters more than ever when specifying steel doors.
They are doorsets designed to reduce heat transfer between internal and external environments. Unlike traditional single-skin doors, insulated doors incorporate thermal insulation within the door leaf and frame to improve energy efficiency and limit heat loss.
In steel applications, insulated doors typically feature:
The result is a doorset that maintains the strength and durability of steel while delivering significantly improved thermal performance.
Heat loss through doors occurs when warm internal air transfers through poorly insulated materials or gaps around the doorset. Insulated doors combat this through a combination of design features working together.
Key elements include:
When correctly designed, an insulated steel doorset significantly reduces heat loss compared to older or uninsulated steel doors.
The thermal performance of insulated doors is measured using a U-value. This indicates how much heat passes through a building element.
For insulated doors, the U-value reflects the performance of the entire doorset, including:
Modern insulated steel doors can achieve U-values far lower than traditional steel solutions, making them suitable for projects where energy efficiency is a priority.
In the UK, the thermal performance of external doors is governed by Part L of the Building Regulations, which aims to reduce energy consumption and heat loss in buildings.
Current Part L requirements set maximum U-values depending on the application:
Specifying insulated doors with strong thermal performance helps ensure compliance – not just today, but as standards continue to evolve.
Thanks to their improved thermal efficiency, insulated doors are now widely specified across a range of sectors, including:
In these environments, insulated steel doors offer a balance of durability, performance, and compliance.
Modern insulated steel doorsets deliver:
This makes insulated steel doors an ideal solution where performance, longevity, and energy efficiency must be achieved together.
Advances in design and manufacturing have led to a new generation of insulated steel doorsets that go beyond minimum regulatory thresholds.
At Design & Supply, this approach is reflected in Therma-Line a range of thermally efficient steel doorsets engineered to achieve U-values as low as 1.3 W/m²K.
Rather than designing to minimum requirements, modern insulated doorsets like these provide valuable performance headroom, helping future-proof projects as regulations continue to tighten.
All Therma-Line doorsets are:
When specifying insulated doors, it’s important to look beyond a single figure. Key considerations include:
A well-specified insulated steel doorset can improve building performance, support compliance, and deliver long-term value.
As thermal performance becomes a central focus in building design, insulated doors are no longer optional – they are essential. With the right specification, insulated steel doors can deliver energy efficiency without compromising on strength, durability, or compliance.
If you’re specifying insulated doors where thermal performance matters, working with a specialist manufacturer like Design & Supply can help ensure confidence from design through to delivery.
