Thermal performance has become one of the most scrutinised areas of building design, and for good reason. As the UK continues working toward ambitious energy-reduction and carbon-neutral goals, Part L plays a central role in shaping how buildings are constructed, insulated, and sealed.
A significant proportion of a building’s heat loss happens through windows and doors. That’s why choosing high-performance timber products isn’t just a design decision; it’s one of the most effective ways to meet (and exceed) thermal efficiency building regulations.
This guide breaks down Part L requirements, explains how U-values are calculated, and shows how Mumford & Wood’s engineered timber windows and doors help projects achieve modern thermal targets with ease. You’ll also find insights into glazing design, seal performance, and case-based examples that demonstrate compliance in real-world installations.
Understanding Thermal Efficiency and Part L (Approved Document L)
Approved Document L sets the standards for energy performance in new and existing dwellings. It outlines how buildings should be insulated, how they should retain heat, and which performance metrics must be met to ensure lower energy use, carbon reduction, and consistent comfort.
Some of the key areas related to windows and doors include:
- U-values (W/m²K): the rate at which heat transfers through a window or door.
- G-values: solar gain properties.
- Air leakage limits: how well the product prevents uncontrolled ventilation.
- Thermal bridging: minimising heat loss around the frame and junctions.
Part L sets specific limiting values, meaning windows and doors must perform below certain thresholds. For most modern residential projects, this typically includes:
- Windows: around 1.4 W/m²K or better (lower numbers = better performance)
- Doors: between 1.4: 1.8 W/m²K, depending on style and glazing area
- Extensions & refurbishments: often require slightly stricter upgraded performance to improve the existing fabric
These targets continue to tighten with each regulatory update and anticipated changes to the Future Homes Standard. As a result, product specification has never been more important.
How U-Values Are Calculated (And Why They Matter)
A U-value measures how well a window or door resists heat transfer. It is calculated using three main components:
- Glazing performance: double or triple glazing units, gas fills, coatings
- Frame construction: materials, joinery, insulating properties
- Warm edge spacers and seals: perimeter technologies that reduce thermal bridging
The final U-value takes into account both the glazing and the frame; this is known as the whole-window U-value, not just the centre-pane figure.
In practice:
- Lower U-values = higher thermal efficiency
- Higher efficiency means reduced heating demand
- Reduced heating demand supports Part L compliance and better EPC ratings
Mumford & Wood products are engineered to deliver exceptionally low U-values through a combination of sustainable timber construction, high-performance glazing, and precision-manufactured seals.
Why Timber Windows and Doors Naturally Excel in Thermal Performance
Timber is one of the most thermally efficient frame materials available. Unlike aluminium or steel, timber has low thermal conductivity, meaning it naturally slows heat transfer.
Engineered timber frames perform especially well because:
- Multi-layered lamination increases stability and reduces movement, helping seals maintain their compression
- Dense timber fibres offer natural insulation, improving whole-frame U-values
- Timber allows deeper, optimised profiles that accommodate larger insulated glazing units
When paired with high-performance glazing systems, timber windows and doors often surpass the thermal performance of PVC or aluminium alternatives, particularly in traditional, heritage, or conservation-sensitive designs where aesthetics are as important as performance.
How Mumford & Wood Achieves Outstanding Thermal Efficiency
Mumford & Wood products are designed, engineered, and manufactured to meet the newest Part L standards and prepare specifiers for future thermal targets. The performance advantages come from several integrated elements:
1. Engineered Timber Construction
Each frame is constructed from multi-layered laminated timber, selected for its strength and thermal resistance. The engineered construction:
- Prevents warping and twisting
- Maintains tight tolerances for seal compression
- Reduces cold bridging through the frame
- Ensures consistent U-values across all product types
This stability is a crucial advantage when meeting airtightness and thermal continuity requirements across the envelope.
2. High-Performance Double and Triple Glazing Units
Mumford & Wood offers a range of insulating glazing units (IGUs) tailored to project needs:
- Low-E soft-coat glass to reflect internal heat back into the room
- Argon or krypton gas fills for enhanced insulation
- Warm edge spacers that reduce perimeter heat loss
- Acoustic-rated glass options that maintain thermal performance
These combinations create consistent results, delivering whole-window U-values suitable for both new builds and Retrofit/Heritage upgrade projects.
3. Multi-Seal Systems for Superior Airtightness
Performance isn’t only about U-values. Airtightness plays a key role in meeting Part L targets.
Mumford & Wood products use advanced multi-seal systems designed to maintain long-term compression and resist environmental wear. These seals:
- Prevent uncontrolled ventilation
- Reduce heat loss at the frame junction
- Support the building’s overall air-pressure test results
Combined with precision joinery, the result is a weather-tight installation that performs reliably year after year.
4. Optimal Frame Depth and Profile Design
Frame design is just as important as glazing. Mumford & Wood’s profiles are engineered to hold insulating glass securely while minimising thermal bridging. Key advantages include:
- Deep frame depths suitable for modern IGUs
- Thermally optimised profile geometry
- Heritage sightlines available without sacrificing performance
This makes compliance straightforward, especially in period or conservation projects where standard PVC or aluminium frames can’t match the aesthetic requirements.
Practical Steps to Ensure Your Project Meets Part L
To make Part L both achievable and repeatable across projects, consider the following practical steps:
Step 1: Start With Whole-Product U-Values, Not Centre-Pane Figures
Some suppliers list only the centre-pane U-value, which is misleadingly low. Using whole-window and whole-door values ensures compliance and gives a more accurate prediction for SAP/SBEM modelling.
Step 2: Prioritise Airtightness and Sealing Strategy
Even the best U-value is undermined by uncontrolled air leakage. Mumford & Wood’s multi-seal systems help, but installation quality is equally important.
Step 3: Minimise Thermal Bridging at Interfaces
Use insulated cavity closers, correct DPC detailing, and airtight membranes to prevent cold bridging around frames.
Step 4: Choose Glazing to Suit the Building Type
For example:
- New builds: typically require modern low-E, argon-filled IGUs
- Retrofit: can often use slimline units to meet performance and conservation needs
- Acoustic or security-led projects: need glazing combinations that achieve thermal and functional targets simultaneously
Step 5: Conduct Thermal Modelling Early in Design
U-values affect SAP scores, overheating risk, fabric energy efficiency (FEE), and target emission rates (TER).Early modelling avoids costlier changes later in the process.
Why Timber Doors Also Contribute Significantly to Thermal Performance
Doors are often overlooked in a thermal strategy, but they can be a significant heat-loss point if not specified correctly.
Mumford & Wood’s timber doors achieve strong standards due to:
- Solid engineered timber cores
- High-performance threshold and weather seals
- High-specification glazing options for doors with lights
- Stable construction that maintains long-term airtightness
Whether you specify a French door, entrance door or bi-fold, the thermal engineering follows the same principles that make the window range so effective.
Why Choosing Mumford & Wood Makes Part L Compliance Easier
Specifying windows and doors is no longer just about aesthetics or material choice. It has become central to meeting regulatory, environmental, and performance expectations.
Key benefits for specifiers include:
- Whole-product U-values designed to exceed Part L
- Engineered timber for strength, stability, and natural insulation
- Custom glazing options for all project types
- Precision seals and airtightness performance
- Profiles suited to both heritage and contemporary architecture
- Proven results demonstrated through real installations
With every product crafted to meet modern thermal standards, Mumford & Wood supports specifiers in delivering energy-efficient, compliant, and future-ready buildings.
Moving Forward With Confidence
As Part L continues to evolve, thermal efficiency will remain at the forefront of building design. Timber windows and doors offer an effective, sustainable, and design-conscious route to compliance, without compromising character or craftsmanship.
If you’re planning a new build, refurbishment, or heritage-sensitive development and want to ensure full alignment with Approved Document L, our team can help you select the right products, model anticipated U-values, and guide you through compliance-ready options.
