Thermal Bridging and Cold Bridges in Extensions and Renovations

A thermal bridge (or cold bridge) is a localised area of the building envelope where heat loss is significantly greater than the surrounding insulated construction. The bridge 'short-circuits' the insulation layer — heat passes through the conducting element rather than being resisted by the insulation.

**The three types of thermal bridge**:

*1. Geometric bridges (shape bridges)*: Occur at corners and junctions where the internal surface area is smaller than the external surface area — meaning the same heat flow is concentrated over a smaller internal area, making the internal surface colder. Classic example: the external corner of a room (internal corner has a smaller area than the external corner; the surface cools proportionally more at the corner).

• *2. Structural bridges*:
• Occur where a structural element with high thermal conductivity passes through the insulation layer. Classic examples:
• •A concrete slab extending through the insulated wall (balcony slab / floor slab at wall junction)
• •A steel column or beam penetrating the insulation layer
• •Timber studs in a timber frame wall (the stud has 3–5× higher conductivity than PIR insulation; a 90mm stud at 400mm centres creates a significant bridge)
• •Mortar joints in insulating block construction

*3. Repeating bridges (regular/systematic)*: Occur at regular intervals through the insulated construction — the timber studs in a timber frame wall are the most common example. These are modelled within the declared U-value of the construction (the U-value calculation includes a correction factor for repeating bridges).

**Why thermal bridges matter**:

*Heat loss*: A significant thermal bridge can add 10–30% to the heat loss through the affected construction element compared to the nominal U-value.

*Surface temperature and condensation*: The inner surface of a thermal bridge is colder than the surrounding surface. Where the surface temperature falls below the dew point of the room air, condensation occurs — producing mould growth, staining, and eventually deterioration of the surface finishes and structural elements.

*The 'linear thermal transmittance' (psi value, Ψ)*: The additional heat loss at a linear junction (e.g., wall-floor junction, wall-roof junction) is quantified as the Ψ value (psi value) in W/m.K — the additional heat loss per metre length of the junction per degree Kelvin of temperature difference. The Ψ values for all significant junctions in a building are combined to calculate the total additional heat loss from thermal bridging (the 'y-factor' in SAP).

**1. Wall-floor junction**: The junction between the extension wall and the floor slab is the most significant thermal bridge in a typical concrete-slab extension. The concrete slab is a good conductor — it sits in direct contact with the cold ground and the inner leaf of the external wall, creating a bridge around the insulation.

How to minimise: install insulation below the floor slab extending to the edge of the slab (perimeter insulation), and ensure the cavity wall insulation extends fully down to the base of the cavity. Accredited junction details (SAP Appendix K or NHBC) specify the correct detailing.

**2. Wall-roof junction (eaves and verge)**: At the eaves of an extension, the insulation in the wall must connect without gap to the insulation in the roof. Where the insulated layer in the wall is a cavity with partial-fill or full-fill insulation and the roof is a warm flat roof with insulation above the deck, there is a potential gap at the eaves where insulation is absent. The warm flat roof insulation must lap down over the wall insulation at the eaves to close the bridge.

• **3. Lintel over windows and doors**:
• Steel lintels over openings are a common thermal bridge. A standard galvanised steel lintel has very high thermal conductivity — it creates a highly conductive path from the outer leaf of the wall to the inner leaf. Solutions:
• •Insulated cavity lintels (Catnic, IG, or equivalent with factory-applied insulation to the soffit)
• •Thermal break lintel designs where the inner and outer sections of the lintel are connected by a low-conductivity element
• •Ensure the cavity insulation extends fully above the lintel (many installers leave a gap above a lintel — a significant bridge)

• **4. Window frame perimeter**:
• The junction between the window frame and the surrounding masonry/insulation is a thermal bridge — the frame replaces insulation at the perimeter, and the fixing mortar or frame-to-reveal contact creates a cold path. Solutions:
• •Install windows with the frame in plane with the insulated layer (the 'warm installation' position — frame in the inner leaf of a cavity wall, with the cavity insulation lapping around the reveal)
• •Insulate the reveal externally with external insulated render returns or internal insulated plaster

• **5. Rafters in a loft conversion warm roof**:
• In a warm roof loft conversion, the PIR insulation between the rafters is interrupted by the timber rafters themselves. A 47mm × 150mm rafter at 400mm centres constitutes approximately 10–12% of the total area at approximately 5× the conductivity of PIR — a significant repeating thermal bridge. Solutions:
• •Continuous layer of PIR below the rafters (counter-battens or service zone below the insulation), creating a thermally continuous layer covering the rafters
• •The below-rafter insulation should be at least 25mm thick to provide meaningful reduction in the bridge effect

**Accredited construction details (ACDs)**: DCLG and BRE publish a set of 'Accredited Construction Details' (ACDs) for common junctions in domestic construction — showing specific wall, floor, and roof junction details that achieve low Ψ values. Where these details are followed exactly, the builder can use the published Ψ value in the SAP calculation without additional analysis.

• ACDs are available for:
• •Floor-wall junctions (cavity, solid, ICF, timber frame)
• •Eaves and gable junctions
• •Ridge junction
• •Opening (window and door) perimeters
• •Corner junctions

Using ACDs avoids the need for a site-specific thermal bridge analysis (a specialist calculation costing £500–£2,000 per junction type).

• **The y-factor in SAP**:
• In the Standard Assessment Procedure (SAP — the UK's standard methodology for calculating a dwelling's energy performance), thermal bridging is accounted for by a 'y-factor' that adds to the overall heat loss:
• •Default y-factor (no accredited details): 0.15 W/m²K — a significant penalty applied to the total external area of the dwelling to account for poorly detailed junctions
• •ACDs followed: 0.08 W/m²K — half the penalty of the default
• •Enhanced/bespoke calculations (psi values calculated for each junction): can achieve 0.05 W/m²K or lower

In practice, using ACDs saves approximately 5–8% in the SAP score — sometimes the difference between a dwelling passing and failing the Part L1B assessment, particularly in larger extensions with significant junction lengths.

**Condensation risk analysis**: Where Part L1B Building Regulations apply to an extension or conversion, the Building Control body may require a condensation risk analysis at critical junctions. The analysis demonstrates that the surface temperature at each junction is above the dew point under design conditions — confirming that condensation will not occur in normal use.

• This is particularly important:
• •At window reveals (common condensation location)
• •At the eaves of a flat-roofed extension
• •At loft conversion roof-wall junctions
• •At any junction where the insulated layers change or where there is a potential gap in the insulation

**The practical design rule**: The fundamental rule for minimising thermal bridging is: keep the insulation layer continuous and uninterrupted around the entire thermal envelope. Every element that penetrates the insulation layer, every gap in the insulation, and every change in insulation type creates a bridge. The detailing of junctions — not the choice of insulation material — is usually the critical factor in achieving a low-bridge construction.