Heat guide
Thermal Bridging in Walls and Building Envelopes
A thermal bridge is a region where heat can bypass the more resistive parts of an assembly through a more conductive path.
Common thermal bridges
Examples include timber or metal studs through insulated cavities, concrete slab edges, balcony connections, window frames, steel beams and mechanical fasteners.
Parallel paths change effective U-value
When heat crosses several paths in parallel, each path carries heat according to its own U-value and area fraction. A simple area-weighted approach can approximate repeated paths such as framing, although detailed junctions often require multidimensional analysis.
Thermal bridges affect more than energy use
Local cold spots can lower interior surface temperatures and increase condensation risk when indoor air is humid. Comfort can also decline near colder surfaces.
Metal framing deserves special attention
Steel conducts heat far more readily than insulation. A thin steel member can therefore have an outsized effect on assembly performance compared with its area alone.
Detailed design can require simulation
Corners, slab edges and structural penetrations can involve two-dimensional or three-dimensional heat flow. Simple one-dimensional R-value addition cannot capture these junction effects accurately.
Thermal bridges also matter when assessing surface condensation
A local bridge can lower the interior surface temperature even when the room-average wall temperature remains comfortable. If that surface falls below the local dew point, condensation or elevated surface humidity can occur around corners, fasteners or structural penetrations before it appears on the rest of the wall.
For this reason, detailed junction analysis often reports both additional heat loss and a minimum internal surface temperature. Energy performance and moisture robustness are related but distinct outcomes of the same bridge geometry.
A repeated-bridge example
Suppose 90% of a wall area has U = 0.20 W/(m²·K) and 10% is framing with U = 1.00 W/(m²·K). A simple parallel-path area weighting gives an effective U of 0.90 × 0.20 + 0.10 × 1.00 = 0.28 W/(m²·K). The framing occupies only one tenth of the area but raises the overall transmittance by 40% relative to the insulated path alone.
This simple method suits repeated one-dimensional paths better than complex junctions such as balconies, corners and slab edges.
Linear thermal bridges use different accounting
Junctions extending along an edge are often represented by a linear thermal transmittance, commonly called a psi-value and expressed in W/(m·K). Point penetrations can use point transmittance in W/K.
These quantities supplement area U-values so that heat flow associated with junction geometry is not lost in a purely one-dimensional calculation.
Surface temperature can be the critical outcome
A bridge may create a local interior surface much colder than the surrounding wall even when its contribution to annual energy use appears modest. That cold spot can affect comfort and moisture risk.
Detailed junction analysis therefore often checks both extra heat flow and minimum surface temperature.
Thermal bridges also change local moisture conditions
A bridge that conducts heat efficiently can depress an interior surface temperature during cold weather. If the local surface approaches the dew point of indoor air, condensation or high surface relative humidity can occur even though most of the wall remains warmer. This is why junction details around balconies, lintels, slab edges and metal fasteners can matter for durability as well as energy use.
The risk cannot be judged from whole-wall U-value alone because an acceptable area-average heat flow may still contain a sharply colder local point. Detailed calculations often examine both added heat loss and the minimum interior surface temperature.
Continuous insulation reduces repeated framing effects
Placing an insulating layer across structural framing can reduce the direct high-conductivity path through studs or other members. The improvement depends on continuity, fasteners, penetrations and junction details, but the principle is straightforward: a continuous resistive layer interrupts repeated parallel bridges that cavity insulation cannot remove by itself.
This does not eliminate every bridge. Corners, window interfaces, shelf angles, service penetrations and structural connections can still require dedicated detailing or two-dimensional analysis.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.