Thermal bridge
A thermal bridge is a localized path where heat flows more easily through a building envelope than through the surrounding insulated assembly. It can occur at framing, balconies, slab edges, fasteners, corners, window perimeters, foundations, and other junctions, reducing comfort and energy performance while increasing condensation risk.
What a thermal bridge is
A thermal bridge is a weak spot in a building's thermal boundary. Heat takes the easier path through a material or junction with lower thermal resistance than the surrounding assembly. In cold weather, that path can pull heat outward and make an interior surface colder. In hot weather, it can bring more heat inward. The bridge may be obvious, such as a concrete balcony slab, or hidden inside framing and connection details.
Why bridges form
Thermal bridges form when insulation is interrupted, compressed, bypassed, or crossed by conductive materials. Wood studs conduct more heat than cavity insulation, and steel or concrete conduct much more. Geometry also matters: corners, parapets, window frames, and slab edges can create extra heat flow because surfaces and layers do not line up as neatly as they do in a flat wall section.
Material and geometric bridges
A material thermal bridge happens when a conductive material penetrates or crosses insulation, such as a steel beam through a wall or metal fasteners through exterior insulation. A geometric thermal bridge happens because of shape, such as an outside corner where heat spreads through a larger exterior surface. Real details often combine both effects.
Energy and comfort
Thermal bridges reduce the effective performance of insulation. A wall can have high nominal R-value in the cavity but much lower whole-wall performance once studs, plates, headers, anchors, and junctions are included. Cold interior surfaces can feel uncomfortable even when the room air is warm, because people exchange radiant heat with nearby surfaces.
Moisture risk
Cold bridge areas can drop below the dew point of indoor air, allowing condensation on or inside assemblies. Repeated wetting can support mold growth, damage finishes, corrode metal, rot wood, or reduce insulation performance. This is why thermal bridge design is tied closely to moisture control, not only to energy efficiency.
Common building examples
Balconies that continue through the exterior wall, exposed concrete slab edges, steel lintels, shelf angles, metal studs, roof parapets, window-wall joints, uninsulated foundation edges, and poorly detailed corners are common examples. Small repeated bridges, such as fasteners or studs, can matter as much as one dramatic bridge because they occur across large areas.
How designers reduce them
Common strategies include continuous exterior insulation, thermally broken balcony connectors, insulated window frames, careful window placement, nonconductive spacers, fewer metal penetrations, better slab-edge detailing, and aligning insulation across walls, roofs, and foundations. The aim is not magic; it is continuity. The thermal control layer should be easy to trace around the whole building.
Measuring and modeling
Thermal bridges can be estimated with whole-wall calculations, two-dimensional or three-dimensional heat-flow modeling, and thermal transmittance values for linear or point details. Infrared thermography can help reveal surface temperature patterns, but it must be interpreted carefully because wind, sun, moisture, emissivity, and indoor-outdoor temperature differences affect images.
Why it matters
Thermal bridges matter because small details can decide whether a high-performance envelope actually performs well. They affect heating and cooling loads, comfort near walls and windows, moisture durability, construction cost, and retrofit strategy. Understanding them helps turn insulation from a product choice into a complete building-detailing problem.