We all are aware of the importance of gaps and openings in our homes and of their impact on our comfort. That's part of common sense... But most people aren't aware of the importance of thermal bridging through walls and other parts of the envelope of the house; in other words: they minimize the importance of heat flow through the materials that make up the walls, floors, roofs and windows.
Construction materials - concrete, wood... - have not the thermal resistance that many people intuitively think they have.
Thermal bridging in walls, floors, ceilings
Even without openings or gaps, heat can flow through the home’s envelope, that is, through the wood or steel studs in the walls, or the glass and the frames of the windows and doors... And that's thermal bridging.
Thermal bridging can be more important than air leakage… It doesn't just involve small openings; it involves very large surfaces, especially the walls.
Image: Dow Chemical
Causes of thermal bridging
Thermal bridging is driven by differences between indoor and outdoor temperatures. If the differences are minimal, thermal bridging will also be minimal. But if the outside temperature is significantly warmer, unwanted heat gains can be very important; or the opposite, when inside temperature is higher.
On the other hand, thermal bridging also varies with the conductivity of the materials in the home’s envelope. Concrete walls are more prone to thermal bridging than wood-frame walls, because concrete has a higher thermal conductivity. Walls with steel frames - instead of wood - are still more prone to thermal bridging (2 to 3 times more); and so on…
Materials such as aluminum (thermal conductivity: 117) or steel (thermal conductivity: 26) have very high heat transmission rates. Common glass (0.65), concrete (0.48), brick (0.36) or wood (0.13) are less conductive. But we should not undervalue thermal bridging through them too.
Windows (with their glass and metal frames…) are a major point of concern, but walls – with their large overall surface – and ceilings (and, in a lesser degree, the floors) can also be important sources of heat flow.
Addressing the thermal bridging problem
Thermal bridging problems in walls, floors and ceilings can only be solved through insulation. There isn't any other way around it.
Common wall sheathing (OBS, plywood…) and siding doesn't avoid thermal bridging. Only insulation materials with their very low conductivity (0,02 and less) can provide proper thermal protection.
In other words: if you want to prevent thermal bridging, or to reduce it to a minimum, you should use a proper layer of insulation all over the envelope of your house.
The framing, the piping and other components of walls, floors and roofs should be properly insulated with a continuous layer of insulation.
Obviously, the issue of thermal bridging in windows is addressed differently: by using special glazing and frames, and thermal breaks and spacers between the window panes, and also insulated frames and shade... (see: Windows Performance Guide)