Heat will transfer first through the point of least resistance. Places where there are weaknesses in the insulated envelope of the house are called cold bridges or thermal bridges, because they literally provide a “bridge” for heat to escape. What is more, these are places where condensation can form, bringing with it all its associated structural and health problems. The better insulated a building, the greater the problem posed by thermal bridges which is why environmental architects obsess on the design details that minimise these bridges, especially around windows.The main areas for thermal bridges are:
- Where there is a break in the insulation. For example, if insulation is only placed between the roof joists, the joists will act as cold bridges
- At corners. Especially where walls meet the ground
- Around windows and doors. Condensation may also form on metal window frame
- Where metal pipes or structural steel cross insulation or a wall cavity. Metal is a very good conductor of heat, and therefore makes a powerful cold bridge. Typically this is a problem where pipes enter a cold loft space (also a problem for draughts).
Thermal bridges can be largely eliminated with careful insulation: ensuring that loft insulation covers joists or rafters; insulating around pipes; adding extra insulation at corners and around the base of external walls when insulating floors.