Thermal bridging is a phenomenon that occurs when there is a direct path for heat to flow from the interior to the exterior of a building, bypassing insulation and reducing the overall energy efficiency of the structure. In homes, this can happen when there is a lack of insulation around windows, doors, and other openings, as well as around exterior walls and roofs.
Why Does it Matter?
Thermal bridging can be a major problem for homeowners, as it can lead to higher heating and cooling costs, and uneven temperatures throughout the home. Additionally, it can contribute to mold and moisture problems, as well as increased risk of damage from ice dams in cold climates. Identifying and addressing thermal bridges is an important step in constructing an energy efficient building.
How Does Heat Move?
There are three types of heat transfer: radiation, conduction, and convection. We will use the example of an bonfire on a cold winter day to understand these concepts.
Radiation - heat traveling through empty space and hitting something. The fire makes your face feel warm, even though the air is cold.
Conduction - heat moving through a solid substance. A metal poker resting in the fire will become hot.
Convection - heat flowing through the movement of air. The cold air is being drawn in the sides of the fire and being expelled out the top as hot air.
Thermal bridging is caused by conduction. The opposite of a thermal bridge is a thermal break. Imagine standing beside that winter bonfire. If you were standing barefoot, your feet would immediately become cold even though you were being warmed by the fire. Insulated winter boots act as a thermal break between the warmth of your feet and the cold of the snow.
Wood Frame Walls
The wall of a wood framed house is made up of vertical studs with plywood sheathing on the outside. Insulation is placed in between studs to increase the R-value (resistance to movement of heat) of the wall, and exterior siding is attached to the outside of the sheathing to create a weatherproof barrier.
Insulation has a relatively high R-value, meaning it resists the movement of heat. Most insulation has an R-value of around R3/inch. Solid wood is a worse insulator, and has a value of only about R1/inch. This means that while insulation resists heat movement quite well, wood studs do not.
Above is an image of a wood framed wall on a very cold day. The areas between the studs where the insulation is have remained cold on the outside, and are covered by frost. The places where the studs are attached to the sheathing conducted heat from the interior to the exterior, melting the frost!
Basements
Concrete is a great conductor of heat. Since most foundations are made from concrete, basements are especially susceptible to thermal bridging. During the winter, heat is constantly flowing through your foundation and into the ground. That is one of the reasons there is often a strip of exposed grass next to a house - the snow has been melted.
Evidence of thermal bridging around a foundation. Notice that even in the cold winter months, the grass next to the concrete is still green and alive!
What Can I Do?
Fortunately, there are several ways to prevent thermal bridging when renovating an existing home or building a new one.
The most effective way is to install a layer of insulation between the sheathing and the siding, and on the inside of the basement foundation wall. This creates a thermal break between the warm interior and the cold exterior, which prevents heat from flowing through the wood studs or concrete foundation, driving up your heating bills.
If you are building a new home, or replacing the siding on your existing home, consider adding exterior insulation (sometimes nicknamed "outsulation"). Also think about adding insulation board to the inside of the foundation, or building with Insulated Concrete Forms (ICF) which feature a layer of insulation on the inside and the outside of the concrete.
Rigid insulation board is applied over plywood sheathing before the siding to prevent thermal bridging through the wall studs.
Key Points
Thermal bridging is caused by heat flowing through solid objects (conduction).
Solid materials like wood and concrete conduct heat from inside to outside.
This can be a major source of heat loss in homes.
Adding insulation in key areas like inside basement foundations and underneath siding can help prevent this issue.