Frosty Instead Of Toasty?
Hidden pathways may be the cause of heat escaping your home
Thermal bridging as one of the main causes of home heat loss has received a lot of attention in recent years but is often explained in an overly technical way. Thank you for reading. If you enjoy the post, please support me by liking it or sharing.
Did you know that something as seemingly insignificant as a gap in your building's insulation can significantly impact heat loss and affect overall comfort? The gap is an example of a thermal bridge.
A thermal bridge is an area that allows heat to pass through it more quickly than the adjacent areas. It is generally undesirable. In winter, it enables heat to escape making your heating system work harder, and increasing your energy costs. Conversely, in summer, a thermal bridge will allow heat to enter your house more quickly, having similar effects. Thermal bridges also impact indoor comfort as they can create drafts, as well as cold and hot spots. They also have a negative effect on the environment.1
Heat transfer in the world occurs in several different ways: conduction, radiation, and convection. (See Repository). A thermal bridge is a specific example of conduction, where heat is transferred through a material that is a good conductor situated between materials that are poor conductors. For instance, consider an aluminum window frame or the studs in an insulated wall. Both aluminum and steel are excellent conductors of heat relative to the surrounding insulation, and they allow heat to pass through them more quickly than the wall itself.
The role of conduction in heat transfer
Thermal conduction is the transfer of heat (in the form of thermal energy) within a material or between materials in direct contact. When two materials are in contact, heat will flow from the warmer material to the cooler one. For example, during winter, heat flows from the warm interior of a house to the colder outdoor air.2 This heat transfer always follows the path of least resistance, which means it prefers to travel through materials that are better conductors. In the wall above, the studs serve as good conductors of heat, allowing heat to pass through them more quickly than the insulation, which is a less effective conductor.
Heat flow or heat flux between two objects, quantified as q is expressed in SI units (aka metric)3 as watts (See Repository). A simple formula, based on Fourier's law or the law of heat conduction, is provided below to help you understand the factors that affect it.
Don’t break your head over this if you don’t like math. All you need to understand is that the thermal conductivity of the material, the temperature difference and the surface area can all increase the rate of heat flow.
Where:
q = Conduction heat transfer or heat flow (W)
k = Material’s thermal conductivity (W/mK)
a = Cross sectional area (m²)
T1 Hot = Higher temperature (°C)
T2 Cold = Colder temperature (°C)
L = Material thickness (m)
Tip: Once you have the heat flow (q), you can also divide it by time to calculate the rate of heat transfer per hour, day, or the entire season. You can then use this rate to determine the energy cost associated with thermal bridges. (See Repository)
How much could the heat loss from skinny studs be?
The rate of heat transfer depends on the thermal conductivity of a material, the surface area, and the temperature difference on either side. The thermal conductivity of insulation is approximately 0.04 W/(mK), whereas for steel (50 W/(mK)), it is about 1000-1500 times higher. As a result, steel studs allow heat to escape at a significantly higher rate.
Stud framing inside walls can account for 25% of the wall area. The heat loss through studs can vary between 33% to 49% of the total heat loss from the building. This means that studs, occupying only 1/4th of the total area, can contribute approximately one-third to nearly half of the total heat loss.
Note: Thermal conductivity is a property that describes a material’s ability to conduct heat. It is measured in Watts per meter Kelvin or W/m·K
The impact of thermal bridges on comfort and efficiency
In the winter, your house acts like a cozy box, with the inside warmer than the cold exterior. The roof, walls, windows, doors, and floor collectively form your building's thermal envelope—the boundary separating the indoors from the outdoors. Heat moves through this thermal envelope at varying rates, depending on the materials involved. Some areas allow heat to escape more quickly, and these are the thermal bridges in your building. During winter, the inner surface of these thermal bridge areas may feel colder than other parts of your home.
Conversely, in the summer, when it's hot outside, heat attempts to enter your home. It infiltrates more rapidly through these thermal bridges. This means that in winter, you lose more warmth, and in summer, you gain more heat in these spots. Unfortunately, this isn't conducive to maintaining your building's comfort and efficiency.
There are two types of heat loss in buildings. First, there's the heat loss stemming from components like walls, roofs, and windows – the kind of heat loss that we can predict with a fair degree of accuracy. This predictable component typically accounts for a substantial portion, approximately 70-80%, of the total heat loss from a building.
It is typically measured in terms of thermal transmittance, often represented as the U-value, which quantifies the rate of heat transfer through a specific area of the building envelope (e.g., walls, roof, windows, doors).
Then, there are thermal bridges, a lesser-known but equally important factor, responsible for the remaining 20-30% of heat loss. These can manifest within walls and, particularly, at junctions where different building components meet. The two kind of thermal bridges are repeating and non-repeating.
Repeating thermal bridges
Repeating bridges are regular interruptions in the building envelope such as studs or wall ties, mortar joints in a wall etc. They are usually accounted for in overall heat loss calculations.
Non-repeating thermal bridges
Non-repeating thermal bridges refer to areas where the insulation is interrupted or less effective, causing a localized increase in heat transfer. Unlike repeating thermal bridges, which occur consistently at regular intervals (such as through-wall studs in a framed wall), non-repeating thermal bridges are irregular. They include:
Junctions between wall and floor, wall and roof, wall and a deck or balcony etc.
Recessed lighting, roof lights etc.
Gaps in insulation
Frames and openings around windows and doors
A thermal imaging camera can show the thermal bridges in a building. The thermal bridges will appear as areas of higher temperature typically appearing as red or orange. Penetrations in the envelope, such as beams, pipes, vents, flues, exterior wall outlets, canopy brackets, etc
On the left a thermal imaging camera shows the heat loss where a steel beam penetrates the building. Image via Schöck
Why thermal bridges are a problem
Energy costs: Thermal bridges can increase your heating and cooling costs. Your heating and cooling system will need to work harder, resulting in higher energy bills to maintain a consistent and comfortable indoor temperature.
Discomfort: Thermal bridges can lead to discomfort, especially near areas like windows where warm air is drawn towards them and then escapes, creating a cyclical temperature variation.
Reduced effectiveness of insulation and energy efficient measures: Thermal bridges can account for up to 30% of heat loss, which can negate the positive effects of insulation and airtightness measures. Similarly, energy-efficient windows must be correctly installed to avoid thermal bridges created by frames and openings.
Condensation, mold, and rot: Warm indoor air often contains moisture from activities like cooking, bathing, and breathing. A thermal bridge can cause condensation when it allows warm, moist indoor air to come into contact with a cold surface, causing the air to cool and reach its dew point, leading to the condensation of water vapor. This moisture can lead to issues like mold growth and structural rot. Condensation within walls is particularly concerning as it is not easily visible from either the interior or exterior of the building.
How to fix them
Fixing thermal bridges, whether they are repeating or non-repeating, is essential for improving energy efficiency and comfort. While many breaks can be easily identified through visual inspection, thermal imaging can help prioritize areas that are losing the most heat to perform targeted fixes. Here are some common strategies for addressing thermal bridges:
Improve Insulation: One of the most effective ways to mitigate thermal bridges is to add or improve insulation in the affected areas. This can involve using materials with higher thermal resistance (higher R-values) or adding additional layers of insulation to cover the bridge.
Thermal Breaks: Installing thermal breaks, which are materials with low thermal conductivity (e.g. insulating material), can interrupt the path of heat transfer. These are often used at junctions, such as balcony connections, to prevent heat from escaping or entering.
Improved Sealing Techniques: Ensuring proper construction techniques, such as sealing gaps and cracks, can reduce thermal bridging. This includes using air sealing products and ensuring that insulation is installed without gaps or compression.
Advanced Framing Techniques: In the case of repeating thermal bridges, advanced framing techniques can reduce the number of studs or framing members, thereby decreasing the thermal bridging effect.
Continuous Exterior Insulation: In some cases, it may be feasible to add insulation to the exterior of a building to create a continuous insulation layer, reducing or eliminating thermal bridges through the wall assembly.
Window and Door Installation: Proper installation of windows and doors with insulated frames and seals can minimize thermal bridging around these openings.
Materials fabricated with thermal breaks: In some construction elements, such as lintels, using materials designed specifically as thermal breaks can prevent heat transfer. These materials are placed between components to disrupt the thermal bridge.
Design Considerations: Early in the design phase, architects and engineers can work together to minimize or eliminate potential thermal bridges through thoughtful design and detailing.
Addressing thermal bridges may require a combination of these strategies, depending on the specific situation and the building's design. Consulting with a building professional can be helpful.
Related content:
Why My House Is Still Freezing
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Queen’s Repository
Heat transfer
Heat can be transferred in three ways: by conduction, by convection, and by radiation. Conduction is the transfer of heat energy by direct contact, convection is the movement of heat by actual motion of matter; radiation is the transfer of energy with the help of electromagnetic waves. You have experienced conduction if you have warmed your feet on a sand beach or burned yourself on the hot handle of a pan.
Metal is a particularly conductive material, and steel has very high thermal conductivity. This can be a useful in cookware, packaging and electronics, but it also can be challenging to manage in exterior building materials, such as fenestration (windows and doors) and framing.
Law of Thermodynamics
The laws of thermodynamics are a set of fundamental principles that govern the behavior of energy and matter in the universe. There are four laws, but the second governs heat transfer and states that heat flows from hot to cold. This principle is known as the law of heat transfer, and it helps explain why a hot cup of coffee cools down in a cooler room. Heat transfer from hot to cold is a one-way process, and it doesn't occur in the reverse direction without external work or energy input.
Fourier’s Law
The second law of thermodynamics establishes the direction in which heat transfer naturally occurs, and Fourier's Law provides a mathematical description of this process. Fourier's law states
"The rate at which heat flows through a material is directly proportional to the temperature difference across the material and inversely proportional to the material's resistance to heat flow."
Heat flow is measured as q in the equation above.
In simple terms
Heat flows from hot to cold: Heat naturally moves from areas with higher temperature to areas with lower temperature.
The faster the temperature difference: The greater the temperature difference between two points in a material, the faster heat will flow between them.
It depends on the material: Different materials have different abilities to conduct heat. Some materials, like metals, are good conductors and allow heat to flow quickly, while others, like insulators, are poor conductors and slow down heat transfer.
Link: How to calculate home heat energy.
Thermal conductivity and thermal resistance
Thermal conductivity is a property that describes a material’s ability to conduct heat.
Thermal resistance is the inverse of thermal conductivity and it is how much a material resists heat flow. Thermal resistance is measured in different ways and one of them is the R-value (used for insulation) which is thermal resistance of unit area of a material.