In reaching high standards of insulation, thermal bridging takes a more prominent role in building design. Elrond Burrell, architect and author of the popular blog 'Passivhaus in Plain English & More' examines the importance of reducing thermal bridging and ways of achieving it.
What is Thermal Bridge Free Construction?
Thermal bridges (sometimes referred to as "cold bridges") in the building envelope have a measurable impact on energy efficiency and thermal comfort. The impact can be relatively low on buildings that are not very well insulated. However, with buildings that are well insulated and energy efficient, the relative impact of thermal bridging is significant.
Building regulations and codes are now starting to recognise this and in some places, it is required or recommended that thermal bridging be minimised.
The Passivhaus Standard recognises the importance of thermal bridges and the significant impact they can have on the high-performance Passivhaus building envelope. The Passivhaus Standard requires a continuous thermal envelope: this means thermal bridge free construction.
This guide answers the following questions:
• What is a thermal bridge?
• What are the different types of thermal bridges?
• Why are thermal bridges a problem?
• What is thermal bridge free construction?
The Passivhaus Standard requires thermal bridge free construction to ensure a robust high-quality building envelope that delivers radical energy efficiency and exceptional comfort.
What is a Thermal Bridge?
A thermal bridge is an element or location with less insulation, or reduced insulation performance, relative to the adjacent areas of the thermal envelope. This means the element or location provides a path of least resistance (a “bridge”) for heat to move through the building envelope. In cold climates, this means additional heat will be lost through these specific locations. In hot climates, a thermal bridge will allow unwanted additional heat to pass through the thermal envelope into the building.
A thermal bridge allows higher heat transfer than the surrounding thermal envelope.
What are the Different Types of Thermal Bridges?
There are two very distinct types of thermal bridges: geometric thermal bridges and construction thermal bridges.
Geometric thermal bridges
These are where the geometry of the thermal envelope causes increased heat loss in specific locations. Geometric thermal bridges do not form a literal bridge in the way construction thermal bridges do. And they can occur where full insulation thickness and continuity is maintained. Typically a geometric thermal bridge is where the external heat loss area is greater than the corresponding internal area of the thermal envelope. Some examples of this include:
External wall corners.
The eaves junction.
The ground floor and external wall junction.
Around window and door openings.
Geometric thermal bridging is unavoidable. However, geometric thermal bridging increases with the complexity of the building form. Therefore, it can be minimised by keeping the building form simple.
Geometric thermal bridges can be considered a heat loss area correction factor for junctions. The Passivhaus Standard measures heat loss area to the outside face of the thermal envelope, which tends to slightly overestimate geometric thermal bridging. Other methods of measuring heat loss, such as SAP in the UK, use the inside face of the thermal envelope and therefore slightly underestimate geometric thermal bridging. Hence the need for a correction factor in some situations.
Construction thermal bridges
These are the easiest type of thermal bridge to comprehend and visualise. A construction thermal bridge is where there literally is a physical material, a gap or a component that passes through the insulation. The material or component conducts heat better than the insulation and therefore effectively forms a bridge allowing heat to transfer between the inside and the outside. Some examples of this include:
Rafters that pass through the thermal envelope to support the eaves (or for decoration!)
Timber studs or joists within the insulation zone.
Cantilevered structure passing through the thermal envelope.
Lintels that interrupt cavity insulation.
Gaps left between insulation boards.
Construction thermal bridges can usually be avoided or minimised with careful design. Any construction thermal bridges that do occur will contribute a measurable heat loss. For Passivhaus buildings, the heat loss from any construction thermal bridges must be calculated and accounted for.
Combined thermal bridges
In many cases, geometric thermal bridges also include an element of construction thermal bridging. For example, an external wall corner while being a geometric thermal bridge will also tend to have additional structure creating construction thermal bridging. Similar the ground floor and external wall junction often involves a degree of construction thermal bridging.
Subtypes of thermal bridges
Both geometric, construction and combined thermal bridges can be broken down into further subtypes as follows:
Linear thermal bridges: where there is a thermal bridge with a specific length, for example, a lintel.
Point thermal bridges: where there is a thermal bridge at specific points only, for example, masonry wall ties.
Repeating thermal bridges: where there is a thermal bridge that repeats at regular intervals within an element of the thermal envelope, for example, timber studs in an insulated wall.
Non-repeating thermal bridges: where there is a one-off thermal bridge, for example, a structural column in an insulated wall.
In any building, even a Passivhaus building, some thermal bridging is unavoidable. However, good design and construction can minimise the number and effect of thermal bridges.
Why are Thermal Bridges a Problem?
Thermal bridges are hardly an issue in very poorly insulated buildings. However, with a high-performance thermal envelope, thermal bridging is critical. When the building heat loss is very low, thermal bridging can contribute a significant proportion of it. Additionally, a thermal bridge can perform much worse than the adjacent thermal envelope, which creates considerable temperature and moisture risks.
Heat loss Thermal bridges are the weakest point in the thermal envelope and so they can contribute considerable heat loss. This reduces the building energy efficiency and increases heating costs.
Unwanted heat Thermal bridges let heat in as well as out through the thermal envelope. In summer this can introduce further unwanted solar heat gain.
Cold internal surfaces Where heat escapes through a thermal bridge, the internal surface temperature will drop, creating a cold spot. The surface relative humidity will thereby increase. This introduces the risk of condensation on the internal surfaces, which may lead to mould growth. This can be unsightly and a health risk to the people in the building.
Cold spots in the building fabric While thermal bridges can cause internal cold spots, they can equally cause cold spots within the building fabric. The same issues arise – a drop in temperature and an increase in relative humidity. In this case, the risk is interstitial condensation, mould within the building fabric and potential damage to elements of the building fabric. Over a long period, the damage can be considerable.
Risks to comfort and health Cold spots are uncomfortable to be near and can cause draughts. If condensation occurs, or even worse mould, the indoor air quality will suffer, along with the people breathing it. Moisture and mould can both lead to health problems for the people in the building.
Thermal bridges reduce the performance of the thermal envelope and introduce unwanted risks.
What is a Thermal Bridge Free Construction?
The Passivhaus thermal envelope should be continuous. The insulation should pass the pen-test in every drawing. However, neither of these guarantees thermal bridge free construction.
For the Passivhaus Standard, thermal bridge free construction is where calculating the heat loss from all the thermal bridges doesn’t increase the overall building heat loss calculation. This is possible even with some thermal bridging, such as unavoidable geometric thermal bridges, because the Passive House Planning Package (PHPP) slightly overestimates the building heat loss. The slight overestimation ensures PHPP calculations are conservative rather than optimistic and therefore very reliable.
Calculating all the thermal bridges of a building would be very onerous. Instead, the Passivhaus Standard takes a pragmatic approach as follows:
If the external psi-value (thermal bridge loss coefficient Ψe) of a linear thermal bridge is equal or less than 0.01 W/mK there is no need to calculate it. There will still be some losses, but they are negligible even in the Passivhaus high-performance thermal envelope. There are standard details available that have already been calculated as compliant with this and can be utilised without the need to undertake any further calculations. Also, some product manufacturers have verified data indicating that their product is compliant; similarly there is no need to undertake any further calculations.
Point thermal bridges are considered to be thermal bridge free, except if they are a highly conductive material such as steel.
If the two-thirds of the insulation thickness, or the equivalent conductivity, is maintained there is no need to calculate the thermal bridge.
Repeating thermal bridges within elements of the thermal envelope, for example timber studs, are accounted for in the U-value calculation of the element.
Thermal bridges associated with window and door openings are accounted for in the window and door u-value calculations.
Thermal bridges that don’t meet any of these criteria must be calculated individually and as with all thermal bridging, minimised as far as possible.
If thermal continuity has been considered from the outset, it is possible to achieve thermal bridge free construction. If it hasn’t been considered, or other factors influence the design, then each thermal bridge needs close consideration. Some can be easily minimised or eliminated through careful detailing. Others might require expensive and complex thermal break components and detailing. For example, concrete or steel structure passing through the thermal envelope will need a structural thermal break component.
Passivhaus thermal bridge free construction is acheivable with careful and early consideration of thermal envelope continuity.
A High Quality Thermal Envelope is Thermal Bridge Free
The Passivhaus Standard requires thermal bridge free construction in order to deliver what it promises: radical energy efficiency and exceptional comfort.
Thermal bridges can have a serious impact on the energy efficiency and comfort of high-performance buildings. Thermal bridges also increase a number of undesirable risks than can damage the building fabric. Thermal bridge free construction minimises or eliminates these risks.
Thermal bridge free construction ensures a consistent high quality of thermal envelope that is robust and long lasting.
Early consideration of the thermal envelope can eliminate critical thermal bridges by design. Like the Heat Loss Form Factor, thermal bridging is another area where architects can be empowered to influence energy efficiency and cost, as well as comfort and durability. (If you are an architect, understanding thermal bridges might give you a headache, though – see the first point here.)
Thermal bridges increase heat loss and therefore heating costs. And resolving designed-in thermal bridges with complex structural thermal break details is often expensive.
Thermal bridge free construction is essential for the Passivhaus buildings to deliver radical energy efficiency and exceptional comfort. It empowers architects to deliver cost-effective high-performance architecture.
Elrond Burrell is an Architect, Writer & Speaker with over 15 years experience in designing and delivering excellent buildings. His areas of expertise are Sustainable Design, Passivhaus, Timber construction, and Building Information Modeling (BIM). He is a passionate advocate of these subjects and loves writing and speaking about them, as he often does in the UK and around the world. You can connect with Elrond via his website.
Elrond Burrell is an Associate at Architype Ltd, however, this article is written in a personal capacity and represents his personal opinions, not those of his employer.