Thermal Bridges and Clause E3, Internal Moisture
Thermal Bridges and Their Impact on Internal Moisture
Thermal bridges are areas in a building where heat flows more easily compared to the surrounding insulated areas. This increased heat flow results in lower surface temperatures on the inside of the building, which can lead to increased heat loss through transmission. These enhanced transmission heat losses are quantified using a heat loss coefficient, or the Psi (Ψ) Value and at the same time the minimum internal surface temperature factor fRSi.
Linear Thermal Bridges (Ψ-value): These are described by the Greek letter Ψ (Psi) and are measured in W/(m·K). For more information on thermal bridges see our article here.
minimum Surface temperature factor fRSi: A factor derived from the temperature difference between outside and inside, without a unit.
Understanding the fRSi Value in Relation to Internal Moisture
The fRSi value, or the temperature factor for surface condensation, is a critical measure for assessing the risk of mold growth and condensation on interior surfaces. It represents the ratio of the difference between the internal surface temperature (Ti) and the external temperature (Te) to the difference between the internal air temperature (Ta) and the external temperature (Te).
fRSi = (Ti - Te) / (Ta - Te)
- Ti: Internal surface temperature
- Te: External temperature
- Ta: Internal air temperature
Risk Assessment
A higher fRSi value indicates a lower risk of condensation and mould growth, as it suggests that the internal surface temperature is closer to the internal air temperature, reducing the likelihood of moisture forming on the surface. The range of value is between 0 and 1 and has no unit, eg. fRSi 0.7.
Comfort and Health:
Maintaining a high fRSi value helps ensure that the internal surfaces remain warm enough to prevent condensation, thereby protecting against mould growth, which can have adverse health effects and compromise indoor air quality.
Current Standards and Gaps
In the current version of Clause E3 of the New Zealand Building Code (NZBC), there is no quantifiable measurement for the effects of thermal bridging regarding moisture protection. This oversight can lead to issues with mould and condensation in buildings.
While the Standards will probably take some time to catch up here are some tips and tricks to identify and manage thermal bridging for your project.
Typical Areas Prone to Mould and Condensation due to Thermal Bridges
Window Connections
The junctions where windows meet the walls often have higher thermal conductivity, leading to cold spots around the window frame.
Wall-to-Floor/Perimeter Junctions
The connections where walls meet floors are common sites for thermal bridges. This thermal bridge however is now considered under Clause H1, in VM1 and VM2 as a contribution to the floor slab R-Value.
Corners and Edges of Walls
External corners of buildings can have increased heat flow due to the larger exterior surface area compared to the interior. On top of the geometric challenge the corners of walls are often not properly insulated between studs and amplify the effects.
Balconies and Overhangs
The projection of structural elements through the building envelope can create cold surfaces where condensation and mould can easily form in the immediate vicinity.
Penetrations for Utilities
Points where pipes, ducts, and other utilities penetrate through the building envelope create thermal bridges. These point entries can add cold surfaces around the entry point but metal ductwork can also bring colder exterior air into the building if there are no dampeners at the entry point. For this reason insulation is typically recommended along ductwork.
Recommendations for Designers Considering Thermal Bridges
Given the current lack of consideration for thermal bridges in Clause E3 of the New Zealand Building Code (NZBC) regarding moisture protection, designers can take several proactive steps to mitigate the risks associated with thermal bridges:
Building Energy Modelling
Employ energy modelling tools during the design phase, which you can also use for H1 compliance, Identify and quantify potential thermal bridges to understand their impact on the building’s overall thermal performance. Use the High Performance Details Handbook if you have similar construction types or engage someone who can calculate these thermal bridges.
Connect your Insulation
Make sure that insulation materials are connected at critical junctions, such as around windows, doors, and where walls meet floors and roofs. Timber, rigid board insulation, fibrous insulation and expanding foam are all great materials to "bridge the gap".
Use Thermal Breaks
Where insulation cant be continuous/ especially around structure - Integrate thermal breaks into the building design. In areas prone to thermal bridging, such as balconies, or cantilevered structures a structural thermal break can make all the difference. A great system is this thermal break from thermal innovations, or the Isokorb from Schoeck.
Optimize Geometric Design
Minimize the complexity of building geometry. Simplify design elements that create additional exterior surface area, such as corners and edges, to reduce the potential for thermal bridging.
Select Appropriate Construction Materials
Choose construction materials with low thermal conductivity for critical areas. For example, using insulated window frames and low-conductivity fasteners can help reduce thermal bridging.
If you still need a calculated fRSi?
We can calculate these for you. Whether you have a Passive House Project, a Homestar Project, or you need a detail for your H1 Report. Just give us a shout!
