Minimising Thermal Bridging - Walls and Junctions

Words by Christopher Day, Tuesday 20 December 2016

BSBG Senior Architectural Technologist, Chris Day (MCIAT), guides us through the complexities of thermal bridging, the need to minimise it and its impact on building performance...

Introduction - What is a thermal bridge?

A thermal bridge (or cold bridge, depending on the climate) occurs when a more conductive or poorly insulated building material interrupts the continuity of the thermal barrier, resulting in heat transfer and energy loss through the building fabric - or thermal bridging.

Weak points between the insulated and non-insulated spaces, which result in a thermal bridge, have a detrimental impact on the overall energy performance of the building and also pose a potential risk of condensation and mould growth. Thermal expansion of materials (Thermal coefficients of expansion) may also cause damage to materials as we will see below. All types of Thermal bridges should be avoided where a viable detail solution can be achieved.

Types of thermal bridges

These fall into two main categories, which are:

Repeating thermal bridges

For example, mortar joints, metal studs, curtain wall mullions and transoms. These types of bridges are unavoidable and are included when determining the total resistance of the wall (Rt) and the U-value performance.

Non-repeating thermal bridges

Non-repeating thermal bridges - Thermal Bridging

Junctions of floors, roofs and poor detailing at window jambs, to name a few. These are avoidable and should be designed out.

The common causes of thermal bridging

Multiple situations and interfaces could lead to unwanted heat transfer. A few examples are: Steelwork detailing - Where steelwork bypasses the building Service penetrations through areas of condition and none condition spaces. Reveals around windows and doors. Junctions between walls and floors, and walls and roofs. If the design team fails to include all suitable allowances that prevent thermal bridging in the design, they are not meeting their due diligence and providing the client with an energy performing design, compliant with the engineer’s thermal model and statutory building regulations.

Key concerns and considerations

Interstitial condensation This is a form of structural dampening that occurs when warm, moist air penetrates inside a wall, roof or floor structure and reaches a dew point and condenses into liquid water. The warm, damp air may penetrate a wall from the exterior of an air-conditioned building in a warm climate where a thermal bridge is found.

The dew point should never be present within the wall make-up. This condensation/temperature difference may allow mould growth, corrosion of metal components and/or a reduction in the thermal insulation’s effectiveness. This may result in structural damage without visible indications until significant harm has occurred.

Key consideration - The mechanical engineer’s performance targets should be reflected in the detail and monitored throughout all stages of the design process.

The U-value and dew point calculations should then be run-through with the assistance of a specialist insulation consultant to develop a composition which will prevent such an occurrence. Additional concerns with weak/thermal bridge points are:

Coefficients of thermal (thermal expansion) in a material can cause breaks and cracking to the materials surrounding it. We have to ensure we pay attention to the thermal expansion between different building elements and allow for compressible joints and split ties where required.

Over specifying insulation to outweigh the repeating thermal-bridging - although this may maintain the performance targets, this will still cause localised problems that must be addressed in the detail.

How to minimise thermal/heat transfer

Think about the detail! It is important to spot a thermal bridge and design ways to prevent this early in the design. A keen eye and a good thought process are required, keeping in mind a need for a continuous, insulated line. Imagine a swimming pool filled with water. If there is a weak point/damage in the lining, the water would find its way out. By having a continuous material forming a sealed box, the water will remain and prevent expensive running costs to refill constantly to meet the requirements. This works similar to a building. A weak point will leak energy and require extra operation requirements to achieve the target environment.

A continuous thermal barrier is needed in the building enclosure; the location of this barrier for most buildings should be outboard of highly conductive materials. All penetrations through this line should be broken with a thermal break pad.

Lapping of insulation where direct continuity is not possible can mitigate thermal bridges (eg: EFIS on blockwork where this meets a column. Good design would prevent this at the design stage but the column cannot be left exposed, so a stepped EFIS detail is required).

Window-to-wall interfaces create additional challenges that need to be carefully reviewed for energy considerations and condensation risk due to positioning of the fenestration within the rest of the assembly.

Thermal breaks built into to the structure will help to stop the flow of heat. These thermal breaks are thin layers of insulated material (such as aerogel strips) which can be adhered directly to metal beams, effectively 'breaking' the heat bridge.

Most building materials are permeable so the common method of controlling interstitial condensation is to control indoor moisture. The majority of buildings control this by using the ventilation systems with dehumidification through the ventilation system and also can be controlled by an impermeable barrier on the warm side of the wall.

Christopher Day

Director

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