How to Bridge a Cavity Wall below a Door Threshold without creating a Thermal Bridge

Cavity-wall construction creates a particular challenge where a door, sliding door or large glazed element needs to be supported across the cavity.

This situation is especially common at doors leading onto balconies, terraces or other external areas, where the threshold may extend across the inner and outer masonry leaves and the cavity between them.

The support below the door has to perform several functions at the same time:

• transfer the door loads safely into the supporting structure;

• bridge the cavity without excessive deflection;

• maintain thermal continuity through the threshold junction;

• provide sufficient height for the final door position;

• allow external waterproofing to be connected correctly;

• leave enough space for drainage and external finishes.

The important point is:

Structural strength alone is not enough.

A solution can be perfectly capable of supporting the door and still create a significant thermal bridge.

Why Traditional Structural Solutions Can Create a Problem

Historically, the main question at this junction was often simply:

How do we support the door across the cavity?

This can lead to solutions selected primarily for their structural strength.

A steel plate or steel angle spanning the cavity is an obvious example.

Structurally, steel can provide excellent support. Thermally, however, an unbroken metal connection across an otherwise insulated cavity creates a highly conductive path between inside and outside.

That works directly against the objective of maintaining a continuous thermal envelope.

For modern energy-efficient construction, the support beneath a door therefore needs to be considered as both:

a structural component and a thermal component.

A Structural Insulation Board Is Not Automatically a Good Thermal Detail

Replacing steel or concrete with a structural insulation board is an important step in the right direction.

But that alone does not guarantee good thermal performance.

Different structural insulation products can have very different thermal conductivities. A material may provide extremely high compressive strength while also transmitting considerably more heat than another product that still provides sufficient structural capacity for the application.

Once the required structural load can be transferred safely, additional material strength does not necessarily improve the threshold detail.

The better question is:

What structural performance does the door actually require, and how can we achieve it with the least possible thermal compromise?

The thermal conductivity of the material is only one part of this assessment.

The Ψ-value belongs to the complete junction, not to the individual board.

It is influenced by factors including:

• thermal conductivity and thickness of the structural insulation;

• cavity width;

• bearing arrangement;

• surrounding masonry;

• floor and wall insulation;

• position of the door threshold;

• geometry of the complete construction.

Two details that appear structurally similar can therefore perform very differently thermally.

Independent Thermal Modelling Shows Why This Matters

Prodomo commissioned independent thermal modelling of a representative cavity-wall door-threshold arrangement in accordance with EN ISO 10211.

In this simulation, a structural insulation board from another supplier was used to bridge the junction.

The material was modelled with a thermal conductivity of:

λ = 0.0761 W/(m·K)

The completed threshold junction produced a DEAP/NEAP linear thermal transmittance of approximately:

Ψ = 0.118 W/(m·K)

and a calculated surface-temperature factor of:

fRsi = 0.688

Using the residential surface-temperature criterion applied in the assessment, the thermal modeller classified this particular junction as not suitable for residential use.

This is an important result.

The product modelled was a structural insulation material intended to provide load-bearing support. Structurally, the concept appeared reasonable.

Thermally, however, the complete junction did not achieve satisfactory performance in the assessment.

This demonstrates an important principle:

A structurally suitable cavity bridge is not automatically a thermally satisfactory cavity bridge.

The result applies only to the specific materials and geometry used in that thermal model and should not be interpreted as a general assessment of every structural insulation product or every possible threshold construction.

Why We Use MultiPro

For comparison, the blaugelb MultiPro Structural Insulation Board used in our recommended cavity-bridging solution has a nominal thermal conductivity of:

λ10 = 0.0403 W/(m·K)

MultiPro is designed to provide high structural performance while maintaining comparatively low thermal conductivity.

The material λ-value alone does not determine the final Ψ-value of a junction. The complete geometry and surrounding construction still need to be considered.

However, starting with a structural material that transfers substantially less heat provides a better basis for maintaining thermal continuity through this critical part of the building envelope.

For this reason, our recommended starting point for bridging a cavity beneath a door or large glazed element is:

Minimum 60 mm MultiPro Structural Insulation Board

as the main structural bridging element.

The 60 mm thickness should be regarded as our practical minimum for this application.

Actual suitability still depends on factors such as:

• cavity width;

• available bearing;

• supporting masonry;

• door loads;

• position of concentrated loads;

• overall construction geometry.

The structural requirements of the door system must always be considered as part of the project design.