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Avoiding Thermal Bridges with Bosig Phonotherm

Joe Fitzgerald

By Joe Fitzgerald

Sunday 8th January 2023

The avoidance of thermal bridging is an important consideration in all building projects when planning for insulation. The issues that can arise from thermal bridging can lead to additional heat loss, resulting in higher energy costs and problems with condensation, mould and building deterioration.

Avoiding Thermal Bridges with Bosig Phonotherm Avoiding Thermal Bridges with Bosig Phonotherm

What is a thermal bridge?

A thermal bridge can be seen as a weakness in a building’s outer thermal envelope. Ideally, a building will have uniform thermal resistance everywhere, however in the real-world insulation can become compromised, or interrupted, leading to heat escaping quicker from certain areas. This mostly occurs when the insulation layers surrounding a building become separated by a more conductive material (for example, structural steel), or by a sudden reduction in thickness.

Heat can escape from a building or cold air can enter as a result of a thermal bridge, and this is why they are often called ‘cold bridges.’ Heat will always move from hot to cold, which is why it feels cold when we touch thermally conductive materials like metal. The cold sensation is caused by the heat moving via conduction from our warm hands to the cooler metal, and our skin temperature drops as a result. Thermal bridging in buildings is no different, and if we allow heat to escape due to weak points in the insulation, the internal building surfaces will become colder as a result. When a surface temperature falls below a certain point the result is likely to be condensation, mould, or more serious structural damage which negatively affects a building’s durability. Humidity too, is often quite high in our buildings just from living in them. If a surface is cold enough comes into contact with moist air the outcome will be condensation, think of breathing onto a cold window for example.

Bosig Phonotherm used around the window reveal of a newbuild
Bosig Phonotherm used around the window reveal of a newbuild

How are thermal bridges measured and calculated?

Thermal imaging showing cold bridges underneath windows
Thermal imaging showing cold bridges underneath windows

Thermal bridges are numerically calculated using specialist software. This is done by building a simulated model of the detail which analyses the heat flow. Depending on the type of thermal bridge being assessed (linear or point thermal bridge), the result will give us the amount of heat being lost expressed either as Psi-Value (W/mK), for linear heat loss, or as a Chi Value (W/K), for single penetrations through the insulation. Following the software calculations, the overall amount of heat loss can be found by multiplying along the length of each junction, or simply by counting up the sum total of point thermal bridges.

U-Values are a way to describe how good a building element is at preventing heat loss. However, a U-value only measures heat loss in a very one-dimensional way. It tells us very little about the 2nd or 3rd dimensions for heat losses that occur in buildings (linear thermal bridging or point thermal bridging). In theory we could have a wall, floor and roof in a building with outstanding u-values, but the building could be terrible at holding heat if there are gaps between those elements, or if structural penetrations pierce through the building from inside to outside. This can be a reason why a heavily insulated building can still be cold despite having a lofty energy rating. In the building industry, this is sometimes referred to as a ‘performance gap’.

Note: Thermal bridges are calculated by a qualified person in accordance with conventions contained in EN ISO 10211 'Thermal Bridges in Building Construction - Heat Flows and Surface Temperatures - Detailed Calculations' and 'BR 497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors'.

An NSAI approved thermal modeller or a suitably qualified person can carry out a thermal bridge assessment for specific junctions, highlighting how to best offset the effects of thermal bridging.

Where do thermal bridges occur?

Thermal bridges are most common where insulation is missing or separated in some way. A very common thermal bridge occurs when structural elements, like steel beams of columns, breach through a building’s insulation. These are called ‘point thermal bridges’. 

A ‘repeating thermal bridge’ often occurs around timber elements, for example in a cold pitched roof where the insulation sits between the joists

Thermal bridging is also very likely to occur where one building element meets another, for example where a wall meets a floor or where a wall meets a roof. These building junctions often present a linear thermal bridge along the entire length of each junction. Another common area for linear thermal bridges is at the openings of a building around the doors and windows or roof lights. 

The reason new windows and doors must have high thermal resistance and great u-values is because essentially, they are filling up the holes made in the building envelope which represents a lot of missing insulation. The connection between new doors or windows back to the insulated building is all-too-often a very poor one, meaning heat usually finds a way out between the new frames and the building itself. This is why mould often occurs in the reveals of our windows and doors. This happens below large sliding doors too, or on building elements which require high compression resistance and structural support. A large lift-and-slide door commonly requires a layer of concrete beneath it to support the weight, as conventional insulation would not have the compression resistance to support it. This could lead to a significant thermal weakness between the high performing door and the cold concrete it rests on.


A window corner junction showing Bosig Phonotherm and Aerosana Visconn airtight liquid membrane
A window corner junction showing Bosig Phonotherm and Aerosana Visconn airtight liquid membrane
Bosig Phonotherm used underneath and around a porch door
Bosig Phonotherm used underneath and around a porch door

How does Bosig Phonotherm help?

Bosig Phonotherm is a dense, rigid insulated board which provides a much higher level of structural support than conventional insulation. In fact, it can have as much as 46 x more compression strength than conventional PIR insulation which is commonly used in floor construction. By simply comparing Bosig Phonotherm to an available insulation slab on the market it’s very quick to see the difference:

  • Bosig compressive strength, 7000kPa
  • Commonly used PIR used as floor insulation, >150kPa

Bosig Phonotherm also has a high level of physical stability and compression resistance, allowing it to provide structural support at load-bearing points and prevent point thermal bridges. This helps reduce heat loss around door thresholds or reveals in openings. The boards are manufactured from 100% upcycled polyurethane and have an Environmental Product Declaration (EDP), along with European Technical Assessment (ETA) certification.

Despite this inherent strength it strikes the right balance in terms of thermal quality also. It optimises junctions where thermal bridges occur and reduces heat loss, offering much greater thermal performance than steel or timber beams. This minimises thermal bridging at locations where additional structural support is required.

Bosig boards are also 100% water resistant, with no risk of swelling or decay, offering constant material thickness and stability. This is really useful as it’s often required to pre-line the building openings with Bosig Phonotherm prior to the installation of new windows or doors, meaning it is subjected to the weather until the new unit has been installed and made weathertight.

3D visual of a balcony door using Bosig Phonotherm as a thermal break
3D visual of a balcony door using Bosig Phonotherm as a thermal break
3D visual of a threshold using Bosig Phonotherm as a thermal break
3D visual of a threshold using Bosig Phonotherm as a thermal break

What is Bosig Phonotherm made from?

Bosig Phonotherm is a breathable, 100% upcycled polyurethane thermal insulation. It is formaldehyde free and utilises raw materials such as up-cycled polyurethane, whose raw material is sourced as a by-product from white goods, and from the motor and thermal insulation industries. 

From an ecological perspective, Bosig Phonotherm 200 utilises raw materials which would otherwise end up in landfill. Circularity of material resources have therefore been achieved instead of the using the wasteful environmental model of make, use, and dispose. The manufacturing process is carried out in accordance with ISO 14001 and the boards have a full Environmental Product Declaration (EPD).

Up-close product shot of Bosig Phonotherm

Working with Bosig Phonotherm

A new build which has Bosig Phonotherm around doors and windows

Bosig Phonotherm is very easy to work with and can be cut, drilled, screwed, nailed, or machined on site - just like timber. Carbide tipped power tools or well maintained (sharp) hand tools can be used for installation. Bosig can also be plastered or tiled directly, with a ‘hatched’ profile version available which is easy to plaster over and conceal at floor or window-to-wall junctions.

If Bosig Phonotherm is being used to support a heavy load such as a steel component or a large slider door, then it should have adequate support beneath the board. Although it is compressively very strong, it shouldn’t be cantilevered out from a structure without support from beneath and expected to support large weight. Care should be taken to ensure the product is understood and an Engineer’s advise should be sought where necessary.

Need help reducing thermal bridging on your project?

For more information on reducing thermal bridging in your eco-friendly or Passive House building project, our expert team are here to help. Contact us to find out more.

Blog author

Joe Fitzgerald

Joe Fitzgerald

Technical Specification Manager

Joe is a trained Energy Manager and graduated from TUS with an Degree in Sustainable Construction and an Honours Degree in Energy Management. Following this he went on to complete a Postgraduate Diploma in Green Engineering and joined Ecological shortly after. In addition to his academic studies, Joe has also spent many years as a tradesman and has completed the Passive House Contractor exam. Most recently, Joe qualified as an Advanced Thermal Modeller and Hygrothermal Risk Assessor from Dublin Technological University.

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