Thermal bridges

Heat makes its way from the heated space towards the outside. In doing so, it follows the path of least resistance.
A thermal bridge is a localised area of the building envelope where the heat flow is different (usually increased) in comparison with adjacent areas (if there is a difference in temperature between the inside and the outside).

The effects of thermal bridges are:

• Altered, usually decreased, interior surface temperatures; in the worst case this can lead to moisture penetration in building components and mould growth.
• Altered, usually increased, heat losses.
  

Both effects of thermal bridges can be avoided in Passive Houses: the interior surface temperatures are then so high everywhere that critical levels of moisture cannot occur any longer – and the additional heat losses become insignificant. If the thermal bridge losses are smaller than a limit value (set at 0.01 W/(mK)), the detail meets the criteria for “thermal bridge free design”.

If the criteria for thermal bridge free design are adhered to everywhere, the planners and construction manager don't have to worry about cold and damp spots any more – and less effort will have to be made for calculating the heat energy balance.

Thermal bridge free design leads to substantially improved details; the durability of the construction is increased and heating energy is saved.

This illustration shows a completely thermal bridge free building envelope, as implemented in the “Lummerland” Passive House estate in Hannover Kronsberg by the architects Grenz and Rasch of Büro Faktor 10. Literature: CEPHEUS report No. 18, “Climate-neutral Passive House Development in Hannover Kronsberg”, which can be obtained free of charge from http://www.passiv.de/ (Literature → Brief reports and technical literature about the Passive House → Final Reports: Climate-neutral Passive House Development in Hannover-Kronsberg)

A building envelope is considered to be thermal bridge free if the transmission losses under consideration of all thermal bridges are not greater than the result calculated using the external surfaces and regular U-values of the standard building elements alone. (It's a good idea to include the 'regularly ocurring structures' {which might also create regularly occuring thermal bridges - like periodic studs} in standard building elements within the regular U–values [AkkP 16] ). This is summarised using formulas as follows.

The overall temperature-specific heat loss is characterised by the transmission conductance H_T. It comprises the regular losses of all areas A with their regular heat transfer coefficient U

U⋅A

This regular loss does not include the thermal bridge contributions (Ψ ⋅ l) and neither the punctiform contributions χ (Ψ is the linear, χ the punctiform thermal bridge loss coefficient). As the punctiform contributions are generally insignificant, they will not be discussed in detail here.

“Thermal bridge free design” can be defined as follows: the contributions provided by the thermal bridge contributions are smaller than or equal to zero:

∑ Ψ ⋅ l + ∑ Χ ≤ 0
[defined as thermal bridge free]

It will then be admissible to omit the thermal bridge effects, thus simplifying the calculation quite considerably. This is equivalent to the following statement

Δ U_WB ≤ 0

where Δ U_WB is the thermal bridge correction addend (as used in the German energy saving regulation for example)

In the Passive House the heat losses due to thermal bridges are also reduced, so much that the thermal bridge losses do not have to be taken into account.

Example of a thermal bridge free connection of the external wall rising at the insulated floor slab (with a representation of the reference dimensions; calculations based on external dimensions). For this detail the thermal bridge coefficient has been calculated as a function of the base point block used (according to [AkkP 16] ).

Reviewal using this definition of thermal bridge free design would imply that all details would have to be calculated in a multi-dimensional way. Therefore, simplified criteria for “thermal bridge free design” should be devised. It was found that for ordinary building geometries, the “thermal bridge free” requirement was almost always sufficiently fulfilled for all linear disturbances only if

Ψ ≤ 0.01 W/(mK) [TbCrit].

Thermal bridges which comply with [TbCrit] can still lead to positive contributions to a certain extent, which would be considered as “negligibly small” even within the context of Passive Houses.

Besides, the remaining contributions are compensated to a certain extent by other connections where there are negative thermal bridge loss coefficients. The [TbCrit] requirement is enough for all structures which affect connections, edges, and individual interruptions in consistent areas. Recurring interruptions in consistent areas must already be taken into account when giving the regular heat transfer coefficient U_reg (for example recurring shafts in a wood-stud or panel construction; the connection thermal bridge during the installation of a window is also appropriately included in the regular window U-value, this has already been applied in the PHPP and makes for less work).

With the simplified criterion, planning and construction become significantly easier: for a particular category of connection details, it only has to be verified once in advance that the [TbCrit] criterion has been met. This can be done, for example, by calculating all the relevant details for building envelopes. Many system manufacturers have already followed this approach and have ensured that their products comply with this criterion. If the designer uses these details, he can simply omit the thermal bridge unit (therm) during the planning of the Passive House and thus save himself a lot of work during calculations.

At the web page of the Passive House Institute there are many examples of construction systems for which all connection details that are normally required have been certified as “thermal bridge free”.

Correlation between the thermal bridge coefficient (Ψ-value) and the thermal conductivity λ of the base point block. If the block's thermal conductivity is smaller than λ < 0.25 W/(mK), then Ψ ≤ 0.01 W/(mK) - the detail is considered thermal bridge free. The criterion is marked with a horizontal blue line. With “normal” blocks (λ >0.8 W/(mK)) considerable thermal bridge losses can result (from [AkkP 16] ).

This example shows that “thermal bridge free design” can often be achieved with a few simple changes in the details and without incurring high costs. However, this must be recognised early and taken into account during the planning. Making subsequent changes in the completed building, although technically possible, is unjustifiably expensive. This is why significant thermal bridges still remain in this area even after modernisation of old buildings using Passive House components.

The following principle demonstrates this clearly: the insulation layers should be planned in such a way that one should be able to outline the minimum insulation thickness (20 cm in the Passive House) of the whole external envelope within the insulation layers using a pencil, without a break. The following figure illustrates this principle using a sectional drawing. The critical points can be easily identified in this way, e.g. the base point of the brick wall on the basement ceiling.

Sectional drawing: plan insulation layers so that the minimum insulation thickness (20 cm in the Passive House) of the whole external envelope within the insulation layers can be outlined using a pencil, without a break.

The purpose of “thermal bridge free design” is to substantially improve the details. A slightly more expensive substantial improvement of the connection details is preferable to recalculation in detail of less appropriate connections, which can be just as costly.

There have been positive experiences with numerous construction systems in which the principle of “thermal bridge free design” has already been applied. There are complete catalogues of thermal bridge free details now available for

• Massive constructions with solid bricks,
• Massive constructions with low-conductivity blocks (e.g. porous concrete blocks),
• Timber constructions (solid wood beams as well as lightweight beams),
• Constructions using formwork elements,
• Constructions using prefabricated lightweight concrete elements.

Details for massive constructions, timber constructions and formwork constructions can be found in the Protocol Volume [AkkP 16] . Timber construction details can be found in the Timber Construction Handbook [Kaufmann 2002] .

The Passive House Institute provides advice for manufacturers regarding the development of thermal bridge free constructions.

In a new building, placing the porous concrete blocks in the lowest row is quite easy. The picture shows a slight difference in the colour.

In the Protocol Volume 16 “Thermal bridge free design” that has often been mentioned, numerous other details for building envelopes with thermal bridge free connections have been presented apart from the one shown here.

If a new building is not built in accordance with the thermal bridge free principle, remaining thermal bridges can cause considerable heat losses. There was an increase in the annual heating demand of up to 14 kWh/(m²a) in various examples of building projects. For a construction project, careful planning with regard to thermal bridges can therefore be decisive for achieving the Passive House standard at all.

The thermographic image of the base point of the external wall in a Passive House from the inside shows that there are no cold spots any more. The separation using the porous concrete block has proved to be successful. (Image: PHI, in the Passive House in Darmstadt Kranichstein).

This is what can be seen if there is no thermal separation: a cold strip (blue) appears alongside the skirting board. Moisture dama- ge often occurs due to this. By designing in a thermal bridge free way, this can be prevented – with practically “zero” extra costs in new buildings. (Photograph: Klaus Michael, Director of the Low-energy Institute in Detmold and Head of the Working Group at the Passive House Conference on a regular basis).

Minimising thermal bridges in existing buildings

Improving thermal bridges and airtightness in existing buildings

[AkkP 16] Wärmebrückenfreies Konstruieren ; Protokollband Nr. 16 des Arbeitskreises kostengünstige Passivhäuser, 1. Auflage, Passivhaus Institut, Darmstadt 1999 (Thermal bridge free design; Protocol Volume No. 16 of the Working Group for Cost-effective Passive Houses, 1st Edition, Passive House Institute, Darmstadt 1999 (see PHI's list of publications).

[Kaufmann 2002] The Passive House – Energy-efficient Construction, Timber Construction Handbook Series 1: Design and Construction Section 3: Residential and administrative buildings Part 10: Passive House – Energy-efficient Construction, DGfH Innovation and Service GmbH, 2002 PDF