Minimizing thermal bridging

Thermal weak spots, as compared to the average heat transfer coefficient of an external construction element, are called thermal bridges. These spots must be analysed in terms of heat losses. The differential value is the coefficient of thermal bridging loss (Ψ) in W/mK.

Structure geometry involves a risk of thermal bridging at projections or corners, for example. However, if the insulation is drawn around the corner at almost the mean insulation thickness, this results in a "negative" thermal bridge.

This means a small plus when the heat losses through the external construction elements are calculated.

Interior corners always involve additional thermal bridging due to geometry.

To minimize the thermal bridging at windows/surrounds in all types of configuration, the insulation must be drawn over the window frame as far as possible.

Depending on the type of structure, the following additional considerations apply:

Timber stud/timber frame structures

In U value calculation, the first thing is to compare the timber and insulation shares of the structure; both enter into the U value of the area overall. In addition, thermal bridging at the dado, at the intersecting ceilings, at the roof transition and, if applicable, at intersecting inner walls must be taken into account.

Non-frame timber structures with external insulation

If the thermal insulation is on the outside, intersecting construction elements do not generally form thermal bridges. Dado and roof intersection usually enter the calculations of heat flow as negative thermal bridges, due to the geometry of outside corners. Penetrations of the insulation layer to the outside, such as spot thermal bridges for anchors, are disadvantageous.

External wall structures with composite thermal insulation systems (ETICS)

As far as thermal bridging is concerned, these are similar to non-frame timber structures: no problems with intersections of inner walls and ceilings; advantages at outside corners such as dado and roof connection, if the insulation is drawn around the corner at full thickness. In recent years systems to minimize thermal bridging have been developed for mounting construction elements on the outside of the wall; these should be included in the thermal bridging balance sheet.

External wall structures with curtain wall

Basically the same applies as for external walls with ETICS. But since there is a load-bearing scaffold for the facing on the outside, this has to be taken into account in the U value calculation. On the other hand attaching lightweight construction elements to the façade is not a problem, because they can be fixed to the load-bearing scaffold. Spot thermal bridges due to structurally relevant penetrations must be assessed individually.

Single-shell external wall structures

All construction elements with deviating coefficients of thermal conductivity must enter the balance sheet as thermal bridges if they intersect the external wall. This is the case for ceilings, due to the necessary load transfer, and possibly for walls because of soundproofing requirements. On the other hand, dado and roof connections can usually be implemented with negative heat transfer coefficients, which means they yield a small bonus in the energy balance sheet.

Double-shell external wall structures

Basically the same applies to these as to external wall structures with a curtain wall.