In a Passive home the heat losses of thermal bridges are significantly reduced, too. The reduction is made to a degree that the losses through thermal bridges become negligible. If the thermal insulation is not disturbed at any place of the envelope, the heat loss calculated with the U-values and the external surface areas of the building will be higher than the actual losses (including all thermal bridges).

An example: The thermal bridge at the joint of the interior masonry wall withthe slab-on-grade can be avoided almost completely if a porous concrete block (yellow) is used for the first row of bricks.

It is not difficult to use highly insulating porous concrete blocks for the first row at the building site. In this photo a difference in color of the blocks can be seen.

This is an infrared picture documenting that the thermal separation of the external wall from the concrete basement floor is working: There is no significant temperature drop (IR-photo taken by PHI).

This is how it looks if no care is taken about the thermal separation: a cold band (blue) all along the baseboard is visible (Photo: Klaus Michael, Director of the Institute for Low Energy Construction, Detmold, Chairman in workshops of the 10th conference on Passive Houses). This will often cause damages by high humidity. It can be eliminated by a design for avoiding thermal bridges - for new construction which is not cost prohibitive, see the three illustrations above this last IR-photo.

Heat will flow the easiest path from the heated space to the outside - the path with the least resistance. And this will not necessarily be the path perpendicular to the surfaces. Very often heat will “short circuit” through an element which has a much higher conductivity than surrounding material. In such cases the experts call this a "thermal bridge".

Typical effects of thermal bridges are:

• Decreased interior surface temperatures; in the worst cases this can result in high humidity in parts of the construction
• Significantly increased heat losses.

Both can be avoided in Passive Houses: The internal surface temperatures are high enough that a critical humidity can not occur at any place, and the additional heat losses will be negligible. If the thermal bridge coefficient (which is an indicator of the extra heat losses of a thermal bridge) is lower than 0.01 W/(mK), the detail is said to be “Thermal Bridge Free”.

If this criterion of avoiding thermal bridges is fullfilled throughout the thermal envelope, neither the designer nor the builder has to worry about cold and humid parts in the construction - and it will be far much simpler to calculate the heat energy balance.

At the exhibition during the 11th Conference on Passive Houses the manufacturers of components of the building envelope in most cases have a certificate from the Passive House Institute which documents that all important joints are Thermal Bridge Free.

This section illustrates a building envelope avoiding thermal bridges end-to-end. These houses have been built in the Passive Houses settlement "Lummerlund" in Hannover Kronsberg. The architects are Grenz and Rasch from the office "Factor 10". Literature: PEP-report #1, "Climate neutral Passive House estate in Hannover-Kronsberg: Construction and measurement results", to be downloaded for free, see www.proklima-hannover.de.

(updated: 2006-09-23 thanks to Dylan Lamar for proof reading the first edition 
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