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Thermal-Bridges Building Analysis Software Simulation

Thermal bridges are locally limited areas of constructions with an increased heat flux density, which can result both from geometric (corners) and structural influences (presence of construction material with increased thermal conductivity). Due to the locally increased heat flow, the surface temperature will decrease on the side with the higher temperature (interior side of building element). This particularly results in two problem areas in connection with thermal bridges:
1. Increased transmission heat loss via the exterior building element.
2. Increase of the relative humidity due to decrease of the surface temperature.
In particular, the fact mentioned at last can cause another negative effect: formation of mould. Since mould only needs high relative humidity but no condensation water for the germination of spores,

special attention should be paid to the prevention of high relative humidity at surfaces of building elements. As a basic principle, thermal bridges can be separated into two groups:
1. Geometric thermal bridges.
2. Material thermal bridges.
In practice, overlap of both types can often be found, the „pure“ type is rather seldom. A typical factor for a geometric thermal bridge is an exterior corner. In the undisturbed wall, the surface which absorbs heat on the interior side has the same size as the exterior surface, which releases this heat again. Due to the geometry, the exterior surface of the corner is larger, resulting in a more intense cooling down of the interior surface, often particularly of the interior edge. The material thermal bridges in a building can be primarily found at surfaces and spots, where material with

increase load bearing capacity must be used due to requirements of support structure planning (e.g. arrangement of a reinforced steel pillar as bracing inside the brickwork) and/or wherever the individual load-bearing systems of a building interlock (e.g. support of ceilings on the brickwork).Geometric junctions and connections between elements typically provide a thermally conductive bypass route for heat loss and must be reduced or eliminated wherever possible. The picture top left shows some common areas which thermal bridges can occur. Careful construction detailing is required to ensure the junctions do not create unnecessary heat loss paths. The use of external insulation provides a major advantage in reducing thermal bridges at geometric junctions. Strategic placement of insulation in and

around junction details helps to reduce connection heat loss paths. To avoid unnecessary heat loss one optimal house should be thermally bridge free, in practice, this means that any linear ( two dimensional )thermal bridges should have a psi (Ψ) value of ≤ 0.01 W/mK. In one optimal House the heat loss areas and thermal bridges are calculated relative to the external boundary layer, and with good detailing it is possible to achieve negative psi values in some cases. A negative psi value implies that a junction is so well insulated that the two-dimensional heat flow through the junction is less than the respective one-dimensional heat flows.

We just said above that since mould only needs high relative humidity but no condensation water for the germination of spores, special attention should be paid to the prevention of high relative humidity at surfaces of building elements so the calculation of the surface temperature, only the location with the minimum temperature is of interest for the mentioned verification;it must be ensured under the mentioned boundary conditions that the so-called temperature factor ƒRsi does not fall below the value of 0.7, which for an outside temperature of -5 °C and an inside temperature of 20 °C leads to a surface temperature of at least 12.6 °C. The correlation is fRsi = (θsi-θe)/(θi-θe) where fRsi = Dimensionless temperature

factor; θsi = Room-side surface temperature; θi = Inside air temperature; θe = Outside air temperature. If the temperature factor is maintained, this requires that the humidity of 80 % which is critical for mould formation will not be reached on the surface.Due to the definition that all surfaces under verification are to be determined with the external dimension, taking into account EN ISO 13789, also the Ψ values shall be calculated with the external dimension, which may possibly lead to negative Ψ values (e.g. for corners of external walls).The use of bespoke construction details will necessitate accurate modeling of individual thermal bridges using specialist software. Two and three-dimensional thermal bridging calculations can be carried out by our technical engineering office to confirm the psi (Ψ) value of any junction and ƒRsi of the surface temperature, and expert advice given on ways to improve construction detailing. Once built thermal bridges may be identified through the use of infra-red thermography, however, at this stage, it is usually too late to do anything about them.

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