Linear thermal bridges

Many thermal bridging effects in building structures may be treated as a linear thermal bridge. Examples are lintels (in any material), details around openings, junctions between plane elements (walls/roofs and walls/floors), or any similar detail that can be specified primarily in terms of its length. The extra heat loss due to the presence of the thermal bridge is accounted for by means of the linear thermal transmittance, which is the heat loss coefficient per unit length of the bridge. The value of ¥ for a specific construction detail (such as a lintel or a wall/floor junction) may be found by laboratory measurement, but is more often obtained by calculation. Because the heat flow is two dimensional, the calculation procedure normally requires a numerical modelling method. Consider a detail, such as a lintel of length L and height y, as shown in Fig. 15.2.

Fig. 15.2 Lintel detail.

The total fabric heat loss through this detail, including the bridging effect, may be written as:

where ^2Dis the two dimensional heat transfer coefficient, determined from numerical modelling of the whole detail.

This heat loss may be considered in two parts: the heat loss that would have occurred if the bridge was not present, and the extra heat loss due to the presence of the bridge. The first of these is found from the U-value of the basic structure and its area, Ly. The second is found from the linear thermal transmittance:

where U is the U-value that the detail would have in the absence of the thermal bridge

¥ is the linear thermal transmittance of the detail.

Table 15.1 Maximum ¥ values.

Type of junction detail in external wall

Maximum T,W/mK

Metal box lintel

O.3O

Other lintels

O.21

Sill

O.O4

Jamb

O.O5

Ground floor

O.16

Intermediate floor within a dwelling

O.O7

Intermediate floor between dwellings (apply half of the ¥ value to each dwelling)

O.14

Balcony within a dwelling (externally supported, not a continuation of floor slab, so that wall insulation is continuous and not bridged by balcony)

O.OO

Balcony between dwellings (externally supported, not a continuation of floor slab, so that wall insulation is continuous and not bridged by balcony; apply half of the ¥ value to each dwelling)

O.O4

Eaves (insulation at ceiling level)

O.O6

Eaves (insulation at rafter level)

O.O4

Gable (insulation at ceiling level)

O.24

Gable (insulation at rafter level)

O.O4

Corner (normal)

O.O9

Corner (inverted)

-O.O9

Party wall between dwellings (apply half of the ¥ value to each dwelling)

O.O6

Adapted with permission from BRE IP 17/01 [53]

Adapted with permission from BRE IP 17/01 [53]

Rearrangement of this equation provides the means for evaluating W:

Further details on linear thermal bridges are given in BS EN ISO 10211-2: 2001 [18] and in IP 17/01 [55]. Typical maximum values of W for several generic types of linear thermal bridge are given in Table 15.1.

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