Building Regulations: Technical Standards

Appendix B: Thermal bridging at the edges of openings

Summary

B1 This Appendix gives a procedure for establishing whether:

a. there is an unacceptable risk of condensation at the edges of openings; and/or

b. the heat losses at the edges of openings are significant.

B2 The procedure involves the assessment of the minimum thermal resistance between inside and outside surfaces at the edges of openings. This requires identification of minimum thermal resistance paths, and calculation of their thermal resistance, taking into account the effect of thin layers such as metal lintels.

B3 These minimum thermal resistances are then compared with satisfactory performance criteria to see whether corrective action is indicated.

Minimum thermal resistance path

B4 The minimum thermal resistance path through a thermal bridge is that path from internal surface to external surface which has the smallest thermal resistance, R_min. Diagram B1 illustrates this for a section through a window jamb.

The minimum resistance path in this case is from the internal surface at A to the external surface at B.

Diagram B1 Minimum thermal resistance path

R_min is equal to the total length from inside to outside (AB) divided by the thermal conductivity of the material of the jamb.

Additional calculation for thin layers such as metal lintels

B5 For details containing thin layers of thickness not exceeding 4 mm (such as metal lintels), a second modified calculation of minimum thermal resistance (R_mod) is made wherein the effective thermal conductivity of the thin layer is taken as the largest of 0.1 W/mK and the thermal conductivities of the materials immediately on either side of it. An example of this more complex calculation is given in BRE Information Paper IP 12/94.

Risk of surface condensation

B6 The risk of surface condensation and mould growth at the edges of openings can be assumed to be negligible if:

a. for edges containing thin layers of thickness not exceeding 4 mm:

R_min (rounded to two decimal places) is at least 0.10 m²K/W; and
R_mod (rounded to two decimal places) is at least 0.45 m²K/W; or

b. for other edge designs:

R_min (rounded to two decimal places) is at least 0.20 m²K/W.

Note: These criteria do not apply to cases where internal surface projections are used to avoid surface condensation, eg curtain walling.

B7 In the event of an unacceptable risk being identified, marginal cases could be more rigorously analysed using numerical calculation methods, but in any case modification to improve the design should be considered.

Additional heat loss

B8 For the purposes of regulations for the conservation of fuel and power, the additional heat losses at the edges of openings may be ignored if:

a. for edges containing thin layers of thickness not exceeding 4 mm,

R_mod (rounded to two decimal places) is at least 0.45 m²K/W; or

b. for other edge designs:

R_min (rounded to two decimal places) is at least 0.45 m²K/W.

Compensating for additional heat loss

B9 Where the additional heat losses around the edges of openings cannot be ignored they can be taken into account in calculations as follows:

a. for dwellings the Target U-value method could be used with the average U-value increased by the following amount,

b. for other buildings the calculation procedure could be used with the rate of heat loss from the proposed building increased by the following amount:

c. compensating measures, such as reducing the U-value of one of the elements of the construction, should then be incorporated so that:

i. for dwellings, the average U-value does not exceed the Target U-value; or

ii. for other buildings, the rate of heat loss from the proposed building does not exceed that of the notional building.

Example

Diagram B2 shows a window jamb in a masonry cavity wall with the blockwork returned towards the outer leaf at the reveal. By inspection it can be seen that ABCD is the minimum resistance path.

Diagram B2 70 mm cavity wall showing window jamb with blockwork returned at the reveal

Table B1 to Part J Appendix B: Thermal conductivity of materials in Diagram B2

Calculation of R_min

Using the thermal conductivities from Table B1, Table B2 gives the resistance R for each segment of the path ABC. R for each segment is obtained by dividing the length of the path segment in metres by its thermal conductivity in W/mK. R_min is the sum of the resistances of each path segment.

Avoidance of the risk of surface condensation and mould growth

Referring to paragraph B6, R_min in this example is greater than 0.20 m²K/W and so the risk of surface
condensation and mould growth is acceptably low.

Table B2 to Part J Appendix B: Thermal resistance path in Diagram B2

Additional heat loss at the edge detail

Referring to paragraph B8, R_min in this example is less than 0.45 m² K/W, and so the additional heat loss at this edge should not be ignored.

Improving the edge design

Instead of returning the blockwork at the reveal the cavity could be closed using an insulated cavity closer, as in diagram B3.

Diagram B3 Window jamb showing cavity closed with an insulated cavity closer

The revised calculation of the minimum resistance is shown in Table B3. R_min is now greater than 0.45 m²K/W and so the additional heat loss can be ignored.

Table B3 to Part J Appendix B: Minimum resistance path with insulated cavity closer

Alternative method

B10 A heat loss factor for a particular detail could be obtained by a numerical method and used to modify the calculation of the average U-value or the total rate of heat loss. A calculation procedure for deriving such loss factors is given in BRE Information Paper IP 12/94.