Annex 6.D Thermal bridges at the edges of openings
6.D.0 Introduction
6.D.1 Calculation method
6.D.2 Example

annex 6.D

Thermal bridges at the edges of openings
[Appendix D]

6.D.0 Introduction

This Annex gives a procedure for establishing whether:

1. There is an unacceptable risk of condensation at the edges of openings; and/or
2. The heat losses at the edges of openings are significant.

The procedure involves the assessment of the minimum thermal resistance between inside and outside surfaces at the edges of openings. Minimum thermal resistance paths should be identified, and their thermal resistance calculated, taking into account the effect of thin layers such as metal lintels.

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

Minimum thermal resistance path

6.D.1 Calculation method

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 1 illustrates this for a section through a window jamb.

Additional calculation for thin layers such as metal lintels

The minimum resistance path in this case is from the internal surface at A to the external surface at B. R_min is equal to the total length from inside to outside (AB) divided by the thermal conductivity of the material of the jamb. An example calculation is given on the following page.

For details containing thin layers of thickness not exceeding 4 mm (such as metal lintels), a second modified calculation of minimum thermal resistance (R_rnod) is made wherein the effective thermal conductivity of the thin layer is taken as the largest of 0.1 W/mK or the thermal conductivities of the materials immediately on either side of it. An example of this more complex calculation is given in BRE IP 12/94: ‘Assessing condensation risk and heat loss at thermal bridges around openings’.

Risk of surface condensation

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, e.g. curtain walling.

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

For the purposes of Section 6, 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

For non-domestic buildings, where the additional heat losses around the edges of openings cannot be ignored they can be taken into account in calculations. The calculation procedure could be used with the rate of heat loss from the proposed building increased by the following amount:

0.3 x total length of relevant opening surrounds (W)

Compensating measures, such as reducing the U-value of one of the building elements, should then be incorporated so that the rate of heat loss from the proposed building does not exceed that of the notional building.

6.D.2 Example

Diagram 2 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.

Note: Numbers denote materials in Table 1 on the next page.

Table 1: Thermal conductivity of materials in Diagram 2

Referring to paragraph "Risk of surface condensation", 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 2: Thermal resistance path in Diagram 2

Instead of returning the blockwork at the reveal the cavity could be closed using an insulated cavity closer, as in Diagram 3.

The revised calculation of the minimum resistance is shown in Table 3. R_min is now greater than 0.45 m²K/W and so the additional heat loss can be ignored.

Table 3: Minimum resistance path with insulated cavity closer