General

This Appendix deals with the assessment of discrete thermal bridging not taken account of in the calculation of the U-values of plane building elements, e.g. at junctions and around openings such as doors and windows. It gives guidance on:

avoidance of mould growth and surface condensation, and
limiting factors governing additional heat losses.

The guidance is based primarily on “BRE IP 1/06: Assessing the effect of thermal bridging at junctions and around openings”.

D.2 Mould growth and surface condensation

The key factor used in assessing the risk of mould growth or surface condensation in the vicinity of thermal bridges is the temperature factor (fRsi). The temperature factor (fRsi) is defined as follows: -

fRsi = (Tsi – Te) / (Ti – Te)

where:

Tsi = minimum internal surface temperature,
Te = external temperature, and
Ti = internal temperature.

For dwellings, the value of fRsi should be greater than or equal to 0.75, so as to avoid the risk of mould growth and surface condensation. For three-dimensional corners of ground floors this value may be reduced to 0.70, for all points within 10 mm of the point of lowest fRsi.

D.3 Linear thermal transmittance and additional heat loss

The linear thermal transmittance (ψ) describes the heat loss associated with a thermal bridge. This is a property of a thermal bridge and is therate of heat flow per degree Kelvin per unit length of bridge that is not accounted for in the U-values of the plane building elements containing the thermal bridge. The transmission heat loss coefficient associated with non-repeating thermal bridges is calculated as: -

HTB = Σ(Lx ψ) (W/K)

where L is the length of the thermal bridge over which ψ applies.

D.4 Calculation procedures

The calculation procedure to establish both temperature factor (fRsi) and the linear thermal transmittance (ψ) is outlined in BRE IP 1/06.Details should be assessed in accordance with the methods described in I.S. EN ISO 10211:2017. These calculations of two dimensional or three dimensional heat flow require the use of numerical modelling software. To be acceptable, numerical modelling software should model the validation examples in I.S. EN ISO 10211:2017 with results that agree with the stated values of temperature and heat flow within the tolerance indicated in the standard for these examples. Several packages are available that meet this requirement.

Detailed guidance on decisions regarding specific input to the modeling software and the determination of certain quantities from the output of the software is contained in BRE Report BR 497 Conventions for calculating linear thermal transmittance and temperature factors. This guidance should be followed in carrying out modeling work so that different users of the same software package and users of different software packages can obtain correct and consistent results.

D.5 Treatment of thermal bridging in DEAP calculation

Heat loss through thermal bridging is taken account of in the DEAP calculation. Two alternative methods of accounting for heat loss are possible:

(a) Heat loss through thermal bridging can be accounted for in terms of a fraction (y) multiplied by the exposed surface area of the building. Where Acceptable Construction Details for sections 1 to 6 in the 2011 edition of the document “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” are used for all key junctions the value of (y) can be taken as 0.08. Where this is not the case, but this method of accounting for thermal bridging is used, the default value of (y) is taken to be 0.15.

(b) Values of ψ can be determined from the results of numerical modeling, or they can be derived from measurement. The linear transmission heat loss coefficient (HTB) can then be calculated directly and included in the DEAP calculation. Values of ψ are given in Tables D1 to D6 of this Appendix for Acceptable Construction Details and for Appendix 2 of the 2011 edition “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” to allow this approach to be used for these details. Where Section 3 - internal insulation details are used the psi values given in Table D6 may be used.

Table HL15A - Section 1 - Cavity wall insulation - Extract from TGD L

Table HL15B - Section 1 - Cavity wall insulation (contd...) - Extract from TGD L

ψ values for a Target U-value for the wall of 0.18 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.13 to 0.16 W/m2K;
Flat Roof = 0.16 to 0.20 W/m2K;
Ground Floor = 0.16 to 0.21 W/m2K.

ψ values for a Target U-value for the wall of 0.15 W/m2K can be used for a range of U-values from of 0.12 W/m2K to 0.17 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling 0.11 to 0.16W/m2K;
Flat Roof = 0.11 to 0.17 W/m2K;
Ground Floor = 0.12 to 0.18 W/m2K.

Where two building elements have one U-value above its target while the other is below its target U-value, the aggregate percentage change from the respective target U-values in the table should not exceed +20 % for the Psi (ψ) value to be valid, i.e. if for the 0.15 U-value wall, if the U-value was increased by 10 % above the wall target U-value (from 0.15 to 0.165), then the roof U-value could be at most 10 % below the roof target U-value (from 0.14 to 0.126), because the aggregate change would then be 20 %.
This is an externally supported balcony (the balcony slab is not a continuation of the floorslab) where the wall insulation is continuous and not bridged by the balcony slab.
Value of Ψ is applied to each dwelling.
Psi value is for whole junction. Half the value should be applied to each dwelling on either side of the junction

Table HL16 - Section 2 - External insulation - Extract from TGD L

ψ values for a Target U-value for the wall of 0.18 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.13 to 0.16 W/m2K;
Flat Roof = 0.16 to 0.2 W/m2K;
Ground Floor = 0.16 to 0.21 W/m2K.

ψ values for a Target U-value for the wall of 0.15 W/m2K can be used for a range of U-values 72 from of 0.12 W/m2K to 0.17 W/m2K for theconstruction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling 0.11 to 0.16 W/m2K;
Flat Roof = 0.11 to 0.17 W/m2K;
Ground Floor = 0.12 to 0.18.

Where two building elements have one U-value above its target while the other is below its target U-value, the aggregate percentage change from the respective target U-values in the table should not exceed +20 % for the Psi (ψ) value to be valid, i.e. if for the 0.15 U-value wall, if the U-value was increased by 10 % above the wall target U-value (from 0.15 to 0.165), then the roof U-value could be at most 10 % below the roof target U-value (from 0.14 to 0.126), because the aggregate change would then be 20 %.
This is an externally supported balcony (the balcony slab is not a continuation of the floorslab) where the wall insulation is continuous and not bridged by the balcony slab.
Value of Ψ is applied to each dwelling.
Psi value is for whole junction. Half the value should be applied to each dwelling on either side of the junction.

Table HL17 - Section 3 - Internal insulation - Extract from TGD L

Table HL18A - Section 4 - Timber frame construction - Extract from TGD L

ψ values for a Target U-value for the wall of 0.18 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.13 to 0.16 W/m2K;
- Flat Roof = 0.16 to 0.2 W/m2K;
- Ground Floor = 0.16 to 0.21 W/m2K.

ψ values for a Target U-value for the wall of 0.15 W/m2K can be used for a range of U-values 75 from of 0.12 W/m2K to 0.17 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling 0.11 to 0.16W/m2K;
Flat Roof = 0.11 to 0.17 W/m2K;
Ground Floor = 0.12 to 0.18.

Where two building elements have one U-value above its target while the other is below its target U-value, the aggregate percentage change from the respective target U-values in the table should not exceed +20 % for the Psi (ψ) value to be valid, i.e. if for the 0.15 U-value wall, if the U-value was increased by 10 % above the wall target U-value (from 0.15 to 0.165), then the roof U-value could be at most 10 % below the roof target U-value (from 0.14 to 0.126), because the aggregate change would then be 20 %.
This is an externally supported balcony (the balcony slab is not a continuation of the floorslab) where the wall insulation is continuous and not bridged by the balcony slab.
Value of Ψ is applied to each dwelling.
Psi value is for whole junction. Half the value should be applied to each dwelling on either side of the junction.

Table HL19A - Section 5 - Steel frame construction - Extract from TGD L

ψ values for a Target U-value for the wall of 0.18 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.13 to 0.16 W/m2K;
Flat Roof = 0.16 to 0.2 W/m2K;
Ground Floor = 0.16 to 0.21 W/m2K.

ψ values for a Target U-value for the wall of 0.15 W/m2K can be used for a range of U-values from of 0.12 W/m2K to 0.17 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling 0.11 to 0.16 W/m2K;
Flat Roof = 0.11 to 0.17 W/m2K; 77
Ground Floor = 0.12 to 0.18.

Where two building elements have one U-value above its target while the other is below its target U-value, the aggregate percentage change from the respective target U-values in the table should not exceed +20 % for the Psi (ψ) value to be valid, i.e. if for the 0.15 U-value wall, if the U-value was increased by 10 % above the wall target U-value (from 0.15 to 0.165), then the roof U-value could be at most 10 % below the
roof target U-value (from 0.14 to 0.126), because the aggregate change would then be 20 %.
This is an externally supported balcony (the balcony slab is not a continuation of the floorslab) where the wall insulation is continuous and not bridged by the balcony slab.
Value of Ψ is applied to each dwelling.
Psi value is for whole junction. Half the value should be applied to each dwelling on either side of the junction.

Table HL20A - Section 6 - Hollow block construction - Extract from TGD L

ψ values for a Target U-value for the wall of 0.18 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.13 to 0.16 W/m2K;
Flat Roof = 0.16 to 0.2 W/m2K;
Ground Floor = 0.16 to 0.21 W/m2K.

ψ values for a Target U-value for the wall of 0.15 W/m2K can be used for a range of U-values from of 0.12 W/m2K to 0.16 W/m2K for the construction type specified. The U-values of the flanking elements to the wall can vary from the flanking element target U-value as follows:

Pitched roof insulation on slope, insulation on ceiling = 0.11 to 0.16 W/m2K;
Flat Roof = 0.11 to 0.17 W/m2K;
Ground Floor = 0.12 to 0.18.

Where two building elements have one U-value above its target while the other is below its target U-value, the aggregate percentage change from the respective target U-values in the table should not exceed +20 % for the Psi (ψ) value to be valid, i.e. if for the 0.15 U-value wall, if the U-value was increased by 10 % above the wall target U-value (from 0.15 to 0.165), then the roof U-value could be at most 10 % below the roof target U-value (from 0.14 to 0.126), because the aggregate change would then be 20 %.
This is an externally supported balcony (the balcony slab is not a continuation of the floorslab) where the wall insulation is continuous and not bridged by the balcony slab.
Value of Ψ is applied to each dwelling.
Psi value is for whole junction. Half the value should be applied to each dwelling on either side of the junction.

Table HL21 - Example calculation of (y) value for DEAP for a 3 bed semi-detached house using the actual lengths of the internal ACD key junctions and their psi-values from Appendix D - Extract from TGD L

Appendix D - Thermal bridging