U-values
4.1 U-values should be assessed using the methods and conventions set out in the Building Research Establishment’s BR 443. U-values should be assessed for the whole thermal element (e.g. in the case of a window, the combined performance of the glazing and the frame).
4.2 The U-value of a window should be assessed using one of the following methods.
a. Calculated using the actual size and configuration of the window.
b. For windows in buildings similar to dwellings, calculated for a standard window 1.23m (±25%) wide × 1.48m (–25%) high and the actual configuration of the window.
c. For windows in buildings similar to dwellings, calculated for a standard window 1.23m (±25%) wide × 1.48m (–25%) high and one of the following standard configurations. Standard configurations should not be used for commercial windows.
i. For a casement window, a central vertical divider with one opening light and one fixed light.
ii. For a vertical sliding sash window, a central horizontal divider with two opening lights.
iii. For a roof window, no divider.
d. Measured using the hot-box method as set out in BS EN ISO 12567-1 for windows and BS EN ISO 12567-2 for roof windows.
NOTE: For domestic-type window construction, to be used in buildings similar to dwellings (see Table 4.1), the default value from the Standard Assessment Procedure (Table 6e) may be used if there are no test data or calculated performance values.
4.3 The U-value of a door should be assessed using one of the following methods.
a. Calculated using the actual size and configuration of the door.
b. Calculated using one of the following standard sizes.
i. 1.23m (±25%) wide × 2.18m (±25%) high, for doors ≤ 3.6 m².
ii. 2.00m (±25%) wide × 2.18m (±25%) high, for doors > 3.6 m².
NOTE: When a single U-value is calculated for a product range of doors, the configuration of the door chosen for the calculation should be the worst performing in the product range.
c. Measured using the hot-box method as set out in BS EN ISO 12567-1.
4.4 To correctly assess whether an element meets the limiting U-value, the U-value must be calculated for the element in the appropriate plane – either horizontal or vertical. For windows and roof windows, U-values should be calculated based on a vertical position. For rooflights, U-values should be calculated based on a horizontal position. If the data available for the element is in the incorrect plane, it should be adjusted according to the guidance in the Building Research Establishment’s BR 443.
NOTE: These orientations should only be used when calculating U-values to check that they meet the limiting standards outlined in paragraphs 4.5 to 4.8 below. They should not be used in the energy calculations in Sections 1 and 2, where the U-value of each element is calculated based on the plane in which it is constructed or installed.
Limiting standards for new or replacement elements
4.5 New fabric elements should meet the limiting standards in Table 4.1. This includes all of the following.
a. Elements in new buildings.
b. New elements in extensions to existing buildings.
c. New or replacement elements in existing buildings.
Guidance on when a new or replacement element in an existing building must meet the requirements in this table is given in Section 10.
4.6 If windows or fully glazed pedestrian doors cannot meet the requirements of Table 4.1 in an existing building because of the need to maintain the character of the building, either of the following should be met.
a. These fittings should not exceed a centre pane U-value of 1.2W/(m2·K).
b. Single glazing should be supplemented with low-emissivity secondary glazing.
Table 4.1 Limiting U-values for new or replacement elements in new and existing buildings and air permeability in new buildings
Limiting standards for renovated and retained elements
4.7 Existing elements that are being renovated should meet the limiting standards in Table 4.2.
Guidance on when an existing element should meet the standards in Table 4.2 is given in Section 11.
Elements that should meet the standards include both of the following.
a. Thermal elements being renovated in existing buildings. Renovated elements should achieve the U-values in Table 4.2, column (b).
b. Elements being retained in existing buildings, for example following a material change of use or change to energy status (see Section 11). Retained elements with a U-value that is higher than the threshold value in Table 4.2, column (a), should be upgraded to achieve the U-values in Table 4.2, column (b).
4.8 If achieving the U-value in Table 4.2, column (b) either:
a. is not technically or functionally feasible or
b. would not achieve a simple payback of 15 years or less
then the element should be upgraded to the lowest U-value that both:
a. is technically and functionally feasible and
b. can achieve a simple payback not exceeding 15 years.
Generally, a thermal element once upgraded should not have a U-value greater than 0.7W/(m2·K). A lesser standard for the thermal element may be acceptable where work complies with Part C of the Building Regulations on protection from the harmful effects of interstitial and surface condensation.
Table 4.2 Limiting U-values for existing elements in existing buildings
Continuity of insulation
4.9 In new and existing buildings both of the following should apply.
a. The insulation should be reasonably continuous across newly built elements.
b. Thermal bridging, including at the party wall, should be reasonably limited.
NOTE: Any solution to edge sealing or thermal bridging in new buildings should take particular account of Part E of the Building Regulations.
4.10 To avoid air movement within thermal elements in new and existing buildings, either of the following measures should be implemented.
a. The insulation layer should abut the air barrier at all points across newly built elements.
b. The space between the air barrier and the insulation layer should be filled with solid material.
4.11 Thermal bridging should be addressed in the design and construction of a building by either of the following means.
a. Using construction joint details calculated by a person with suitable expertise and experience, which can then be used in the building primary energy rate and building emission rate calculations. Construction joint details should be calculated using both of the following.
i. The guidance set out in the Building Research Establishment’s BR 497.
ii. A process flow sequence that has been provided to the building control body indicating the way in which the detail should be constructed.
NOTE: Evidence of suitable expertise and experience for calculating linear thermal transmittance would be to demonstrate that the person has been trained in the software used to carry out the calculation, has applied that model to the example calculations in the Building Research Establishment’s BR 497 and has achieved results within the stated tolerances.
b. Using construction joints with no specific quantification of the thermal bridge values. In such cases, the generic linear thermal bridge values given in the Building Research Establishment’s Information Paper 1/06 and increased by 0.04W/(m·K) or 50%, whichever is greater, should be used in the building primary energy rate and building emission rate calculation.
4.12 To calculate linear thermal transmittances and temperature factors in support of the approaches in paragraph 4.11a, follow the guidance in the Building Research Establishment’s BR 497. Specified construction details should achieve a temperature factor that is no worse than the performance set out in the Building Research Establishment’s Information Paper 1/06.
4.13 To support the approaches in paragraph 4.11a, it should be demonstrated to the building control body that an appropriate system of site inspection is in place to give confidence that the construction procedures achieve the required standards.
4.14 When thermal elements are replaced or renovated, a report should be produced, signed by a suitably qualified person, which confirms all of the following.
a. Appropriate design details and building techniques have been specified.
b. The specified details, as constructed, provide adequate protection against surface condensation using the guidance in the Building Research Establishment’s Information Paper 1/06 and BR 497.
Airtightness in existing buildings
4.15 When carrying out work in existing buildings, care should be taken to reduce unwanted heat loss through air infiltration by doing all of the following.
a. When installing pipework or services, taping and sealing around openings and service penetrations.
b. When installing or renovating thermal elements, the element being installed should be draughtproofed and air-leakage gaps should be filled.
c. When installing controlled fittings, the controlled fitting should be well fitted and reasonably draught-proof.
NOTE: Particular attention should be paid to guidance in Approved Document F and Approved Document J when making an existing building more airtight.
Limiting the effects of solar gains in summer
4.16 In new residential buildings, as defined in Table 0.1 of Approved Document O, solar gains should be limited in summer in accordance with the guidance in Approved Document O.
4.17 The guidance in paragraph 4.18 applies to all other buildings not covered in paragraph 4.16, irrespective of whether they are air-conditioned. The intention is to limit solar gains during the summer, in order to either:
a. reduce the need for air-conditioning
b. reduce the capacity of any air-conditioning system that is installed.
4.18 For each space in the building that is occupied or mechanically cooled, the solar gains through the glazing – aggregated from April to September inclusive – should be no greater than would occur through the relevant reference glazing systems in Table 4.3 with a defined total solar energy transmittance (g-value) calculated according to BS EN 410. In this context, an occupied space means a space that is intended to be occupied by the same person for a substantial part of the day. This excludes circulation spaces and other areas of transient occupancy, such as toilets.
Table 4.3 Reference glazing systems for solar gain calculation
Limiting heat losses and gains from building services
Direct hot water and heating pipework
4.19 Hot water pipework should be insulated in all areas inside and outside the building unless it can be demonstrated that the heat is ‘always useful’.
4.20 Insulation should be designed so that the permissible heat losses in BS 5422 for hot water services in non-domestic buildings are not exceeded. Meeting the standards in Table 4.4 is one way of demonstrating that this has been achieved for low temperature systems.
4.21 Insulation thickness should be calculated in accordance with BS EN ISO 12241.
NOTE: in most cases, manufacturers will be able to supply information and thicknesses for their specific products. However, Tables 4.4 and 4.5 give indicative thicknesses for typical applications.
Table 4.4 Minimum thickness of pipework insulation for low temperature hot water space heating applications in non-domestic buildings
Table 4.5 Minimum thickness of pipework insulation for domestic hot water services in non-domestic buildings
Cooling pipework
4.22 Cooling pipework should be insulated along its whole length. Control should be maximised and heat gain to uninsulated pipes should only be permitted where the proportion of the cooling load relating to distribution pipework is less than 1% of the total load.
4.23 Insulation should be designed so that the maximum permissible heat gains in Table 10 of BS 5422 are not exceeded.
4.24 Provision should also be made for control of condensation by following the Thermal Insulation Manufacturers and Suppliers Association’s HVAC Guidance for Achieving Compliance with Part L of the Building Regulations.
Insulating ductwork
4.25 Ductwork that carries warm or cold air should be insulated throughout its whole length to achieve heat transfer no greater than that given in Table 4.6. Table 4.6 also gives indicative insulation thicknesses, which offers one way of demonstrating that the heat transfer value has not been exceeded.
Condensation should also be controlled by following the Thermal Insulation Manufacturers and Suppliers Association’s HVAC Guidance for Achieving Compliance with Part L of the Building Regulations.
Table 4.6 Maximum heat losses and gains for ducts delivering air for heating and/or cooling
Domestic hot water
4.26 Domestic hot water storage vessels should meet either of the following.
a. Maximum heat losses in Table 4.7.
b. Maintenance consumption values in BS EN 89.
Table 4.7 Maximum heat losses from domestic hot water storage vessels
