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 fabric 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. Calculated for a standard window 1.23m (±25%) wide × 1.48m (–25%) high and the actual configuration of the window.

c. Calculated for a standard window 1.23m (±25%) wide × 1.48m (–25%) high and one of the following standard configurations.

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.

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 ≤ 3.6m².

ii. 2.00m (±25%) wide × 2.18m (±25%) high, for doors > 3.6m².

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 Alternatively, for doors or windows, the default value from the Standard Assessment Procedure Table 6e can be used.

4.5 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: This does not apply to Standard Assessment Procedure calculations, where the U-value of each element is calculated based on the plane in which it is constructed or installed.

Limiting standards in new dwellings

4.6 Insulating fabric elements in new dwellings should meet the limiting standards in Table 4.1.

general G6
Table 4.1 Limiting U-values for new fabric elements and air permeability in new dwellings

Limiting standards in existing dwellings

New and replacement elements

4.7 New fabric elements in existing dwellings should meet the limiting standards in Table 4.2.

4.8 The U-value of a replacement fabric element in an existing dwelling should both:

a. be no worse than that of the element being replaced

b. meet the limiting standards in Table 4.2.

4.9 Guidance on when a new element must meet the standards in Table 4.2 is given in Section 10.

Elements that should meet the standards include both of the following.

a. Elements in extensions to existing dwellings.

b. New or replacement elements in existing dwellings.

4.10 If windows or fully glazed external pedestrian doors cannot meet the requirements of Table 4.2 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.

general G6
Table 4.2 Limiting U-values for new fabric elements in existing dwellings

Renovated and retained elements

4.11 The U-value of an existing thermal element that is being renovated should both:

a. be no worse than that of the element before it was renovated

b. meet the limiting standards in Table 4.3.

4.12 Guidance on when an existing element should meet the standards in Table 4.3 is given in Section 11.

Elements that should meet the standards include both of the following.

a. Thermal elements being renovated in existing dwellings. Renovated elements should achieve the U-values in Table 4.3, column (b).

b. Elements being retained in existing dwellings, for example through a loft or garage conversion.

Retained elements with a U-value that is higher than the threshold value in Table 4.3, column (a) should be upgraded to achieve the U-values in Table 4.3, column (b).

4.13 If achieving the U-value in Table 4.3, 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.

NOTE: Examples are given in Appendix C.

NOTE: When renovating thermal elements, the work should comply with all the requirements in Schedule 1, but particular attention should be paid to Parts B, C, F and J.

general G6
Table 4.3 Limiting U-values for existing elements in existing dwellings

Continuity of insulation

4.14 Gaps in insulation can have a significant impact on heat loss and thermal bypass and create a risk of condensation and mould. The building fabric should be constructed so that the insulation is reasonably continuous across newly built elements.

4.15 To ensure continuity of insulation in new dwellings, all of the following apply.

a. Drawings should identify the insulation layer. The designer and installer should review drawings to ensure the insulation layer is continuous, buildable and robust.

b. Before elements are concealed by subsequent work, an on-site audit should be undertaken to confirm that the designed details have been constructed. Photographs of the details should be taken in line with the guidance in Appendix B.

c. Floors and foundations: insulation should be installed tight to the structure, without air gaps between insulation panels and at edges.

i. Perimeter insulation should be continuous and have a minimum thickness of 25mm.

ii. Moisture-resistant insulation should be fitted below damp-proof course level and extend to the foundation block/structure.

d. Windows and doors: should be installed in such a way that the thermal integrity of the insulated plane is maintained.

i. Tolerance around a window or door unit and the surrounding opening should be minimal and be in accordance with BS 8213-4.

ii. Position: window or door units should be located with an overlap between the inner face of the unit and the inner face of the external leaf – for windows an overlap between 30mm and 50mm, and for doors 50mm – so that the window or door unit is contiguous with the insulation layer of the external wall.

iii. Fully insulated and continuous cavity closers should be used, installed tight to the insulation and cavity apertures.

For door units, install perimeter insulation within the threshold zone or use a reinforced cavity closure.

e. Walls: insulation should be fitted without any air gaps and tight to the structure, cavity closers, lintels and cavity trays. Mortar snots should be removed to ensure a tight fit with the structure and cavities cleared of all debris. Where fire-stopping socks are required, these should fully fill the areas where they are fitted, including at the heads of cavities.

f. Roofs: insulation should be installed tight to the structure, without air gaps, and should extend to the wall insulation. For roofs insulated at ceiling level, the long-term protection of the insulation layer should be considered: boarded areas should be provided above the insulation to give access for maintenance.

g. Rigid insulation boards: should only be used on flat surfaces. Boards should be fitted to the structure to avoid any gaps between board edges and between the board facings. The use of boards with lapped or tongue and groove edges should be considered. Any unavoidable gaps between boards should be infilled using compressible tape (e.g. for boards within roof rafters) or low expansion foam (e.g. for boards within wall cavities).

h. Penetrating elements include steel beams, incoming services, meter boxes and sub-floor vents.

Designs should clearly indicate means to limit disruption to the insulation. For recessed meter boxes on the cold side of the construction, insulation should be installed behind the enclosure. For incoming services, insulation should fit tightly around ducts, pipes, etc.

Thermal bridging

4.16 Thermal bridges occur when an area of a building has significantly higher heat transfer than the surrounding parts. Breaks in insulation, reduced insulation or more conductive materials can contribute to thermal bridge effects. The building fabric should be constructed so that thermal bridging, including at the party wall, is reasonably limited.

Thermal bridging in new dwellings

4.17 To limit thermal bridging in new dwellings, all of the following apply.

a. Drawings should be provided for junctions. The designer and installer should review drawings to check that junctions are buildable and to ensure construction sequencing is carefully considered for each detail. Complex details should be avoided wherever possible.

b. Before elements are concealed by subsequent work, an on-site audit should be undertaken to confirm that the designed details have been constructed. Photographs of the details should be taken in line with Appendix B.

c. Product specification: opportunities should be considered to use products that help to reduce thermal bridges. Options include both of the following.

i. Masonry construction: lightweight blockwork in the inner leaf of a cavity wall or both leaves of a party wall can help to reduce thermal transmittance, particularly at junctions, such as the ground floor to wall junction.

ii. Timber construction: the use of insulated plasterboard on the inside of the frame can help to reduce bridging at various junctions.

d. Product substitution: the products used should be those shown in the original design. If a product is substituted, the revised specification should be reflected in the SAP calculation and report in the Building Regulations England Part L compliance report (BREL report).

e. Foundations: wherever possible, blocks below the damp-proof course should be the same as those specified in the design for the above-ground main wall element (in masonry construction).

f. Ground floors and external walls: the wall-to-floor junctions should be detailed to achieve continuity of insulation.

i. Perimeter floor insulation should abut or extend the full depth of the main floor insulation.

ii. Masonry construction: external or cavity wall insulation should extend below the dampproof course (where applicable) and be at least the equivalent of one full block height (215mm) below the underside of the floor structure/slab and beyond the depth of the floor insulation.

iii. Timber construction: insulation between boards/within sheathing should extend to the floor plate. The wall insulation and the floor perimeter insulation should abut.

g. Intermediate floors: floor-to-wall junctions should be detailed to ensure that insulation in the external wall is continuous. For a timber frame where the intermediate floor structure breaches the external wall insulation, further insulation – of the same thickness as the insulation used in the external wall – should be included within the depth of the intermediate floor structure.

h. Windows: designs should minimise thermal bridging.

i. Lintels: consider using independent lintels with an insulated cavity closure between the inner and outer lintel. For common leaf lintels, the base plate should not be continuous and the lintel core should be insulated.

ii. Insulated cavity closers should be used for all construction types. Additionally, insulated plasterboard should be used in reveals to abut jambs and should be considered within reveal soffits.

i. Roofs: continue the insulation across the wall-to-eaves and wall-to-gable junctions.

i. Wall insulation should be installed to the top of the wall plate; in some places, such as the eaves, this may be above the cavity closure or barrier. In all cases, roof insulation should be continuous with wall insulation.

ii. Roofs insulated at ceiling level: loft insulation at the eaves should extend beyond the wall insulation without any reduction in thickness due to the pitch of the roof. The roof insulation should be installed when the eaves are still accessible. At gables and party walls, insulation should extend to the wall; if the space between the wall and joist is less than 100mm, perimeter insulation may be required.

iii. Roofs insulated at rafter level: at the eaves, insulation should extend to the top of the external wall. Voids between insulation at the top of the external wall and the cavity wall/timber frame insulation should be fully filled with insulation.

NOTE: Any solution to edge sealing or thermal bridging in new dwellings should take account of Part E of the Building Regulations.

4.18 Thermal bridges should be assessed in a new dwelling using one of the following methods.

a. Use construction joint details calculated by a suitably competent person following the guidance in the Building Research Establishment’s BR 497 and the temperature factors set out in the Building Research Establishment’s Information Paper 1/06.

b. Use junction details from a reputable non-government database containing independently assessed thermal junction details, such as Local Authority Building Control’s Construction Details library.

c. Use the values in the Standard Assessment Procedure, Table K1. A mixture of known and default values may be used.

d. Use a default y-value of 0.20W/(m2.K).

NOTE: A mix of approaches may be used for different elements on the same dwelling. When using the approach in (a) or (b) above, an appropriate system of site inspection should be in place.

Thermal bridging in existing dwellings

4.19 When carrying out work in existing dwellings, care should be taken to reduce unwanted heat loss through thermal bridging. Thermal bridges can be limited in an existing dwelling by following the junction details from a reputable non-government database containing independently assessed thermal junction details, such as Local Authority Building Control’s Construction Details library.

Follow the guidance in paragraph 4.17 where appropriate.

Airtightness

Airtightness in new dwellings

4.20 The minimum standard for air permeability of a new dwelling is given in Table 4.1. When carrying out work in new dwellings, care should be taken to reduce unwanted heat loss through air infiltration.

4.21 To ensure airtightness in new dwellings, all of the following apply.

a. Drawings: all relevant drawings should be provided to clearly identify the position, continuity and extent of the air barrier. Drawings should be reviewed by the designer and installer and should include specifications for key materials.

b. Incoming services: ducts, and cables wherever possible, should be grouped to minimise how often the air barrier is penetrated, while ensuring sufficient space to allow adequate screed flow between ducts. (Use temporary supports for services during floor works.) Grommets or flexible collars should be used around incoming services and sealed to the air barrier with air-sealing tape or sealant.

c. Internal building services: where services penetrate the air barrier, holes should be as small as possible and should be core drilled to limit damage. The penetrating services should be sealed to the air barrier using proprietary grommets or collars with air-sealing tape or sealant. Where membranes are penetrated, careful detailing should be used to achieve a robust and durable seal at these penetrations.

d. Structural penetrations need to be effectively sealed for airtightness. Timber joist hangers should be considered as an alternative to penetrating through the inner leaf.

e. Cavity walls: the inner block leaf mortar joint should be fully filled and pointed within the cavity.

Where dense aggregate blocks have been used, plaster, parge coat or liquid membranes should be applied internally to reduce air permeability. Internal plasterboard linings are not appropriate for use as an air barrier solution.

f. Timber frame: the vapour control layer should overlap at seams and junctions and be taped where it forms the airtightness barrier. Any damage, such as tears, should be repaired before boarding. Where sheathing board forms the air barrier, air-sealing tape should be applied at junctions and edges.

g. Fixings: care should be taken to ensure that fixings do not damage the airtightness barrier.

h. Windows and doors: to ensure continuity of the air barrier, window and door units should connect to the primary air barrier and window and door frames should be taped to surrounding structural openings, using air sealing tape. Compressible seals or gun sealant may be used to supplement taping.

i. Loft hatches: where the roof is insulated at ceiling level, hatches should be suitably designed and installed to ensure optimum airtightness.

4.22 To avoid air movement within thermal elements, either of the following measures should be implemented.

a. The insulation layer should abut the air barrier at all points in the building envelope.

b. The space between the air barrier and the insulation layer should be filled with solid material.

Airtightness in existing dwellings

4.23 When carrying out work in existing dwellings, 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 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 windows, roof windows, rooflights or doors (all of which are controlled fittings), the controlled fitting should be well fitted and reasonably draught-proof.

NOTE: Particular attention should be paid to Approved Document F and Approved Document J when making an existing dwelling more airtight.

Limiting heat losses and gains from building services

Hot water and heating pipework

4.24 In a new system, all of the following new pipework should be insulated.

a. Primary circulation pipes for heating circuits where they pass outside the heated living space, including where pipework passes into voids.

b. All primary circulation pipes for domestic hot water.

c. All pipes that are connected to hot water storage vessels, for at least 1m from the point at which they connect to the vessel.

d. All secondary circulation pipework.

4.25 In an existing system, when a boiler or hot water storage vessel is replaced, any accessible pipes in the dwelling should be insulated.

4.26 Heat losses from insulated pipework should not exceed those given in BS 5422 for hot water services at 60°C, regardless of the actual design temperature. Meeting the standards in Table 4.4 is one way of demonstrating that heat losses will not exceed those given in BS 5422.

general G6
Table 4.4 Minimum thicknesses of pipework insulation for hot water services and space heating applications using high performance insulation

External pipework for district heat networks

4.27 Pipework for district heat networks should be insulated to meet either of the following.

a. The standards in BS EN 253 for pre-insulated pipes.

b. An equivalent performance for conventionally insulated pipes.

4.28 Where pipework is above ground, the performance of the pipe insulation should be at least as high as the insulating performance of pipework in the buried part of the system.

Heated water storage for space or domestic hot water

4.29 Vessels that store heated water for a heating or domestic hot water system should have standing losses that do not exceed the heat loss given in Table 4.5 for that system type.

general G6
Table 4.5 Maximum daily heat loss for a hot water cylinder

4.30 Hot water storage vessels should comply with all of the following.

a. Copper hot water storage combination units should comply with BS 3198.

b. Vented cylinders should comply with the heat loss and heat exchanger requirements of BS 1566-1 or BS EN 12897 as appropriate.

c. Unvented hot water storage system products should comply with BS EN 12897.

4.31 Primary storage systems should meet the insulation requirements of the Hot Water Association’s Performance Specification for Thermal Stores.

Heat interface units

4.32 Vessels that store heated water for a heating or domestic hot water system should have standing losses that do not exceed the heat loss given in Table 4.5 for that system type.

Section 4: Limiting heat gains and losses