Building Fabric

General

This section gives guidance on acceptable levels of provision to ensure that heat loss through fabric elements provided by way of material alteration or extension to an existing dwelling is limited insofar as reasonably practicable. Guidance is given on three main issues: -

insulation levels to be achieved by the plane fabric elements (sub-section 2.1.2);
thermal bridging (sub-section 2.1.3); and
limitation of air permeability (sub-section 2.1.4).

Where a material change of use of an existing building to use as a dwelling occurs, the performance of the fabric elements of the newly provided dwelling should also meet the performance levels specified in this subsection.

2.1.1.2 This Part of the Building Regulations applies to the replacement of external doors, windows, or rooflights in an existing building. The average U-value of replacement units should not exceed the value of 1.6W/m^2^K set out in Table 5. In this context, the repair or renewal of parts of individual elements, e.g. window glass, window casement sash, door leaf, should be considered as repair and not replacement.

2.1.1.3 Unheated areas which are wholly or largely within the building structure, do not have permanent ventilation openings and are not otherwise subject to excessive air filtration or ventilation, e.g. common areas such as stairwells, corridors in buildings containing flats, may be considered as within the insulated fabric. In that case, if the external fabric of these areas is insulated to the same level as that achieved by equivalent adjacent external elements, no particular requirement for insulation between a heated dwelling and unheated areas would arise.

Fabric Insulation

2.1.2.1 The derivation of U-values, including those applicable where heat loss is to an unheated space, is dealt with in paragraphs 0.3.4 to 0.3.8 and Appendix A.

2.1.2.2 Acceptable levels of thermal insulation for each of the plane elements of the building are specified in terms of average area-weighted U-value (Um) in Tables 1 and 5 for each fabric element type for extensions (Table 1, Column 2) and for material alterations and material changes of use (Table 5, Column 2). These values can be relaxed for individual elements or parts of elements where considered necessary for design or construction reasons. Maximum acceptable values for such elements or parts of elements are specified in Column 3 of their respective Tables 1 or 5. Where this relaxation is availed of, the average area weighted values given in the Tables continue to apply and compensatory insulation measures may be necessary for other elements or parts of elements of that type to ensure that these are met. Where the source of space heating is underfloor heating, a floor U-value of 0.15 W/m^2^K should generally be satisfactory. Further guidance in relation to insulation of floors where underfloor heating is proposed is contained in the document “Heating and Domestic Hot Water Systems for Dwellings – Achieving compliance with Part L 2011” (to be published).

2.1.2.3 For extensions, reasonable provision would also be achieved if the total heat loss through all the opaque elements did not exceed that which would be the case if each of the area-weighted average U-value (Um) set out in Table 1 were achieved individually. Where this approach is chosen, the values for individual elements or sections of elements given in Table 1 Column 3 apply to each relevant element. For ground floors or exposed floors incorporating underfloor heating, the guidance in paragraph 2.1.2.2 applies.

2.1.2.4 For extensions, the maximum area weighted average U-value for doors, windows and rooflights of 1.60 W/m^2^K given in Table 1 applies when the combined area of external door, window and rooflight openings equals 25% of floor area. However, both the permitted combined area of external door, window and rooflight openings and the maximum area-weighted average U-value of these elements may be varied as set out in Table 6. The area of openings should not be reduced below that required for the provision of adequate daylight. BS 8206: Part 2: 2008 Code of practice for daylighting and CIBSE Lighting Guide LG10 for daylight gives advice on adequate daylight provision.

Table 5: Maximum elemental U value (W/m^2^K)^1,2^ for Material Alterations or Material Change of Use

2.1.2.5 In applying paragraph 2.1.2.4 to an extension to an existing dwelling, the relevant floor area may be taken to be: -

the combined floor area of the existing dwelling and extension; in this case the combined area of external doors, windows and rooflight openings refers to the area of such openings in the extended dwelling, i.e. the opening area of retained external doors, windows and rooflights together with the opening area of external doors, windows and rooflights in the extension; or
the floor area of the extension alone; in this case the combined area of external doors, window and rooflight openings refers to the area of such openings in the extension alone. In this case, the maximum combined area of external doors, windows and rooflights derived using Table 6 can be increased by an area equivalent to the area of external door, window and rooflight openings of the existing dwellings which have been closed or covered over by the extension.

Table 6: Permitted variation in combined areas (A~ope~) and average U-values (U~ope~) of external doors, windows and rooflights.

For extensions which: -

are thermally separated from the adjacent spaces within the building by walls, doors and other opaque or glazed elements which have U-values not more than 10% greater than corresponding exposed areas of the main dwelling, and
are unheated or, if provided with a heating facility, have provision for automatic temperature and on-off control independent of the heating provision in the existing building,

the limitation on the combined area of exposed external door, window and rooflight openings does not apply. In this case the average U-value of these elements should not exceed the value of 1.60 W/m^2^K.

2.1.2.6 This Part of the Building Regulations applies to the replacement of external doors, windows, or rooflights in an existing dwelling. The average U-value of replacement units should not exceed the value of 1.60 W/m^2^K set out in Table 5. In this context, the repair or renewal of parts of individual elements, e.g. window glass, window casement sash, door leaf, should be considered as repair and not replacement.

Thermal Bridging

2.1.3.1 To avoid excessive heat losses and local condensation problems, reasonable care should be taken to ensure continuity of insulation and to limit local thermal bridging, e.g. around windows, doors and other wall openings, at junctions between elements and other locations. Any thermal bridge should not pose a risk of surface or interstitial condensation. See Appendix D for further information in relation to thermal bridging and its effect on dwelling heat loss.

2.1.3.2 Reasonable provision with regard to limitation of thermal bridging for extensions is to adopt Acceptable Construction Details for typical constructions as shown in the document “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” or other details that are similar or have been assessed as limiting thermal bridging to an equivalent level.

2.1.3.3 For material alterations or material change of use lintel, jamb and sill designs similar to those shown in Diagram 2 would be satisfactory and heat losses due to thermal bridging can be ignored if they are adopted. At lintel, jambs and sills generally a 15 mm thickness of insulation material having λ-value of 0.033 W/mK (or equivalent) will generally be adequate. Where advanced levels of insulation are applied a higher thermal resistance may be necessary.

Diagram 2: Lintel, jamb and sill designs

2.1.3.4 For material alterations or material change of use care should be taken to control the risk of thermal bridging at the edges of floors where the floor is being replaced. The insulation should have minimum thermal resistance of 0.7 m^2^K/W (25 mm of insulation with thermal conductivity of 0.035 W/mK, or equivalent).

Air Permeability

2.1.4.1 For extensions to existing dwellings reasonable levels of air permeability can be achieved by adopting the standard details referred to in paragraph 2.1.3.2 above, together with an appropriate performance specification and the on-site inspection regime and related quality control procedures as referred to in sub-sections 1.5.2 and 1.5.3. Alternative approaches to element design, details and quality control procedures may also be acceptable, provided it can be shown that these approaches are equivalent.

2.1.4.2 For material alterations or material change of use infiltration of cold outside air should be limited by reducing unintentional air paths as far as is practicable. Measures to ensure this include: -

sealing the void between dry-lining and masonry walls at the edges of openings such as windows and doors, and at the junctions with walls, floors and ceilings (e.g. by continuous bands of bonding plaster or battens);
sealing vapour control membranes in timber-frame constructions;
fitting draught-stripping in the frames of openable elements of windows, doors and rooflights;
sealing around loft hatches;
ensuring boxing for concealed services is sealed at floor and ceiling levels and sealing piped services where they penetrate or project into hollow constructions or voids.

Diagram 3 illustrates some of these measures. Further information on sealing service penetrations can also be found in the Introduction Section of Acceptable Construction Details.

Care should be taken to ensure compliance with the ventilation requirements of Part F and of Part J of the Building Regulations.

Building Services

General

2.2.1.1 Space and water heating systems provided in the context of material alterations to existing dwellings or extensions to existing dwellings should be energy efficient and have efficient heat sources and effective controls. Similar considerations apply to space and water heating systems provided in the context of a material change of use of an existing building to use as a dwelling. Specifically, Regulation L2(d) provides that oil or gas fired boilers installed as replacements in existing dwellings should have a minimum seasonal efficiency of 90%, where practicable.

This Section gives guidance where the main space and water heating is based on pumped low temperature hot water systems, utilising radiators for space heating and incorporating a hot water cylinder for the storage of domestic hot water, and the fuel used is natural gas, LPG or oil. Guidance is given on three main issues: -

heating appliance efficiency (sub-section 2.2.2);
space heating and hot water supply system controls (sub-section 2.2.3); and
insulation of hot water storage vessels, pipes and ducts (sub-section 2.2.4).

Diagram 3: Air filtration measures

Detailed guidance for dwellings using a wide range of space and water heating systems is contained in a supporting document “Heating and Domestic Hot Water Systems for Dwellings – Achieving compliance with Part L 2011” (to be published).

2.2.1.2 This Section also contains guidance in relation to the energy efficiency aspects of biomass independent boilers (paragraph 2.2.2.2) where provided.

Heating appliance efficiency

2.2.2.1 The appliance or appliances provided to service space heating and hot water systems should be as efficient in use as reasonably practicable. Guidance on appropriate efficiency for various systems and fuels is contained in “Heating and Domestic Hot Water Systems for Dwellings – Achieving compliance with Part L 2011” (to be published). For fully pumped hot waterbased central heating systems utilising oil or gas, the boiler seasonal efficiency should be not less than 90% as specified in the DEAP manual and the associated Home-heating Appliance Register of Performance (HARP) database maintained by the http://www.seai.ie/harp. Effectively this requires the use of condensing boilers. In a limited number of situations involving replacement of existing boilers, provision of a condensing boiler may not be practicable. Detailed guidance on the assessment of specific situations to identify those where provision of condensing boilers is not practicable is given in “Heating and Domestic Hot Water Systems for Dwellings – Achieving compliance with Part L” 2011” (to be published).

2.2.2.2 For fully pumped hot water-based central heating systems utilising a biomass independent boiler, the boiler seasonal efficiency should be not less than 77% as specified in the DEAP manual and the associated Home-heating Appliance Register of Performance (HARP) database maintained by the http://www.seai.ie/harp.

Space heating and hot water supply system controls

2.2.3.1 Space and water heating systems should be effectively controlled so as to ensure the efficient use of energy by limiting the provision of heat energy use to that required to satisfy user requirements, insofar as reasonably practicable. The aim should be to provide the following minimum level of control: -

automatic control of space heating on the basis of room temperature;
automatic control of heat input to stored hot water on the basis of stored water temperature;
separate and independent automatic time control of space heating and hot water;
shut down of boiler or other heat source when there is no demand for either space or water heating from that source.

The guidance in paragraphs 2.2.3.2 to 2.2.3.5 below is specifically applicable to fully pumped hot water-based central heating systems.

2.2.3.2 Provision should be made to control heat input on the basis of temperature within the heated space, e.g. by the use of room thermostats, thermostatic radiator valves, or other equivalent form of sensing device. For larger dwellings, independent temperature control should generally be provided for separate zones that normally operate at different temperatures, e.g. living and sleeping zones. Thermostats should be located in a position representative of the temperature in the area being controlled and which is not unduly influenced by draughts, direct sunlight or other factors which would directly affect performance. Depending on the design and layout of the dwelling, control on the basis of a single zone will generally be satisfactory for smaller dwellings. For larger dwellings, e.g. where floor area exceeds 100 m 2 , independent temperature control on the basis of two independent zones will generally be appropriate. In certain cases, additional zone control may be desirable, e.g. zones which experience significant solar or other energy inputs may be controlled separately from zones not experiencing such inputs.

2.2.3.3 Hot water storage vessels should be fitted with thermostatic control that shuts off the supply of heat when the desired storage temperature is reached.

2.2.3.4 Separate and independent time control for space heating and for heating of stored water should be provided. Independent time control of space heating zones may be appropriate where independent temperature control applies.

2.2.3.5 The operation of controls should be such that the boiler is switched off when no heat is required for either space or water heating, i.e. boiler interlock. Systems controlled by thermostatic radiator valves should be fitted with flow control or other equivalent device to ensure boiler switch off.

Insulation of hot water storage vessels, pipes and ducts

2.2.4.1 All hot water storage vessels, pipes and ducts associated with the provision of heating and hot water in a dwelling should be insulated to prevent heat loss. Hot water pipes and ducts within the normally heated area of the dwelling that contribute to the heat requirement of the dwelling do not require insulation (except those referred to in paragraph 2.2.4.4). Pipes and ducts which are incorporated into wall, floor or roof construction should be insulated.

2.2.4.2 Adequate insulation of hot water storage vessels can be achieved by the use of a storage vessel with factory applied insulation of such characteristics that, when tested on a 120 litre cylinder complying with I.S. 161: 1975 using the method specified in BS 1566, Part 1: 2002, Appendix B, standing heat losses are restricted to 0.8 W/litre. Use of a storage vessel with 50 mm, factory applied coating of PU-foam having zero ozone depletion potential and a minimum density of 30 kg/m^3^ satisfies this criterion. Alternative insulation measures giving equivalent performance may also be used.

2.2.4.3 Unless the heat loss from a pipe or duct carrying hot water contributes to the useful heat requirement of a room or space, the pipe or duct should be insulated. The following levels of insulation should suffice: -

pipe or duct insulation meeting the recommendations of BS 5422: 2009 Methods of specifying thermal insulating materials for pipes, ductwork and equipment (in the temperature range - 40^o^C to + 700^o^C); or
insulation with material of such thickness as gives an equivalent reduction in heat loss as that achieved using material having a thermal conductivity at 40 ^o^C of 0.035 W/mK and a thickness equal to the outside diameter of the pipe, for pipes up to 40 mm diameter, and a thickness of 40 mm for larger pipes.

2.2.4.4 The hot pipes connected to hot water storage vessels, including the vent pipe and the primary flow and return to the heat exchanger, where fitted, should be insulated to the standard outlined in paragraph 2.2.4.3 above, for at least one metre from their point of connection.

2.2.4.5 It should be noted that water pipes and storage vessels in unheated areas will generally need to be insulated for the purpose of protection against freezing. Guidance on suitable protection measures is given in Technical Guidance Document G and Report BR 262, Thermal insulation: avoiding risks, published by BRE.

Existing Dwelling