Building Fabric
General
This section gives guidance on acceptable levels of provision to ensure that heat loss through fabric elements provided by way of extension, material alteration, material change of use to an existing dwelling is limited insofar as is 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.
This Part of the Building Regulations and the European Union (Energy Performance of Buildings) Regulations 2019 applies to the replacement of external doors, windows, or roof lights in an existing building. The average U-value of replacement units should not exceed the value of 1.4 W/m2K 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.
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 infiltration 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.
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.
Fabric insulation
Extensions
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 column 2, Table 1 for each fabric element type for extensions and 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 Table 1. Where this relaxation is availed of, the average area weighted values given in Table 1 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, the maximum floor U-value should be 0.15 W/m2K. 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 and Energy Performance of Buildings Regulations 2019 (to be published).
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 column 3, Table 1 apply to each relevant element. For ground floors
or exposed floors incorporating underfloor heating, the guidance in paragraph 2.1.2.1 applies.
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 LG 10 for daylight gives advice on adequate daylight provision.
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 average U-value of these elements should not exceed the value of 1.40 W/m2K.
Material Alterations and Material Change of Use
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) for material alterations and material changes of use, as specified in column 2, Table 5.
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, Table 5. Where this relaxation is availed of, the average area-weighted values given in Table 5 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, the maximum floor U-value should be 0.15 W/m2K. 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 and Energy Performance of Buildings Regulations 2019 (to be published).
Table HL5 - Maximum elemental U-value (W/m²K) for material alterations or material change of use - Extract from TGD L
Thermal bridging
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.
Reasonable provision with regard to limitation of thermal bridging for extensions is to adopt Acceptable Construction Details for typical onstructions as shown in the document Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details (to be published) and in Annex H of NSAI S.R. 54:2014 Code of Practice for the Energy Efficient Retrofit of Dwellings or other details that are similar or that have been assessed as limiting thermal bridging to an equivalent level.
For material alterations or material change of use, lintel, jamb and sill designs similar to those shown in Annex H of NSAI S.R. 54:2014 Code of Practice for the Energy Efficient Retrofit of Dwellings would be satisfactory.
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 m2K/W (25 mm of insulation with thermal conductivity of 0.035 W/mK, or equivalent).
Air permeability
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.
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:
a) 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;
b) sealing vapour control membranes in timber-frame constructions;
c) fitting draught-stripping in the frames of openable elements of windows, doors and rooflights;
d) sealing around loft hatches; and
e) 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.
Information and illustrations on sealing service penetrations and airtightness membranes can be found in the Introduction Section of Acceptable Construction Details and Diagram 2.
Diagram HL2 - Air infiltration measures - Extract from TGD L
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
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. Guidance is given on three main issues:
a) heat generator efficiency (sub-section 2.2.2);
b) space heating and hot water supply system controls (sub-section 2.2.3); and
c) insulation of hot water storage vessels, pipes and ducts (sub-section 2.2.4).
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 and Energy Performance of Buildings Regulations 2019 (to be published).
This Section also contains guidance in relation to the energy efficiency aspects of biomass independent boilers (paragraph 2.2.2.4) where provided.
Heat Generator efficiency
The appliance or appliances provided to service space heating and hot water systems should be as efficient in use as is 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 and Energy Performance of Buildings Regulations 2019 (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 SEAI 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 and Energy Performance of Buildings Regulations 2019” (to be published).
For fully pumped hot water-based central heating systems heat pumps, the seasonal space heating energy efficiency and water heating energy efficiency should not be less than the minimum requirements in accordance with Ecodesign regulations.
New or replacement storage heaters should have a heat retention not less than 45 % measured according to I.S. EN 60531:2000. They should incorporate a timer and electronic room thermostat to control the heat output that are user adjustable.
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 SEAI http://www.seai.ie/harp).
Space heating and hot water supply system controls
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 is 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; and
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.
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.
Hot water storage vessels should be fitted with thermostatic control that shuts off the supply of heat when the desired storage temperature is reached.
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.
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.
Guidance for controls for heat pumps is provided in Tables 2 and 3.
Insulation of hot water storage vessels, pipes and ducts
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.
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 Annex B of BS 1566-1:2002+A1:2011, 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/m3 satisfies this criterion. Alternative insulation measures giving equivalent performance may also be used.
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:
a) 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 deg. C to + 700 deg. C); or
b) insulation with material of such thickness that 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.
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 paragragh 2.2.4.3 above, for at least one metre from their point of connection.
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.
Major Renovations
Major Renovation means the renovation of a dwelling where more than 25 % of the surface of the dwelling envelope undergoes renovation.
Where a dwelling undergoes major renovation, the energy performance of the whole dwelling should be improved to Cost Optimal level insofar as this is technically, functionally and economically feasible.
The cost optimal level to be achieved is:
a) An energy performance of 125 kWh/m2 /yr when calculated in DEAP as set out in column 2, Table 7 or
b) Implementing the energy performance improvements as set out in column 3, Table 7 insofar as they are technically, functionally and economically feasible.
In order to calculate the percentage of the surface of the dwelling envelope undergoing renovation, Table 6 describes the areas affected by works that should be included.
The surface area of the dwelling thermal envelope means the entire surface area of a dwelling through which it can lose heat to the external environment or the ground, including all heat loss areas of walls, windows, floors and roof.
Appendix F provides examples on how to calculate the percentage of renovation surface area for typical dwellings.
The cost of renovation to a cost optimum level should be technically, functionally and economically feasible. Column 1, Table 7 describes the works and elements that qualify as a Major Renovation.
Where a new extension is added to an existing dwelling and affects more than 25 % of the surface area of the existing dwelling’s envelope, then the final energy performance of the completed dwelling should achieve the cost optimal level.
When undertaking works on, or in connection with, a building that is of architectural or historical interest or permeable traditional construction , the aim should be to improve the building as far as is reasonably practicable. The work should not prejudice the character of the building or increase the risk of long -term deterioration of the building fabric or fittings. Refer to sub - section 0. 6.
Where major renovation upgrades are being carried out, care should be taken to ensure compliance with the ventilation requirements of Part F and of Part J of the Building Regulations. Refer to Clause 10 of S.R 54:2014 Code of practice for the energy efficient retrofit of dwellings.
Table HL7 - Cost optimal works activated by major renovation - Extract from TGD L
