Building standards technical handbook 2020: domestic

The levels set out in the guidance to this standard are robust back-stops and these are necessary for the following reasons:

Non-repeating thermal bridging at the junctions of building elements and around openings in the building envelope form part of the calculation of energy performance in the Standard Assessment Procedure (SAP 2012, see clause 6.1.1). Heat loss through such junctions, if poorly designed and constructed can contribute significantly to the overall heat loss through the insulation envelope.

As fabric insulation levels improve, the rate at which heat is lost through air infiltration through the building envelope (air permeability) becomes proportionally greater. For example, in a typical 1960’s house with poorly fitted windows 20% of the total heat could be lost through air infiltration. If the same building was upgraded to 2002 levels of fabric insulation but no attempt was made to improve the air permeability then the heat loss through infiltration could represent over 40% of total heat losses. When addressing infiltration, the provision of adequate, controllable ventilation is essential if both energy efficiency and good indoor air quality are to be achieved.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirements of this standard in so far as is reasonably practicable, and in no case be worse than before the conversion (regulation 12, schedule 6).

Where a balanced and practical approach is taken to reducing energy demand in new dwellings, a consistent and good level of fabric insulation will limit heat loss through the building envelope. Column (a) of the table below sets out robust backstop measures. In most cases, meeting Standard 6.1 will result in even better levels of thermal insulation unless the design of a dwelling involves extensive use of building -integrated or localised low carbon equipment (LCE).

Localised areas of the same building element may be designed to give a poorer performance. These in turn will need to be compensated by the rest of the element being designed and built to a more demanding level. An example of this would be a meter box set into an external wall. These localised areas should have a U-value no worse than the figures given in column (b) of the table below. This is particularly important with regard to the control of condensation (see Section 3: Environment). Repeating thermal bridges (e.g. timber studs in a timber frame wall) should not be considered as an individual element in this respect, as these are already taken into account within a BS EN ISO 6946: 2007 U-value calculation.

For communal areas refer also to clause 6.2.13.

Notes:

Cavity separating walls - unanticipated heat loss can arise via air movement, within a cavity separating wall, from heated areas to points outwith the insulation envelope. To limit heat loss, a separating wall cavity should have effective perimeter sealing around all exposed edges and in line with insulation layers in abutting elements which separate the dwelling from another building or from an unheated space. This allows a U-value of 0.2 to be assigned to such walls. Further reduction in heat loss can be achieved where the cavity separating wall is also fully filled with a material that limits air movement, allowing a U-value of 0.0 to be assigned.

In considering this issue, it is important that solutions also address the need to limit noise transmission (see Section 5: Noise).

Due to the carbon emissions Standard 6.1, there is no need for guidance on minimum or maximum area for windows, doors and rooflights in new dwellings. The use of a methodology for establishing compliance with Standard 6.1 provides an equitable approach to balancing the issues of heat loss versus solar gain and natural lighting versus artificial lighting.

In certain cases where there is a desire to have a large proportion of glass it may be difficult to demonstrate compliance with Standard 6.1. In such cases, innovative solutions will need to be considered. All relevant standards and guidance should be considered, including Standard 6.6, on avoiding high internal summer temperatures.

Guidance on alterations, extensions and conversions is provided in clauses 6.2.6 to 6.2.13.

Common areas - for communal areas refer to clause 6.2.13.

As insulation values of new buildings improve, the need to limit heat loss through thermal bridging becomes increasingly important. Incorrect detailing at design stage or poor construction work can have a significant adverse effect on building performance.

The insulation envelope of any heated building should be designed and constructed to limit heat loss through thermal bridging. The key areas of concern are:

Whilst repeating thermal bridges are taken into account in the BS EN ISO 6946: 2007 U-value calculation, a separate assessment of non-repeating thermal bridging should be carried out for new buildings which are subject to Standard 6.1. Advice and further information on assessment of the effects of thermal bridging can be found in BRE Information paper IP 1/06 – 'Assessing the effects of thermal bridging at junctions and around openings' http://www.brebookshop.com/.

The SAP calculation tool referred to in the guidance to Standard 6.1 includes an assessment of heat loss arising from non-repeating thermal bridges in new dwellings. The overall heat loss is derived from numerical modelling of individual  (psi) values calculated in accordance with BS EN ISO 10211: 2007 ‘Thermal bridges in building construction - heat flows and surface temperatures - detailed calculations'. Guidance on this process is given in BR 497, ‘Conventions For Calculating Linear Thermal Transmittance and Temperature Factors’.

To determine the value for heat loss arising from non-repeating thermal bridging (transmission heat transfer coefficient or H_tb) for the proposed dwelling, designers should identify the presence of junctions listed in Appendix K of SAP 2012 and assign  values to each junction, based upon the following options:

Note that a combination of  values from these sources can be used to produce a calculated heat loss.

Further commentary on this process and use of other published documents providing sources of pre-calculated values can be found within ‘Accredited Construction Details (Scotland) 2015’ SAP 2012.

Addressing infiltration in new dwellings can significantly reduce heat loss and result in lower carbon dioxide emissions. This can provide flexibility when applying the methodology used to meet the TER for carbon dioxide emissions (see Standard 6.1).

To limit heat loss, any heated building should be designed to limit air infiltration through the building fabric. This is done by providing a continuous barrier that resists air movement through the insulation envelope and limits external air paths into each of the following:

The infiltration rate used for the TER calculation is 7m³/h.m² @ 50 Pa (see clause 6.1.2). Whilst no backstop value is set for uncontrolled infiltration, it is recommended that buildings are designed to achieve a value of 10m³/h.m² @ 50 Pa or better to allow a balanced approach to managing building heat loss.

Where no infiltration rate is specified by the designer, a value of 15m³/h.m² @ 50 Pa will be assigned to the proposed dwelling for the purpose of the DER.

Designing and constructing a building in accordance with the principles set out in BSD’s document ‘Accredited Construction Details (Scotland) 2015’ http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks will assist in limiting air infiltration. Due to the contribution of both detailing and workmanship, it remains difficult to achieve a specified air infiltration rate with any degree of accuracy. To ensure the dwelling will deliver the intended thermal performance without adversely affecting air quality, air tightness testing should be undertaken to verify as-built air infiltration rates (see clause 6.2.5).

Limiting air infiltration to improve energy performance should not compromise ventilation required for:

Lower air infiltration rates, of less that 5m³/h.m² @ 50 Pa, may give rise to problems with internal air quality and condensation unless this is addressed through planned ventilation. Accordingly, where design infiltration rates are proposed below this rate, reference should be made to additional measures needed to ensure air quality under Standard 3.14, on the provision of ventilation within dwellings.

Similarly, work to improve an existing dwelling which includes measures which reduce infiltration should also consider the impact of such work on condensation risk and moisture movement within affected construction elements (see clause 6.2.10).

Common areas - in building consisting of dwellings, common areas which need particular consideration to limit air infiltration include common stair entrances and shafts which extend through most of the floors (e.g. lift and common stair enclosures).

Low air infiltration rates will contribute to energy performance but should not be so low as to adversely affect the health of occupants or the building fabric. There is, therefore, a need to establish dwelling performance by test to demonstrate compliance in both these respects.

Evidence from testing of dwellings, constructed to the 2007 and 2010 Accredited Construction Details (Scotland) and of similar constructions elsewhere in the UK, indicates that air-tightness levels of 5 to 7m³/h.m² @ 50 Pa or better are readily achievable and can be exceeded unintentionally. Air-tightness testing should be carried out on new dwellings to demonstrate that air infiltration rates deliver both the stated design level under this guidance and that the proposed ventilation strategy remains appropriate (see Section 3: Environment).

Frequency of testing dwellings - testing of completed dwellings should be carried out on at least 1 in 20 dwellings or part thereof. The verifier may request that the frequency of testing be varied as considered appropriate to ‘reasonable inquiry’ and in response to previous test results within a development.

In larger developments, it is advisable to test more than one example of the same dwelling type and form, completed at different stages in the overall development, to help establish consistency in quality of construction. In smaller developments, the proportion of dwellings tested may also need to increase, dependent on the range of type and form of dwellings present, to ensure a representative sample is taken.

Normally, for a development of one dwelling, an air-tightness test should be carried out as it will not be possible to obtain comparative data on the quality of construction from similar dwellings.

Alternatively, for any single dwelling or number of dwellings, where a default design value of 15m³/h.m² @ 50 Pa is stated in demonstrating compliance under Standard 6.1, testing need not be carried out.

Testing should be in accordance with BS EN 13829: 2001 – ‘Thermal performance of buildings - determination of air permeability of buildings - fan pressurization method’. Practical advice on procedure for pressure testing is given in the ATTMA publication ‘Measuring Air Permeability of Building Envelopes'.

Testing should be carried out by persons who can demonstrate relevant, recognised expertise in measuring the air permeability of buildings. This should include membership of a professional organisation which accredits its members as competent to test and confirm the results of testing.

Further advice on these matters can be found in chapter 5 of the BSD publication ‘Sound and Air-tightness Testing’, 2015 Edition http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/techbooks/ast2015.

A building that was originally designed to be unheated has, in most instances, the greatest void to fill in terms of energy efficiency. The introduction of heating to such buildings will, if not accompanied by fabric insulation, result in disproportionate heat loss and wasteful use or fuel and power.

Where conversion of an unheated building (e.g. a barn) or part of a dwelling is to be carried out, or heating is introduced to a building that was previously designed to be unheated, the building should work to achieve the same standards to those for an extension to the insulation envelope by following the guidance in clauses 6.2.9 and 6.2.10, meeting the U-values in column (b) of the table to clause 6.2.9.

In this context, existing buildings where heating is provided solely for the purpose of frost protection (rated at a maximum of 25W per m² of floor area) shall be treated as unheated buildings.

Conversion of part of a dwelling - examples of work which involve conversion of part of a dwelling are; changing a roof space, an unheated garage or a deep solum space into an apartment:

In the case of a building that was previously designed to be heated, the impact on energy efficiency as a result of the conversion, may be detrimental but could be negligible, or in some circumstances even an improvement.

A less demanding approach than identified in clause 6.2.6 is recommended which at the same time still ensures that some overall improvements are being made to the existing building stock.

Where an extension or conservatory is formed and/or alterations are being made to the building fabric at the same time as the conversion, the guidance given in clauses 6.2.9 to 6.2.12 should also be followed.

Where conversion of a heated building is to be carried out, the existing insulation envelope should be examined and upgraded following the table below:

Additional information:

With historic, listed or traditional buildings, the energy efficiency improvement measures that should be invoked by conversion can be more complex.

Whilst achieving the U-values recommended in clause 6.2.6 and 6.2.7 should remain the aim, a flexible approach to improvement should be taken, based upon investigation of the traditional construction, form and character of the building in question and the applicability of improvement methods to that construction. Provisions under other legislation (e.g. planning consent for listed buildings or those within conservation areas, where there is a need to maintain character, form or features) are also relevant. The manner in which proposed improvements may affect moisture movement or the permeability of existing construction will also require assessment to address the risk of adverse consequences.

For all buildings, it would be advisable to consider the feasibility of upgrading fabric to at least the U-values given in column (c) in clause 6.2.9 (individual element U-values). In many cases, specialist advice will be helpful in making an assessment to ensure that, in improving energy efficiency, there is no other, adverse effect to the building fabric.

Accordingly, each building will have to be dealt with on its own merits. Improvements to the fabric insulation of the building will often depend on factors such as whether or not improvement work can be carried out in a non-disruptive manner without damaging existing fabric (for example, insulating the ceiling of an accessible roof space), or whether potential solutions are compatible with the existing construction.

In certain cases, buildings are given historic or listed status because of specific features present in certain parts of the building. In these circumstances, it may be possible to make greater improvements to other less sensitive areas.

In all cases the ‘do nothing’ approach should not be considered initially. Innovative but sympathetic and practical solutions to energy efficiency, which are beyond the scope of this guidance, can often result in an alternative package of measures being developed for a building. For example, carbon dioxide emissions can be reduced without affecting building fabric through improvements to the heating system (refer to Standards 6.3 and 6.4), the lighting system (refer to Standard 6.5) or incorporation of low carbon equipment (such as a biomass boiler or heat pump). Consultation on such matters at an early stage with both the verifier and the planning officer of the relevant authority is advised.

Further guidance on issues that merit consideration and potential approaches to improvement can be found in the Historic Scotland Document ‘Guide for Practitioners 6 - Conversion of Traditional Buildings’ http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/hsg6ctb.

Extension of a domestic building is not subject to Standard 6.1. In view of this, measures to limit energy demand and carbon dioxide emissions rely primarily upon the performance of the new building fabric.

As the majority of construction work for an extension will be new, there will seldom be the need to consider construction to a lesser specification as is sometimes the case for conversions and alterations. The exception to this is at the junction between existing and new, for example the need for proprietary metal ‘wall starter’ ties where the existing brickwork stops and new cavity blockwork begins. However other building standards should still be met with regard to such transitional construction elements.

Unlike a new building, an extension to an existing building will not commonly benefit from the provision of an efficient heating system or low carbon equipment (LCE). Therefore, fabric U-values should improve on the new build backstops identified in clause 6.2.1 to limit CO₂ emissions and energy demand to an equivalent level.

Accordingly, where the insulation envelope of a dwelling or a building consisting of dwellingsis extended, the new building fabric should be designed in accordance with one of two levels of elemental U-values for walls, floors, roof, windows, doors and rooflights. The maximum area weighted U-values applicable for new works to an extension is determined by the energy performance of the existing building, assessing both external wall and roof elements:

To limit heat loss through openings, the area of windows, doors, and roof lights within an extension should be limited to 25% of the floor area of the extension plus the area of any existing openings built over within the extensions. This may be exceeded where the compensatory approach (described below) is used to demonstrate that this results in no additional heat loss.

Areas of the same building element may have a poorer than average performance provided the area-weighted average U-value for all elements of the same type is maintained (e.g. by some elements having correspondingly better performance). To reduce the risk of condensation, the maximum individual element U-values should be no worse than the figures given in column (c) of the table below:

Notes:

'Compensatory approach' using a notional extension - a compensatory approach allows U-values for the elements involved in the work to be varied provided that the resulting overall heat loss for an extension is not greater than that of a ‘notional’ extension. The ‘notional’ extension should be the same size and shape as the proposed extension, and have the area weighted average U-values from the relevant column in the table above and have an area of windows, doors and rooflights equal to 25% of the total extension floor area plus the area of built over openings.

In situations where the U-values of the existing dwelling means the extension is to be built to column (a) U-values, the compensatory approach can be extended to give applicants greater flexibility, by allowing the extension to be built to column (b) U-values providing that the further reduction in heat loss is achieved through fabric improvements to the existing dwelling.

Examples of this approach are given in annex 6B.

Whole dwelling approach - where SAP data is available for the existing dwelling, it may be practical to provide a revised SAP calculation to demonstrate compliance of a dwelling, as proposed, including extension, using the target-based methodology (DER not more than TER) set out in guidance to Standard 6.1 (carbon dioxide emissions). This option will generally only be viable where both extension and dwelling are built to the same, current edition of the standards.

Where works to alter, extend or convert a building, the elements involved in the building work should follow the guidance in clauses 6.2.3 and 6.2.4 on limiting heat loss from thermal bridging and air infiltration and reference should be made to the principles set out in the BSD document ‘Accredited Construction Details (Scotland) 2015’ http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks. Calculation of heat loss from linear thermal bridging is not necessary unless the SAP methodology is being used to demonstrate compliance.

In addition, the recommendations within Building Research Establishment (BRE) Report 262 'Thermal insulation: avoiding risks (2002 edition)’ can be followed.

It should be noted that, unless the SAP methodology is being used to demonstrate compliance, air-tightness testing is not necessary for work to existing buildings. In such cases, a default value of 10m³/h.m² @ 50 Pa can be assumed or testing of the extension carried out as identified in clause 6.2.5.

For alterations it is more than likely that the existing construction will be from a different era, in building regulation terms. In many instances each building will need to be considered on its own merits. Some of the guidance given in this clause is written in specific terms, but in certain cases (e.g. historic, listed or traditional buildings), it may be necessary to adopt alternative energy efficiency measures which relate to the amount of alteration work being undertaken.

Alterations that involve increasing the floor area and/or bringing parts of the existing building that were previously outwith the insulation envelope into the heated part of the dwelling are considered as extensions and/or conversions (regulation 4, schedule 2) and reference should be made to the relevant guidance clause for such work.

The extent to which improvement can be delivered will be affected by a range of issues, such as:

Alterations to the insulation envelope of a building should be considered using the guidance in the following paragraphs.

Infill of small openings - the infill of an existing opening of approximately 4m² or less in the building fabric should have a U-value which matches at least that of the remainder of the surrounding element. In the case of a wall or floor however it should not be worse than 0.70W/m²K, and for a roof, not worse than 0.35W/m²K.

Infill of large openings - the infill of an existing opening of greater area (than approximately 4m²) in the building fabric should have a U-value which achieves those in column (b) of the table to clause 6.2.9. Another way would be to follow the guidance in the paragraph above, but compensate for the energy efficiency deficit by improving the overall U-value of other parts of the insulation envelope.

Internal elements which become part of the insulation envelope - alteration can cause an existing internal element of a building to become part of the insulation envelope. This will most likely occur where a part of a building is permanently removed as a phase of the alteration work. Where this occurs, that part of the building (including any infill construction) should have U-values which achieve those in column (b) of the table to clause 6.2.9. Another approach would be to follow the guidance given in the previous paragraph, but compensate for the energy efficiency deficit by improving the overall U-value of other parts of the insulation envelope.

However, where this occurs at a boundary, no upgrading need be carried out if the element is a wall that is exclusively the property of the adjoining building.

Windows, doors and rooflights - where windows, doors and rooflights are being created or replaced, they should achieve the U-value recommended in column (b) of the table to clause 6.2.9. A compensating approach may be used and an example of this is given in annex 6A.

Where the work relates only to 1 or 2 replacement windows or doors, to allow matching windows or doors be installed, the frame may be disregarded for assessment purposes, provided that the centre pane U-value for each glazed unit is 1.2W/m²K or less. For secondary glazing, an existing window, after alteration should achieve a U-value of about 3.5W/m²K.

Areas of windows, doors and rooflights - where additional windows, doors and rooflights are being created, the total area (including existing) of these elements should not exceed 25% of the total dwelling floor area. In the case of a heated communal room or other area (exclusively associated with the dwellings), it should not exceed 25% of the total floor area of these rooms/areas.

Reconstruction of elements - where the build-up of an element forming part of the insulation envelope is to be altered or dismantled and rebuilt, the opportunity should be taken to improve the level of thermal insulation.

Column (b) of the table to clause 6.2.9 gives benchmark U-values and in many cases these can be achieved without technical risk. within the constraints of the existing construction. It is recognised however certain constructions are easier to upgrade than others and these values should be met as far as is reasonably practicable.

A building that was in a ruinous state should, after renovation, be able to achieve almost the level expected of new construction. It may not however be reasonably practicable for a dwelling, which is in a habitable condition, to have its internal space significantly reduced in area or height in order to accommodate insulation; or for excessive enabling alterations to be caused by the fitting of external thermal insulation, unless the owner/occupier of the dwelling intends that these changes are to be made. Other building standards and the impact that they will have when upgrading thermal insulation should be taken into account.

In the majority of cases however, after an alteration of this nature to the insulation envelope, a roof should be able to achieve at least an average U-value of 0.35 and in the case of a wall or floor, 0.70W/m²K.

For older buildings of traditional construction, further guidance to assist in this assessment can be found in the Historic Scotland Document ‘Guide for Practitioners 6 - Conversion of Traditional Buildings’ http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/hsg6ctb.

Thermal bridging and air infiltration - when alterations are carried out, attention should still be paid to limiting thermal bridging at junctions and around windows, doors and rooflights and limiting air infiltration (clause 6.2.10). However, only the work that forms the alteration and the impact of that work on the existing building need be considered.

Conservatories are a common addition to many dwellings. Traditionally used as an ancillary space, occupied for part of the year, conservatories are now often used year-round leading to an increased heating demand. Accordingly, such buildings should, like other heated stand-alone buildings, be constructed to limit energy demand and reduce CO₂ emissions.

Some smaller conservatories can be exempt from both building warrant and building standards (see Section 0). Conservatories of 50m² or more are subject to Standard 6.1 of the non-domestic guidance.

Thermal division - a conservatory should be thermally divided from a dwelling, being outwith the insulation envelope of the dwelling. The dividing elements (e.g. wall, door, window) should have U–values equal or better than the corresponding exposed elements in the rest of the dwelling.

U-values - although conservatories are attached to dwellings, they are stand-alone buildings. Where not exempt, a conservatory (heated or unheated) should be built to the same maximum U-values as any other new work, as listed in columns (b) and (c) of the table in clause 6.2.9. The exception is that glazing and framing elements forming the walls or roof of a conservatory are unlimited in area and should have a maximum area-weighted average U-value of 1.8W/m²K and a maximum individual element U-value of 3.3W/m²K.

U-values of glazing elements forming the roof are usually quoted in the vertical plane and should therefore be adjusted allowing for the angle of the roof. Further guidance and U-value adjustments can be found in BR 443: 2006 ‘Conventions for U-value calculations’.

Varying U-values - 'Compensating U-values for windows, doors and rooflights’ - individual U-values for the glazed and framing elements may exceed 1.8W/m²K provided that the average U-value for all the glazed and framing elements is no greater than 1.8W/m²K. An example of this approach is given in annex 6A.

Thermal bridging and air infiltration - in order to limit air infiltration and thermal bridging at junctions and around windows, doors and rooflights, guidance in clause 6.2.10 should be followed.

If using the Building Standards Division document: 'Conservatories' http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/techconserv2nd, these issues will be considered to have been taken into account. Draught stripping for existing windows and doors which are part of the thermal division between the conservatory and the dwelling should be of a similar standard as the exposed windows and doors elsewhere in the dwelling.

Thermal division of a stand-alone building from the remainder of a dwelling or domestic building is explained in clause 6.2.12.

For heated stand-alone buildings of less than 50m², the fabric values identified in columns (b) and (c) of the table to clause 6.2.9 and clause 6.2.10 should be followed. U-value recommendations should be met, though it should be noted that the area of glazing is not limited. This allows, for example, a dwelling to be extended to create a highly-glazed stand-alone building such as a sunroom, with glazing in excess of the limits identified in clause 6.2.9.

Stand-alone buildings of 50m² or more are subject to Standard 6.1. Reference should be made to clause 6.1.7 and use of the non-domestic calculation methodology to assess carbon dioxide emissions.

Common areas - where the total area of a communal room or other heated accommodation associated with a block of dwellings is less than 50m², these rooms or accommodation should also be treated as a stand-alone building. Elements (including dividing elements) should have U-values equal to or better than those chosen for the rest of the building, as determined in conjunction with the methodology in Standard 6.1. As part of a new building, the area of windows, doors, rooflights and roof windows in these rooms or accommodation should be limited to 25% of the total floor area of these common areas.