6.2 Building insulation envelope
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The levels set out in the guidance to this standard are robust backstops and these are necessary for the following reasons:
to help reduce energy consumption, particularly in new buildings and large extensions, where low carbon equipment (LCE) may reduce carbon dioxide emissions but not energy consumption, and
to ensure that a good level of fabric insulation is incorporated in building work especially to construction elements that would be difficult and costly to upgrade in the future.
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 SBEM calculation tool (refer to 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 by air infiltration through the building envelope (air permeability) becomes proportionally greater. For example, in a typical 1960’s building 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).
6.2.1 Maximum U-values for new buildings
Where a balanced and practical approach is taken to reducing energy demand in new buildings, 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 values for the thermal performance of building elements.
Localised areas of the same type of element may be designed to give 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 or a roof void access hatch. These areas should not be any worse than the figures given in column (b) of the table below. 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.
Table 6.3. New buildings - maximum U-values for building elements
Notes:
For modular and portable buildings, refer to the maximum area-weighted U-values for new buildings identified in annex 6.C.
Excluding separating walls and separating floors between heated areas where thermal transmittance need not be assessed, provided measures are taken to limit heat loss arising from air movement within a cavity separating wall (see below).
Vehicle access doors and similar large doors should have a maximum U-value of 1.5W/m² K.
There is no maximum U-value for display windows (refer to clause 6.2.2).
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 this 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 building from another building or from an unheated space. Further reduction in heat loss can be achieved where the cavity separating wall is also fully filled with a material that limits air movement.
In considering this issue in residential buildings, it is important that solutions also address the need to limit noise transmission (see Section 5 Noise).
A display window is an area of glazing, including glazed doors, intended for the display of products or services on sale within the building, positioned at the external perimeter of the building, at an access level and immediately adjacent to a pedestrian thoroughfare. Glazing that extends to a height of more than 3m above such an access level, or incorporates a fixed or opening light of less than 2m2, should not be considered part of a display window except:
where the size of individual products on display require a greater height of glazing, or
in cases of building work involving changes to the facade (including glazing) and requiring planning consent, where development control officers should have discretion to require a greater height of glazing, e.g. to fit in with surrounding buildings or to match the character of the existing facade.
There is no area limitation for display glazing in new buildings, however heat loss and solar gain through display windows need to be compensated for elsewhere in the building (see clause 6.1.6).
It is expected that display windows will be found in the type of buildings detailed below:
shops including retail warehouse, undertakers, show-rooms, post offices, hairdressers, shops for sale of cold food for consumption off premises
financial and professional services banks, building societies
estate and employment agencies
food and drink restaurants, pubs, wine bars, shops for sale of hot food for consumption off premises.
Due to the carbon dioxide emissions Standard 6.1, there is no need for guidance on minimum or maximum areas for windows, doors and rooflights in new buildings. 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 versus artificial lighting.
There is no area limit for windows, doors and rooflights for a shell building or for a fit-out subject to a continuing requirement (see clause 6.1.8) as this is considered within the TER calculation required under Standard 6.1.
If constructing a new building as a shell under one building warrant with further, separate fit-out work required prior to occupation, a greater emphasis should be placed on heat loss and the performance of the building envelope.
Column (a) of the table below sets out robust backstop values for the thermal performance of building elements. The application of these benchmark values when designing the shell building will give greater flexibility to the subsequent process of showing compliance with Standard 6.1 for fit-out of the building. Limits on individual element U-values, as discussed in clause 6.2.1, should not be any worse than identified in column (b) of the following table:
Notes:
Excluding separating walls and separating floors between heated areas where thermal transmittance need not be assessed, provided measures are taken to limit heat loss arising from air movement within a cavity separating wall (see clause 6.2.1).
Vehicle access doors and similar large doors should have a maximum U-value of 1.5W/m² K.
There is no maximum U-value for display windows (see clause 6.2.2).
It is advisable to consult with the verifier on shell and fit-out issues at an early stage of the proposed development. In some instances it may be advisable to apply instead for a 'staged building warrant'.
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 SBEM 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 buildings and large extensions. The overall heat loss is derived from numerical modelling of individual 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’.
For the ‘actual’ building, heat loss is calculated automatically from the building geometry input by the designer, based upon the following options:
adoption of a default y-value of 0.10 (adding 10% to the calculated heat loss for all planar elements of the building)
input of default values for each junction assigned by SBEM (listed in the NCM Modelling Guide for Scotland) http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/ncmg2015
where construction of a junction follows the ‘Accredited Construction Details (Scotland) 2015’ http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks or other published and substantiated construction detail sets, input of value of the relevant junction(s) from that document, or
input of values calculated by a person with suitable expertise and experience following the guidance set out in BR 497.
Note that a combination of values from (b), (c) & (d) can be used to produce the calculated heat loss.
Further commentary on this process and use of other published documents providing sources of pre-calculated values can be found within the ‘Accredited Construction Details (Scotland) 2015’ http://www.gov.scot/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks.
Addressing infiltration in new buildings 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 inside of the building
the ‘warm’ side of insulation layers, and
spaces between the component parts of exposed building elements, where such parts contribute to the thermal performance of the element.
Areas that need particular consideration in this respect include loading doors, entrance areas and shafts which extend through most of the floors (e.g. lift and stair enclosures).
Where a building warrant application is made for a new building shell only, air permeability should not exceed 7m³/h.m² @ 50 Pa. Where the shell is subject to a continuing requirement under Standard 6.1, testing should be carried out both at completion of the shell and, again when the fit-out is completed (see clause 6.2.7).
In all other cases, no backstop value is set for air permeability. However 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.
Limiting air infiltration to improve energy performance should not compromise ventilation required for:
Lower infiltration rates may give rise to problems with internal air quality and condensation unless this is addressed through the appropriate ventilation strategy. Accordingly, where very low design infiltration rates are proposed, additional measures may be needed to ensure the air quality under Section 3 Environment.
Similarly, work to improve an existing building 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 6.2.12).
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 building performance by test to demonstrate compliance in both these respects.
All new non-domestic buildings and large extensions which are subject to Standard 6.1 (carbon dioxide emissions) should be tested on completion, with the following exceptions:
multiple units, under the same building warrant, of not more than 150m² in floor area and of the same form and construction. A sample of 1 in 20 units or part thereof may be tested as it can be considered that all units will have a similar build standard. The verifier should select the units to be tested by the applicant
modular building of less than 500m² where no site work is needed other than connection of modules, provided test results for similar configuration of modules with the same connection details is available
large extensions subject to Standard 6.1, where a default infiltration rate of 10m³/h.m² @ 50 Pa is declared for the actual building, and
new buildings where due to size or complexity, it is impractical to carry out testing of the building, provided this has been demonstrated as part of the building warrant submission and evidence to support the declared infiltration rate is provided.
Testing should be in accordance with BS EN 13829:2001 – ‘Thermal performance of buildings - determination of air permeability of buildings - fan pressurisation method’. Practical advice on procedure for pressure testing is given in the ATTMA publication ‘Measuring Air Permeability of Building Envelopes’ (http://www.attma.org/).
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 the application of these exceptions and on testing in general is provided in chapter 5 of the BSD publication ‘Sound and air-tightness testing, 2015 Edition’ http://www.scotland.gov.uk/Topics/Built-Environment/Building/Building-standards/techbooks/techhandbooks/ast2015.
Shell and fit-out buildings - where a shell building is subject to a continuing requirement under Standard 6.1, testing should be carried out both at completion of the shell and, again when the fit-out is completed.
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 such a building is converted or heating is introduced to a building that was previously designed to be unheated, the building should achieve a level of performance similar to those for an extension to the insulation envelope of a non-domestic building and follow the guidance given in clauses 6.2.11 to 6.2.12.
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.
6.2.9 Conversion of heated buildings
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 6.2.8 is applied which aims to ensure that some overall improvements are being made to the existing building stock.
Where an extension is formed and/or alterations are made to the building fabric at the same time as the conversion, the guidance given in clauses 6.2.11 to 6.2.13 should be also followed.
Where conversion of a heated building is to be carried out, the insulation envelope should be examined and upgraded following the table below:
Table 6.5. Conversion of heated buildings - maximum U-values for building elements
| Type of element | (a) Area weighted average U-value (W/m2K) for all elements of the same type | (b) Individual element U-value (W/m2K) |
|---|---|---|
| Wall [1] | 0.30 | 0.70 |
| Floor [1] [2] | 0.25 | 0.70 |
| Roof [1] [2] | 0.25 | 0.35 |
| Windows, doors, roof windows and rooflights [3] | 1.6 | 3.3 |
Notes:
Where upgrading work is necessary to achieve the recommended U-values, reference should be made to 'Reconstruction of elements' in clause 6.2.13 and more demanding U-values achieved, where reasonably practicable.
Excluding separating walls and separating floors between heated areas where thermal transmittance need not be assessed, provided measures are taken to limit heat loss arising from air movement within a cavity separating wall (see clause 6.2.1).
There is no maximum U-value for display windows (refer to clause 6.2.2).
6.2.10 Conversion of historic, listed or traditional buildings
With historic, listed or traditional buildings, the energy efficiency improvement measures that should be invoked by conversion can be more complex.
Whilst achieving the values recommended in clauses 6.2.8 and 6.2.9 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 constructions 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 (b) in clause 6.2.11 (individual element U-values). In many cases, specialist advice will help 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 biomass boilers, heat pumps or CHP). 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.
Other than for large extensions where Standard 6.1 applies, measures to limit energy demand and carbon dioxide emissions rely primarily upon the performance of the new building fabric.
As the majority of the construction work for an extension will be new, there will seldom be a 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 building work, 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.
Where the insulation envelope of a building is extended, the new building fabric should be designed in accordance with the following table:
Table 6.6. Extensions - maximum U-values for building elements
Notes:
For extensions to modular and portable buildings, refer to the maximum area-weighted U-values for new buildings identified in annex 6.
Excluding separating walls and separating floors between heated areas where thermal transmittance need not be assessed, provided measures are taken to limit heat loss arising from air movement within a cavity separating wall (see clause 6.2.1).
There is no maximum U-value for display windows (refer clause 6.2.2).
Where the insulation envelope of a building is extended, the new opening areas should be designed in accordance with the table below:
Table 6.7. Extensions - Maximum windows, doors and roof-light areas
| Building Type | Windows and doors as % of the area of exposed wall | Roof-lights as % of area of roof |
|---|---|---|
| Residential buildings, offices, shops and buildings for entertainment and assembly purposes | 40 | 20 |
| Industrial and storage buildings | 15 | 20 |
‘Compensatory approach' using a notional extension - the U-values for the elements involved in the building work may be varied provided that the overall heat loss of all the elements in the extension is no greater than that of a 'notional' extension. The ‘notional’ extension should be the same size and shape as the proposed extension, with U-values for elements as in column (a) of the table above, where the area of openings in the walls (excluding separating walls where it is considered that zero heat loss occurs) and roof of the 'notional' building extension are as the percentages given above. An example of this approach is given in annex 6B.
Alternative approach - an alternative to the use of the area-weighted U-values described above is use of SBEM to demonstrate compliance with Standard 6.1 for:
the extension alone, where assessment of fabric and the energy efficiency of the building services systems can be considered in isolation from the existing building, or
the entire building, as extended where detailed information exists of the original construction and building services. This option will be most viable where both extension and existing building are built to the same, current edition of the standards.
Where works are to alter, extend or convert a building, the elements involved in the building work should follow the guidance given in clauses 6.2.5 and 6.2.6 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 SBEM methodology is being used to demonstrate compliance.
In addition the recommendations within the Building Research Establishment (BRE) report 262 'Thermal Insulation, avoiding risks' 2002 edition, can be followed (http://www.brebookshop.com/).
It should be noted that, unless the SBEM 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 @ 50Pa can be assumed or testing carried out as identified in clause 6.2.7.
6.2.13 Alterations to the insulation envelope
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 or traditional buildings), it may be necessary to adopt alternative energy efficiency measures which are appropriate to the amount of alteration work being undertaken.
The extent to which improvement can be delivered will be affected by a range of issues, such as:
the form and construction of the existing envelope and the scope of works
the extent to which improvement is technically feasible without the risk of adverse consequences, and
the impact of any other statutory requirements to which the building is subject (e.g. listing, conservation area).
Extending the insulation envelope - reference should be made to the guidance on extensions to the insulation envelope (clause 6.2.11) for alterations that involve increasing the floor area and/or bringing parts of the existing building that were previously outwith the insulation envelope into the conditioned part of the building. Examples of such work could be, changing a roof space, part of an unheated warehouse, or a deep solum space into office accommodation:
in the case of a roof space, this will usually involve extending the insulation envelope to include the gables, the collars, part of the rafters and the oxters, as well as any new or existing dormer construction. The opportunity should be taken at this time to upgrade any remaining poorly performing parts of the roof which are immediately adjacent to the alterations, for example, insulation to parts of the ceiling ties at the eaves
in the case of an unheated warehouse, this will usually involve extending the insulation envelope to include the existing floor, perimeter walls and the roof/ceiling to the new office area, and
in the case of a deep solum space, this will usually involve extending the insulation envelope to include, the solum/existing floor and perimeter walls to the new office area.
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 (a) of the table to 6.2.11. 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 meet the maximum U-values recommended under clause 6.2.11. 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 is necessary 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 meet the maximum U-values recommended in clause 6.2.11. An example of a compensatory approach for windows, doors and rooflights is given in annex 6A.
Where the work relates only to 1 or 2 replacement windows, each window may have a centre pane U-value of no worse than 1.2W/m²K. For secondary glazing, an existing window, after alteration should achieve a maximum U-value of 3.5W/m²K.
There are no limits imposed on U-values for display windows (refer to clause 6.2.2).
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 (a) of the table to clause 6.2.11 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 that 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 building 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 building 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 0.35W/m²K 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 sir infiltration - when alterations are carried out, attention should still be paid to limiting thermal bridging at junctions and around windows, doors and rooflights and also limiting air infiltration (clause 6.2.12). As far as alterations are concerned, only the work that forms the alteration and the impact of that work on the existing building need be considered.