Building standards - ministerial view: building insulation envelope (ref. V2023/3)

Ministerial view of a case relating to the Building Insulation Envelope which was referred to Building Standards Division (ref: V2023/3, section 6 – Energy, functional standard 6.2, domestic building).

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Functional Standard 6.2  Building Insulation Envelope

Every building must be designed and constructed in such a way that an insulation envelope is provided which reduces heat loss.


This standard does not apply to:

(a) non-domestic buildings which will not be heated, other than heating provided solely for the purpose of frost protection
(b) communal parts of domestic buildings which will not be heated, other than heating provided solely for the purpose of frost protection, or
(c) buildings which are ancillary to dwellings, other than conservatories, which are either unheated or provided with heating which is solely for the purpose of frost protection.

Clause 6.2.8 - Extensions to the insulation envelope

Extension of a domestic building is not subject to Standard 6.1. In view of this, measures to limit energy demand and greenhouse gas 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. 

Where the insulation envelope of a dwelling or a building consisting of dwellings is extended, the new building fabric should be designed in accordance with the maximum U-values set out in clause 6.2.1.

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 guidance clause 6.2.1.

'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 guidance to clause 6.2.1 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. Examples of this approach are given in Annex 6.A.

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.


City of Edinburgh Council

Technical Context

The building warrant application is for a single storey extension to a house. The extension is 16 m² in area and open to the rear of the existing dwelling. The extension is proposed with floor to ceiling glazing to two elevations, with a declared glazing area of 20.6 m².

The referral relates to mandatory standard 6.2 and to guidance clause 6.2.8 of the February 2023 Domestic Technical Handbook. The applicant proposes an alternative means of compliance with standard 6.2 whereby the heat loss from the new extension, over and above that which would be expected when following the solution within published guidance, would be offset by improvements to the insulation of elements of the existing house.  The intent being that this offsetting would permit a larger area of glazing to the extension.

The applicant is in doubt about whether this solution would be deemed to meet the requirement of the functional standard 6.2. The justification for the alternative means of compliance is that the overall heat loss from the house and extension would be limited to, or improve upon, the heat loss from the unimproved house and an extension which was constructed to achieve the level of heat loss for an extension illustrated by the approach set out in guidance.

The approach set out in published guidance to standard 6.2 is provided in clause 6.2.8 (as above),clause 6.2.1 ‘Maximum U-values for fabric elements of the building’ and in Annex 6.A ‘Compensatory approach - heat loss example’.

The applicant notes that earlier discussion with the verifier had referenced previous guidance on extensions within the 2015 Domestic Technical Handbook, where limited offsetting of heat loss from an extension was an option where the existing dwelling did not meet defined thermal criteria and an extension would otherwise be built to a more challenging set of U-values.  These provisions are no longer cited in published guidance from 1 February 2023.

From the information provided in support, it is noted that the proposed heat loss from the extension is 2.5 times higher than for the notional extension as calculated using Annex 6.A ‘Compensatory approach - heat loss example’ of the 2023 Domestic Technical Handbook.  The applicant proposes to insulate the roof of the existing house to deliver a reduction in heat loss within the existing dwelling that would significantly exceed, and therefore offset, the additional heat loss from the extension.

The view of Scottish Ministers

On behalf of Scottish Ministers, Building Standards Division has considered all the information submitted in this case.

A provision existed within guidance to standard 6.2 within the 2010 and 2015 Handbooks whereby the values for thermal performance for an extension were set based upon the wall and roof U-values of the existing dwelling, with a default set of U-values and a more challenging set of U-values for the extension where the existing dwelling was less well insulated. The option existed to design the extension to the default set of U-values provided the difference in heat loss between the two sets of values was offset by improvement to the existing dwelling. This offsetting of heat loss was limited to that difference and the extension was still expected to meet the default level of insulation to adequately limit heat loss. The ‘compensatory approach’ could be used to illustrate this process.

The 2023 Domestic Technical Handbook no longer cites this approach or the application of two sets of U-values for extensions, which defined that addressable difference in heat loss. 

Building regulations are appliable to the extent of works undertaken.  In considering the element of works which constitute the dwelling, the proposed design has significant glazed areas resulting in more than double the heat loss from the extension when compared to the level of performance identified within published guidance.

Standard 6.2 requires that “Every building must be designed and constructed in such a way that an insulation envelope is provided which reduces heat loss” and clause 6.2.8 illustrates the level of expectation in that respect for an extension to a dwelling.

The proposed solution does not address or approach that level of expectation. Action to improve the thermal performance of an existing building is to be welcomed but this should not be to the significant detriment of the performance of the planned new building work.

The applicant is aware that a solution for designs with significant areas of glazing is already recognised in guidance where the new accommodation is constructed as a stand-alone building in clause 6.2.12 ‘Stand-alone buildings’ in the 2023 Domestic Technical Handbook:

For heated stand-alone buildings of less than 50 m², the fabric values identified in the table to clause 6.2.1. 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 6.2.8.”

This approach which recognises the thermal separation of such a building from the dwelling, was introduced in 2007 to provide greater flexibility on such issues.  A stand-alone building is considered separately from the dwelling in respect of thermal performance.

Having carefully considered all the information submitted in this case, it is the view of Scottish Ministers that the proposals do not meet the requirements of Standard 6.2.

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