In order to comply with standard 6.1 every building must be designed and constructed in such a way that the insulation envelope resists thermal transfer.

Limitation

This standard does not apply to:

a. buildings, other than dwellings, which will not be heated nor cooled, other than heating for the purpose of frost protection;

b. buildings which are ancillary to a dwelling and which will not be heated other than heating for the purpose of frost protection.

6.2.0 Introduction [J8.1]

The guidance in 6.2.1 to 6.2.3 gives three alternative approaches to the design of the insulation envelope of a new non-domestic building. These are:

• Elemental Method
• Heat Loss Method
• Carbon Emissions Calculation Method

With the Elemental Method and Heat Loss Method a building fabric improvement is allowed against heating systems which have a high carbon intensity (see 6.3), but in general the energy efficiency of the building services are considered as separate issues. The Carbon Emissions Calculation Method offers the most design flexibility. The principal advantages of each approach are given in the preamble to each method. It is important to refer to the Elemental Method in the first instance. This is because the "trade-off" arrangements which are allowed by either the Heat Loss or the Carbon Emissions Calculation Methods use a notional building designed to the Elemental Method as a comparison.

6.2.1 The Elemental Method [J8.2] [J8.3] [J8.4] [CRD141]

High carbon intensity heating systems

At an early stage a check should be done to ensure that the form and fabric does not result in excessive cooling being needed for the building (see 6.6.1). Also, if the heating system has a high carbon intensity the U-values will need to be improved (see 6.3.2).

Where heating type is unknown

When the type of heating system for a building is not known at the time of making the building warrant application, for example a staged warrant application, a high carbon intensity for the system should be assumed. U-values that have been modified by following the guidance in 6.3.2 should be adopted.

Objective

To use the Elemental Method, individual exposed building elements and other elements forming the insulation envelope of a building should have U-values which do not exceed the numerical value given in Table 1. Also the percentage areas of windows, doors and rooflights should not exceed those indicated in Table 2. It should be noted that Annex 6.P can be used instead of Table 1, if the building is of limited-life and does not contain sleeping accommodation.

Table 1: Maximum recommended U-values for building elements forming part of the insulation envelope

When reading Table 1, above, the following points should be taken into account:

1. A wall excludes any glazing (including windows), doors and their associated frames.
2. A roof excludes any glazing and associated frames (including roof windows and rooflights). This is not considered applicable however, where such glazing only provides daylight to a roofspace where the thermal insulation material is located between the ceiling ties or collars.
3. Individual U-values for windows, doors and rooflights may vary however the average U-value for all windows, doors and rooflights should not be more than the figure shown. Trade-off should only occur between elements in the same frame classification: i.e. (1) wood and plastic or (2) metal. For method of calculation see Annex 6.E.
4. Metal-framed windows have slimmer frames and therefore give a passive solar benefit and this justifies a less demanding level of U-value.
5. No maximum U-value necessary for vehicle access and similar large doors which for operational reasons, have to be left open when the building is in use.
6. No maximum U-value necessary for shop entrances and shop display windows which are situated at an access level.

Table 2: Maximum recommended percentage areas of windows, doors and rooflights in the insulation envelope

Next stages

When reading Table 2, above, it should be noted that the minimum percentage areas do not apply to:

• Vehicle access and other similar large doors.
• Shop entrances and shop display windows at an access level.
• Separating walls that follow the guidance in 6.0.6.

Once the U-values for the insulation envelope have been established by this method, the following issues should be considered:

• Limiting the effect of thermal bridging at junctions and around openings that occur in the insulation envelope. (see 6.2.4)
• Limiting air infiltration at the insulation envelope. (see 6.2.5)
• Energy efficiency of the space heating and hot water system, including any improvement in the average U-value due to the high carbon intensity of the heating system (see 6.3 and 6.4)
• Energy efficiency of the artificial lighting (see 6.5)
• Energy efficiency of any air conditioning or mechanical ventilation system (see 6.6)
• Commissioning of the building services (see 6.7)
• Written information (see 6.8).

High carbon intensity heating systems

6.2.2 The Heat Loss Method [J8.5] [J8.6]

This approach allows greater flexibility with the design of the insulation envelope than is given with the Elemental Method. The principal benefit of using this Method is that it allows the higher U-values for parts of the insulation envelope to be compensated by lower U-values for other parts of the construction. A worked example of this calculation is given in Annex 6.J. Similar to the Elemental Method, at an early stage a check should be done to ensure that the form and fabric does not result in excessive cooling being needed for the building (see 6.6.1). Also, if the heating system has a high carbon intensity, the U-values will need to be improved (see 6.3.2). This adjustment is done by using more demanding U-values in the notional building that is formed as a part of the calculation process.

Objective

To use the Heat Loss Method, the insulation envelope of a non-domestic building should have a total rate of heat loss which is not greater than that which would occur from a notional building (of the same size and shape) which follows the guidance given for the Elemental Method.

Proposed building

In the first instance, use a copy of Worksheet 6.J.4 (see Annex 6.J) and in conjunction with the table below, establish by calculation the total rate of heat loss from the proposed building.

Maximum recommended U-values when trading off between building elements in Heat Loss Method

Note:

• If the total area of windows, doors and rooflights in the proposed building is less than the total maximum area recommended in Table 2 to 6.2.1, then the average U-value of the roof, wall or floor should not exceed the appropriate value given in Table 1 to 6.2.1 by more than 0.02 W/m²K. Annex 6.P can be used instead of Table 1 to 6.2.1, if the building is of limited-life and does not contain sleeping accommodation. This simple constraint helps to reduce the incidence of buildings being designed without regard to natural daylight and the energy efficiency implications from excessive use of artificial lighting.

• Only a half of the permitted rooflight area in Table 2 to 6.2.1 can be converted into an increased area of window and doors.

Notional building

After the total rate of heat loss has been established for the proposed building, this process should be repeated but this time demonstrating the total rate from the notional building. To do this, the appropriate U-values obtained from Table 1 to 6.2.1 (or if appropriate Annex 6.P) should be inserted into a further copy of Worksheet 6.J.4.

Where heating type is unknown

When the type of heating system for a building is not known at the time of making the building warrant application, for example a staged warrant application, a high carbon intensity for the system should be assumed. The U-values inserted into Worksheet 6.J.4 should be those, that have been modified by following the guidance in 6.3.2.

When creating the notional building insulation envelope for comparison purposes, the maximum percentage area for windows, doors and rooflights in Table 2 to 6.2.1 should be used. In establishing the total rate of heat loss from the notional building, the following constraints should be applied:

1. The insulation envelope of the notional building is to be the same size and shape as that for the proposed building
2. Vehicle access/similar large doors which for operational reasons, have to be left open when the building is in use and also shop entrances and shop display windows at an access level should be of the same area and U-value as those in the proposed building. This is principally because the percentage area of such openings and their U-value are unrestricted in the elemental method.
3. If, with no added insulation, the U-value of a floor next to the ground in the proposed building is less than the value given in Table 1 to 6.2.1 (or if appropriate Annex 6.P), then that lower value should be used for the notional building. This prevents an inequitable trade off being obtained, which would be based solely on the fortuitous "footprint" size of the building.

Comparison

The total rates of heat loss from both the proposed and the notional buildings, should be compared. If the proposed building total rate is less than or equal to the notional building, the design is satisfactory.

Next stages

Once the U-values for the insulation envelope have been established by this method, the following issues should be considered:

• Limiting the effect of thermal bridging at junctions and around openings that occur in the insulation envelope. (see 6.2.4)
• Limiting air infiltration to the insulation envelope. (see 6.2.5)
• Energy efficiency of the space heating and hot water system, including any improvement in the average U-value due to the high carbon intensity of the heating system (see 6.3 and 6.4)
• Energy efficiency of the artificial lighting (see 6.5)
• Energy efficiency of any air conditioning or mechanical ventilation system (see 6.6)
• Commissioning of the building services (see 6.7)
• Written information (see 6.8).

6.2.3 The Carbon Emissions Calculation Method [J8.7][J8.8] [J8.9]

This method allows most flexibility in the design of the building and enables the energy efficiency of the building services to be taken into consideration. The annual carbon emissions for the proposed building are compared with those from a notional building. This method can be used to take advantage of solar heat gains and electricity generated from low carbon technologies that are either building integrated or perhaps exclusive to the building or group of buildings, photovoltaic cladding or wind turbines, for example. An integrated approach can be adopted and the following parts of this guidance may be ignored for the design of the proposed building:

• Energy efficiency of the space heating and hot water system (see 6.3 and 6.4)
• Energy efficiency of the artificial lighting (see 6.5)

Energy efficiency of any air conditioning or mechanical ventilation system (see 6.6)

Objective

To use the carbon emissions calculation method, the annual carbon emissions of a non-domestic building should be no greater than that which would occur from a notional building. This notional building should comply with the Elemental Method and also the guidance given on energy efficiency of the building services systems. It should also be of the same size and shape as the proposed building.

Proposed building

Although this method allows greater freedom of design, there are constraints placed on the insulation envelope these are:

• The guidance in this document for limiting thermal bridging at junctions and around openings and for limiting air infiltration should be applied.
• The U-values in the table below should not be exceeded.

Maximum recommended U-values when trading off between building elements in the Carbon Emissions Calculation Method

Notional building

The following constraints should be applied to the notional building created using the Elemental Method:

1. The insulation envelope of the notional building is to be the same size and shape as that for the proposed building
2. Vehicle access/similar large doors which for operational reasons, have to be left open when the building is in use and also shop entrances and shop display windows at an access level should be of the same area and U-value as those in the proposed building. This is principally because the percentage area of such openings and their U-value are unrestricted in the elemental method.
3. If, with no added insulation, the U-value of a floor next to the ground in the proposed building is less than 0.25 W/m²K, then that lower value should be used for the notional building. This prevents an inequitable trade off being obtained, which would be based on the fortuitous "footprint" size of the building.

Calculation procedure

The Carbon Emissions Calculation Method can be carried out by using a calculation procedure that:

1. has been tested satisfactorily against available benchmark tests as described in CIBSE Application Manual ‘Building Energy and Environmental Modelling’, AM11, 1998; and
2. has been accepted by the submitting organisation as having satisfied their in-house quality assurance procedures, for example by submitting with the calculations a completed and signed copy of Annex B to AM11: ‘Checklist for choosing BEEM software’, showing that the software used is appropriate for the purpose to which it has been applied.

Next stages

Once the U-values for the insulation envelope and the energy efficiency of the building services have been established by this method, the following issues should be considered:

• Limiting the effect of thermal bridging at junctions and around openings that occur in the insulation envelope. (see 6.2.4)
• Limiting air infiltration to the insulation envelope. (see 6.2.5)
• Commissioning of the building services (see 6.7)
• Written information (see 6.8).

6.2.4 Limiting the effect of thermal bridging at junctions and around openings [J9.1]

The insulation envelope of the non-domestic building should be constructed in such a way that there are no substantial thermal bridges or gaps where the layers of insulation occur. Significant in-use energy consumption can occur, through incorrect detailing at the design stage or careless construction on site. The key areas of concern are:

1. Within building elements
2. At the junction between building elements
3. At the edges of building elements where openings in the structure are formed.

www.bre.co.uk

One approach to addressing these issues would be to follow the advice given in the Building Research Establishment (BRE) Report 262 — ‘Thermal insulation, avoiding risks’ 2002 edition. Another way, could be to demonstrate equivalent performance by calculation. Annex 6.D gives a procedure for establishing condensation risk and heat loss at the edges of openings.

6.2.5 Limiting air infiltration [J10.1]

All building fabric will allow a certain degree of air leakage. It is widely recognised that it is impossible to make the insulation envelope 100% airtight. Where it is desirable to either vent or ventilate the building fabric to the outside air (to allow moisture due to either precipitation or condensation to escape), this should be designed into the construction. Reliance on fortuitous ventilation should be avoided. Measures should be introduced however, to reduce unwanted air leakage and thereby prevent an increase in energy use within the heated or cooled part of the building. The guidance given here should not be used to compromise ventilation required for:

• Health of the building occupants
• Any smoke control system
• Combustion appliances

The main principle of limiting air infiltration is to provide a continuous barrier to air movement around the insulation envelope and thereby reduce external air paths into each of the following:

• The inside of the building
• The internal building elements
• The "warm" side of the insulation
• Spaces between the component parts of exposed building elements, where such parts contribute significantly to the thermal performance of the element.

Other areas of the building that need consideration are entrances to the building and shafts which extend through most of the floors (e.g. lift or stair enclosures).

Correct cavity barrier design for the purposes of structural fire precautions, with airtight materials, can often contribute to achieving this objective.

One approach to addressing these issues would be to follow the advice given in the Building Research Establishment (BRE) Report BR 448 — 2002 edition — ‘Airtightness in Commercial and Public Buildings’.

BR 448 gives examples of appropriate design details and construction practice, but alternative details can be used if equivalent performance can be demonstrated.