Information for SuperHeat users:

Target U-Value Method Requirements - Domestic buildings

(England, Wales & Scotland: different rules apply to Northern Ireland)

SuperHeat User Page . Infomation Pages Home Page

General

You can show compliance using the Elemental Method, Target U-Value Method or Carbon Index Method. You do not have to comply with all three, just one.

The Target U-value method sets a target for the average U-value of exposed elements of a dwelling. Any semi-exposed walls or floors (e.g. those adjoining an integral garage) are not included in the calculation but their U-value has to be not greater than 0.7 if this method is being used to show compliance. This method can be used to justify a glazing area in excess of that permitted by the Elemental method, provided that the rest of the structure is insulated to a higher standard. Alternatively it lets you construct one part of the building to a lower standard of insulation (within limits) if other parts are insulated to a higher standard (for example the provision of additional wall insulation could allow you to use an uninsulated solid ground floor).

The formula used to calculate the target uses the total floor area which is entered in the SuperHeat SAP Dimensions dialog.

• Target U-value = 0.35 - 0.19(AR/AT) - 0.10(AGF/AT) + 0.413(AF/AT)

or in Scotland (assuming a pitched roof with insulation between joists - different formulae apply to other roof types)

• Target U-value = 0.30 - 0.14(AR/AT) - 0.05(AGF/AT) + 0.425(AF/AT)

where AT = total area of exposed elements; AR = roof area; AGF = ground floor area; AF = total floor area For a simple two storey house (AGF=AF/2; AR=AF/2) the formula simplifies to Target U-value = 0.35 + 0.268(AF/AT)

Unpacking the basic Target U-value formula (E&W)

Transposing the AD formula gives the following:

• Target UV x AT ( i.e. max permissible heat loss/K ) = 0.35AW + 0.16AR + 0.25AGF + 0.413AF

You'll recognise the first three values to the right of the equals sign as being the Elemental U-values, and the last represents the additional heat lost through doors, windows and any rooflights. For a pitched roof dwelling built to the exact Elemental standards with wood frame doors and windows (so the additional heat loss/m2 = 2.2-0.35 = 1.85W/K) the last term equates to a permissible door and window area of 22.3% of the floor area (100x41.3/1.85). This reduces to 17.5% for a dwelling with flat roof (U-value=0.25) - in such cases it may be necessary to establish compliance using the Elemental or Carbon Index methods.

The heating system efficiency can affect the target

If the heating system does not meet the requirements of §1.2 the Target U-value is divided by 1.15 (i.e a more onerous standard is applied) The Target U-value must be decreased if the heating system has an efficiency less than that required by Table 2 (e.g. 78% for boilers using mains gas) and when checking to Part L, but not Part J, can be increased where the efficiency is greater than this. SuperHeat identifies such cases automatically. Within SuperHeat the heating system is selected in the SAP dialog, Primary heating system page. Note that the Table 2 reference efficiencies have been increased from April 1 2005 (to 86% for gas boilers etc.).

Benefiting from solar gains

Where the windows of a dwelling have metal frames the target can be multiplied by 1.05 to take account of the additional solar gain (§1.23). If the area of south (±30°) facing glazing exceeds that facing north (±30°), the target can be increased (§1.24); where applicable, SuperHeat will make the necessary adjustment provided you enter the orientations correctly. Limits on the relaxation of U-values The extent to which the Target U-Value and Carbon Index methods may be used to justify the use of elements that have higher U-values than those required by the Elemental method is limited by §1.29 which states that exposed walls and floors should have U-values of not more than 0.7 and roofs not more than 0.35. SuperHeat will indicate whether the specified forms of construction meet these requirements.

Top