The elemental method for dwellings

In the elemental method, compliance is demonstrated by specifying:

• Maximum U-values for walls, floors and roofs

• Maximum area-weighted U-values for windows, doors and rooflights

• Maximum combined area of windows, doors and rooflights

• Minimum boiler efficiencies for the heating system boiler (the SEDBUK rating, or Seasonal Efficiency of a Domestic Boiler in the UK).

The maximum U-values allowed by the elemental method are shown in Table 2.1, and are illustrated in Fig. 2.2.

If an element is exposed to the outside through an unheated space (e.g. garage, atrium, etc.) the unheated space can be ignored, provided it is assumed that the element itself is exposed to outside air, i.e. the element is assigned its normal U-value. Alternatively, the unheated space may be included in the U-value calculation for the element, as demonstrated in Chapter 5. The slightly higher permitted U-value for openings in metal frames is intended to allow for the fact that, because the frames themselves are usually more slender than wood or UPVC and therefore have a higher proportion of glazing, there is on average additional solar gain to offset the heat load of the dwelling.

The area weighted U-value of the windows, doors and rooflights, Uwdr The maximum values in Table 2.1 refer to the area weighted average of all the windows, doors and rooflights in the dwelling; this can be calculated from the formula at the top of page 15:

Table 2.1 Maximum U-values for the elemental method.

Exposed element

Maximum U-value, W/m2K

Comment

Pitched roof, insulation between

0.20

Any part of a roof having a

rafters

pitch of 70° or more can be

Pitched roof with integral insulation

0.25

considered as a wall.

Pitched roof, insulation between

0.16

joists

Flat roof

0.25

Roof of pitch 10° or less

Walls, including basement walls

0.35

Floors, including ground floors and

0.25

basement floors

Windows, doors and rooflights,

2.20

Rooflights include roof

glazing in metal frames, area-

windows. The higher U-value

weighted average

for metal frames allows for

Windows, doors and rooflights, glazing in wood or PVC frames,

2.00

extra solar gain due to greater glazed proportion

area-weighted average

Njrotc Uniform

0.25

Fig. 2.2 Maximum U-values allowed by the elemental method for dwellings.

0.25

Fig. 2.2 Maximum U-values allowed by the elemental method for dwellings.

Aw + Ad + Ar where AW, Ad and Ar are, respectively, the areas of windows, doors and rooflights

UW, Ud and Ur are their U-values. Maximum areas of windows, doors and rooflights

The elemental method allows a maximum combined area, called the standard area provision, which is equal to 25% of the total floor area. If the average U-value, Uwdr of the windows, doors and rooflights, does not exceed the value given in Table 2.1, and also their combined total area does not exceed 25% of the total floor area, then the requirement is met.

Minimum boiler efficiencies

The minimum boiler efficiencies allowed by the elemental method are shown in Table 2.2. There is no provision to use lower values. The SEDBUK efficiency (also called the SAP seasonal efficiency) of a large number of boilers can be found on the internet (www.SEDBUK.com). If the SEDBUK of the proposed boiler is not available, it is permissible to use the appropriate seasonal efficiency value as given in SAP 2001.

Table 2.2 Minimum boiler seasonal efficiencies (SEDBUK) for the elemental method.

Central heating system fuel

SEDBUK efficiency, %

Mains natural gas

78

LPG

80

Oil

85

Oil-fired combination (as calculated using the SAP-9

8 method) 82

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