How SuperHeat 5 checks for compliance with Elemental, Calculation and Heat Loss methods

Checking for compliance with Elemental, Calculation and Heat Loss methods using the 2002 Part L and Part J documents is more complicated than before, because different U-value standards are applied to openings with metal and non-metal (wood or uPVC) openings. A decision has to be made as to how to handle cases where there is a mixture of frame types. This page sets out the methodology we have introduced into SuperHeat 5.00d (revised in 5.01). We do not pretend that this is the only way of approaching this subject and will gladly consider any alternative approaches.

Elemental method

First consider a house with a floor area of 100m². The combined area of windows and rooflights may not exceed 25% of the floor area (25m²) as long as the average U-values of metal and non metal framed openings are 2.2 and 2.0 or less respectively (the heating system is assumed to come within Table 1).

Calculation/Heat Loss method (buildings other than dwellings)

The Calculation Method in earlier Part L AD's no longer officially has this name but it is still there as in L2 §1.15 as a variation on the Elemental Method, and renamed as the Heat Loss method in Part J (Scotland). In each case the actual building is compared with a notional building of the same shape and size as the actual building with solid elements constructed to Elemental Method standards (with the caveat that where the U-value of an uninsulated ground floor of this size and shape is lower, the lower figure is used.

The total area of door, window and roof light openings allowed by the Elemental Method (15/30/40% of external wall area + 20% of roof area) determines the door/window and rooflight areas in the notional building - see the table below - and (except in NI) whether a check has to be made on the average U-values of opaque elements. If (except in NI) the total opening area is below the Elemental Limit, the average U-value of each element has to be no more than the Elemental limit + 0.02. So in such cases you can trade off (for example) a piece of wall with a U-value of 0.4 against a similar area with a U-value of 0.3, or use windows with a higher U-value than the Elemental figure, but cannot use the reduced opening area to allow a higher U-value for opaque elements or use a higher than minimum level of insulation in the walls to compensate for an uninsulated solid floor with a U-value of more than 0.27. Our understanding is that these rather convoluted requirements are based on the fact that later insulation improvements to walls and floors (and some roofs) may be almost impossible, whilst replacing windows with ones of a higher standard is much easier in most cases.

A further complication (except in NI) is that the Elemental U-value figures for doors, windows and rooflights vary depending on whether wood/uPVC or metal frames are used. Where the actual building has a mixture of frame types we take the proportions and use a weighted average for the U-value of the openings in the notional building: for example if the building has windows of 50% of each frame type, the notional building window U-value is taken as 2.1 (2.0x50% + 2.2x50%)

The effect of these various assumptions is that if you enter a building that just meets the Elemental requirements with the doors, windows and rooflights using any mixture of metal and non-metal frames and with the rooflights distributed between different roof types in any proportion, the calculated heat loss for the notional building will be exactly the same as for the actual building and this is our justification for approaching the problem in this way.