AIRCRETE

4.3.3

Chapter 4. Design

Page 5 of 18

4.3.3

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4.3.2.1.1 Examples of U-values of floors

The heat loss through any form of ground floor is significant unless it is adequately insulated.

The U-value of a floor is influenced by the area and shape of the floor so floors of small area or long narrow shape will require more added insulation than, say, large square floors.

For a given size and shape, beam and Aircrete block floors need less added insulation than other forms of slab or structural floor.

Using solid Aircrete perimeter walling in the foundations reduces the perimeter heat losses compared with heavier wall construction.

4.3.3 Conservation of fuel and power
The Approved Document gives three methods of showing compliance with Part L of the Building Regulations both for dwellings, AD L1, and for buildings other than dwellings, AD L2.

4.3.3.1 Dwellings
For dwellings, the three methods of showing compliance with the regulations are:

a) An Elemental Method
b) A Target U-value Method
and
c) A Carbon Index Method

The Target U-value Method is preferred for new dwellings whereas the Elemental Method needs to be used for alterations and extension work to avoid having to take the whole of the existing building into the calculation.

The Target U-value Method and the Carbon Index Method provide more flexibility in design than the Elemental Method. The Elemental Method cannot be used where fuel is electricity.

The U-value of an element for use in any of the three methods is calculated by applying the following steps:

Calculate the upper resistance limit (R upper) by combining in parallel the total resistances of all possible heat-flow paths (i.e. sections) through the plane building element.

Calculate the lower resistance limit (R lower) by combining in parallel the resistances of the heat flow paths of each layer separately and then summing the resistances of all layers of the plane building element.

Calculate the U-value of the element from U = 1 / RT,
where RT = R upper + R lower

Adjust the U-value as appropriate to take account of metal fasteners and air gaps.

4.3.3.1.1 Poorest acceptable U-values and limiting air
	leakage for methods b) and c)

Local U-values of exposed walls and floors should be limited to being not worse than 0.7 W/m

K and the building fabric constructed so there are no significant gaps or thermal bridges in the insulation provided. The design should avoid details which might introduce risks of condensation on inner surfaces or within the fabric of the building.

Reasonable provision should be made to reduce unwanted air leakage. Both the above aims should be achieved by limiting the U-values as described and following the Robust Details.

U-values of widows and doors are some 5 to 10 times worse than those for external walls so the better the window type the bigger the contribution to obtaining the best thermal performance for the building.

4.3.3.1.2 Elemental Method for new dwellings,
	extensions and alterations

The Elemental Method is suitable for alteration and extension work and the requirement will be met for new dwellings by selecting construction elements that provide maximum U-value performance shown in Fig 4. The Elemental Method is also limited to a minimum boiler efficiency, which in turn, depends upon the type of central heating system and fuel used.