A “U-value” is a measure of the rate of heat flow through an element.
It is expressed in W/m2k, and shows the amount of heat lost in watts (W) per square metre of material (for example wall,
roof, floor etc.) When the temperature (k) outside is at least one degree lower.
Lower U-values indicate better thermal insulation, i.e. an element with a U-value of 0.3 W/m2K loses heat at half the rate of an element with U-value of 0.6 W/m2K.
Building elements are normally built up from a number of layers, so U-values must take into account the performance of each layer.
We set out below the minimum U-values for the key building elements in existing dwellings, as
set out in Approved Document L of the Building Regulations – Conservation of fuel and power,
Volume 1: Dwellings.
Table 4.2 Limiting u-value for new fabric elements in existing dwellings
Element type Minimum U-Value W/(M2K)
Window 1.4 or Window Energy Rating B
Roof light 2.2
Doors with >60% of internal face glazed 1.4 or Window Energy Rating B
Other doors 1.4 or Window Energy Rating B
For new buildings however, targets are tighter
- Walls 0.18 – this can be achieved with a full-filled 150mm cavity and thermal blocks on the inner-leaf
- Floors 0.13 – this can be achieved with a PIR insulation of at least 150mm insulation
- Cold Roof 0.11 – this can be achieved by insulating between joists and above with at least 400mm total thickness
- Warm Roof 0.11 – this can be achieved by insulating between the rafters and at least 150mm of rigid board insulation over top.
In addition air-tightness, thermal bridging and heating efficiencies have been uprated:
- Airtightness – typical maximum result of 5.0
- Thermal bridging – all bridges require measuring however defaults are available (target Y-value of 0.05)
- Heating – if mains gas is to be used, a 92% ErP is required with interlock and compensator.
Offer considerations include:
- Holistic fabric-first approach to becoming net-zero by 2050 (by the implementation of low/zero carbon technologies and onsite generation)
- Appropriate ventilation strategies
- Internal comfort: overheating analysis and background noise levels
- Water useage
- Lighting designs
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