‘U’ values W/m² K
The measure of heat-loss through the fabric of a building is expressed as a ‘U’ value with limiting values set out in the building regulations. However, to fully comply with the building regulations, the overall dwelling emission rate (DER) must be lower than the target emission rate (TER) when calculated in SAP.
‘U’ value W/m² K
Note: ‘U’ values are based on NJB1 beam
centres. Actual house-type values will vary
dependent on floor layout.
To achieve this, lower ‘U’ values may be required.
The new improved Jetfloor provides a range of ‘U’ value options that will help improve the overall DER. This table demonstrates this range based on specific Perimeter/Area Ratios and insulation material lambda values.
(‘Psi’) values (W/m K) and
‘Y’ values (W/m² K)
Heat is also lost through thermal bridges or junctions in a building and is expressed as a ‘Y’ value. Its units of measure are the same as the ‘U’ value W/m² K.
To calculate the ‘Y’ value the length of the thermal bridge must be multiplied by the Psi value of the junction of the building being considered i.e. the wall to floor junction.
The Psi value is the measure of the thermal transmittance at the thermal bridge and is calculated using thermal modelling, its units of measure is W/m K.
Jetfloor incorporating the new ‘Psi-Block®’ has resulted in significantly improved Psi values as shown on the table opposite.
(1) Values based on wall U-value of 0.28 W/m2 K
floor U-value of 0.15 W/m2 K.
(2) From SAP table K1.