The U value (W/m2K) is the thermal transmittance of a building element (wall, roof, floor or window). It includes the thermal resistances of all layers (including air cavities) and surfaces resistances at both surfaces of the element. The surfaces resistances account for both convective and long wave radiation between the element surface and the surroundings.
The U value is the heat flow Q (W) through the whole area of the element divided by the total area of the element and temperature difference between external and internal environments. It is, therefore, used in building energy calculations (i.e. SAP, SBEM) to estimate the total heat loss through the building fabric.
The R value (m2K/W) is the thermal resistance of a building element and is the reciprocal of the U value (R=1/U)
The psi value or linear thermal transmittance is the heat transferred through the junctions of the elements and it is the extra heat which cannot be accounted by the use of the U or R value
In order to build energy efficient buildings it is very important to understand, predict and calculate accurately the heat flow through the building envelope. The heat transferred through solid structures is estimated by the use of U or R and psi values. The other two heat transfer 'paths' through the building envelope are the solar radiation through windows and convection which is the heat carried by intentional air flows (ventilation) or unintentional air leakages.
These free online U value R value and psi value calculators follow the method described in EN ISO 6946:2007 and EN ISO 13370:2007 (walls, roofs and floors) and EN 673:2011 (glass) and BR497 (linear thermal trasmittance) and include:
-The effect of thermal bridges.
-Upper and lower limits of the total thermal resistance.
-Heat paths and percentages.
-Correction for fixings and air gaps.
-Ground floor U value.
-Centre of glazing unit (but not the whole window U value including frame and spacer).
-External full fill cavity wall – ground floor (insulation below slab) linear thermal trasmittance.
These U value, R value and psi value calculators carry out U and R value calculations for walls, roofs, ground floors and glass and psi value calculations for external wall - ground floor junction. They are a free tool, no need of registration or download, you will need only to have installed Silverlight plugin in you browser freely available from Microsoft. Please click here to access the calculators.