Cold-Bridging
Why is it important in a timber frame if timber is not a conductor?
Cold bridges occur when thermal resistance through part of the fabric of the building is less than the rest of the construction. They happen particularly around openings and at junctions of walls/floors and walls/roofs. Although concrete and steel framed buildings are particularly prone to cold-bridging, unless these elements are individually insulated, it can occur in timber frame buildings as the studs are less thermally conductive relative to the insulated void between them. This can increase heat loss through the external wall construction by as much as 10%. The use of the Pavatex system of cladding, with its bespoke system of fasteners, eliminates cold bridging across studs to the outside of the frame, which would result in a sometimes undesirable increase in wall thickness and insulation cost if it were to be compensated for by additional insulation.
As thermal standards have risen so cold-bridging has become more significant, and even small elements such as roof fixings and metal cavity ties can significantly affect performance. Where a rendered finish is required the use of Pavatex Diffutherm not only avoids cold bridging across studs but also eliminates the need for metal frame ties into exterior block work, a further source of cold bridging.
Average U-Values
The effect of Cold-Bridging is to reduce the effectiveness of any insulation. Many countries allow for this in the calculation of the maximum allowable rate of heat loss (U Value) or minimum Thermal Resistance (R Value).
This is done by averaging out the effects of the cold-bridge over the whole structure. The Average U-value/R-Value takes account of all these losses and this value should be used in calculating envelope heat losses/gains during the year.
U-Values vs R-Values
U Values are commonly used in UK and R values in France. In essence they refer to the insulation performance of the structure. They are loosely but not accurately inversely related. The U Value is the thermal conductance of the structure and is the rate of flow of heat across a specific construction.
The R Value is the resistance to the flow of heat across a material or at an interface between two materials.