Heat loss through thermal bridging is not accounted for in the U-value calculation for the plane building elements containing the thermal bridge and therefore must be evaluated separately. It is usually expressed in terms of a fraction known as the y factor. In order to determine the value of y to be used in an energy rating calculation the following may be used;
a) Use 0.15 where no calculations have been performed and where Acceptable Construction Details have not been used;
b) Use 0.08 where the Acceptable Construction Details have been used in all key junctions
c) Use a y factor which can be determined through calculation using Psi values for details in Table D1 to D6 or combined with details with certified Psi values for all key junctions
d) Use a y factor which can be determined using certified Psi values for all key junctions
The y factor is derived using the linear thermal transmittance or Psi (Ψ) value. The Psi value is a property of a thermal bridge and is the rate of heat flow per degree per unit length of bridge.
The Psi value can be obtained in the following ways:
a) the specific Psi value for Acceptable Construction Details from Building Regulations 2011 TGD-L (Dwellings) Tables D1to D6 depending on the construction type.
b) certified Psi values for other details which are assessed in accordance with the BRE IP1/06 “Assessing the effects of thermal bridging at junctions and around openings” and BRE Report BR 497 “Conventions for calculating linear thermal transmittance and temperature factors” in accordance with Appendix D of Building Regulations 2011 TGD-L (Dwellings) ^1^. Certification should be provided by a a third party certification body such as Agrement or equivalent or certified by a member of an approved thermal modellers scheme or equivalent..
c) or they can be derived from measurement by a third party certification body.
The transmission heat loss coefficient (HTB) can then be calculated from:
HTB = Σ(Lx Ψ) W/m^2^K
Where: L is the length of the thermal bridge over which the thermal bridge applies.
Ψ is linear thermal transmittance as defined in a to b above
The y factor can then be derived using the formula:
HTB = y ×ΣAexp.
where ΣAexp. is the summed area of exposed elements, in m^2^.
^1^.Key junction details that are not as recommended in Acceptable Construction Details it is necessary to determine their temperature factor.
Where Σ(l ×Ψ) is not explicitly calculated, the default transmission heat loss coefficient associated with thermal bridges at junctions and around openings can be expressed as a fraction y multiplied by the total exposed surface area of the building. That is, the default transmission heat loss coefficient for junctions can be taken to be: H~TB~ = y×ΣAexp. where ΣAexp is the summed area of exposed elements, in m^2^ .
SEAI have developed an online Thermal Bridging Application for the purpose of performing the above calculation.
Sample calculations
For a 3 bed semidetached house below using the actual lengths of the internal junctions the y factor for use in DEAP can be calculated as follows.
House details
House details
Ground floor external perimeter – 23 m
Intermediate floor – 23 m
Eaves – 14 m Gable (insulation at ceiling level) - 9m
External Corners – 10.20m
Party wall corners with external – 10.20m
Party wall junction with floor- 9m
Party wall junction with ceiling – 9m
Rising walls-9m
Exposed Area
Exposed area – 243.3 m^2^
Construction:
Roof: Pitched tiled roof, insulation laid on attic floor, part between joists and part over joists. Walls: Cavity wall (dense concrete blocks) rendered externally, with partial fill insulation in the cavity and 50mm cavity retained.
Floor: Concrete slab-on-ground floor with insulation under slab
Example y factor calculation
Example y factor calculation
Look up table for R – Values
Insulation thickness required to achieve specified thermal resistance
Table body: Insulation thickness [mm]
Table Thermal Resistance and Insulation Thermal Conductivity
