By continuing to view this resource, you acknowledge that Housebuild is not liable for any injury incurred as a result of the information provided on this page.
Calculation of y factor for use in DEAP with Example
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;
Use 0.15 where no calculations have been performed and where Acceptable Construction Details have not been used;
Use 0.08 where the Acceptable Construction Details have been used in all key junctions
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
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) Specific Psi value for Acceptable Construction Details from Building Regulations 2021 TGD-L (Dwellings) Tables D1 to D6 may be used, 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 2021 TGDL (Dwellings). Certification should be provided by a third party certification body such as Agrément or equivalent or certified by a member of an approved thermal modellers scheme or equivalent. NSAI operate a third party certification scheme for thermal modellers
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(L x Ψ) W/m2K
Where:
L is the length of the thermal bridge over which the thermal bridge applies.
Ψ is linear thermal transmittance as defined in a) and b) above.
The y factor can then be derived using the formula:
HTB = y × ΣAexp.
Where:
ΣAexp. is the summed area of heat loss elements (walls, floors and roofs), in m2.
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 (all dimensions are internal)
Construction:
Roof: Pitched tiled roof, insulation laid on attic floor, part between ceiling joists and part over ceiling joists. U-value = 0.14 W/m2K.
Walls: Cavity wall (dense concrete blocks) rendered externally, with partial fill insulation in the cavity and internal insulation. U-value = 0.15 W/m2K.
Floor: Concrete slab-on-ground floor with insulation under slab. U-value = 0.15 W/m2K. Key junctions and their lengths.
Ground floor/external wall ……….…. 23.0m
Ground floor/separating wall ………. 9.8m
Ground floor/int. partition walls ….. 12.8m
Intermediate floor/external wall ….. 23.0m
Roof (eaves)/external wall …………… 14.0m
Roof (ceiling)/gable wall ………..……. 9.0m
Roof (ceiling)/separating wall ….…... 9.8m
External wall/ext. wall corners …….. 10.2m
External wall/separating wall .……… 10.2m
External wall/masonry partition …... 9.0m
External wall/stud partition …………. 12.3m
External wall/jambs ……………………... 23.4m
External wall/lintels ……………….....…. 11.7m
External wall/window cills ………….... 10.8m
Non-Key Junctions
Roof (ceiling)/stud partition wall
Ground floor/stud partition wall built off insulated ground floor slab
Threshold to front and back doors
Exposed Surface Area:
Total heat loss surface area (floor, walls and roof) …………………………………………… 243.3 m2
