Appendix A

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.

SEAI have developed a Thermal Bridging Application which can be used to calculate a nondefault thermal bridging factor for use in the Dwelling Energy Assessment Procedure (DEAP). This can be downloaded from the SEAI website: https://www.seai.ie/data-andinsights/forms/Thermal-Bridging-Application.xlsm

SAMPLE CALCULATION

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

Y-FACTOR CALCULATION