Thermal Modelling Design Services

Dara Stewart

Dip Arch, B.Arch. Sc, MSc, MRIAI

NSAI Registered Thermal Modeller

IHER Energy Services Ltd.

Thermal Bridging

Thermal bridging

• occurs where the continuity of the insulation is interrupted

• typically occurs at the junctions between plane building elements, e.g. at wall/roof

Effects of Thermal Bridging

Poorly designed junctions

• Increase heat loss through the building fabric

• Pose a risk of condensation due to the low internal surface temperatures

• Create occupant discomfort due to draughts

Part L & Thermal Bridging

TGD L requirements

• ensure continuity of insulation & limit local thermal bridging

• take account of heat loss within the energy demand calculations (DEAP) – affects EPC and CPC compliance

• assess risk of surface condensation & mould growth

Accounting for heat loss in DEAP

In DEAP (Dwelling Energy Assessment Procedure)

• In DEAP the fabric heat loss calculation includes the heat loss due to TB

• This is done by multiplying the exposed surface area of a dwelling by a thermal bridging factor (y-value)

• y-value is a penalty applied to the building envelope area

Fabric heat loss in DEAP:

Heat loss via plane elements (W/K) = Σ A*U (i)

Heat loss via TB (W/K) = Σ Aexp * y-value (ii)

Total fabric heat loss (W/K) = (i) + (ii)

Thermal bridging factors in DEAP

In DEAP you can apply the following y-values:

y-value

0.15 default y-value (alternative details, existing buildings)

0.08 default y-value(ACDs)

or calculate y = Σ (l * Ψ) Σ A exp

Calculating a y-value

Ψ-values:

• default values from Tables D1 to D6 of TGD L (ACDs)

• from certified details

Certification

• certified by a third party certification body

• or by certified thermal modeller (e.g. NSAI)

y-value = Σ (l * Ψ ) Σ A exp

Temperature factor (fRsi)

• Appendix D of TGD L sets out the requirements in relation to avoiding risk of surface condensation and mould growth

• ACDs are deemed to comply with the requirements

• Any alternative details must be assessed by calculating the temperature factor, (fRsi)

fRsi= (Tsi – Te ) / (Ti – Te) Tsi = minimum internal surface temperature

• For dwellings

fRsi ≥ 0.75

Tsi ≥ 15°C at the junction

Calculating Ψ-value & fRsi

Effect of y-value in DEAP

Y-value = 0.15 Heat loss due to TB = 73 W/KPlane heat loss = 130 W/K Fabric heat loss = 203 W/K

Y-value = 0.08Heat loss due to TB = 39 W/K Fabric heat loss = 169 W/K

Y-value = 0.05Heat loss due to TB = 24 W/KFabric heat loss = 154 W/K

-17% fabric heat loss

-24% fabric heat loss

NZEB MPEPC = 0.302

2011 MPEPC = 0.40

Substantiating y-values in DEAP

Set of drawings highlighting all key junctions

When using y-value < 0.15 in DEAP - must provide supporting documentation to the BER Assessor

Substantiating y-values in DEAP

Dwelling Type: Type A (2-storey, 3-bedroom, semi-detached )

Dwelling Address: 5 Park Avenue, New Street, Co. Dublin

Thermal Bridge Corresponding ACD / certified detail

V.001 1.27.1, 2011

V.002 1.06.1, 2011

V.003 1.08, 2011

V.004 1.27.2, 2011

V.005 1.06.1, 2011

V.006 1.27.1, 2011

V.007 1.01a

V.101 1.06.1, 2011

V.102 1.27.1, 2011

V.103 1.11.2

V.104 1.26

V.105 1.23.2

Signature: _________________ (Site Engineer)

Date: _________________

List of all key junctions and their corresponding details signed by Architect / Engineer / Builder / Developer

Substantiating y-values in DEAP

Copies of all relevant ACDs / certified details

Limitations of the ACDs

In order to use a y-value of < 0.15

• All key junctions must be identified and assigned a detail (acceptable / certified)

• Range of junctions within the set of ACDs is quite limited

• ACDs represent the minimum requirements in terms of limiting thermal bridging for new construction

• Psi-values for some of the junctions are particularly high

Limitations of the ACDs

Thermal modelling services

Main services currently being sought from us:

• Developers compiling their own set of certified details which exceed standards of the ACDs – view to rolling out across a number of developments

• Architects identifying junctions within their own designs that are not featured within ACDs - seeking psi-values and fRsi calculations

• Developers identifying the junctions within ACDs with relatively high psi-values and looking at ways to improve on those

Case Study – Looking towards NZEB

• 3-bed semi-d & mid terraced

• fabric U-values: 0.15 W/m2K

• airtightness: 1.5 m3/hr/m2

• MVHR

• PV

• y-value = 0.08 (ACDs)

• how is NZEB achieved?

Case Study – Looking towards NZEB

• Default psi-values for party wall junctions - high

• Already insulating their party wall junctions to a higher standard than the ACDs

• Capture the benefit of the reduced heat loss due to thermal bridging in the BER of the dwelling

Case Study – Looking towards NZEB

Party wall / external wall

Party wall /ceiling

Party wall / ground slab

Default Ψ 0.155 0.229 0.240

Calculated & Certified Ψ 0.093 0.149 0.157

Thermal Bridging Factor 0.08 0.04

EPC 0.326 NZEB 0.301

Thermal Modelling Design Services

Calculating heat loss at junctions will

become an essential part of the NZEB solution

Thank You

Dara Stewart

dara@iher.ie