Thermal Bridge Analysis

The definition of the calculation model according to the Standards consists of 3 parts:

Basics of the Modeling

Boundary Conditions

Geometry

Materials

Using flixo: either by DXF-Import, by connecting predefined components or by drawing directly inside flixo

Model: Geometry

To check› The model must be large enough, cut-off planes

should be positioned concerning the central element as follows (EN ISO 10211):

› In general at least 1 meter or if the thickness of the flanking element is greater than 33 centimeter, 3 times of the thickness of the flanking element

› At the line of symmetry, if one is present› Omit the cladding and the air layer if the air layer is

well ventilated (EN ISO 6946)

Using flixo: either by DXF-Import, by connecting predefined components or by drawing directly inside flixo

Model: Geometry

PHI recommendations:• Length of components should be 4‐5 x width, 

when measured using exterior dimensions• Some components (with steel and concrete 

especially) may need even longer lengths.• The goal is to achieve stable isotherms before 

they reach the cutoff plane.  • Inserting a cut‐off plane before isotherms 

have achieved a stable pattern introduces error.

Using flixo: either by Drag & Drop materials from the material database or the material list or using the «Assign Property»-Tool

Model: Materials

PHI recommendations:• For unventilated air layers, use the still air U‐

value  calculator built into PHPP U‐values tab.• Or use the database contained in Flixo

(organized by direction of heat flow and offering large range of thicknesses)

Using flixo: either by Drag & Drop materials from the material database or the material list or using the «Assign Property»-Tool

Model: Materials

To check› Proper «Air»-Material:

› Air cavities: equivalent conductivities accordingto EN ISO 10077-2

› Air layers: equivalent conductivities accordingto EN ISO 6946

› Well ventilated air layers: boundary conditions accordingto EN ISO 6946

› Fillings of glazing: equivalent conductivities accordingto EN ISO 673 or by using the glazing wizard

Using flixo: Defining the start points by using the «Boundary Condition»-Tool.The boundary conditions will be applied counter clockwise up to the next start point

Model: Boundary Conditions

The boundary conditions depend on the type of analysis 1) Energy balance calculation (e.g. Uf-value, Psi-value)2) condensation risk analysis

Model: B.C. for Energy Balance Calculation

Surface resistances Rsi / Rse [m2K/W]

Model: B.C. for Condensation Risk Analysis

Surface resistances Rsi / Rse [m2K/W] Note:• These surface film resistances change 

if conducting a condensation risk analysis (frsi)

Model: B.C. for Energy Balance Calculation

Surface Temperatures for modeling constructions

Surface Temperatures for modeling windows

Exterior temperature: ‐10 Celsius (14F)Interior temperature: 20 Celsius (68F)

Exterior temperature: 0 Celsius (32F)Interior temperature: 20 Celsius (68F)

Note:The different BC temperatures can cause some headaches when calculating psi‐install 

values for windows.  Must use surface temperatures for modeling constructions, but heat flux for the window may have been calculated with 0C exterior.  If so, window heat flux needs to be recalculated at ‐10C.

The influence of periodic thermal bridges are considered in a special U-value:equivalent U-value

Using flixo: Ueq-value object (U-value tool)› Click 1 and click 2: start- and end point

of the internal or external surface counter clockwise> Click 3: position of the dimension line

U-Value: Periodic Thermal Bridges

Φ ⋅ Δ ⋅ ⟹ Φ

Δ ⋅

Recommendation:• Calculate a Ueq for any inhomogenous

layer (that is, a layer interrupted by periodic thermal bridges ‐ studs, rafters)

U-Value: Periodic Thermal Bridges

PHI recommendations:• For most of the 

inhomogenous layer, use a different material representing a combination of studs and insulation, with a lambda value calculated using Ueq

• Only insert the actual periodic bridging elements at the intersection/junction

U-Value: Periodic Thermal Bridges

Recommendations:Ueq calculation for inhomogenous layer

To create this model:1) Use one bay width, with 

stud centered2) End BCs are adiabatic3) Assign one side interior 

conditions (20C), the other exterior (‐10C)

4) Make sure the surface film resistances are set to 0!

U-Value: Periodic Thermal Bridges

Recommendations:• Once Ueq is known, calculate 

lambda value for the inhomogenous layer.

• Ueq x thickness (m) = λ• Go back to the TB model 

and create a new material with this lambda value. 

-Psi value calculation: Above grade junction

Note:• Flixo needs homogenous 

layers to calculate the U‐values used in its psi value calculation.

› Calculation according to EN ISO 10211› Often several 2dimensional calculations are needed› Often the U-value of the base constructions are unknown or can’t

be applied› 3 standard cases

-Psi value calculation: Ground

No Cellar Unheated cellar Heated cellar

› Calculation according to EN ISO 10211› Often several 2dimensional calculations are needed› Often the U-value of the base constructions are unknown or can’t

be applied› 3 standard cases

-Psi value calculation: Ground

No Cellar Unheated cellar Heated cellar

Not covered today:

-Psi value calculation: Ground

Here’s the problem: What is the U‐value of the ground?

-Psi value calculation: Ground

2 thermal bridge calculations are needed1. Heat flow through the slab + ground alone (this allows

determination of the ground effect)2. Heat flow through the complete construction (slab +

footing + wall + ground) (this allows determination of the thermal bridge effect)

Model 1 Calculation› The heat flow through the floor without thermal bridge effect has to be

calculated in a separate 2dimensional calculation – just the slab plus ground› This model is built using slab and insulation thickness as designed, but

placed on top of the ground block. The edges are given adiabatic BCs, so all heat flow is to the ground.

› All elements of the footing or thickened edge are removed.

-Value calculation: Ground, no cellar

Model 2 Calculation› 2nd model is built showing the complete junction.› Rather than using a U value for the slab, Flixo uses the heat flux calculated from the

first model.

-Value calculation: Ground, no cellar

A-E-C, *

=

T-

T- U

2·b

2=

50.49830.000

-33.93530.000

- 0.278·1.410 = 0.160 W/(m·K)

Model: Ground - Geometry

20m

20m

4m

Recommendations:• For most models, dimension 

b = 8m is acceptable• 4m is then the exact 

distance from the edge of the slab insulation to cut‐off plane

• 20m x 20m is the size of the ground block

b: effective floor dimension if known, 8 meter if the floor dimension is unknown or greater than 8 m

Model: Ground - Geometry

Material› -value of soil is 2.0 W/mK, if no other values are known

(EN ISO 10211)Temperatures

› External Temperature› Surface temperatures: -10 C

› Interior Temperature› Surface temperatures: 20 C

Surface Resistances› Adiabatic condition applies to all cut-off planes and edges

of ground block (except top)› Top of ground block is standard Rse - 0.04› Interior and exterior surface resistances of the assemblies

as normal

Model: Ground - Material, B.C.

Model: Interior footing

Linear footings away from perimeter› For interior footings, ground block is not modeled.› BC on bottom of insulation is 10°C, with no film resistance.

Post footings away from perimeter› Modeled same as the linear footing, but entered in PHPP

with length of 1m

AB

CD

E 15 15

11 11 13 13 17 17 19 19

C-G-D

=

T- U

1·b

1- U

2·b

2=

5.20110.000

- 0.117·2.250 - 0.117·2.250 = -0.005 W/(m·K)

A-B

= 5.201 W/m 88.58

U-= 0.117 W/(m

2

·K)

88.58

U-= 0.117 W/(m

2

·K)

Replacing glazing by insulation panel (EN ISO 10077-2)

d: effective thickness, if the frame is designed for a specific thickness,24 mm for double glazing, 36 mm for triple glazing otherwise

Model: FrameGeometry: Uf-Value Calculation, -Value Wall-Frame

Replacing glazing by insulation panel (EN ISO 10077-2)

Model: FrameGeometry: Uf-Value Calculation, -Value Wall-Frame

Notes:• To calculate Uf and install :• Solid “glazing panel” always 

used instead of glass.• Glazing panel λ = 0.035• Length of panel has to be at 

least 190mm starting from the sightline

• Thickness of panel = actual thickness of glass.  Or 36mm for triple glazing, if unknown.

Model: Frame Boundary Conditions: Uf-Value Calculation, -Value Wall-Frame

Temperatures> Ti=20°C, Te=0°C

Resistances> Internal

> Normal (R=0.13 m2K/W) (shown in red)> Reduced radiation/convection (R=0.20 m2K/W) (shown in yellow)

> External (R=0.04 m2K/W) (shown in blue)

Note:• Exterior temperature for    

Uf calculation = 0°C• Exterior temperature for 

install calculation =  ‐10°C

Model: Frame - Cavities

Unventilated cavities> Completely closed> Slits ≤2 mm

Slightly ventilated cavities> Slits between 2 mm and 10 mm

Well ventilated cavities and grooves> Slits greater 10 mm

Note:• Flixo will automatically 

detect type of cavity and assign associated properties accordingly

Model: Frame - Cavities

Determining factors> Geometry of the cavity> Temperature distribution in the cavity and on the cavity surfaces> Emissivity of the surfaces

Uf-value according to EN ISO 10077-2Uf-Value: Frame

Uf x bf +  Up x bp = φ/ΔT

Using flixo: Uf-value object (U-value tool)

> Click 1: Select the result object

> Define the width of the frame bf

> Flixo calculates Upand φ, then solves for Uf

A

B

Uf A,B

=

T- U

p·b

p

bf

=

5.39220.000

- 0.649·0.192

0.123= 1.19 W/(m

2

·K)

-= -5.392 W/m

191.50 122.60

U-= 0.649 W/(m

2

·K)

Boundary Condition q[W/m2] [

oC] R[(m

2·K)/W]

Epsilon 0.9 0.900Exterior, frame 0.000 0.040 Interior, frame, normal 20.000 0.130 Interior, frame, reduced 20.000 0.200 Symmetry/Model section 0.000

Material [W/(m·K)]

Aluminium (Si Alloys) 160.000 0.900EPDM (ethylene propylene diene monomer) 0.250 0.900Elastomeric foam, flexible (1) 0.050 0.900Panel 0.035 0.900Silicone, pure (1) 0.350 0.900Slightly ventilated air cavity Softwood 500, typical construction timber 0.130 0.900Unventilated air cavity

2 thermal bridge calculations are needed1. Heat flow through the frame + glazing panel alone (at -10C)2. Heat flow through the complete construction (wall + frame

+ panel) with the same boundary conditions

install value

install value

Model 1 Calculation› The heat flow through the

frame + glazing panel

› Exterior BC is now -10C› Flux value is higher

A

B

Boundary Condition q[W/m2] [

oC] R[(m

2·K)/W]

Epsilon 0.9 0.900Exterior, normal -10.000 0.040 Interior, frame, normal 20.000 0.130 Interior, frame, reduced 20.000 0.200 Symmetry/Model section 0.000

Material [W/(m·K)]

Aluminium (Si Alloys) 160.000 0.900EPDM (ethylene propylene diene monomer) 0.250 0.900Elastomeric foam, flexible (1) 0.050 0.900Panel 0.035 0.900Silicone, pure (1) 0.350 0.900Slightly ventilated air cavity Softwood 500, typical construction timber 0.130 0.900Unventilated air cavity

A-B

= 8.1 W/m

install value

Model 2 Calculation› 2nd model is built

showing the complete junction (wall + frame + panel).

› Rather than using a U value for the frame + panel, Flixo uses the heat flux calculated from the first model.

A B

C D

E

0

0 10 -8 -4 4 8

12

14

16 18

18

18

Material [W/(m·K)] Aluminium (Si Alloys) 160.000 0.900EPDM (ethylene propylene diene monomer) 0.250 0.900Elastomeric foam, flexible (1) 0.050 0.900Fiberglass batt Lambda 0.044 0.044 0.900Gypsum plasterboard (1) 0.250 0.900Panel 0.035 0.900Plywood Lambda 0.12 0.120 Silicone, pure (1) 0.350 0.900Slightly ventilated air cavity Softwood 500, typical construction timber 0.130 0.900Timber 450 kg/m3 (softwoods) 0.120 0.900Unventilated air cavity XPS Lambda 0.029 0.029

Boundary Condition q[W/m2] [

oC] R[(m

2·K)/W]

Epsilon 0.9 0.900Exterior, normal -10.000 0.040 Exterior, ventilated, horizontal -10.000 0.130 Interior, frame, normal 20.000 0.130 Interior, frame, reduced 20.000 0.200 Interior, heat flux, downwards 20.000 0.170 Interior, normal, horizontal 20.000 0.130 Symmetry/Model section 0.000

A-E-C, *

=

T-

T- U

2·b

2=

14.16130.000

-8.060

30.000- 0.168·1.029 = 0.030 W/(m·K)

A-C

= -14.161 W/m 3

14.1

0 1

028.

70

U-= 0.168 W/(m

2

·K)

-Psi value calculation: Reference Point – Exterior Corner

0

0

15

15

-9

-9

-9

-9

-6

-6 6

6

18 18Reference Point

> Choose the reference point based on exterior dimensions

> Exterior dimensions should be taken from the outside face of thermal envelope

> Flixo will automatically measure from the cut-off planes to the reference point.

Exterior

Interior

0

0 15

15

-9

-9 -9

3

6

6 9

12

18

18

-Psi value calculation: Reference Point – Interior Corner

Reference Point> Choose the reference

point based on exterior dimensions

> Exterior dimensions should be taken from the outside face of thermal envelope

> Flixo will automatically measure from the cut-off planes to the reference point.

Exterior

Interior

0

0 10

10

-8

6

16

-Psi value calculation: Reference Point – Parapet

Exterior

Interior

Reference Point> For parapet TB, do not

include any part of the parapet above the top surface of the roof’s thermal boundary

-Psi value calculation: Reference Point – Perimeter

Exterior Interior

Reference Point> For perimeter TB, do not

include any part of the footing below the lowest surface of the slab’s thermal boundary

0

0

15

15 -9

-9 6

6

9

9

-Psi value calculation: Reference Point – Rim joist

Exterior Interior

Reference Point> For rim joist, position of the

reference point doesn’t matter much if the two wall assemblies are the same.

0

0

15

15 -9

-9

6

-Psi value calculation: Reference Point – Rim joist

Exterior

Interior

Reference Point> For rim joist with two

different assemblies, the reference point needs to be aligned according to the assembly dimensions used in PHPP.

Notes:• Does the above grade wall 

extend to here in PHPP?

0

0

15

15 -9

-9

6

-Psi value calculation: Reference Point – Rim joist

Exterior

Interior

Reference Point> For rim joist with two

different assemblies, the reference point needs to be aligned according to the assembly dimensions used in PHPP.

Notes:• Or here?

0

0 10 -8 -4 4 8

12

14

16 18

18

18

-Psi value calculation: Reference Point – Windows

Exterior

Interior

Reference Point> For the window install psi

value, the reference point is aligned with the bottom of the window frame, even if the frame is overinsulated.

Model: Standards

Thermal Bridge, Model› EN ISO 10211 (geometry, mesh, accuracy, ground)› EN ISO 10077-2 (frame U-value, edge -value)› EN ISO 12631 (Ucw-value, 3D elements like screws)

Materials› EN ISO 10456 (general materials)› EN ISO 6946 (air layers)› EN ISO 10077-2 (frame)› EN ISO 673 (filling of glazing)

Boundary Conditions› EN ISO 6946 (general)› EN ISO 13788 (mold, condensation)› EN ISO 10211 (ground)› EN ISO 10077-2 (frame)

ggff bUbUT

-value: Spacer

2 calculations are neededa) Uf-value calculation with insulation panel instead of glazingb) Heat lost calculation with glazing and spacer

Using flixo: Psi-value Edge (context menu)> Click 1: selection of the result object> Select the file with the Uf-value calculation