Thermal Bridges – Sandwich Panel Constructions

Markus Kuhnhenne

Edinburgh 23.10.2008

2

RWTH Aachen – Institute for Steel Structures

• Application areas (integrated approach)

Steel Structures

Light Weight Structures

Wind Engineering

Structural Glazing

Composite Structures

Building Physics & LCE

3

RWTH Aachen – Institute for Steel Structures

• Building Physics and

Life Cycle EngineeringConsultancy, experimental and numerical investigations, expertises, etc.

– Fire safety concepts

– Sound-, thermal- and moisture performance

– Air-tightness

– Whole energy performance and efficiency

– Thermal comfort and Daylight use

– Integration of renewable energy resources

– Life Cycle Engineering

4

Why energy efficient building design?

• New energy saving requirements in EU member states in recent years– limitation of the building energy use

• EU-Directive “Energy performance of Buildings”– limitation of the building energy use for heating, cooling and

lighting

– introducing energy certification of buildings

• Energy performance of buildings becomes an important aspect of innovative and cost-effective building design.

• Especially the optimization of the thermal protection of building envelopes is a way to improve the overall energy performance.

5

Building envelope

• Building envelopes using metal cladding systems have to be improved regarding their thermal performance

6

Building envelope

Workmanship

Structural stability

Building Physics

Fire safety

Thermal performance

Moisture proofing

Durability

Cost-effectiveness

Weather protection

Sound proofing

7

Building envelope

• Thermal performance of sandwich elements and constructions

3 main aspects:

– Air-tightness

– Minimum requirements to avoid condensation and mould growth

– Transmission heat transfer

8

Building envelope – Air-tightness

• Building envelopes have to be air-tight according to regulations and requirements (vary from country to country)

– Whole building air-tightness performance

– Air-tightness of building components

and joints

9

Building envelope – Condensation and mould growth

– θsi,min internal surface temperature, below which mould growth and condensation problems can be expected (under well defined circumstances of humidity),

– Internal temperature: θi = 20 °C

– External temperature: θe = -5 °C

– Internal thermal transfer resistance: Rsi = 0,25 (m²·K)/W

– External thermal transfer resistance: Rse = 0,04 (m²·K)/W

– Relative air humidity: ϕi = 50%

– Relative air humidity close to surface: ϕsi = 80%

• Minimum requirement at thermal bridges to avoid condensation and mould growth (Example Germany):

Csi °= 6,12min,θ⇒][7,0min, −≥−−

=ei

esiRsif θθ

θθ

10

Building envelope – Transmission heat transfer

gDT HHH +=

Transmission heat transfer via the ground acc. to EN ISO 13370gH

• Transmission heat transfer according to EN ISO 13789

Direct transmission between internal and external environmentsDH

11

Transmission heat transfer according to EN ISO 13789

• Direct transmission between internal and external environments

∑∑∑ +⋅Ψ+⋅=j

jk

kki

iiD lAUH χ

– Ui is the thermal transmittance of element i of the building envelope

– Ai is the area of element i of the building envelope

– Ψk is the linear thermal transmittance of thermal bridge k, calculated according to EN ISO 10211

– lk is the length of linear thermal bridge k

– χj is the point thermal transmittance of point thermal bridge j, calculated according to EN ISO 10211, in W/K (point thermal bridges which are normally part of plane building elements and already taken into account in their thermal transmittance shall not be added here).

12

Transmission heat transfer according to EN ISO 13789

• Sandwich panel construction - Direct transmission

∑∑∑ +⋅Ψ+⋅=j

jk

kki

iiD lAUH χ

• Plane elements– Roof

– External Walls

• Thermal bridge junctions– Corner

– Eaves

– etc.

• e.g. Penetrations

13

Transmission heat transfer – Sandwich elements

• Plane elements (U-values)

fjSEnSEd UUUU Δ+Δ+= ,,

• Nominal value depending on– profile shape

(trapezoidal, corrugated, lined)

– thermal conductivity of insulation

– thickness

• Joint performance

(Ψj - value) depending on– joint

geometry

– thermal conductivity of sheets

• Fastener performance

(χf - value) depending on– number of

fasteners per m²

– thermal conductivity of fasteners

Indices:

• d design value

• n nominal value

• j joint

• f fastener

• SE Sandwich element

14

Transmission heat transfer – Sandwich elements

• Nominal U-value of sandwich elements

sed

csi

SEn

Red

RU

+Δ+

+=

λ

1,

– Rsi and Rse surface resistance acc. to EN ISO 6946

– dc minimal thickness of thermal insulation

– λd thermal conductivity of thermal insulation (design value)

Δe-value

– Δe-value: numerical calculations

acc. to EN ISO 10211

15

Transmission heat transfer – Sandwich elements

• Influence of fasteners on the U-value of sandwich elements

• 3D numerical investigations according to EN ISO 10211 necessary,but influence of stainless steel fasteners on Ud,SE-value normally negligible (not more than 1,2 fasteners per m²)

16

Transmission heat transfer – Sandwich elements

• Influence of joints on the U-value of sandwich elements– Types according to EN 14509

– Numerical investigations recommended

Type 1Type 3

2D numerical investigations

3D numerical investigations

Type 2

17

Transmission heat transfer – Sandwich elements

• Influence of joints on the U-value of sandwich elements– Types according to EN 14509

– Numerical investigations recommended

Type 4Type 5

2D numerical investigations

BU jj

Ψ=Δ – Ψj Numerical calculations acc. to EN ISO 10211

– B Width of sandwich element

18

Sandwich constructions – Thermal bridge junctions

• Example: Verge

Ventilatedair-cavity

Part of U-value calculation (Δe-values)

19

Sandwich constructions – Thermal bridge junctions

• Example: Verge

Air-tightness layer

Insulation layer

• Thermal bridge effects– Geometry

– Material

20

Sandwich constructions – Thermal bridge junctions

• Example: Eaves (Standard Detail)

Air-tightness layer

21

Sandwich constructions – Thermal bridge junctions

0,350,57480

fRsi [-]Ψ [W/(m·K)]dd [mm]

• Example: Eaves (Standard Detail)

22

Sandwich constructions – Thermal bridge junctions

• Classifying thermal bridge effects

very important effectimportant effectpoor effectnegligible effect

C4

Ψ ≥ 0,50

C3

0,25 ≤ Ψ < 0,50

C2

0,10 ≤ Ψ < 0,25

C1

Ψ < 0,10

Classes of thermal bridge effect, based on the evaluation of the ψ-value

Source: Practical guide for the hygrothermal evaluation of thermal bridges

23

Sandwich constructions – Thermal bridge junctions

• Example: Eaves (Enhanced Detail)

24

Sandwich constructions – Thermal bridge junctions

0,83

with cut

0,029

with cut without cutwithout cut80

0,700,138

fRsi [-]Ψ [W/(m·K)]dd [mm]

• Example: Eaves (Enhanced Detail)

25

Transmission heat transfer – Industrial Building

• Thermal Bridges Junctions (Ψ-values)

l

b

h

A

O

N

M

L

K

J

IH

G

F

D

BC

E

SillO

Window SillN

Window JambM

Window HeadL

Door JambK

Door HeadJ

Large Door JambI

Large Door HeadH

Lateral joint external wallG

CornerF

EavesE

VergeD

Lateral joint roofC

RooflightB

RidgeA

JunctionDetail

26

Transmission heat transfer – Industrial Building

• Calculation procedure applied in Germany

]/[ KWLAUHk

kkii

iT ∑∑ ⋅Ψ+⋅=

• Option 1:

• Plane elements– Roof, External Walls (Ud,SE-values)

– Rooflights, Windows, Doors

– Basement

• Thermal bridge junctions– Corner

– Eaves

– etc.

27

Transmission heat transfer – Industrial Building

• Calculation procedure applied in Germany

]/[ KWAUAUHi

iTBii

iT ∑∑ ⋅Δ+⋅=

ΔUTB is the global additional value for transmission heat losses of thermal bridge junctions

ΔUTB,EnEV = 0,1 W/(m²·K) for sandwich constructions (EnEV = German legislation)

• Option 2:

28

Transmission heat transfer – Variation Industrial Building

• Calculation procedure applied in UK

• UK regulations require a maximum of transmission heat transfer via thermal bridge junctions of 10 % of the transmission heat transfer through plane elements.

][1,0 −≤⋅

⋅Ψ=∑∑

iii

kkk

cal AU

Lα

29

Thermal performance – Sandwich constructions

• Lightweight metal envelope constructions have to be– planned

– designed

– assembled

with respect to building physics requirements

• Three main aspects regarding thermal performance– Air-tightness

– Risk of condensation and mould growth

– Transmission heat transfer

• Thermal performance regulations are depending on– National legislations and requirements

– Climatic conditions

30

Thermal performance – Sandwich constructions

• Tasks for all– manufactures

– assembling companies

– associations

• Tasks– Research and Development

– Further training and education

– Quality Control on building sites

31

Thermal performance – Sandwich constructions

• Modular Research Building in Steel: “Sandwich-Demo-House”

– European and German research projects

– Education of manufactures, architects, engineers, assemblers, students

32

Energy performance of buildings

• Development of requirements in Germany

33

Energy performance of buildings

• Thank you for your attention!

... and yourbuilding energyuse per m² ?