Chris SandersChris Sanders Glasgow Caledonian University ... · Chris SandersChris Sanders Glasgow Caledonian University 23/10/08. Centre for Research on Indoor Climate and Health

Centre for Research on Indoor Climate and Health

Heat loss though twin skin metal walls and roofswalls and roofs

Chris SandersChris SandersGlasgow Caledonian University

23/10/08


Heat loss through the fabric is an important component of the energy demand of all buildings gy g


Three important parameters

U-value, Ψ-value, f-valueWhy do these matter?How do we find them?How do we find them?What affects them in metal constructions?


Fabric heat loss

Combination ofPlane areas with an area and U-valueJoints between them with length and ψ-value


Thermal properties of the building fabric

U-value - W/m2K - heat loss through plane areas of fabricareas of fabricΨ-value - W/mK - heat loss through thermal bridges at edges between plane areasf-value - surface temperature factor atf value surface temperature factor at thermal bridge - gives risk of condensation


Total Fabric TransmittanceU value in W/m2K Area in m2U-value in W/m2K, Area in m2

Ψ-value in W/mK Length in mΨ value in W/mK, Length in m

Total fabric heat loss from buildingTotal fabric heat loss from building

( ) ( )TTlUA ∑∑( ) ( )outin TTlUA −×Ψ×+×= ∑∑


Calculation of U-values Pre 1995

A series of uniform layersA series of uniform layers

With 1 D Heat flow


Material thermal conductivitiesMaterial Conductivity W/mK

Metals Steel 50 Stainless steel 17Aluminium 160 Copper 380 Other materialsOther materialsTimber 0.14 PVC 0.17 Polyester resin 0.20 Polycarbonate 0 20PolycarbonatePolythene thermal break

0.200.37

Insulants Glass fibre 0 040Glass fibre 0.040Mineral wool 0.037 Expanded polystyrene 0.035 Urethane 0.022


Repeated thermal bridgesCombined Method : BS EN 6946Combined Method : BS EN 6946


Metal components distort heat flow soMetal components distort heat flow so much that simple rules do not work


Need to use software to build 3DNeed to use software to build 3D models to calculate heat flows


What should be included in the model?

Any high conductivity materials that crossesmaterials that crosses the insulation layerAnything that affects the insulation thicknessinsulation thicknessShould a feature be

i d t th U lassigned to the U-value or an adjacent ψ-value?


Model should be representative ofrepresentative of repeat distance of i t t f timportant features


Many structures contain air cavities

Thermal properties ofThermal properties of cavity depends on:

O i t tiOrientation

Dimensions

Emissivity of surfaces


Grid definitionGrid definition


U value found from calculated heatU-value found from calculated heat loss through model


S i lifi d l h bSome simplified rules have been worked out for some structures

Zed spacer walls and roofsBRE IP 10/02: Metal cladding: assessing the thermalBRE IP 10/02: Metal cladding: assessing the thermal performance of built-up systems which use ‘Z’ spacers, BRE 2002

Rail and bracket spacer walls and roofsSCI Technical Information Sheet P312 Metal cladding: gU-value calculation, SCI 2002

BRE U-value calculator available from:BRE U value calculator, available from: http://www.brebookshop.com/details.jsp?id=139470


BRE U-value calculator


Fabric heat loss

Combination ofPlane areas with an area and U-valueJoints between them with length and ψ-value


Ψ- value Linear thermal transmittance

roofroofwallwalloutin

LULUTT

Q×−×−

−=Ψ


U-values for Ψ-values

Because the calculation of the Ψ-value depends on the difference between the heat flow term and the U-value terms, it is very sensitive to ythe U-value used. Therefore, the U-value of the plane elements should never be assumed, but always calculated for each plane element from the output of the model.the output of the model.

roofroofwallwalloutin

LULUTT

Q×−×−

−=Ψ

It is essential that the U-values used to calculate the Ψ-value, are those that apply to the plane areas in the model of the thermal b id th b li htl diff t f th l l t d f

outin

bridge; these may be slightly different from those calculated from a representative model of the plane area. Even though this difference might be small it may be sufficient to change the Ψ-value significantly.


N d d id h h fNeed to decide whether a feature contributes to the U-value or Ψ-value


f-value - surface temperature factor

TTiTeTsf −

=TeTi−

Te)(TifTeTs −×+=

If Ti = 18 Te = -5If Ti 18, Te 5

Ts = 13.9


Why do low surface temperatures matter? 1 71.7

kPa

100 9080 70

60Relati e H midit %

condensation mould1.5

sure

: k

50

Relative Humidity %

O

1.3

r Pre

ss

40

1.1

Vapo

ur

40

0.910 12 14 16 18 20 22 24

V

10 12 14 16 18 20 22 24

Temperature : C


Mould and condensation

In housing:

Absorbent surfacesMetal cladding

Non absorbent surfacesMould at RH>80%

Health Problems

Non absorbent surfaces

Condensation RH=100%

D i iHealth Problems Dripping?


Point thermal bridges


StandardsBS EN ISO 6946 :2007BS EN ISO 6946 :2007Building components and building elements —Thermal resistance and thermal transmittance —Thermal resistance and thermal transmittance Calculation method

BS EN ISO 10211-1:1996 -Thermal bridges in building construction — Calculation of heat flows and surface temperatures — Part 1: General methods (under revision)




Guidance for calculations – BR497



RidRidge

Provided the insulation is complete across the ridge and there are no extra fixings that have not been taken into account in the U-value of the roof surface, the only thermal bridge occurs because of the larger area on the outside surface compared to the inside The ψ-value and f-value will vary slightly with the roof slopecompared to the inside. The ψ-value and f-value will vary slightly with the roof slope, however they can be assumed to be: Ψ = 0.01 W/mK f = 0.95


Hip

Essentially the same as the ridge Ψ = 0.01 W/mK f = 0.95


Valley Gutter

Potential severe thermal bridge at gutter head


Eaves

If the roof cladding liner is taken back to the wall liner, and the air gap filled with insulation, the Ψ-value and f-value can be assumed to be: Ψ = 0.02 WmK, f = 0.95

If one of the liners fully crosses the insulation to touch the outer sheet, Typical values will be: Ψ = 0.16 WmK, f = 0.84


Verge

Essentially the same as the eaves Ψ = 0.02 WmK, f = 0.95


Corner

If the roof cladding liner is taken back to the wall liner, and the air gap filled with insulation, the Ψ-value and f-value can be assumed to be: Ψ = 0.02 WmK, f = 0.95

If one of the liners fully crosses the insulation to touch the outer sheet, typical values will be: Ψ = 0.16 WmK, f = 0.84


Drip Sill – cladding meets masonry or concreteWhat should we model?

Ψ = 1.10 WmK, f = 0.61

Ψ = -0.02 WmK, f = 0.94


Window surrounds

Jamb Head

Sill

All Ψ=0 05 f= 0 95All Ψ=0.05, f= 0.95


Garbage inGarbage inGarbage out

Charles Babbage

Difference Engine 1822Difference Engine 1822

Babbage said:On two occasions I have been asked,—"Pray, Mr. Babbage, if , y, g ,you put into the machine wrong figures, will the right answers

come out?" I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question

Documents

Chris SandersChris Sanders Glasgow Caledonian University ... · Chris SandersChris Sanders Glasgow Caledonian University 23/10/08. Centre for Research on Indoor Climate and Health