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
Centre for Research on Indoor Climate and Health
Heat loss through the fabric is an important component of the energy demand of all buildings gy g
Centre for Research on Indoor Climate and Health
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?
Centre for Research on Indoor Climate and Health
Fabric heat loss
Combination ofPlane areas with an area and U-valueJoints between them with length and ψ-value
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
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×+×= ∑∑
Centre for Research on Indoor Climate and Health
Calculation of U-values Pre 1995
A series of uniform layersA series of uniform layers
With 1 D Heat flow
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
Repeated thermal bridgesCombined Method : BS EN 6946Combined Method : BS EN 6946
Centre for Research on Indoor Climate and Health
Metal components distort heat flow soMetal components distort heat flow so much that simple rules do not work
Centre for Research on Indoor Climate and Health
Need to use software to build 3DNeed to use software to build 3D models to calculate heat flows
Centre for Research on Indoor Climate and Health
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?
Centre for Research on Indoor Climate and Health
Model should be representative ofrepresentative of repeat distance of i t t f timportant features
Centre for Research on Indoor Climate and Health
Many structures contain air cavities
Thermal properties ofThermal properties of cavity depends on:
O i t tiOrientation
Dimensions
Emissivity of surfaces
Centre for Research on Indoor Climate and Health
Grid definitionGrid definition
Centre for Research on Indoor Climate and Health
U value found from calculated heatU-value found from calculated heat loss through model
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
BRE U-value calculator
Centre for Research on Indoor Climate and Health
Fabric heat loss
Combination ofPlane areas with an area and U-valueJoints between them with length and ψ-value
Centre for Research on Indoor Climate and Health
Ψ- value Linear thermal transmittance
roofroofwallwalloutin
LULUTT
Q×−×−
−=Ψ
Centre for Research on Indoor Climate and Health
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.
Centre for Research on Indoor Climate and Health
N d d id h h fNeed to decide whether a feature contributes to the U-value or Ψ-value
Centre for Research on Indoor Climate and Health
f-value - surface temperature factor
TTiTeTsf −
=TeTi−
Te)(TifTeTs −×+=
If Ti = 18 Te = -5If Ti 18, Te 5
Ts = 13.9
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
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?
Centre for Research on Indoor Climate and Health
Point thermal bridges
Centre for Research on Indoor Climate and Health
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)
Centre for Research on Indoor Climate and Health
Centre for Research on Indoor Climate and Health
Centre for Research on Indoor Climate and Health
Guidance for calculations – BR497
Centre for Research on Indoor Climate and Health
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
Hip
Essentially the same as the ridge Ψ = 0.01 W/mK f = 0.95
Centre for Research on Indoor Climate and Health
Valley Gutter
Potential severe thermal bridge at gutter head
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
Verge
Essentially the same as the eaves Ψ = 0.02 WmK, f = 0.95
Centre for Research on Indoor Climate and Health
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
Centre for Research on Indoor Climate and Health
Drip Sill – cladding meets masonry or concreteWhat should we model?
Ψ = 1.10 WmK, f = 0.61
Ψ = -0.02 WmK, f = 0.94
Centre for Research on Indoor Climate and Health
Window surrounds
Jamb Head
Sill
All Ψ=0 05 f= 0 95All Ψ=0.05, f= 0.95
Centre for Research on Indoor Climate and Health
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