Research on improving the thermal
performance of traditional windows and
thermal performance monitoring
Dr Paul Baker
Centre for Research on Indoor Climate & Health
School of Engineering & the Built Environment
Glasgow Caledonian University
Introduction
Summarise recent work on improving the thermal performance of
tradition windows including field trials of ‘slim-profile’ double
glazing replacement panes in Georgian sash windows in
Edinburgh.
Presentation of methodology for in situ thermal performance
testing.
Comparison of measured results with calculation methods.
Traditional Windows
•
There are c.44 million single-glazed sash windows in the UK
(Source: English Heritage)
•
20% of domestic properties in UK are pre-1919, i.e. traditional
buildings (Source: Scottish House Condition Survey)
•
Single glazing & poor window insulation can account for 20%
of a home’s heat loss (Source: Energy Saving Trust)
•
An average house with 7 timber single-glazed sash windows
can lose £211 a year through these windows (Source:
www.energyratedwindows.co.uk)
•
72% of heat loss from a single-glazed window occurs
through the glazing (Source: Historic Scotland Technical Paper 1:
Thermal performance of traditional windows, 2008)
Traditional Windows
Easy option for replacement with modern double glazing?
Draughty, prone to condensation and hard to maintain.
Major problem – poor thermal performance of single glazing:
• U-value> 5 W/m2K
But, with good care and maintenance traditional windows can
outlast modern replacements, define the ‘look’ of a building and
can be considered as a sustainable resource.
Traditional Windows
Secondary glazing – seen as most effective option to preserve
existing traditional windows and reduce heat loss.
There has been little information on the performance of more
traditional (and cheaper) methods of reducing heat loss, such
as, draught proofing, shutters, blinds and curtains.
Research carried out for Historic Scotland & English Heritage has
sought to quantify the benefits of
• Draught-proofing
• Blinds, curtains, & shutters
• Secondary glazing
• Replacing single glazed panes with double glazing
Lab based tests using environmental chamber
Heavy Curtains
Traditional Shutters
Insulated Shutters with Spacetherm
Modern Roller Blind
Blind with low-e foil
applied
Victorian Blind
Honeycomb Blind
Metallised interior of
honeycomb
Low-e Secondary Glazing
U-values
Effect of Shutters
Draughtproofing
80
HS WINDOW AS RECEIVED
70
HS WINDOW AFTER DRAUGHT-PROOFING
HS WINDOW DRAUGHT-PROOFED + 2ndry GLAZING
AIR FLOW [cu.m/h]
60
50
40
Professional draught proofing
reduces air leakage by 86%
compared with the as-received
condition
30
20
10
0
0
5
10
15
20
25
30
35
PRESSURE DIFFERENCE DP [Pa]
40
45
50
Slim-profile double glazing trial
Lister Housing Co-operative, Edinburgh
Changeworks/Historic Scotland
Changeworks
Slim-profile double glazing trial
•
Georgian (1820s) tenements
•
Traditional sash & case windows with single glazing
•
World Heritage Site/‘B’ listed
•
DG not currently permitted
•
Social housing
•
High fuel bills/Electric heating
•
Excessive condensation, etc.
Slim-profile double glazing trial
DG unit thicknesses 9,11, 12, 16mm.
Low-e coated glazing
Gas fill:
• Air
• Argon
• Krypton
• Xenon & Krypton
Quoted U-values:
• 1.8 – 2.6 W/m2K
Vacuum Glazing
http://www.pilkington.com/resources/pilkingtonspaciaenglish1.pdf
Beauty is in the eye of the beholder
In Situ U-value Measurements
Measured U-values
Heat loss through whole window
Slim-profile double glazing trial - conclusions
Two years on ... second series of measurements indicates no
significant changes in thermal performance.
Vacuum glazing superior to conventional slim-profile DG.
Secondary glazing generally superior to conventional slim-profile
DG.
Design of slim-profile units not systematic in terms of cavity width
and gas type – manufacturers should follow standard
calculation procedures to optimise thermal performance.
Some concerns about deterioration of conventional edge seal
materials due to reaction with putty. No such concerns with
vacuum glazing with glass edge seal.
Suggested reading for improving energy efficiency of
traditional windows
Historic Scotland Technical Paper 1 (to be revised)
www.historicscotland.gov.uk/thermal_performance_of_traditional_windows_2010.pdf
Historic Scotland Technical Paper 9
www.historic-scotland.gov.uk/slim-profile_double_glazing_2010.pdf
English Heritage - Improving the Thermal Performance of Traditional
Windows
www.climatechangeandyourhome.org.uk/live/research_generic.aspx
In situ Thermal Performance Testing
Main approach has been to measure the in situ U-values of building
envelope elements using heat flux sensors.
In situ U-value Measurements
RoomTemperature
ExternalTemperature
Heat Flux Meter
Interior Surface Temperature
External Surface Temperature
Analysis of in situ heat flux data – averaging method
•
Generally, the heat flux and wall surface temperatures are measured
over a suitable period: a minimum of 14 days is usually sufficient.
Averaging over the monitoring period is used to obtain the U-value:
Ut 
1
it
 Tsi  Tse
i
0
i
it
Q
0
Temperature difference
across element
i
 rint  rext
Heat Flux
Can also use “parameter identification” techniques
Use full dynamic data.
Take into account the thermal capacity of the wall.
Appropriate if there are large diurnal swings in external conditions as
may be experienced during spring, or changes in the weather
pattern during the test period.
Example of a wall modelled as
a network of conductances and
capacitances
Checking result....cumulative average U-value
Use of in situ values
Traditional buildings may have unknown construction details or
material properties – difficult to estimate U-value using U-value
Calculators.
Comparison of in situ measurement with calculated values have
shown that calculated values often overestimate U-value (SPAB,
HS & EH projects).
– Databases of U-value calculators are focussed on modern
materials.
– Lack of information on traditional material properties.
Stone rubble wall
?
?
+voids?
Stone rubble wall
The influence on the calculated Uvalue of the assumed proportion of
mortar in a 600mm sandstone wall
with 25mm lime plaster on the hard.
Sandstone =2 W/mK
Lime Mortar =0.7 W/mK
Use of in situ values
Recommend that where possible measured in situ U-values should
be used as data for input into performance assessments – e.g.
SAP.
Acceptable as an alternative to default values used in RDSAP?
No current standard for measurement.
Use in situ U-value & airtightness measurements
Measure Heat Loss through building
envelope
In situ U-value measurements in
representative locations for walls,
floors & ceiling. Centre-of-pane for
glazings.
Airtightness – background ventilation
Blower door tests
Air leakage rate @ 50Pa
Calculate Element Areas from
Building dimensions
A1…etc.
Calculate UxA values
For windows and doors use centre-ofpane values in BRE U-value calculator
to first calculate whole window Uvalues.
Use same procedure as
SAP ‘Heat losses..’
worksheet to calculate
Heat Loss Coefficient,
etc.
Use in SAP
Measured in situ
Heat loss
Heat loss
coefficient, parameter,
2
W/K
W/m K
Rating
SAP2005 using
default U-values
366
4.9
D56
SAP2005 using
in-situ U- values
318
4.2
D60
In Situ Thermal Performance Testing - Conclusions
A robust method of measuring the in situ thermal performance of
building elements has been developed over a number of
campaigns, producing new data on our traditional buildings.
The tests have shown the need for improved material
properties/default values for calculation methods.
Where feasible, recommend in situ U-value measurements to
improve accuracy of energy performance assessment,
particularly before upgrading insulation in traditional buildings.
Impact on Green Deal?
Standard or Guidelines on in situ measurements required.
Historic Scotland Technical Paper 10
www.historic-scotland.gov.uk/hstp102011-u-values-and-traditionalbuildings.pdf
Thank You