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Dr Paul Baker
Centre for Research on Indoor Climate & Health
School of Engineering & the Built Environment
Glasgow Caledonian University
Summarise recent work on improving the thermal performance of traditional timber windows including field trials of ‘slim-profile’ double glazing replacement panes in Georgian sash windows in Edinburgh.
Condensation risk?
Comfort?
• 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)
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.
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
Metallised interior of
honeycomb
http://www.conservationglazing.co.uk/
0
10
20
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40 45 50
PRESSURE DIFFERENCE DP [Pa]
AIR
FL
OW
[cu.m
/h]
HS WINDOW AS RECEIVED
HS WINDOW AFTER DRAUGHT-PROOFING
HS WINDOW DRAUGHT-PROOFED + 2ndry GLAZING
Professional draught proofing
reduces air leakage by 86%
compared with the as-received
condition
Acoustic secondary glazing (New Lanark)
Single glazing + low-e secondary glazing (Edinburgh)
Single glazing + Vacuum secondary glazing (New Bolsover)
Single glazing + Shutters (Edinburgh)
Insulated blind (Glasgow)
2.0 1.6 0.8 2.2 2.5
Changeworks
• 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.
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
http://www.pilkington.com/resources/pilkingtonspaciaenglish1.pdf
Two years on ... second series of measurements indicates no significant changes in thermal performance (HSTP 20 will present results).
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.
Historic Scotland Technical Paper 1 (under revision)
www.historic-scotland.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
Changeworks – Energy Heritage – A guide to improving energy efficiency in traditional and historic homes
www.changeworks.org.uk/uploads/83096-EnergyHeritage_online1.pdf
Singled glazed timber windows
50% RH = 6g after 5 hours 60% RH = 83g after 5 hours
As the overall U-value improves the internal surface temperature of original glazing decreases, indicating an increased risk of condensation.
However, in a realistic situation, the likelihood of condensation will depend on, for example, how well the shutters, blinds and curtains fit the window thereby reducing airflow, the change in inside and outdoor temperatures and humidity within the room, the room ventilation rate, etc.
In a realistic situation, the likelihood of condensation will depend on, for example,
• How well the shutters, blinds and curtains fit the window thereby reducing airflow,
• The change in inside and outdoor temperatures
• Radiation exchange with external environment (buildings, sky, etc.)
• Humidity within the room, the room ventilation rate
• etc.
Most at risk are edge of glazing areas...
With 2-D or 3-D modelling can estimate dew point temperature for given set of internal & external conditions.
Higher risk unless well sealed to prevent warm air entering gap and condensing on original pane.. Lab tests...
Condensation collected on panes of sash and case window after 5 hours with room conditions at 22oC & 60% relative humidity and external temperature of 2oC
Baker PH, Research into the thermal performance of traditional windows – Timber sash windows, English Heritage, October 2009, http://www.climatechangeandyourhome.org.uk/live/content_pdfs/580.pdf
Condensation collected,
grams
Window without secondary
glazing
33
1. Window and secondary
glazing closed
0 2. Window closed &
secondary glazing open
36 3. Window and secondary
glazing open
48 4. Window open &
secondary glazing closed
0
Replacement double glazed panes? Minimal risk?
Higher risk – worse thermal performance of frame.
Secondary glazing can be effective if tightly fitting.
Unpublished work carried out for EH.
Better sealed windows will reduce unwanted draughts, however consider impact on overall ventilation requirements..
“Build tight, ventilate right!”
Any option from curtains to replacement double glazed panes will increase room facing surface temperature
Improved radiant temperature balance in room can be a key factor in improving comfort.
Bonus impact on energy saving if reduce asymmetry?
Replacement double glazed panes in traditional timber window.
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