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.