4       Windows

In cooperation with AEE

4.1       General principle of windows

The main functions of windows are to create a sight connection from inside to the ambient environment and to provide the premises with sufficient daylight, which both are essential for the well-being and health of the users of the building.

In old buildings the large windows ensure the illumination of typical generous rooms in summer and in the winter the solar gains help reducing the heat demand.

However, windows do not only have positive effects on the indoor environment of a building.

Since windows have by nature higher U-values than wall constructions, they rate among the weak points of a building envelope from thermo-technical point of view. Building materials and techniques to guarantee air tight junctions between window frames and wall construction in order to avoid infiltration or unwanted ventilation heat losses were not commonly applied until the last decades, with the reduction of energy consumption in buildings becoming more and more important. Therefore, especially in winter months, old and leaky windows in historic buildings cause energy losses through transmission and infiltration.  These energy losses also have a big influence on the thermal comfort for the users of the building. Because of the low heat insulation capacity of old windows the heat energy in conditioned rooms is conducted outside through the windowpanes. This heat flow causes an uncomfortable cold surface of the windowpanes, which is recognized as cold radiation by the users staying near the window (see Part 2 – Chapter 1 Indoor user comfort).

Uncontrolled ventilation or infiltration heat losses because of leak windows cause a cooling down of the air respectively of the wall construction in the window area. The chilled air can be experienced as cold draft, which results in discomfort for users situated in areas near the window. Air humidity might condense on cool wall surfaces or window frames. This condensate can lead to construction damages and the development of mould.

In summer large solar gains can lead to an overheating of the building. Despite historic buildings usually have a large thermal mass, especially in warm climates with long hot periods it is important to avoid overheating in summer by limiting incoming light, through shading or radiation screening. Once historic buildings have been overheated, due to their thermal mass, they cool down very slowly.

According to the level of technical progress and economical situation in the year of construction, historic buildings are equipped with different window types. Depending on the applied technology retrofitting measures has to be developed.

The following window types in Table 3 should be retrofitted, modernized or replaced:

Table 3: Development of windows in historic buildings [ORT05]

Windows with metal frames U-value: 3,5 – 5,0 W/m²K					1920  - 2008
Coupled windows U-value: 2,3 – 2,9 W/m²K					1910 - 1980
Box-type windows U-value: 2,2 – 2,5 W/m²K				1850 - 1940
Single glazed windows with additional window in the façade level U-value: ~ 3,0 W/m²K			1820 - 1900
Single window with single glazing U-value: ~ 4,6 W/m²K	- 1850

 

The easiest and most effective intervention may result from be the replacement of the whole windows. In most historic buildings windows cannot be changed because of monumental protected facades. In this case the historic frames have to be retrofitted. They have to be tested for the exclusion of moistness or rotting and for the capability of their casements to take up more loads for the case they will be equipped by heavy, high efficient double glazing.

Different retrofit solutions for windows are described in the following subchapters.

 

4.2       Integration of high efficient glazing and frames

Generally the restoration of historic windows should be preferred to their replacement. After physical evaluation of the windows concerning their soundness and, in consultation with the local monument protection authorities, different solutions to integrate high efficient glazing and frames into historic buildings can be implemented.

The result of the physical evaluation could be that the historic float glazing is damaged or thermally inefficient, but that the frame is in a good and stable condition, for example only slight wood and painting works have to be performed. In this case, the glazing can be replaced by high efficient insulating glazing, mostly consisting of two windowpanes with 10 to 16 mm distance. Attention has to be paid on the load bearing capacity of the frame and fittings, because high efficient glazing is considerable heavier than historic float glazing. The space between the windowpanes can be filled with inert gases like argon or krypton. A weak point of this solution is the junction between the glazing and the wooden glazing-bar, which means a constructive heat bridge along the glazing margins, where heat losses and condensation of humidity may occur.

If the physical evaluation of the windows reveals that the sashes cannot be restored mostly because of moisture damages of the wood construction, a new reconstructed high efficient sash can be installed. In this case the wooden glazing-bar do not have to penetrate the high efficient glazing, but the sash can be glued onto the glazing. Another possibility would be that the glazing-bars inside the two planes of the insulating gas-filled glazing consist of heat conductive materials like plastics or precious metals. Through this measure the effect of the heat bridge can be minimized.

In both cases the installation of light-selective glazing can be considered, if this is compatible with the requirements of the monumental protection authorities. Light-selective glazing is coated with materials, which influence the translucence of the glass. The less translucent the glazing, the less heat energy from sun radiation can enter the room. This means an advantage in summer months, while in winter less solar gains can be registered than through glazing with high translucence.

Figure 28 shows a historic building, which upper window is kept in its original shape. Its balcony door below is upgraded by high efficient glazing and frame. The characteristic re-entrant corners as seen in the upper area of the original window could not be reconstructed in the high-efficient copy of the balcony door [GAM01].

Figure 28: Original and reconstructed window respectively balcony door in a historic building [GAM01]

Another solution to improve the thermal quality of single-glazed windows is to create a box-type window by presetting an additional window with high efficient glazing and frame inside the original historic window. With this measure the outside appearance of the building front will not be changed, though the thermal and soundproofing quality of the building will be improved considerably. Attention has to be paid on the dimension of the interior sash. If the exterior sash opens inwards, the additional interior sash has to be larger than the exterior, so that both sashes can be entirely opened. In the case that the exterior sash opens outwards, both sashes can be constructed in the same dimension. It has to be noted, that through the installation of an additional sash, the usage of the windowsill will be limited. Figure 29 shows a historic window, which is completed by an additional interior sash in opened (left picture) and closed (right picture) position [GAM01].

 

               

Figure 29: Historic window with addition interior sash [GAM01]

4.3       Box-type window constructions (buffer and change of interior sash)

Box-type windows in historic buildings have a positive influence on the indoor environment. In comparison to historic single glazing windows they have good thermal and soundproof insulation qualities. Though box-type windows have a minor U-value of about 2.5 W/(m²K), their deep structures are due to a temperature profile in the wall-window connection area, that does not generate risk of mould or humidity condensate.

The thermal quality of a box-type window can be improved by replacing the glazing into high efficient insulation glazing. It is recommended to replace the glazing and to improve the air tightness of only the interior sash and to keep the original exterior sash. Through this measure possible humidity can penetrate the window and doesn’t condensate on the colder exterior windowpane.

Table 4 shows the effect of glazing variations on the U-value of a box-type window:

Table 4: U-values of box-type windows [GAM01]

Window type	U-value of box-type window [W/m²K]
Exterior sash (Single glazing Ug=5,70 W/m²K)	2,43
Interior sash (Single glazing Ug=5,70 W/m²K)
Exterior sash (Single glazing Ug=5,70 W/m²K)	1,84
Interior sash (High efficient glazing Ug=3,00 W/m²K)
Exterior sash (Single glazing Ug=5,70 W/m²K)	1,24
Interior sash (High efficient glazing Ug=1,30 W/m²K)

 

Figure 30 shows a historic box-type window, which interior sash was replaced by a high efficient tilt and turn window without glazing-bars. The exterior sash shows the original slight glazing-bars typical for historic windows.

Figure 30: Box-type window with high efficient interior sash [GAM01]

Table 5 shows different temperature profiles in the window and wall construction in the case of retrofitting a box-type window (1) by improving the thermal performance of the window through exchanging the exterior glazing into high efficient glazing (2) and by replacing the box-type window into a simple window with high efficient glazing (3). Under the conditions that the exterior temperature is –12°C and the interior temperature is +20°C, as expected the temperature of the inner surface of the window is 4.2°C in case (1), 13.1 °C in case (2) and 13.8°C in case (3). Special attention has to be paid on the junction point between wall and window. In case (1) and (2) the temperature of the junction point between wall and window does not fall under 15.1°C, where in case (3) it is only 11.6°C, which means a high risk for condensate and mould formation.

As a conclusion it can be stated, that energetically and from user comfort point of view it is more advantageous to improve box-type windows by reconstruction measures, like the installation of high efficient glazing in their exterior wings, than to replace them into simple windows with high efficient glazing.

Table 5: Temperature profile after different retrofit measures [GAM01]

1	2	3
Original state	Retrofit measure – improvement	Retrofit measure – replacement
Box-type window with float glazing	Box-type window with exterior high efficient glazing	Simple window with high efficient glazing

 

4.4       How to improve the air tightness of windows

Leaking, not airtight wooden windows can be sealed against unwanted infiltration energy losses by implementing a silicone or natural rubber sealing stripes. The stripes are fixed in milled gaps in the window sash, without using any kind of glue. Through this measure not only the thermal behaviour of the window can be improved, but its sound proofing capacity too.

 

4.5       Integration of external shadowing

In general, historic buildings have very large heat masses, which help to avoid overheating of the building - especially in cold and moderate climates. In warm climates, with long heat periods and few cooling downs at night, it can be necessary to implement sun protection devices. Once overheated, thermal storage masses of historic buildings cool down very slowly.

4.5.1    Protected façades

Exterior shading devices can be implemented in box-type windows, which exterior sash opens outwards. Figure 31 shows a window with outwards opening exterior sashes in winter case. Double windowpanes create a buffer and a better temperature profile on cold winter days. Figure 32 shows the exterior sashes replaced by shutters with flexible blades, in order to obtain an individual shading of the rooms [GAM01]. In summer month shutters with flexible blades can shield up to 70 % of the entering sun radiation (shading factor = 0.30).             

		Figure 31: Winter window [GAM01]	Figure 32: Summer window [GAM01]

4.5.2    Non protected façades

In most cases only the front façade of a historic building is listed under monument protection. On non-protected facades in the backyard, windows can easily be changed and external shading devices can be implemented without any problem. Especially in south- and west-orientated windows on backyard facades have to be equipped with shading devices in order to avoid glare and overheating of the rooms. 

The ideal shading device should obstruct the incidence of sun radiation, while allowing the passage of daylight and the view outside. It can be useful to apply flexible shading devices, because they can be adapted to the actual requirements, which are affected by varying weather situations, as it is known especially in spring and autumn.

The most common exterior shading devices are (Figure 33):

§  Window shutters are made of wooden blades, which can be fold in front of the window (1).

§  Rolling blinds consist of horizontal plastic (or wooden) blades, which can be rolled up an d let down along guide rails within the window reveals (2)

§  Textile blinds can be rolled up and down horizontally or sloped in combination with a vertical limb (3).

§  Rotating vertical blades made of wood or metal can be adapted according the actual sunset (4).

                    

			(1)	(2)
			(3)	(4)

Figure 33: Exterior shading devices for non-protected facades

 

4.6       References

 

[ORT05]          Ortler A. et al. “Energetische Sanierung in Schutzzonen“, Project Report within the „Haus der Zukunft“ Program, Innsbruck/Austria, March 2005

[GAM01]         Gamerith H. et al. „Untersuchungskriterien zur Bestimmung der Erhaltenswürdigkeit historischer Fenster“, Amt der Steiermärkischen Landesregierung, Graz/Austria, January 2001