Insulation, windows

Insulation, windows

Many rural homes and buildings in Europe are equipped with sub-optimal window that are a significant source of heat losses during winter. Energy efficient, high-performing windows maximise daylight penetration while minimizing heat losses and may have the ability to prevent overheating in summer depending on the glazing solutions chosen.

Modern windows with double or triple glazing reduce heat losses while providing solar heat into buildings. Thereby they can achieve a positive energy balance (the window will provide more energy that it loses). High-performing windows increase the comfort of a building for its inhabitants and significantly reduce outside noise.

Payback periods vary widely depending on conditions such as climate, size of windows and the energy performance of windows being substituted. However, when appropriately sized, designed and placed, payback periods are shorter than their lifetime. Windows should be chosen, designed and applied in accordance to the energy efficiency standards of the whole building envelope i.e. the standard of the insulation elsewhere in the building.

General Info

What is it?

The purpose of having high-performing windows is to maximise daylight penetration while minimizing heat losses. They may have the ability to prevent overheating in summer depending on the glazing solutions chosen. Nowadays, windows with double or triple glazing, with low emissivity1 coatings and inert gas-filling reduce heat losses through the window and can achieve a positive energy balance, i.e. the window will provide more energy than it loses.

In case a room or building gets overheated during summer, windows should ideally be equipped with solar control glass, which coatings reflects and radiates away a large degree of the sun’s heat, while allowing daylight into buildings.

What are the benefits?

Because most of European buildings are equipped with sub-optimal window products, existing windows are a significant source of heat losses in buildings during winter, increasing heating needs, and can lead to overheating in summer, thereby creating additional need for air- conditioning. Modern double and tripled-glazed windows fitted with the adequate glazing solutions (i.e. low-E or a combination of low-E and solar control glass) substantially improve the energy performance of the building.

Double or tripled-glazed windows also improve the comfort of a building for its occupants. It reduces cold spots surrounding windows at night, which may cause draughts, lowers the draw of heat away from inhabitants when sitting next to windows and reduces condensation.

Another additional benefit is that they give a better “acoustic” rating, especially where outside noise is an issue.

Recently adopted European building regulations have set stringent rules for new and existing buildings in Member States. Chiefly, the Energy Performance of Buildings Directive (EPBD2) that sets minimum energy performance requirements for buildings or building units. For example, as of 31 December 2020 new buildings in the EU will have to consume 'nearly zero' energy and the energy will be 'to a very large extent' from renewable sources. Also, minimum energy efficiency requirements for components such as insulation and windows are introduced for all replacements and renovations.

How does it work?

Double or triple glazed windows have two - or three layers of glass separated by a spacer bar and filled with air or gas such as Argon. While older versions of double (and triple) glazed windows used aluminium spacer bars, currently available windows often use a so-called warm edge spacer bar made from a non-metal component that insulates better. The space in between the layers of glass reduces the heat transfer through the window. Using a gas such as Argon instead of air slightly improves energy efficiency further.

Low-emissive (Low-E) glass has been treated with an invisible metal or metallic oxide coating, creating a surface that reflects ultraviolet radiation (heat), while allowing visible light to pass through. In winter, Low-E glass reduces the amount of heat loss through windows while allowing passive solar heat gains, keeping heating costs down.

In warm climates where overheating in summer is a major source of energy consumption via air-conditioning use, low emissivity coating can be combined with a solar control glass coating. The latter reflects and radiates away a large degree of the sun’s heat, without blocking visible light. Solar control glass helps reducing air-conditioning needs, improve thermal comfort all year round and save significant amount of energy.

Suitability/applicabil ity

Windows in all types of buildings can be replaced by energy efficient and high performing windows. However, when replacing existing windows with high-performing double or triple glazed windows one should first of all take into account their energy balance.

The energy balance of a window is the difference between heat losses (measured by way of the Ug-value) and solar heat gains (measured by way of the g-value). It is this balance that determines the most appropriate glazing solution for a given window. It should be calculated with an equation that factors in the heat gains and heat losses and is weighted by the climatic conditions. Hence, one should consider a range of Ug and g values according to the specific needs of each building.

For instance in climates with cold winter and mild summer, one should opt for windows with the highest possible insulation (very low Ug-value,) and high heat gains (high g-value) to maximise passive solar heating during the cold seasons. On the contrary, in climates with hot summer and mild winter, it is best to minimize heat gains by choosing window with a very low g-value while still providing good insulation (low Ug- value).

Double and triple glazed windows currently available on the market have improved much over the years and show significant energy efficiency gains, compared to first generation double and single glazed windows. Therefore, replacing e.g. first generation double glazing with currently available modern double glazing (with low E-values and a warm edge spacer bar), can improve the energy efficiency of a building significantly.

Windows are a system made of glazing and a frame. Today’s state-of- the-art glazing products are either double or triple (low-e coating and inert-gas filled), and always insulate better than the frame. Therefore, when replacing existing single or sub-optimal double-glazed solutions by state-of-the-art ones while keeping an inefficient frame does not make much sense, especially if the existing frame does not have any thermal bridge-cut off. When a glazing is changed, it is recommended to also change the frame. The frame must be performing and durable.

Detailed Info

Costs, Savings, Earnings

Costs, savings and earnings of efficient windows depend on a number of factors:

  • The climate, country or region where they are installed;
  • The size of the windows as well as the total window surface of the building;
  • The energy performance of the windows being substituted;
  • The energy performance standard the building or in other words, of the insulation elsewhere in the building. Additional annual energy savings could be little while the payback time for triple glazing (like advanced double glazing) is high compared to other energy- efficiency improvements.

Payback periods vary widely depending on these conditions. However, when appropriately sized, designed and placed, payback periods are shorter than the expected lifetime of the window.

Usually, modern double glazed windows are sufficient for most houses and buildings and the additional added value of triple glazing is small. Triple-glazed windows are not always better than double-glazed

windows. Triple glazing is generally more expensive (although costs are coming down), but increasingly applied in very efficient buildings (so- called near or zero energy buildings).

In the context of recently introduced European energy performance standards of buildings regulation, it is expected that more European countries will introduce obligatory energy performance certificates or labels for buildings that are being sold or rented-out. Proper insulation, including windows, is a major contributor towards having a good energy label, which will increase the value of a property. This will increase the benefits of installing energy efficient windows further.

In a number of countries, high performance glazing is supported by government grants or low-interest loans and other incentives.

Environmental Impacts

There are no significant negative environmental impacts associated with the replacement of old windows with more efficient ones. By contributing to the overall energy efficiency of a building, the energy needs of a building are reduced, which in turn lowers the environmental impact of the building.


Not applicable.

Commercial Maturity

Double and triple glazed windows, as well as low-E and solar control glass are commercially mature technologies. Experienced installers are widely available.

Level of Maintenance

VERY LOW: Energy efficient windows usually require no maintenance (except for periodic cleaning). Wooden frames typically require more maintenance than aluminium, steel or composite frames.

Technical Details

Typical heat transmission (expressed in Ug-value in W/m²K) and heat gains values (expressed in g-value in % of transferred solar energy) for different types of windows in Europe. For heat transmission lower values indicate less heat conductivity and thus less heat loss.

Single clear glass: Ug-value 5.4 W/m²K; g-value from 39% to 86% Double glazing with 12mm air gap: 2.8 W/m²K; g-value from 27% to 77%

Double glazing with 20 mm air gap and Low-E coating: 1.7 W/m²K; g-value from 26% to 73%

Double glazing with 16 mm argon filled gap and Low-E coating : 1.3 W/m²K; g-value from 26% to 74%

Triple glazing with 14mm argon-filled gaps and Low-E coating: Ug value 0.75W/m²K; g-value from 19% to 63%

(EC, 2010. Green Public Procurement: Windows Technical Background Report Windows, Glazed Doors and Skylights. Available from:

Regional variations

Windows with double or triple panes with Low-E insulated glass and inert gas filling (either argon or krypton) are usually the best option for cold weather situations.

In warmer climates, overheating needs to be avoided to reduce air- conditioning needs during the hot seasons. In these cases it becomes important to consider glass types (e.g. solar control glass) that will reduce the radiation that enters the building through the window conversely reducing energy consumption of cooling systems without reducing access to natural daylight. It is also worth combining solar control properties with a low-emissivity coating on a double glazed window with inert-gas filing in order to provide sufficient level of insulation all year round.

Trade associations

Glass for Europe