Window Replacement (Triple Glazing)

Replacing old single- or double-glazed windows with modern triple glazing in a thermally broken frame.

How does window replacement with triple glazing work?

Per square metre, windows are the weakest components of the building envelope. A double-glazed window from the 1980s has a Uw-value of about 2.8 W/(m²·K), an old single-glazed timber window as much as 4 to 5 W/(m²·K). The Uw-value describes the heat transfer of the whole window, glass and frame. Modern triple low-emissivity glazing combines three panes with noble gas filling and low-e coatings in a thermally broken frame of timber, timber-aluminium or uPVC, achieving Uw-values of 0.90 W/(m²·K) and below. Professional installation is decisive for the effect: the window is positioned in the insulation plane, the reveal is insulated, and the connection joint is made airtight on the inside and driving-rain-tight on the outside according to ÖNORM B 5320. Only then do thermal bridges at the window edge remain small.

Which buildings benefit from window replacement?

Window replacement is an option for all building types in which single-glazed or older double-glazed windows are installed. It is particularly worthwhile in combination with external wall insulation, because window position and insulation plane can then be coordinated. Since new windows close far more tightly than old ones, the previous uncontrolled air exchange through gaps is eliminated. A ventilation concept, possibly with mechanical ventilation, is therefore a prerequisite to avoid moisture damage and mould. In listed buildings with original windows worth preserving, refurbishing the box-type windows is the more compatible alternative.

What does window replacement actually deliver?

Heating energy savings are 15 to 25 %. For an apartment building with 24 windows and 43.2 m² of window area, the saving is around 7,100 kWh of natural gas per year, equivalent to about 1.4 t of CO2. Costs are 550 to 900 €/m² of window area including installation, with a payback of 20 to 35 years over a 35-year service life. In purely energy terms, window replacement therefore pays back more slowly than insulation measures. The comfort gain, however, is substantial: the inner pane surface stays above 17 °C, cold air draughts are eliminated, and sound insulation improves significantly.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme, Strom → Erdgas, Heizöl, Pellet, Fernwärme, Strom

Savings potential

15-25 %

Worked example

In an apartment building, 24 windows of 1.8 m² each, 43.2 m² in total, are to be replaced. The existing double-glazed windows from the 1980s have a Uw-value of 2.8 W/(m²·K). The new triple-glazed windows with thermally broken frames achieve 0.85 W/(m²·K). The building is heated by a gas boiler with 90 % efficiency, and the location has 3,500 heating degree days. With a correction factor of 0.90 for installation and use, the result is: 43.2 × (2.8 − 0.85) × 3,500 × 0.024 / 0.90 × 0.90 = 7,075 kWh/a. The building thus saves around 7,100 kWh of natural gas per year and about 1.4 t of CO2. Added to this is a substantial comfort gain from draught-free rooms and better sound insulation.

Investment & payback

550-900 €/m² Fenster inkl. Einbau · Service life: 35 a · Payback: 20-35 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does window replacement with triple glazing cost?

New windows with triple low-emissivity glazing cost 550 to 900 €/m² of window area including removal of the old windows and installation. A standard window of 1.8 m² therefore comes to about 1,000 to 1,600 €. Timber-aluminium frames, special shapes, solar shading and complex reveal work increase the price. The premium for triple over double glazing is now small and almost always justified in energy terms.

Do new airtight windows cause mould?

Not the windows themselves, but insufficient ventilation after window replacement. Old leaky windows provided constant air exchange through gaps; new windows close tightly, so moisture from cooking, showering and breathing stays in the room. Without regular purge ventilation or mechanical ventilation, it condenses on cold wall surfaces. A ventilation concept is therefore a prerequisite, and the window reveal should be insulated so that no cold spot forms there.

What U-value is required for new windows in Austria?

OIB Guideline 6 requires a Uw-value of no more than 1.1 W/(m²·K) for windows in refurbishments in Austria. State of the art for triple glazing is 0.8 to 0.9 W/(m²·K); Passive House windows achieve 0.7 W/(m²·K). The requirements apply to the complete window of glass and frame; the Ug-value of the glazing alone is typically 0.5 to 0.7 W/(m²·K) for triple glazing.

Is window replacement worthwhile without external wall insulation?

Yes, if the old windows have a Uw-value above 2.5 W/(m²·K). In terms of building physics, note that the new windows are then the warmest surface of the external wall, and condensation may form on uninsulated wall areas and reveals. A ventilation concept and insulated reveals are therefore essential. Combining both measures is ideal, because window position and insulation plane can then be coordinated and no duplicate connection work is needed.

What is the difference between Uw-value and Ug-value?

The Ug-value describes the heat transfer of the glazing alone, the Uw-value that of the entire window including frame and edge seal. Because frame and edge seal insulate less well than the glass, the Uw-value is always higher than the Ug-value. The Uw-value is what counts for the energy balance and for subsidies. When comparing quotes, the Uw-value for the specific window size should therefore always be requested.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Beim Fenstertausch oft Anlass für Lüftungskonzept - hier passt CO2-/Feuchtemonitoring in Wohnungen ideal als Datengrundlage für richtigen Lüftungsbetrieb.

All measures