Box Window Refurbishment (Inner Sash with Insulating Glazing)

Keeping the historic outer sash, replacing the inner sash with double or triple glazing and improving the seals.

How does refurbishing box-type windows work?

Box-type (double-casement) windows consist of two window planes one behind the other, the outer and inner sashes, with an air space between them. In refurbishment, the historic outer sash that shapes the streetscape is retained and merely overhauled. The inner sash is replaced with a new timber window with double low-emissivity glazing (Ug = 1.1 W/(m²·K)) or vacuum glazing (Ug = 0.5 W/(m²·K)), and the seals of both planes are renewed. The Ug-value denotes the heat transfer of the glazing alone. The air space between the sashes acts as an additional buffer. The refurbished box-type window thus achieves a Uw-value of about 1.1 to 1.4 W/(m²·K), compared with 2.5 to 3.0 W/(m²·K) in the existing state. It is important that the inner sash closes more tightly than the outer sash, so that room moisture does not condense inside the box.

Which buildings benefit from box-type window refurbishment?

The measure is the classic choice for late-19th-century (Gründerzeit) apartment buildings, listed buildings and properties in conservation zones, where complete window replacement is not an option for design or legal reasons. An intact existing stock is a prerequisite: the outer sashes must be in good condition or repairable at reasonable cost. With severely damaged windows showing rot, warped frames or defective fittings, a replica of the box-type window is more economical. In Vienna and other cities with historic building stock, consultation with the heritage authority before refurbishment is standard and often a prerequisite for subsidies.

What does box-type window refurbishment actually deliver?

Heating energy savings are 10 to 18 %. For a Gründerzeit apartment building with 35 m² of box-type window area, the saving is around 4,200 kWh of natural gas per year. Costs are 300 to 550 €/m² of window area, well below a heritage-compliant replica of the windows. Payback is 20 to 30 years over a 30-year service life. Alongside the energy saving, the surface temperature of the inner sash rises, which reduces draughts and condensation, and the sound insulation of the double window plane improves further.

Energy carrier

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

Savings potential

10-18 %

Worked example

A Gründerzeit apartment building in a conservation zone has 35 m² of original box-type windows whose outer sashes are to be retained. The existing windows have a Uw-value of 2.7 W/(m²·K). The inner sashes are replaced with new timber windows with insulating glazing and the seals renewed, giving the refurbished box-type window 1.3 W/(m²·K). The building is heated by gas at 88 % efficiency, and the location has 3,500 heating degree days. With a correction factor of 0.9, the result is: 35 × 1.4 × 3,500 × 0.024 / 0.88 × 0.9 = 4,211 kWh/a. The building thus saves around 4,200 kWh of natural gas per year without any change to the external appearance of the facade. The measure is therefore heritage-compliant and eligible for subsidies.

Investment & payback

300-550 €/m² · Service life: 30 a · Payback: 20-30 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How much does it cost to refurbish a box-type window?

Refurbishing box-type windows with a new inner sash, insulating glazing and renewed seals costs 300 to 550 €/m² of window area. A typical Gründerzeit window of 2 m² therefore comes to about 600 to 1,100 €. Retaining the outer sash makes refurbishment considerably cheaper than a heritage-compliant replica. Additional costs arise if outer sashes have to be sanded, repaired and repainted.

Can box-type windows be replaced in listed buildings?

In listed buildings, replacing original box-type windows is generally not permitted; refurbishment with retention of the outer sash is the approvable route. In Austria, the Federal Monuments Office (Bundesdenkmalamt) is responsible, and in conservation zones the municipality as well. The inner sash may usually be replaced because it is not visible from outside. Early consultation with the authority is also advisable for subsidies.

What U-value does a refurbished box-type window achieve?

A refurbished box-type window achieves a Uw-value of about 1.1 to 1.4 W/(m²·K), compared with 2.5 to 3.0 W/(m²·K) in the unrefurbished state. With double low-emissivity glazing in the inner sash (Ug = 1.1), you are at the upper end; with vacuum glazing (Ug = 0.5) at the lower end. In many cases, the refurbished box-type window therefore meets the OIB Guideline 6 requirement of 1.1 W/(m²·K) for refurbishments.

What is the difference between box-type window refurbishment and window replacement?

In window replacement, the entire window is replaced with a new triple-glazed unit achieving Uw-values of around 0.85 W/(m²·K). In box-type window refurbishment, the historic outer sash is retained and only the inner sash is renewed; the result is 1.1 to 1.4 W/(m²·K). Refurbishment is cheaper, heritage-compatible and preserves the sound insulation of the double plane; window replacement delivers the higher energy saving.

Why does condensation form in box-type windows and how do you prevent it?

Condensation forms when humid room air enters the space between the sashes and cools on the cold outer pane. After refurbishment, the inner sash must therefore close more tightly than the outer sash, so that the box remains slightly ventilated to the outside air and moisture can escape outwards. New seals on the inner sash and deliberately less tightly sealed outer sashes are therefore part of professional execution.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Oberflächentemperatursensor am Innenflügel zeigt Komforterfolg (höhere Oberflächentemperatur, Verringerung Kondensatrisiko).

All measures