Finding the Most Sustainable Path in Energy Renovation

When energy renovation is discussed, the windows almost always end up centre stage. They are visible, they shape the indoor experience, and they influence energy performance as well as the building’s overall character. Yet the choice between installing new high-performance windows and renovating and upgrading the existing ones is rarely straightforward. Many assume that replacement automatically delivers the most effective outcome, but that view has become far more nuanced as research, industry studies and economic analyses have accumulated.

In many Swedish multi-residential buildings, existing windows account for a significant share of heat losses. Older double-glazed units may have U-values around 2.6–3.0 W/m²K, which makes them considerably weaker than today’s triple-glazed windows that often achieve 1.0–1.2. At the same time, both international field studies and Swedish experience show that renovated windows equipped with interior energy glass, secondary panes or low-e panels can reach U-values as low as 1.6–2.0. This means that a substantial share of the potential energy savings can be realised without replacing the entire window assembly. Importantly, much of the improvement stems not only from reduced heat transfer but from lower infiltration. Draughts, leaky frames and poor junctions between window and wall often have a larger impact on heating demand than many expect, and better sealing, adjustment and insulation can deliver unexpectedly strong results.

One of the strongest arguments in favour of renovation rather than replacement is the climate impact. Life-cycle assessments from recent years show that renovating timber windows can reduce the climate footprint by 50–90 per cent compared with full replacement, since much of the embodied energy already resides in the existing materials. When renovation is paired with energy glass or interior heat-reflective panels, the energy performance is often good enough to meet modern requirements, especially in projects where extremely low U-values are not the primary target. As property owners increasingly incorporate climate calculations into their projects, this factor weighs heavily.

There are, however, equally clear arguments for replacement. New windows are justified when technical condition is poor: rot, warping, recurring condensation between panes or functional problems that cannot be corrected. Projects with stringent requirements for sound insulation, safety or fire performance may also require new windows to achieve the correct classification. And when a façade is being refurbished anyway, coordination benefits can be substantial. A new window with a U-value around 1.0 W/m²K also provides the largest direct reduction in energy use, even if the financial payback time is often long when calculated solely on energy savings.

When these two main pathways are compared, a third factor sometimes disappears in the discussion: comfort. Occupants’ experience of draughts, downdraught and radiant cold often matters more than energy use. Renovated windows, particularly when supplemented with secondary panes, can handle this surprisingly well. Reductions in infiltration of thirty to fifty per cent are not uncommon, and the indoor environment becomes markedly more stable. In many projects, property owners report that improvements in comfort are just as significant as the gains in energy performance.

This is where BIM Energy can play an important role. By modelling the building’s existing window performance, airtightness and solar loads, it becomes possible to compare two or more scenarios and immediately see how energy use, peak loads and indoor temperatures are affected. In practice, this means that a renovated window with U ≈ 1.8 and improved airtightness can be tested alongside a new window with U ≈ 1.1 and very high tightness, making the differences in energy use and comfort visible during both cold and hot conditions. The effect of solar shading can also be built directly into the calculation, which is often crucial for summer temperatures in dwellings with large glazed areas.

Since BIM Energy can also assess the economics, profitability and climate impact of energy-saving measures, it provides a decision base that captures both sides of the equation: long-term finances and climate integrity. Many property companies already work this way, and the pattern is clear: renovation and upgrading of existing windows is often the most cost-effective and climate-sustainable option, particularly when the units are in technically sound condition. Full replacement should primarily be reserved for projects where technical demands, façade reconstruction or an ambition to reach very low energy levels outweigh embodied emissions and higher investment costs.

The conclusion is that window decisions should never be made in advance, as a matter of routine. They must be analysed with data and assessed against the building’s performance objectives. This is precisely where BIM Energy functions as a neutral, transparent decision tool: by simulating realistic scenarios, project teams can see differences clearly and justify their choices to both clients and residents. At a time when energy efficiency, climate responsibility and good indoor environments must be achieved simultaneously, such clarity is invaluable