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WINDOW RETROFITS
WINDOWS OF OPPORTUNITY: A CASE STUDY
By Eamonn Ryan
As the HVAC industry accelerates toward energy efficiency, electrification and renewable integration, one building element continues to be underestimated: windows. technical
Often treated as an architectural detail rather than an energy system, glazing can be one of the most powerful levers for reducing heating and cooling loads – particularly in existing commercial buildings.
This was the central message of an ASHRAE Journal podcast, which explored a case study on how window upgrades can unlock deeper HVAC efficiency, enable electrification and significantly improve occupant comfort. The panel consisted of: Jamie Kono, P. E., associate member ASHRAE; Robert Hart, P. E.; Laura van Rietema, P. E., associate member ASHRAE.
The discussion centred on a large commercial office building in San Francisco, but the lessons are highly relevant to South Africa’ s ageing commercial building stock and its push toward lower energy intensity and reduced carbon emissions.
THE SCALE OF THE OPPORTUNITY Commercial buildings represent a massive retrofit opportunity globally. In the US alone, there are approximately six million commercial buildings, covering close to 9.3 billion square metres of floor area. Around 40 % still rely on single-glazed windows, which are among the weakest elements of the building envelope.
Even in new buildings, windows typically account for only about 12 % of the envelope area, yet they are responsible for around 50 % of heat loss or gain through the envelope. In existing buildings, the impact is even more pronounced.
For HVAC designers and energy consultants, this imbalance matters. Oversized heating and cooling systems are often compensating for envelope losses – particularly through glazing – leading to higher capital costs, higher operating energy use and greater difficulty when attempting to electrify systems.
CASE STUDY: ONE MARITIME PLAZA One Maritime Plaza is a 25-storey, Class A commercial office building completed in 1967, with a total floor area of approximately 55 700m ². Like many buildings of its era, it was designed around a high-temperature heating system, using perimeter induction units supplied with hot water at 82 – 93 ° C.
The building owner set a long-term goal of decarbonisation and electrification, prompting a detailed feasibility study. Initial analysis showed that a direct switch to electric boilers or heat pumps was not viable. The heating load was simply too high, and the building’ s electrical infrastructure – as well as roof space and structural capacity – could not support the required plant.
At the start of the study, the building’ s peak heating demand was approximately 5.9MW. Replacing this capacity with electric systems would have required extensive electrical upgrades and rooftop plant installations, making the project impractical.
RACA Journal I July 2026
Rather than focusing solely on HVAC equipment, the project team adopted a whole-building approach. The guiding question shifted from“ How do we replace the boilers?” to“ How do we reduce the demand they are serving?”
Several measures were implemented or planned:
• Control upgrades, including hot water temperature resets and improved zone control.
• System right-sizing, eliminating excess capacity created by conservative original designs.
• Envelope insulation improvements, particularly at spandrel panels.
• Window upgrades, which ultimately delivered the largest reduction in heating demand.
Control improvements alone reduced the heating requirement from 5.9MW to approximately 2.5MW. However, it was the window retrofit that unlocked the final step, reducing the heating load to just over 1.5MW – roughly a 75 % reduction from the original condition.
The existing façade consisted primarily of single-glazed windows with applied film. While the frames were still in serviceable condition, the glazing performance was poor, resulting in high heat loss, perimeter discomfort and excessive reliance on induction units.
Instead of full window replacement – which would have been costly, disruptive and slow – the project team selected secondary glazing. This approach involves installing an additional glazed unit into the existing frame, effectively converting a single-glazed window into a high-performance double-glazed system.
In this case, the upgrade reduced the window thermal transmittance( U-value) from approximately 5.7W / m ²· K to around 1.6W / m ²· K, while also lowering solar heat gain. The result was a dramatic reduction in perimeter heat loss and improved indoor comfort.
Critically, secondary glazing systems typically cost around one-third of a full window replacement, making them far more feasible for large-scale retrofits – a key consideration in costsensitive markets like South Africa.
ENABLING ELECTRIFICATION THROUGH ENVELOPE PERFORMANCE
With the reduced heating demand, the building could shift away from perimeter induction units and instead use internal VAV reheat systems supplied with lower-temperature water at around 55 ° C. This temperature range is well within the capabilities of modern air-to-water heat pumps.
The project now follows a phased electrification strategy, aligned with tenant turnover. As floors are refurbished, induction
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