Building performance in existing assets: how to design, model and prove it early

ESG obligations and decarbonisation deadlines have shifted retrofitting from a long-term consideration to an immediate priority for commercial property owners. The decisions that shape performance need to be made early but knowing that and knowing how to guarantee the outcome are two different things. How do you take an existing building with all its constraints and unknowns, and engineer performance back into it in a way you can predict, model and stand behind before construction starts?

Existing buildings come with unique constraints from day one: unknown services, ageing structure, limited space, tight slab heights, legacy layouts that weren’t designed for modern demands. At the same time, the questions coming from tenants have changed. It used to be about location and spec. Now they want to know how the building is going to perform, what it's going to cost to run, and whether it can support their own ESG obligations.

Scott Caldwell, Associate Director and Head of Sustainability, talks about the real challenge: understanding how the building will actually behave once people start using it.

“We would call it upgrading rather than recovering. What was considered acceptable 20 years ago would not be acceptable now. The standards have all improved and regulations have moved on.” - Scott Caldwell

On retrofit projects, what shows on drawings and what happens in operation can be two very different things.

Existing buildings require a different approach

Existing buildings bring limitations that cannot simply be designed away. Structural grids, slab heights, riser sizes and façade constraints all influence what is possible.

Gary Quinn, Executive Director and Senior Mechanical Engineer, points to 280,000 sq.ft. commercial retrofit project Irish Life HQ as an example:

“That was a building that was designed in 1976. Slab-to-slab heights were lower. Air conditioning wasn’t even really a concept in Ireland back then.” - Gary Quinn

Trying to fit modern HVAC, ventilation and plant into those spaces creates immediate design constraints.

“Fast forward 40 years… it’s very difficult to make all our systems work to the latest standards when you’ve got vintage buildings being retained.” - Gary Quinn

Modern buildings require bigger risers because ventilation systems now move larger air volumes at lower speeds for efficiency. That means larger ducts, increased fresh air provision, and additional plant space required.

And that pressure has to be resolved before detailed design progresses.

Standard compliance modelling is no longer enough

Traditional Building Energy Rating (BER) Pathway and Part L calculations still play an important role in design. They help establish compliance and provide a standardised benchmark for regulated energy performance. But BER only tells part of the story.

“A BER model is very prescribed. You’re only considering regulated energy uses.” - Scott Caldwell

Standard compliance models mainly account for lighting, heating, ventilation, and air conditioning. But that doesn’t capture many of the real operational demands that drive energy use in modern commercial buildings: EV charging, catering kitchens, workstation equipment, monitors and laptops, plug loads, and tenant IT equipment.

That gap is why more occupiers and developers are now asking deeper questions.

“A tenant might ask the landlord: what’s the MPG of the building? What are we able to achieve in terms of energy performance?” - Scott Caldwell

The conversation has shifted beyond basic compliance. Clients increasingly want to know how the building will perform in the real world once occupied. And that is where Operational Energy Modelling (OEM) changes the nature of the decision - because you are no longer committing based on assumptions, you are committing based on what the building is actually projected to do.

“Operational Energy Modelling is far more detailed. You can account for anything you want in that model.” - Scott Caldwell

OEM builds on BER assumptions by modelling how the building is expected to operate in reality.

On owner-occupier projects like Irish Life, we work directly with the client's FM team, IT team and key stakeholders to build a shared picture of how the building will actually be used before any major decisions are locked in.

Scott says the team works directly with client stakeholders to understand exactly how the building will be used.

That includes modelling:

• occupancy profiles
• IT and workstation loads
• EV charging demand
• catering equipment
• operating hours
• tenant-specific requirements

“It’s a lot of work with their IT team and FM team to understand what each workstation is going to have.” - Scott Caldwell

That detail is then programmed into the model, and the more accurate the inputs, the more accurate the forecast.

“The result is only the same quality as the input.” - Scott Caldwell

Proving performance before you commit

On a large commercial  retrofit, the modelling work became part of how the client made operational decisions - not something delivered at the end of the design process, but something that shaped it.

Our OEM strategy identified the catering kitchen as a significant energy consumer. Scott explains:

“When they saw how much it was consuming, they decided that on certain days they would not operate it fully because there was a big saving opportunity there.”

Originally, the kitchen was intended to operate throughout the week. But once the OEM broke down the projected consumption, the client shifted strategy. Rather than running full hot food service every day, they reduced kitchen use on lower-demand days.

That decision happened before occupation, before operational budgets were set and before contracts were signed with catering suppliers. The modelling made it visible in time to act on it - and that is what early OEM actually gives you: the information to make better decisions before they become expensive to reverse.

Performance doesn’t stop at handover

Scott says Metec is increasingly looking at post-occupancy reviews to compare live performance against original modelling assumptions.

The aim is to review completed buildings quarterly and assess how they are actually operating.

“We want to compare the modelling against the actual figures, dig into the data, understand why there are differences, and refine it so we can understand how the design is working in reality.” - Scott Caldwell

Projects such as Fifteen George's Quay, one of Ireland’s highest-scoring LEED v4 certified buildings at the time, reflect the level of performance now expected across premium commercial developments.

As standards continue to rise, that inspection will only increase. Future projects will face an even tougher benchmark.

“LEED v5 will be harder. People need to be aware the thresholds are changing.” - Scott Caldwell

For developers, that means: “The earlier the better. While the building shape, layouts and massing are still in flux is the best time for us to get involved.” - Scott Caldwell

At that stage, teams can influence:

• façade strategy and solar gain
• shading requirements
• plant sizing
• ventilation strategy
• roof plant and PV allowances
• water harvesting systems
• energy reduction measures

Once those decisions are fixed, opportunities to improve performance reduce significantly.

Underperforming to predictable

When upgrading an existing building, performance is something you design, test and validate from the very beginning. Because once construction starts, the major decisions are already locked in. Especially in retrofit work, where guessing becomes expensive very quickly.

Planning an office retrofit or repositioning strategy?

Existing buildings are complex, but the approach to upgrading them doesn't have to feel uncertain. When the right people are involved early, when the building is properly understood before decisions are locked in, and when performance is modelled against how the building will actually be used rather than how a generic template assumes it will be - the outcomes become a lot more predictable.

That is what Scott and the Metec team work towards on every project. If you are weighing early-stage decisions on an existing building and want to understand what that process looks like in practice, Scott can be reached at scaldwell@metec.ie

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About the author: Scott Caldwell, Associate Director, Head of Sustainability (BEng (Hons), LEED Accredited Professional BD+C, Non-Domestic BER Assessor, SEAI Energy Auditor, DEC Assessor) works with clients and design teams to oversee and deliver on sustainability and ESG objectives. The projects where his expertise in this area have really been brought to bear include 1 George's Quay, Cumberland House (Twitter), AIB HQ Molesworth Street, and Farranlea Student Accommodation in Cork.