In short: A building retrofit assessment tool is only as honest as the costs it counts. For an office retrofit, counting heating alone favours mineral wool (6.4-year payback) and rules out vacuum insulated panels. Counting the floor space VIPs saves you space; up to 89.67 m² in our case study, worth about £90,000 a year in London rent and VIPs pay back in as little as 1.3 years. Count embodied carbon and mineral wool wins again (Alam, Picco and Resalati, 2022).
Retrofit decisions are usually presented as material questions: mineral wool or rigid foam, conventional or advanced, cheap-and-thick or expensive-and-thin. Research from the University of Brighton, co-authored by one of the FREDS founders Marco Picco makes an unsettling counter-claim: in commercial retrofit, the decisive variable is often not the material at all, but the accounting method used to judge it. Change what you count, and the winning insulation changes with it (Alam, Picco and Resalati, 2022, published in Building and Environment).
The study is the cleanest demonstration we know of why retrofit appraisal needs to be holistic and of how much money and carbon hides in the gap between a narrow assessment and an honest one.
The contenders and the case
The research applied a comprehensive assessment methodology to a typical office building requiring retrofit, with insulation added internally; the common situation when external walls cannot be altered. Three materials competed:
- Mineral wool: the conventional default: cheap, proven, but thick for a given performance.
- XPS (extruded polystyrene): the conventional upgrade: better resistance per centimetre, higher cost.
- Vacuum insulation panels (VIPs): the advanced insulation option: a fumed-silica core held in vacuum, delivering roughly 5–8 times the thermal resistance of conventional materials at a fraction of the thickness, and a price to match.
The building was simulated dynamically under different climate conditions to establish annual energy performance with each option, then assessed financially and environmentally. The crucial move was running the financial appraisal under two different logics.
Accounting method one: count the heating
The traditional retrofit business case counts energy saved on heating against insulation cost, and under that lens the verdict looks closed: mineral wool pays back in as little as 6.4 years. Cheap material, solid savings, short payback; case dismissed, and with it any argument for paying a premium for advanced materials. This is, more or less, how most retrofit investment decisions are actually made.
The study identifies what this lens leaves out and shows the omissions are not rounding errors. The heating-only approach, in the authors’ words, might be lacking in correctly identifying energy savings, leading to overestimating the case for traditional materials. Two omissions matter most.
Cooling exists. Dynamic simulation across climates captures what a heating-season calculation cannot: insulation changes the building’s summer behaviour too, and an office full of internal gains does not only lose heat it also has to shed it. Counting heating alone misstates the true energy consequence of every option.
Floor space has a price. Internal insulation eats lettable area from every retrofitted wall and here the thickness difference between materials becomes a financial variable. Mineral wool achieving a target U-value might demand many centimetres along every external wall; VIPs achieve it in a sliver. In commercial property, where each square metre carries rent, the “expensive” thin material is quietly handing the owner sellable space value the heating-only ledger never records.
Accounting method two: count everything
Re-run the appraisal holistically heating and cooling loads from the dynamic simulation, plus the monetary value of floor area saved and the ranking inverts. Once the space saved carries a high enough value, VIPs pay back in 1.3 to 4.2 years across the locations investigated, beating the 6.4-year figure that had made mineral wool look unassailable. Same building, same materials, same physics; what changed is that the assessment stopped ignoring two things the building owner genuinely experiences.
This is the headline worth sitting with: the appraisal method swung the verdict by a factor of several, in a decision worth real capital. “Which insulation is best?” turned out to mean “which costs and benefits did you admit into evidence?” and the conventional method had been ruling the advanced option out on a technicality. The flip has a clear condition attached, which the study states plainly: it favours VIPs as soon as the space reaches high enough value. In a low-rent location, mineral wool’s case strengthens again. The method does not crown a universal winner; it makes the verdict honest about its circumstances which is exactly what a building retrofit assessment tool is for.
The carbon twist
Then the study adds the third ledger, and complicates the victory lap. Alongside money, it counts embodied carbon emissions invested in manufacturing each material against the operational emissions each option saves.
Here the ranking flips back: mineral wool shows a significantly lower carbon payback time than VIPs, whose energy-intensive fumed-silica cores carry a heavy manufacturing footprint.
So the honest summary of one retrofit decision reads: financially, VIPs win where space is valuable; environmentally, mineral wool wins today; and no single-metric assessment would have told you either half. That is not an unsatisfying conclusion, it is what a real decision looks like when the analysis stops flattering one answer. A client can weigh rent against carbon explicitly; what they could not do, before the holistic appraisal, was know the trade existed.
What a building retrofit assessment tool changes about practice
Three working rules fall out of the research:
- Simulate dynamically before appraising financially. Heating-degree-day shortcuts misprice every option that changes summer behaviour. The energy line of the business case should come from the building’s full annual, hourly behaviour which is precisely the analysis modern tools deliver in minutes rather than weeks.
- Put floor space on the balance sheet. For internal retrofit in commercial property, thickness is money. Any appraisal comparing materials of different thicknesses without valuing the area difference is structurally biased toward the thick and cheap.
- Run the carbon ledger alongside the financial one. Embodied carbon flips rankings just as space value does. With whole-life carbon moving from voluntary to expected in UK practice, an appraisal silent on it is already out of date.
The deeper point generalises past insulation. Every retrofit decision, glazing, heating systems (see our Victorian hall case study for the same lesson in a heritage setting), envelope strategy sits at the mercy of its assessment boundary. Draw the boundary narrowly and you get a confident wrong answer; draw it honestly and you get a real decision. The job of analysis is not to bless an option. It is to make the trade-offs visible while they are still cheap to act on.
FREDS makes the holistic appraisal affordable dynamic whole-building simulation of retrofit options in minutes, with the consultancy team behind it for the decisions that carry real capital. Assess your retrofit
Related reading
Frequently Asked Questions
What is a building retrofit assessment tool?
Are vacuum insulated panels worth it for retrofit?
Do vacuum insulated panels have a downside?
Why does the assessment method change which insulation wins?
Internal or external wall insulation, which should a retrofit use?
References
Written by Marco Picco, PhD co-founder of FREDS4Buildings and a building-physics lecturer at the University of Lancashire, whose work on building energy simulation has been published in Energy & Buildings , Building and Environment and the IBPSA Building Simulation conference proceedings. About Marco Picco
Reference: Alam, M., Picco, M. and Resalati, S. (2022). Comparative holistic assessment of using vacuum insulated panels for energy retrofit of office buildings. Building and Environment, 214, 108934. View paper
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