Modelling for Retrofit Choices

Modelling earns its keep in retrofit when it compares options on the same basis. Start with a baseline model of the home as it stands, then change one measure at a time and record how the calculated result moves. That discipline matters because measures interact: insulation reduces the heating load, which changes the size and economics of a later heating system, and draught-proofing changes ventilation, which affects indoor air quality and moisture.

A model used this way is a ranking aid, not a shopping list. It can show that, for a particular home, roof insulation moves the calculated result more than a glazing upgrade, or that heating controls offer a larger modelled benefit than a more expensive system change. It cannot, by itself, decide whether a measure suits the building fabric, the occupants or the budget, and those judgements need separate attention.

Order matters in retrofit, and modelling should respect it. Fabric measures that reduce heat loss generally come before heating system changes, because a smaller load may allow a smaller, cheaper system and because a new system sized for an uninsulated home will be oversized once insulation is added. Ventilation must be considered alongside airtightness improvements, so that a tighter home still receives enough fresh air. Moisture risk must be checked where insulation changes how a wall dries, particularly in older solid-wall construction.

A useful modelling exercise therefore tests packages, not just single measures. Test the fabric package first, then the heating package on top of the improved fabric, then controls and ventilation adjustments. The package results often differ from the sum of single-measure results, and the difference is informative: it shows where measures reinforce each other and where they overlap.

Ask the modeller to show a table of options against the same baseline, with the input changed for each option stated plainly. Check that the baseline matches the home as surveyed, that each option is technically plausible for that construction, and that ventilation and moisture implications are noted where relevant. If an option looks strikingly good, ask which input drives the benefit and whether that input is realistic for this home.

Also ask about costs in the right form. A model gives energy figures, not prices, so a cost comparison needs separate, current quotations for the work, including any associated work such as scaffolding, redecoration or ventilation upgrades. A modelled energy benefit divided by an incomplete cost gives a misleading payback, so keep energy modelling and cost estimation as two linked but distinct exercises.

Retrofit modelling conventions differ by place in the same way that rating methods do. In the UK, SAP-based modelling is commonly used for EPC-related retrofit advice, while a deeper retrofit plan may use additional analysis for moisture and ventilation. In EU member states, national methods and renovation roadmaps under the EPBD framework frame retrofit advice differently, and some countries provide staged renovation plans that sequence measures over years. The common thread is sequencing and interaction, even where the paperwork differs.

Where a home is rented, listed or in a conservation area, retrofit options may also be constrained by rules that a model does not know about. A model can rank technically possible measures, but permission, tenancy and heritage constraints decide which of those measures can actually proceed. Check those constraints early, before modelling effort is spent on an option that cannot be implemented.

A common mistake is to pick measures by headline modelled saving alone, without checking fabric suitability. External wall insulation on a wall with rising damp, or internal insulation on a wall that needs to dry to the inside, can create problems that a simple energy model does not show. Another mistake is to install a new heating system first and insulate later, leaving an oversized system that cycles inefficiently.

A further mistake is to ignore ventilation when tightening a home. Draught-proofing, new windows and insulation all reduce uncontrolled air leakage, which is good for energy but requires a deliberate ventilation strategy. A retrofit model that shows only energy, without a ventilation note, is an incomplete basis for a decision.

Keep the baseline model, the options table and the quotations together, and record the sequence you intend to follow. When a later installer proposes a different order, that bundle lets you ask what changes in the modelled result and in the practical risks.

Questions to ask include: which package gives the best modelled improvement for this home, what does each package assume about ventilation and moisture, and what happens to the heating system size if fabric work comes first. Those questions turn a model into a plan rather than a sales aid.