Model Inputs That Matter Most

Not every model input matters equally. In most homes, a small number of inputs drive most of the result: the construction and area of the walls, the roof and floor, the size and type of windows, the heating system and its controls, and the ventilation rate. Everything else still counts, but an error in a minor input rarely changes a decision, while an error in a major one can reverse it.

The reason is scale. Heat loss depends on area multiplied by how easily heat passes through a surface, so a large, poorly insulated wall matters far more than a small, well insulated one. Similarly, a heating system that runs without effective controls can waste energy all winter, while a lighting assumption barely moves an annual total. Checking effort should follow that logic, not the order items appear on a form.

Wall construction deserves first attention, especially in older homes where solid walls, cavity walls and later insulation layers are easily confused. A surveyor who cannot see inside a wall must rely on age, type and any documentary evidence, and that judgement should be recorded as an assumption rather than presented as observed fact. Where the wall type is uncertain, a second opinion or an intrusive check by a qualified person may be worth more than any refinement elsewhere in the model.

Windows come next, because both area and type matter: a large area of single glazing loses far more heat than the same area of modern double glazing, and frame type and draught sealing add further variation. Heating controls are equally influential, since a system with a working programmer, room thermostat and thermostatic radiator valves is modelled very differently from one controlled by a single on-off switch. Ventilation, including trickle vents, extract fans and unintended draughts, completes the influential set.

Work through the model inputs in order of influence. Start with wall, roof and floor construction and areas, then glazing area and type, then heating and hot water system type and controls, then ventilation. For each, compare the modelled value with what is visible or documented, and mark anything that rests on a default or an assumption. If a major input cannot be verified, say so in your notes and treat the model result as provisional.

Pay special attention to insulation recorded as present. Loft insulation depth, cavity fill and floor insulation are often assumed from age or from a previous certificate rather than observed, and an assumed layer that is absent in reality flatters the result. Photographs taken during a survey, invoices for past work and building records are useful evidence here, and they are worth filing with the model.

In the UK, SAP conventions define defaults for unobserved items, so a SAP input schedule will often show where defaults were used. In EU member states, national methods have their own default rules, and the items that default most often may differ, for example where a national method treats heating system age or ventilation differently. The principle is the same in both systems: defaults are a transparent fallback, not a measurement, and a reader should know which inputs are defaults.

Newer homes tend to have fewer uncertain fabric inputs, because construction is documented and insulation levels are known, but system and control inputs still vary. Older homes show the opposite pattern: systems may be simple, while fabric uncertainty dominates. Adjust your checking effort to the home in front of you rather than applying one checklist identically to every property.

A common mistake is to polish small inputs while leaving a large one wrong. Spending time refining a lighting assumption while the wall construction is misrecorded produces a precise-looking result that is still wrong in the way that matters. Another mistake is to accept a model that lists insulation as present with no evidence, especially when the home's age or a previous survey suggests otherwise.

A related mistake is to treat floor area as a minor detail. Floor area scales many results, so a measurement error, for example including an unheated garage or excluding a heated extension, distorts the whole model. Check measured areas against the home as it is used today, not against an old plan.

Keep a short input-check note with the model: which major inputs you verified, which rest on defaults, and which remain uncertain. Share that note with any assessor or installer who later revises the model, so they correct the same items rather than starting from a fresh set of guesses.

Questions to ask include: which three inputs move this result most, what evidence supports each of them, and what would the result be if the most uncertain input were different. If the answer to the last question changes the decision, resolve that input before deciding.