Modelling for Retrofit Packages

A retrofit package is more than the sum of its measures, because measures interact. Insulation reduces heating load, which changes heating system size and economics. Draught-proofing reduces infiltration, which changes ventilation needs. A new heating system changes the value of further fabric work, because a more efficient system saves less, in absolute terms, from each additional insulation step. Modelling measures one by one and adding their modelled savings therefore overstates the package benefit, sometimes substantially, and the honest approach is to model the package as a whole.

Whole-package modelling starts from a baseline model of the home as it stands, then applies the full package of measures together and records the resulting demand, rating and cost indicators. That package result is the headline to compare against the baseline, not the sum of single-measure results. The difference between the package result and the sum of parts is informative: it shows where measures overlap, where they reinforce each other, and where one measure makes another less necessary or more necessary.

Staging turns a package into a plan. Most packages cannot be delivered at once, for cost, disruption or practical reasons, and a staged model tests each stage in order: fabric stage first in most cases, then ventilation adjustments, then heating and controls sized for the improved fabric. Each stage's model should use the previous stage as its baseline, so that the benefit of each stage is measured against the home as it will be at that stage, not against the original home. That staged accounting prevents double-counting and reveals stages that add little once earlier stages are done.

Non-energy interactions must be modelled or at least noted alongside energy. Ventilation, moisture, overheating, heritage and usability all constrain package design, and a package that maximises modelled energy benefit while creating damp, stuffiness or overheating has failed. A package model should therefore be accompanied by ventilation and moisture notes, and by a check that each stage leaves the home safe, comfortable and maintainable.

Before committing to a package, ask for a package table: baseline, each stage in order with its inputs, and the full package result, all on the same method. Check that each stage's inputs are technically plausible for the building, that ventilation is addressed as airtightness improves, and that heating system sizing follows fabric stages rather than preceding them. If a stage looks strikingly beneficial, ask which input drives it and whether that input is realistic.

Also ask about costs and disruption by stage, not merely for the package as a whole. A package that is affordable in total may be unaffordable in its first stage, or may disrupt occupation in a way that makes staging essential. Cost and disruption planning by stage turns a modelled package into a deliverable plan, while a single total cost hides the staging problem.

By place, package approaches reflect national retrofit frameworks. In the UK, whole-home retrofit frameworks, including PAS 2035 where applicable, formalise staged package design with assessment, coordination and evaluation roles. In EU member states, renovation passports and national renovation plans provide staged package frameworks under the EPBD, with varying formality. The common principle is staging and interaction, even where the paperwork differs.

For multi-unit and rented buildings, package staging must also consider collective decisions and tenancy. Common elements, such as roofs and facades, need collective agreement and timing, while unit-level measures need tenant coordination. A package model that ignores those governance and tenancy dimensions may be technically sound and practically undeliverable.

A common mistake is to add single-measure savings together and to budget or promise on the inflated total. Another mistake is to deliver a package in convenience order, such as heating first and fabric later, leaving an oversized system and a missed opportunity to size correctly.

A further mistake is to treat a package as fixed once modelled. Buildings reveal surprises during work, costs change, and household plans change, so a package should be reviewed at each stage, with the model updated to reflect what was actually done, before the next stage is committed.

Keep the baseline model, the package table, the staged plan, cost and disruption notes, and completion evidence together. That file supports each stage, future sales and any query about what the package achieved.

Questions to ask include: what is the whole-package result, how does each stage contribute in order, and what ventilation and moisture notes accompany each stage. Those questions make package modelling honest and usable.