, Building
A pair of 1930s semis in Rochdale get external wall insulation in the same month, by the same installer, funded by the same scheme. One is fine. The other has black mould in both bedroom corners by February, and the tenant is told to open the windows.
The difference was not the render. It was that the second house had trickle vents painted shut, a bathroom fan that vented into the loft, and a family of five drying washing on the radiators. The insulation did what it was supposed to do. It raised the temperature of the wall surface and dropped the ventilation rate, and the moisture that had previously escaped through a cold draughty fabric stayed inside and condensed on the coldest remaining surface.
PAS 2035 exists because the previous approach did not work
Through the 2010s, a series of government-funded insulation programmes installed measures at scale, measure by measure, house by house. A proportion of them caused damage: damp, mould, failed renders, and homes whose energy bills did not fall as promised. The 2016 Each Home Counts review looked at why, and the answer was that nobody was responsible for the house as a whole.
PAS 2035 is the standard that came out of that. It was published in 2019 and updated since, and it sets out the framework for retrofitting existing dwellings for energy efficiency. PAS 2030 is its companion, covering the installation work itself. The pair are mandatory for work funded under the government schemes, which in recent years has meant the Energy Company Obligation, the Great British Insulation Scheme and social housing decarbonisation funding, and which requires the installer to be registered with TrustMark. Privately funded work is not legally obliged to follow PAS 2035, and following it anyway is the difference between a retrofit and a gamble.
The central idea is whole-dwelling assessment. Before anything is specified, someone assesses the building: its construction, its condition, its ventilation, its moisture risk, its occupancy, and its heritage or exposure constraints. From that comes a medium-term improvement plan, a sequence of measures in the right order, even if only the first of them is being funded now.
The order matters. Insulating a wall before fixing a leaking gutter traps water in the fabric. Fitting a smaller boiler before insulating oversizes the system. Airtightness before ventilation causes the Rochdale problem.
The roles are named, and one of them is accountable
PAS 2035 defines a set of roles, and the reason it names them is that the previous system had nobody who owned the outcome.
The retrofit assessor carries out the whole-dwelling assessment and produces the survey, the condition report and the occupancy assessment.
The retrofit coordinator is the accountable role. They take the assessment, oversee the design, manage the risks, ensure the measures are compatible with each other and with the building, and sign off the project. On the more complex risk paths they are required from the start.
The retrofit designer specifies the measures in detail, including the ventilation strategy and the detailing at junctions, reveals and eaves where heat loss and condensation concentrate.
The retrofit installer does the work to PAS 2030. The retrofit evaluator checks a sample of completed projects afterwards.
PAS 2035 sorts projects into risk paths, broadly by the complexity of the building and the extent of the work. A single straightforward measure on a conventional house sits on the lowest path with the lightest process. A traditionally constructed or protected building, or a deep retrofit with several interacting measures, sits on a higher path with more assessment, more design and more oversight. Pretending a solid-walled Victorian terrace is a simple job is the most common way the standard gets subverted.
Ventilation is a design output, not an afterthought
This is the technical heart of it, and it is where small contractors most often get into trouble.
A house produces moisture continuously: breathing, cooking, washing, drying. Older leaky buildings dealt with it accidentally, through draughts, at enormous cost in heat. Every measure that improves the fabric reduces that accidental ventilation. If you do not replace it deliberately, the moisture stays.
So any insulation or airtightness work has to be accompanied by a ventilation strategy that meets the requirements of Approved Document F of the Building Regulations. In practice that means checking and upgrading extract fans in kitchens and bathrooms, making sure they actually vent to outside rather than into a void, providing background ventilation such as trickle vents, and on deeper retrofits considering continuous mechanical extract or mechanical ventilation with heat recovery.
Measuring matters more than specifying. A fan rated at the right extract rate, fitted with four bends of flexible ducting, delivers a fraction of its rating. Test it.
Then there is the fabric itself. Solid walls of brick or stone were built to get wet and dry out. Wrapping them in a vapour-closed insulation can stop that drying, and the water accumulates where you cannot see it. On traditional buildings, breathable systems and a properly considered moisture risk assessment are the safer route, and the British Standard on moisture in buildings is the reference the assessment works from.
Detailing is the other half. Insulation that stops at a window reveal, a roof edge, a party wall or a floor junction creates a cold bridge, and a cold bridge in a now-humid house is where the mould grows. It shows up in the corner of a bedroom, which is exactly the place a surveyor will look.
What this means for a small firm taking the work
Retrofit is a growing market and it is not a straightforward one to enter.
If you want funded work, you need TrustMark registration and PAS 2030 certification for the measures you install, and you have to work within a coordinated project rather than turning up and fitting. That is a genuine cost in time and fees, and it is a barrier that keeps the price of compliant work above the price of the alternative.
If you work privately, you can do as you like, and you carry the risk. The risk is not theoretical. Damp and mould claims have become considerably more serious in recent years, particularly in rented housing, where the Social Housing (Regulation) Act 2023 introduced requirements for landlords to deal with damp and mould hazards within set timescales, with the intention of extending similar duties to the private rented sector. A retrofit that causes mould is now a problem for a landlord who will look for someone to blame.
The defensible position is boring and cheap. Assess the whole house before you specify anything. Fix the water ingress first. Design the ventilation and measure it after installation. Photograph the junctions. Write down what you found and what you recommended, including the recommendations the customer declined.
Where this shows up in the game
Construction arrives in Business Baron with the Property & Construction Track, an add-on to the full game. Brand trust separates competence from integrity and care, and competence is simply whether the work works. A job that passes on the day and fails two winters later damages the part of your reputation that takes longest to rebuild.