Retrofit Ventilation Systems That Fit Existing Homes: Cascade Supply and Room-by-Room Extract

Why Ventilation Trips Up So Many Retrofits

Samuel Caillou of Buildwise, the Belgian building research body, presented findings from a social housing project in Brussels at an INIVE event. The homes were ordinary terraces and flats, some more than a century old, that had been upgraded piecemeal over decades. That history should sound familiar to any UK Retrofit Assessor walking into a pre-1919 solid-wall terrace.

The headline measurement came from a bedroom in one of the unrenovated dwellings with no ventilation system. Overnight CO2 peaked at 5,200 ppm. The house as a whole was leaky, but earlier window replacements had made the bedroom tight enough that night-time occupancy overwhelmed any incidental air change.

Did you know? A building can have poor whole-house airtightness and still produce dangerous CO2 build-up in individual rooms. Airtightness is not evenly distributed, and bedrooms with new windows are the usual culprits.

This is the ventilation trap at the heart of PAS 2035. Every fabric measure reduces uncontrolled air leakage. Without a matching ventilation strategy, indoor air quality falls and moisture risk rises. The damp, mould and condensation guide covers what happens next.

The Installation Problem

Buildwise compared the standard systems used in new build across Belgium and most of Europe: mechanical extract ventilation with natural supply inlets, and balanced supply and extract with heat recovery. Both perform well in new homes. Both are awkward in existing ones.

Barrier	Why it matters in retrofit
Space	Duct runs and the ventilation unit need voids or boxing that old homes lack
Invasive work	New ductwork and service shafts disrupt occupied dwellings
Supply inlets	Trickle vents or wall inlets are needed in every habitable room, and replaced windows rarely have them
Partial systems	Simplified or incomplete installs are common but underperform
Cost	Full systems are expensive relative to the fabric budget

UK assessors will recognise every line. The intermittent extract fan with no trickle vents is the British equivalent of the incomplete system Caillou warns about.

System One: Balanced Ventilation in Double Cascade

The first innovation takes a conventional balanced system and changes the airflow strategy rather than the components. Mechanical supply goes only to the bedrooms. The living room receives no direct supply. Instead, fresh air transfers from bedrooms through hallways and corridors into the living room, then on to the kitchen, bathroom and utility rooms where it is mechanically extracted.

The result is that the living room, often the hardest space to duct in an occupied home, needs no supply ductwork or terminal at all.

Key features Caillou described:

• Standard components. Ordinary ducting, terminals and a heat recovery unit, all available off the shelf.
• Lower design flow rate than a conventional balanced system for the same dwelling, which helps energy performance.
• Flexible control. A central CO2 sensor in the living room can modulate the whole system, with a permanent minimum flow to bedrooms. Time control can be layered on top.
• Smaller unit and less ductwork, so lower installation cost.
• Heat recovery remains possible.
• Not sensitive to poor airtightness. Because supply and extract are both mechanical, indoor air quality holds up even where the envelope is leaky or untested.

Tip: Cascade systems depend on air actually moving between rooms. Door undercuts or transfer grilles are part of the design, not an afterthought.

System Two: Mechanical Extract in Every Room

The second approach starts from a fan-assisted extract system but changes two things. First, extract terminals are installed in every room, including bedrooms, not just wet rooms. Second, there is only one natural supply opening, located in the main hallway, rather than inlets in each habitable room.

This directly solves the trickle-vent problem. Where windows have already been replaced without ventilators, nothing needs cutting into frames or external walls in the bedrooms.

The design flow rate is higher than a conventional extract system, but demand control brings it down sharply in use. Each room can be regulated independently: CO2 in bedrooms, humidity in bathrooms. Caillou’s performance-based simulations showed indoor air quality matching a balanced system because every room has a controlled extract rate, which conventional extract systems cannot guarantee.

Additional advantages he listed:

1. No sensitivity to poor envelope airtightness.
2. Low energy use through demand-controlled flow.
3. Better comfort, since bedrooms receive no direct cold outside air, meaning less draught and less external noise.

System Three: Cascade Extract for Flats

The final system combines the two ideas for apartments. Outside air enters only at the bedroom, transfers through the corridor to the living room and open-plan kitchen, and is extracted from service rooms. A single supply opening serves the whole flat. Where the kitchen is separate from the living room, a mechanical extract point is added in the living room as well.

It suits both centralised block systems using existing risers and small individual units per flat. One caveat matters a great deal for assessors: this system is sensitive to airtightness. It needs a good or very good envelope to perform.

System	Supply	Extract	Airtightness sensitivity	Best fit
Balanced double cascade	Mechanical, bedrooms only	Mechanical, wet rooms	Low	Houses
Extract in every room	One natural inlet in hall	Mechanical, all rooms	Low	Houses with replaced windows
Cascade extract	One natural inlet at bedroom	Mechanical, service rooms	High	Flats with good airtightness

What This Means for the Retrofit Assessor

PAS 2035 places the assessor at the front of the process. The retrofit assessment records the existing ventilation provision, its condition and effectiveness, and any evidence of condensation, damp or mould. That record is what the Retrofit Coordinator uses to specify ventilation alongside fabric measures. The Belgian findings sharpen several things to look for on site.

• Record ventilation room by room. Note which windows have been replaced and whether trickle vents were fitted. A bedroom with new sealed windows and no vent is a flagged risk even in a draughty house.
• Identify duct routes and unit locations. Lofts, cupboards, risers and boxing opportunities determine which system is practical. Note where there is no route at all.
• Assess airtightness honestly. If the envelope is leaky or unknown, systems that tolerate leakage are safer specifications. Cascade extract in a flat needs test evidence.
• Check internal air paths. Door undercuts, transfer grilles and open-plan layouts decide whether a cascade strategy can work.
• Look for the incomplete system. An extract fan on a pull cord with no background ventilation is not a ventilation strategy.

Remember: Ventilation is a measure in its own right under PAS 2035, not a side effect of insulation. Assessors who document it thoroughly make the Coordinator’s job possible.

The day in the life of a Retrofit Assessor post describes how these site observations feed into the assessment report, and the building physics revision guide covers the moisture and air movement principles behind them.

How RetrofitReady Fits Into This

Ventilation questions in the Retrofit Assessor exam test exactly the judgements above: recognising inadequate provision, linking fabric changes to ventilation risk, and knowing what the assessment must record. The Retrofit Assessor app covers this with 480 questions across eight syllabus areas, including multi-answer and diagram-based practice where you identify ventilation paths and defects from plans and photographs.

Use the ventilation and building physics topics together. Answer a short set, read every explanation for the questions you missed, and return to the same topic later in the week. The multi-answer question guide is worth reading first, because ventilation questions often ask you to select several correct observations at once.

The full RetrofitReady suite offers 1900+ questions across 4 apps, so Coordinators, DEAs and Installers studying alongside you can revise the same ventilation principles from their own role’s perspective.

Start with the free quizzes or browse the full RetrofitReady app suite.