Over-ventilating a new building before occupants arrive effectively reduces the emissions of some materials, but not all: its effect depends heavily on the type of source, and beyond a certain airflow, ventilating more adds nothing.
Flush-out means over-ventilating a new building for a period before occupancy, to speed up the off-gassing of materials and clear pollutants before occupants arrive. It is a practice credited by many frameworks — BREEAM (Hea 02), LEED, HQE, WELL, the INTAIRIEUR® label, the ECRAINS method — and the French ADEME project DETOX (2022) is entirely devoted to it.
"Ventilate hard to empty the materials faster" — the intuition is only partly right. That's the important point.
A material's emission is most often limited by internal diffusion of the pollutant towards the surface, not by ventilation. As a result (Liu et al., Environ. Sci. Technol. 2024):
Beyond a certain ventilation rate, increasing ventilation further no longer changes the material's emission rate.
Ventilation acts on the air concentration (which it lowers), not on the emission rate of the material (driven by diffusion). Over-ventilating at 10 air changes/h rather than 4 empties the air faster, but does not pull the pollutant out of the material any faster. So the real effect of flush-out on the material is more limited than commonly believed. Its two concrete benefits:
So the material emits at its full rate, instead of being slowed by saturated air in a closed building.
The absorbing surfaces that would otherwise slowly re-emit after occupancy.
On thin, fast sources that emit strongly at first: paints, varnishes, solvents, adhesives. Their emittable reserve is largely cleared during the flush period.
On deep, slow reservoirs: thick panels, dense PVC. They release only a negligible fraction over a few weeks, and are barely sensitive to ventilation anyway.
| Material | Thickness | Reduction |
|---|---|---|
| Paint / thin coat | 0.2 mm | ~100% |
| Vinyl flooring | 2 mm | ~32% |
| Plasterboard | 12.5 mm | ~0% |
| Dense PVC | 2 mm | ~0% |
Practical consequence: a flush-out is relevant mainly when the project's dominant sources are applied products (paints, adhesives, varnishes) — not when they are thick panels or floors.
For a vinyl floor: ~10% at 3 days, ~32% at 14 days, ~79% at 60 days. The DETOX project recommends using the building's permanent ventilation during the pre-acceptance operations, for at least 28 days.
Beyond the threshold (on the order of a few air changes/h depending on the emitting surface), increasing the airflow barely improves the reduction. No need to aim for extreme rates: the permanent ventilation is generally enough.
Over-ventilation at ambient temperature. Effect limited by the threshold above: ventilation cleans the air, but does not force the pollutant out of the material any faster.
You heat the building (30–40 °C) for several days, with ventilation in support. Heat genuinely speeds up diffusion: studies report −60 to −90% of total VOCs. But it is a thermal lever, more energy-intensive, and outside the model's current scope (fixed temperature).
For a given project, the model estimates, material by material and pollutant by pollutant, the emission reduction a flush-out delivers — depending on its duration and airflow — and therefore helps decide whether it's worth it.
This reduction is a property of the material and the pollutant: it does not depend on the initial quantity, only on permeability, thickness, airflow and duration. You can thus compare a scenario with and without flush-out before committing to the works.
The real benefit of flush-out on already-loaded air and on absorbing surfaces belongs to the whole-building simulation (the i4o solver), not to the emission model alone. Bake-out (acceleration by heat) is not modelled. Finally, the reduction depends on the ratio of material surface to room volume: a loading representative of the real room must be used.
Over-ventilating doesn't empty materials "at will": beyond a threshold, more airflow no longer accelerates emission.
Effectiveness depends on the material: strong for applied products (paints, adhesives, varnishes), weak for thick, slow reservoirs.
Duration matters more than airflow; DETOX recommends ≥ 28 days with the permanent ventilation.
Bake-out (heat) is more effective but is a different lever, not modelled here.
The model lets you compare with / without flush-out by material and by pollutant, to guide the decision.
i4o estimates the expected emission reduction, material by material and pollutant by pollutant, to compare a scenario with and without flush-out. No credit card required.
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