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Laundry Ventilation Calculator — Moisture Removal
Calculate moisture release from clothes drying and the extract ventilation rate needed to remove it. Covers indoor drying, vented dryers, and condenser dryers.
Internal rooms need mechanical extract
For indoor rack drying extract rate
Vented tumble dryer only — for the indicative duct pressure-drop estimate
Each 90° bend ≈ 1.5 m of equivalent straight duct
Vented tumble dryer duct — larger diameter lowers the pressure drop. 100 mm is standard
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How We Calculate This
This calculator estimates the moisture released from clothes drying and determines the extract ventilation rate needed to remove it within a specified time, preventing condensation and mould problems.
Moisture release rates
- Indoor drying rack: ~2.5 kg water per 5kg load
- Vented tumble dryer: 0 kg (vented externally)
- Condenser dryer: ~0.3 kg residual
- Washer-dryer: ~0.5 kg residual
Ventilation rates
Approved Document F (2021) requires 30 L/s for an intermittent extract fan in a utility room (Table 1.1), or 8 L/s for a continuous (humidistat MEV) system (Table 1.2). For indoor rack drying this calculator sizes a boosted intermittent rate to clear the moisture within your target drying time, with the 30 L/s intermittent figure as the floor. The extract fan should ideally be humidity-controlled with boost capability.
Duct pressure drop
For a vented tumble dryer the calculator gives an indicative duct pressure-drop estimate that responds to duct length, number of 90° bends and duct diameter (a wider duct lowers velocity and so the pressure drop). It is an order-of-magnitude guide only — confirm the actual fan/duct combination against CIBSE Guide C friction charts or the manufacturer's data.
Frequently Asked Questions
A typical 5kg wash load retains approximately 2.5 kg of water after spinning. Drying on an indoor rack releases all of this directly into the room air over 4-8 hours. A vented tumble dryer sends it outside via the duct. A condenser dryer captures most of it but releases ~0.3 kg as residual humidity. Four loads per week can add 10+ litres of moisture to a home.
Approved Document F (2021) sets two different rates depending on the type of system. An intermittent extract fan — the normal fan you switch on or that runs on a timer/humidistat boost — must deliver at least 30 L/s in a utility room (Table 1.1). A continuous (always-on) mechanical extract system, such as a humidistat MEV unit, runs at a lower 8 L/s (Table 1.2). If clothes are dried indoors on racks, a higher boosted rate may be needed to clear the moisture quickly and prevent condensation and mould. A humidity-controlled fan that boosts automatically when moisture rises is ideal for utility rooms.
Use rigid 100mm or 150mm diameter duct from the dryer to an external wall vent with a gravity flap. Keep the run as short as possible (under 3m ideally) with minimal bends. As a rule of thumb each 90° bend adds the equivalent of roughly 1.5m of straight duct. Never vent into a loft, wall cavity, or under the floor — this causes severe condensation and rot. Use a lint trap at the dryer connection.
Yes, condenser dryers (especially heat pump models) are preferred for flats as they need no external duct. They collect moisture in a reservoir or drain to a waste pipe. However, they still release some heat and residual moisture, so adequate room ventilation is still needed. Heat pump condenser dryers use 50% less electricity than vented dryers.
Yes, regularly drying clothes indoors without adequate ventilation is a major cause of condensation and mould in UK homes. Each load releases ~2.5 litres of water into the air. In a poorly ventilated room, this raises relative humidity above 70%, creating perfect conditions for mould growth. Always use mechanical extract ventilation when drying indoors.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.