How much moisture does drying clothes release?

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.

Do I need an extract fan in a utility room?

Approved Document F requires 8 L/s minimum extract ventilation for utility rooms. If clothes are dried indoors on racks, a higher rate may be needed to prevent condensation and mould. A humidity-controlled fan that boosts automatically when moisture levels rise is ideal for utility rooms.

How should I vent a tumble dryer to outside?

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. Each 90° bend adds equivalent of ~1m. 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.

Is a condenser dryer better than a vented dryer for flats?

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.

Can drying clothes indoors cause mould?

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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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.

Drying Method

Determines moisture release and ventilation type

Loads Per Week

Room Volume

Window Present

Internal rooms need mechanical extract

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

ADF requires 8 L/s minimum for utility rooms. For indoor drying, the rate is increased to remove moisture within the target drying time. The extract fan should ideally be humidity-controlled with boost capability.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.