Dehumidifier vs PIV System: Which One Actually Solves Condensation?

Dehumidifiers and positive input ventilation (PIV) systems are both frequently recommended for condensation and mould problems — sometimes by people who conflate the two, and sometimes by installers who recommend their own product regardless of what the situation actually calls for. Understanding the difference, and when each is genuinely appropriate, saves money and solves problems.

As an independent damp surveyor covering Derby, Nottingham, and Leicester, I recommend ventilation solutions based on what the building actually needs — with no financial interest in which product you buy.

Dehumidifiers: What They Do and When They Help

A dehumidifier draws air from the room, passes it over cold coils where moisture condenses out, collects the water in a tank or drains it via a hose, and returns the drier air to the room. A typical domestic unit running in a living room will extract 10–20 litres of water per day depending on conditions.

When a dehumidifier is genuinely useful:

• During the drying-out period after a flood or major water ingress event, to speed up structural drying. In this context a large refrigerant or desiccant unit is appropriate, often on hire.
• In isolated rooms with very high moisture production where ventilation alone isn’t managing the humidity — a utility room with no external vent, for example, or a basement with no ventilation provision.
• As a short-term measure while structural issues or ventilation deficiencies are being addressed.
• In properties where tenants have limited control over the building’s ventilation (no opening windows, no extraction) and the landlord hasn’t yet upgraded the system.

When a dehumidifier is not the right answer:

It doesn’t fix the cause of moisture. A dehumidifier running in a living room with a blocked gutter emptying against the external wall is fighting a battle it can’t win. It treats the symptom — excess humidity — but the symptom will keep returning until the source is addressed.

Dehumidifiers are also relatively energy-intensive to run — typically 300–700W — and require regular emptying unless continuously drained. For long-term condensation management in a well-occupied property, they’re not an efficient solution.

PIV Systems: What They Do and When They Help

A positive input ventilation system consists of a fan unit installed in the loft space (or in a wall-mounted unit in flats) that draws fresh air from outside, filters it, and gently introduces it into the property at a continuous low flow rate. This creates a slight positive pressure that dilutes indoor humidity and pushes moist air out through natural gaps in the building fabric.

A typical loft-mounted PIV unit moves around 50–70 litres of air per second — enough to provide the whole-building air change rate recommended by Building Regulations Approved Document F for a standard house. Running cost is modest: typically 10–30W, around £20–£50 per year.

When PIV is genuinely useful:

• Whole-house condensation problems that persist despite tenants following reasonable ventilation habits — the building is simply too airtight without mechanical assistance.
• HMOs and multi-occupancy properties where individual room ventilation is difficult to manage and moisture loads are high.
• Properties where tenants are reluctant or unable to ventilate adequately — PIV runs continuously without requiring occupant action.
• As part of a package with upgraded room extraction in bathrooms and kitchens, where whole-house ventilation needs to be improved systematically.

When PIV is not the right answer:

PIV doesn’t fix structural moisture problems. Water entering through a failed flashing, a leaking pipe, or a bridged DPC will not be resolved by running a PIV unit. In a poorly-sealed building with significant air leakage, the positive pressure effect is diminished.

PIV is also sometimes oversold as a solution for properties with significant structural damp issues — I’ve seen surveys where a PIV installation has been recommended alongside (or instead of) addressing obvious penetrating damp or plumbing leaks. That’s not appropriate.

Combining Both

For serious condensation problems in a property with high occupancy and limited natural ventilation, combining a whole-house PIV system with room-level extraction (humidity-controlled fans in bathrooms and kitchens) is often the most effective approach. The PIV handles background whole-house ventilation; the room extraction deals with peak moisture events from showering and cooking.

A dehumidifier might then be used temporarily during the transition period while the building adjusts.

Cost Comparison

• Portable domestic dehumidifier: £150–£400 purchase; £50–£150/year running costs
• Loft-mounted PIV system (supply and install): £500–£900; £20–£50/year running costs
• Wall-mounted PIV unit (for flats): £400–£700 installed
• Humidity-controlled bathroom extractor fan: £100–£200 installed

📞 07983 550 662
✉️ richard.bull@dampdetectives.co.uk
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Richard Bull MISSE, ACIEH — Independent & Unbiased — No Sales Pressure