Does my loft need ventilation?

The principle is simple. A loft insulated at ceiling level — which is nearly all of them — is meant to be at outside temperature, and it stays dry only if air moves through it. Air comes in low at the eaves, rises, and leaves high at the ridge, carrying moisture with it. Interrupt either end and the roof space gets damp.

In practice it is the low end that gets blocked, and insulation is nearly always the reason. Every energy efficiency improvement of the last thirty years has meant more insulation in the loft, and it is usually pushed hard into the eaves where the roof meets the wall — which is exactly where the air was getting in. The house gets warmer and the loft gets wetter.

The fix at that end is two things: rafter trays, which are simple channels holding the insulation clear so a gap remains, and eaves or over-fascia vents to let outside air into that gap. Neither is expensive, and both can usually be done without touching the covering — which makes this one of the best-value jobs on any roof.

The high end is ridge ventilation. A dry ridge system ventilates as it goes, because the ventilated roll under the mechanically fixed units lets air out along the whole ridge line. Where the ridge is staying as it is, individual tile vents or slate vents near the ridge do the same job.

The other half of the answer is reducing what goes up there. Seal the loft hatch, seal around pipe and cable penetrations and around downlights, and check where the bathroom and kitchen extractors actually discharge. A depressing number of them terminate a foot inside the loft rather than outside, which pumps the wettest air in the house straight into the roof space every time somebody showers.

One thing to be clear about: a breathable membrane reduces condensation risk substantially but it does not remove the need for a ventilation strategy, and it certainly does not make it acceptable to block the eaves. Anybody telling you a breathable underlay means no ventilation is needed is overstating it.