Does Spray Foam Cause Damp in the Loft?

How a cold loft normally stays dry

In an ordinary loft with insulation laid on the floor, the roof space is cold and ventilated. Air enters at the eaves, passes under the underlay and leaves at the ridge or through tile vents. Any moisture that finds its way up from the house is carried away by that moving air. The system looks untidy, but it works because the roof timbers and the underlay sit in a space that is allowed to dry.

Spray foam works on a different principle. It is applied to the underside of the roof covering, between and over the rafters, so the insulation line moves up to the slope and the loft becomes part of the heated envelope. That is a legitimate design called a warm roof, and our page on warm loft versus cold loft compares the two. The risk is that foam applied to a roof built for a cold loft shuts off the airflow it was designed around.

Where the damp comes from

There are three sources. The first is condensation. Warm, moist air from kitchens, bathrooms and breathing leaks into the loft through light fittings, hatches and gaps around pipes. If it meets a cold surface, such as the underside of the roof covering at the edge of the foam, it condenses. With no air to dry it, that moisture can sit on the felt and the battens for months.

The second is a leak. A slipped tile, a cracked slate or a failed flashing admits rain, and in a normal loft you would see the drips or stains on the insulation. Under foam, the water is held against the timber and is out of sight. The third is the foam itself. If it was applied to damp timber, or in conditions that trapped water, the moisture was sealed in from day one. Trading Standards guidance has pointed out that foam can reduce roof-space ventilation and so raise the risk of condensation, mould and potentially timber rot.

Timing is a clue. Damp that appears in the first winter after foaming often points to air leakage or wet timber sealed in at the time. Damp that develops years later is more likely a leak that has built up slowly behind the foam. Either way, the pattern of stains on the ceiling, and whether they follow rafter lines, helps trace the source.

Signs of damp under spray foam

The warning signs show on the room side as well as in the loft. Look for a musty smell, black or green mould on the foam face or on bare timber, dark patches or tide marks on the ceiling below, condensation on the hatch and on the inside of upstairs windows, and rusting on nails or fixings. On the outside, check for slipped tiles, damaged flashings or blocked gutters, since these are the commonest ways water gets in. The most worrying sign is no sign at all: damp timber can sit hidden behind the foam. We cover this in more depth on our page about whether spray foam can rot roof timbers.

Roof design affects the risk. A modern roof built as a warm roof from the start, with the right membranes and ventilation detail, is a different case from a Victorian or 1930s house where the felt is old, the tiles are open-jointed and the whole system depended on air moving. In the older house, foam can change the balance in ways the original builders never allowed for, which is part of why surveyors treat it with caution. Bathrooms and kitchens without good extraction add to the humid air that finds its way up.

Finding out what is happening

The test is to read the timber, not to guess. A moisture meter on exposed rafter ends gives a first picture, and a small inspection opening shows the condition of the wood behind. We check the roof covering and flashings first, since a leak is the easiest cause to fix and the one most often missed. We photograph everything, including the loft conditions, and compare them with what a dry foamed roof should show.

The remedy depends on the cause. A leak is mended and the wet area dried. Poor air sealing between the house and loft is improved. Where there is moisture trapped behind the foam, or a surveyor or lender is concerned, removal and a ventilation check are part of the usual fix, as the RICS consumer guide sets out. After the survey we write a fixed price, and you can read more on our spray foam insulation problems page. If the foam is in good order and the roof is dry, we tell you to leave it be.

The wider service is described on our spray foam service in Basingstoke.