A wet loft with a dry roof is a ventilation problem, and roofing work will not touch it. Here is how the moisture gets there and how to get it out.
Where the water comes from
A household of four puts several litres of water vapour into the air every day through cooking, showering, drying washing and simply breathing. Warm air holds that moisture until it meets a cold surface, and the coldest surface in the house is the underside of the roof.
Modern insulation makes this worse, not better, by design. Thicker loft insulation keeps heat in the house, so the loft above it is colder than it used to be, and cold lofts condense more. The answer is not less insulation, it is more ventilation.
The faults we find most often
Insulation pushed hard into the eaves is the number one cause. It blocks the airflow path at exactly the point where air is supposed to enter the loft. Fitting eaves trays restores the gap in an afternoon.
Bathroom and kitchen extract fans ducted into the loft rather than through the roof or wall come a close second. That is a fan whose entire job is to pump warm wet air into a cold space. Downlights punched through the ceiling and an unsealed loft hatch both let moist air up too.
The rest of the list is short and repetitive: non breathable bitumen felt with no vents fitted anywhere, gaps around service pipes and the loft hatch, recessed downlights leaking warm air upwards, and washing dried indoors through the winter with no background ventilation running.
Telling it apart from a leak
Condensation appears across a wide area rather than at one point, it is worst on cold clear nights, and it is often accompanied by black mould on the cold side of the timbers and on the underside of the felt.
A leak is localised, tracks down a specific rafter or felt lap, and coincides with rain. If the loft is wet after a cold dry week, the roof is not leaking.
The fixes and roughly what they cost
Clearing insulation from the eaves and fitting eaves trays runs 200 to 450 pounds. Over fascia or eaves vents fitted along the roofline are 350 to 900. Tile or slate vents let air out near the ridge and cost 120 to 250 pounds each fitted, and a typical roof needs two to four.
Re routing an extract fan properly through the roof or wall with an insulated duct is 250 to 600 pounds and often solves the whole problem on its own. A dry ridge system with ventilation, fitted during other roof work, adds ventilation at the top of the roof where it is most effective.
Is loft condensation dangerous?
Not immediately, but sustained damp rots roof timbers, ruins the performance of insulation and grows mould that can affect the air in the rooms below.
Should I add a vapour barrier?
In a cold loft the priority is stopping moist air getting up there and letting what does get up escape. Sealing the ceiling helps, but only alongside proper ventilation, never instead of it.
Why did condensation start after I had extra insulation fitted?
Because the loft is now colder and, very often, because the new insulation was pushed into the eaves and blocked the airflow. It is the single most common trigger we are called to.
Do I need to remove the old bitumen felt?
Not usually. Fitting tile vents and eaves ventilation achieves the airflow without stripping the roof. Replacing felt with a breathable membrane happens naturally at re roof time.
Will a loft fan solve it?
Occasionally, in difficult roofs, but passive ventilation at the eaves and ridge is cheaper, silent and needs no power. We would exhaust the passive options first.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.