Quick answer: Loft condensation happens when warm, moist air from your living space gets into a cold loft and has nowhere to go, so it settles on the coldest surfaces – usually the underside of the roof felt, nails and rafters. Over time that moisture feeds black mould. The fix is two-sided: cut down the moist air getting into the loft, and make sure the loft can ventilate.
We see loft condensation and mould more than almost any other problem in Cardiff and the surrounding valleys. A mild, damp climate, older housing and well-meaning improvements that block airflow all play a part. The good news is that most cases have a clear cause, and once that's dealt with the problem usually stops.
Why South Wales lofts get condensation
Condensation needs two things: moist air and a cold surface. A loft in winter is full of cold surfaces, so the question is always where the moisture is coming from and why it isn't leaving.
A damp climate and long wet spells
South Wales gets plenty of rain and a lot of mild, humid days. Outside air is often already damp, so a loft that relies on a small amount of ventilation has less margin. A loft that copes fine in a dry spell can start dripping after a run of wet, still weather.
Moist air rising from the house
Every home produces water vapour from cooking, showers, kettles and breathing. Warm air rises, and it finds its way into the loft through gaps around the hatch, recessed ceiling lights, pipe and cable holes, and the tops of internal walls. In many houses this is the main source of loft moisture.
Drying clothes indoors
Drying washing on radiators or an airer puts a large amount of water into the air. With the windows shut in winter, a lot of that ends up upstairs and then in the loft. If you can, use a vented or condenser dryer, or dry clothes in one room with the door closed and the window open a little or a fan running.
Extractor fans venting into the loft
This one catches people out. A bathroom or kitchen extractor fan should be ducted all the way to the outside – through a wall, a soffit or a roof vent. We regularly find ducting that stops short in the loft, has come loose, or was never connected at all. That fan is then pumping steam straight onto your rafters every time someone has a shower.
Blocked eaves
Most pitched roofs ventilate at the eaves, with air coming in low and leaving higher up. When loft insulation is pushed right into the eaves, that airflow stops. It's one of the most common faults we see after a rushed insulation job. Boarding laid directly on the insulation can also squash it into the eaves and seal off the gap.
Non-breathable roofing felt
Many roofs from the 1960s onwards have traditional bitumen felt under the tiles, which doesn't let vapour through. With that felt, the loft depends on its ventilation openings. Modern breathable membranes let some vapour pass out through the roof, but they still need to be fitted properly. Plenty of older Victorian and Edwardian terraces were built with no felt at all, and some of those lofts started to suffer only after a reroof with non-breathable felt and no added vents.
Condensation or a roof leak? How to tell
Both leave wet timber and stains, so it's worth knowing the difference before you call a roofer or a loft specialist.
| What you see | More likely condensation | More likely a leak |
|---|---|---|
| Where the damp is | Spread over a large area, often a whole slope (frequently the north-facing one) | One patch, often near a chimney, valley, vent pipe, skylight or the ridge |
| When it appears | Cold mornings, especially in winter, whether or not it rained | During or after rain, worse in wind-driven rain |
| What it looks like | Droplets across the felt, wet or rusty nail tips, frost on nails in a cold snap | Water runs or drips from one point, with tide-mark stains spreading out |
| Mould | Black speckles across rafters, sarking boards or the felt | Usually staining and rot in one area rather than widespread spotting |
| Insulation | Damp on the top surface across a wide area | A soaked patch directly under the source |
A simple check: go up on a cold, dry morning with a torch. If the underside of the felt is beaded with water and the nails are wet, that's condensation. If you can see daylight through the roof, or the wet patch only appears after rain, call a roofer. It is possible to have both at once.
How to fix loft condensation
There's rarely a single fix. We work through it in this order.
- Cut the moisture at source. Make sure extractor fans are working and ducted to the outside. Use them with the door closed and leave them running for a while after a shower. Avoid drying clothes indoors where you can.
- Seal the ceiling. Fit an insulated, draught-stripped loft hatch and seal gaps around pipes, cables and the soil stack where they pass into the loft.
- Open up the eaves. Pull insulation back from the eaves and fit eaves ventilators or rafter trays so there is a clear air path from the soffit into the loft. This is a standard part of how we install loft insulation.
- Add ventilation higher up if needed. Where there is non-breathable felt and not enough airflow, tile or slate vents, or ridge vents, let moist air escape near the top of the roof.
- Treat the mould. Once the cause is dealt with, the timbers can be treated with a fungicidal wash to kill the existing growth.
- Replace wet insulation. Mineral wool that has been soaked, has mould on it or has been flattened won't perform properly and should be replaced.
If you store things in the loft, the way it's boarded matters too. Boards should sit on raised legs with a gap above the insulation, not pressed onto it. Our guide to boarding a loft without crushing the insulation explains why.
Is loft mould dangerous?
Mould in a loft you rarely visit is less of a direct worry than mould in a bedroom, but it shouldn't be ignored. UK government guidance on damp and mould says it can produce allergens, irritants and spores that affect the airways and lungs, with children, older people and anyone with a respiratory condition at greater risk. Loft air also leaks back down into the house. Long-term damp will also weaken timber and rust nails and fixings.
Wear gloves and a dust mask when you're up there, and don't try to dry-brush mould off timber – that just spreads spores around.
When to call someone in
Small amounts of seasonal condensation that clear when the weather changes can often be managed with the source fixes above. It's time to get help if:
- water is dripping off the felt or nails onto the insulation or ceiling
- black mould is spreading across the rafters or sarking boards
- the insulation is damp, matted or has visible mould on it
- the problem started after new insulation, boarding or a reroof
- timber feels soft or crumbly when pressed (this needs a roofer or carpenter)
- you've done the obvious fixes and it's still happening
Our loft mould treatment covers both sides of the job: we find and fix the cause – usually ventilation, extractor ducting or blocked eaves – then treat the timbers and replace any spoiled insulation. If we find a roof leak or rotten timber, we'll tell you plainly and point you to a roofer rather than treating the symptom. You can book a free loft survey and we'll take a look.