Question 1

Why is my loft insulation damp?

Accepted Answer

Almost always condensation rather than a leak — warm moist air from the house rising into a cold roof space that has no way to vent it. Damp insulation performs far worse than dry, so it is costing you heat as well as rotting the timbers, and the fix is ventilation rather than roofing. Damp loft insulation is one of the clearest signals of a ventilation problem, because a leak wets one area and condensation wets everything. What is happening is straightforward. Cooking, showering, drying washing and simply breathing put several litres of water into a house's air every day. That moist air rises, gets into the roof space through the loft hatch and around pipes, cables and downlights, and meets the cold underside of the roof. There it condenses — onto the felt, onto the rafters, and onto the insulation. The consequences are worse than they look. Wet mineral wool loses a substantial part of its thermal performance, so the insulation you paid for stops doing its job at exactly the time of year you need it. The moisture rots battens and rafter feet over time. And the black mould that comes with it is a genuine air quality problem in the house below when the hatch is opened. The trigger, more often than not, is the insulation itself. Adding depth makes the loft colder — which encourages condensation — and insulation laid right out to the eaves blocks the ventilation gap that was keeping the space dry. It is not that insulating was wrong; it is that the ventilation path was not maintained when it was done. Old bitumen felt makes everything worse. It is effectively vapour-closed, so moisture that gets past it cannot escape and collects underneath. A great many local roofs from the postwar decades have that build-up, and replacing it with a breathable membrane is one of the genuine benefits when a roof is eventually stripped. The remedy is almost always ventilation. Rafter trays to hold the insulation clear at the eaves, eaves or over-fascia vents to let air in, and ridge or tile vents at high level to let it out — plus sealing the routes moist air is using to get up there in the first place. It costs a small fraction of anything involving the covering, and it works permanently.

Question 2

Wet loft insulation

Accepted Answer

Almost always condensation rather than a leak — warm moist air from the house rising into a cold roof space that has no way to vent it. Damp insulation performs far worse than dry, so it is costing you heat as well as rotting the timbers, and the fix is ventilation rather than roofing. Damp loft insulation is one of the clearest signals of a ventilation problem, because a leak wets one area and condensation wets everything. What is happening is straightforward. Cooking, showering, drying washing and simply breathing put several litres of water into a house's air every day. That moist air rises, gets into the roof space through the loft hatch and around pipes, cables and downlights, and meets the cold underside of the roof. There it condenses — onto the felt, onto the rafters, and onto the insulation. The consequences are worse than they look. Wet mineral wool loses a substantial part of its thermal performance, so the insulation you paid for stops doing its job at exactly the time of year you need it. The moisture rots battens and rafter feet over time. And the black mould that comes with it is a genuine air quality problem in the house below when the hatch is opened. The trigger, more often than not, is the insulation itself. Adding depth makes the loft colder — which encourages condensation — and insulation laid right out to the eaves blocks the ventilation gap that was keeping the space dry. It is not that insulating was wrong; it is that the ventilation path was not maintained when it was done. Old bitumen felt makes everything worse. It is effectively vapour-closed, so moisture that gets past it cannot escape and collects underneath. A great many local roofs from the postwar decades have that build-up, and replacing it with a breathable membrane is one of the genuine benefits when a roof is eventually stripped. The remedy is almost always ventilation. Rafter trays to hold the insulation clear at the eaves, eaves or over-fascia vents to let air in, and ridge or tile vents at high level to let it out — plus sealing the routes moist air is using to get up there in the first place. It costs a small fraction of anything involving the covering, and it works permanently.

Question 3

Black mould in the loft

Accepted Answer

Almost always condensation rather than a leak — warm moist air from the house rising into a cold roof space that has no way to vent it. Damp insulation performs far worse than dry, so it is costing you heat as well as rotting the timbers, and the fix is ventilation rather than roofing. Damp loft insulation is one of the clearest signals of a ventilation problem, because a leak wets one area and condensation wets everything. What is happening is straightforward. Cooking, showering, drying washing and simply breathing put several litres of water into a house's air every day. That moist air rises, gets into the roof space through the loft hatch and around pipes, cables and downlights, and meets the cold underside of the roof. There it condenses — onto the felt, onto the rafters, and onto the insulation. The consequences are worse than they look. Wet mineral wool loses a substantial part of its thermal performance, so the insulation you paid for stops doing its job at exactly the time of year you need it. The moisture rots battens and rafter feet over time. And the black mould that comes with it is a genuine air quality problem in the house below when the hatch is opened. The trigger, more often than not, is the insulation itself. Adding depth makes the loft colder — which encourages condensation — and insulation laid right out to the eaves blocks the ventilation gap that was keeping the space dry. It is not that insulating was wrong; it is that the ventilation path was not maintained when it was done. Old bitumen felt makes everything worse. It is effectively vapour-closed, so moisture that gets past it cannot escape and collects underneath. A great many local roofs from the postwar decades have that build-up, and replacing it with a breathable membrane is one of the genuine benefits when a roof is eventually stripped. The remedy is almost always ventilation. Rafter trays to hold the insulation clear at the eaves, eaves or over-fascia vents to let air in, and ridge or tile vents at high level to let it out — plus sealing the routes moist air is using to get up there in the first place. It costs a small fraction of anything involving the covering, and it works permanently.

Question 4

Damp roof timbers but no leak

Accepted Answer

Almost always condensation rather than a leak — warm moist air from the house rising into a cold roof space that has no way to vent it. Damp insulation performs far worse than dry, so it is costing you heat as well as rotting the timbers, and the fix is ventilation rather than roofing. Damp loft insulation is one of the clearest signals of a ventilation problem, because a leak wets one area and condensation wets everything. What is happening is straightforward. Cooking, showering, drying washing and simply breathing put several litres of water into a house's air every day. That moist air rises, gets into the roof space through the loft hatch and around pipes, cables and downlights, and meets the cold underside of the roof. There it condenses — onto the felt, onto the rafters, and onto the insulation. The consequences are worse than they look. Wet mineral wool loses a substantial part of its thermal performance, so the insulation you paid for stops doing its job at exactly the time of year you need it. The moisture rots battens and rafter feet over time. And the black mould that comes with it is a genuine air quality problem in the house below when the hatch is opened. The trigger, more often than not, is the insulation itself. Adding depth makes the loft colder — which encourages condensation — and insulation laid right out to the eaves blocks the ventilation gap that was keeping the space dry. It is not that insulating was wrong; it is that the ventilation path was not maintained when it was done. Old bitumen felt makes everything worse. It is effectively vapour-closed, so moisture that gets past it cannot escape and collects underneath. A great many local roofs from the postwar decades have that build-up, and replacing it with a breathable membrane is one of the genuine benefits when a roof is eventually stripped. The remedy is almost always ventilation. Rafter trays to hold the insulation clear at the eaves, eaves or over-fascia vents to let air in, and ridge or tile vents at high level to let it out — plus sealing the routes moist air is using to get up there in the first place. It costs a small fraction of anything involving the covering, and it works permanently.

Why is my loft insulation damp?

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