Insulation and ventilation are one subject, not two, and treating them separately is behind most of the damp lofts we are called to look at. Insulating a roof changes where the cold surface sits, and if the air path does not change with it, moisture from the house condenses somewhere it never used to. This guide explains the two basic approaches, where each belongs, and the specific mistakes that turn a well meant energy improvement into wet timbers.
Cold loft: insulation at ceiling level
This is the standard arrangement in most houses. Insulation sits between and then across the ceiling joists, so the living space below is warm and the loft above is cold, and the loft is ventilated so that any moisture that gets up there is carried away.
It is cheap, effective and easy to top up. The two things it demands are a clear air path at the eaves and a serious effort to stop warm moist air leaking up from the house in the first place: loft hatch seals, sealed downlight penetrations, and bathroom and kitchen extract ducted out through the roof rather than dumped into the loft.
Note that once the loft is properly cold, anything up there needs protecting. Water pipes and tanks should be lagged on all sides and left uninsulated underneath so a little heat still reaches them.
Warm roof: insulation at rafter level
If the loft is a room, or is going to be, the insulation has to follow the slope instead of the ceiling. That can be done between and under the rafters from inside, or above the rafters from outside during a re-roof.
Insulating from above, sometimes called a sarking or over rafter system, gives the best continuity because there are no rafters interrupting the layer, and it keeps every millimetre of headroom inside. It raises the roof line slightly, so verges, abutments and flashings all need re-detailing, which is exactly why it is done as part of a full re-roof rather than on its own.
Insulating from below is far less disruptive and does not touch the covering, but it eats headroom and requires care to avoid thermal bridging at the rafters and gaps at wall plates and purlins.
- Over rafter: best performance and continuity, done during a re-roof
- Between and under rafter: cheaper, no external work, costs headroom
- Both need a vapour control layer on the warm side
- Both need a decision about whether any ventilated void remains
Ventilation is not optional
A household produces a surprising quantity of water vapour every day. Cooking, washing, drying clothes and simply breathing all add moisture to the air, and that air rises. In a cold loft the job of ventilation is to give that moisture a route out before it meets a cold surface and condenses.
Ventilation is usually provided at the eaves, with cross flow across the loft and often additional venting at the ridge. Insulation stuffed hard into the eaves is the single most common fault we find, and it is usually done with the best of intentions by someone trying to eliminate a cold spot.
Where the roof design cannot deliver enough airflow, or where the loft is converted, the answer is a breathable membrane and a designed ventilation strategy at ridge and eaves rather than hoping.
The mistakes we see most often
None of these are exotic. All of them are common, and all of them produce damp that owners then blame on the roof covering.
- Insulation pushed into the eaves, closing the air path completely
- Extractor fan ducts terminating inside the loft instead of outside
- Boarding laid straight onto the joists, squashing the insulation flat
- Downlights left unsealed so warm damp air pours up through the ceiling
- Insulation laid over the top of an old, unventilated bitumen felt underlay
- No vapour control layer on the warm side of a rafter level installation
Do it while the scaffold is up
The most cost effective moment to improve a roof thermally is when it is already stripped. The membrane is off, the rafters are visible, ventilation can be built in at ridge and eaves properly, and there is no second mobilisation to pay for.
If a re-roof is on the horizon in the next few years, it is worth planning the insulation strategy now rather than doing a partial job from inside and then undoing it later. We will talk that through as part of any replacement quotation.
Common questions
How much loft insulation should I have?
Current guidance for a cold loft is considerably deeper than the levels found in most older houses, and topping up an existing layer is one of the cheapest improvements available. Lay the top layer across the joists rather than between them so the joists themselves are covered.
My loft is damp since I insulated it. Why?
Almost always because the ventilation path was blocked or the insulation made the loft colder without stopping moist air getting up there. Check the eaves are clear, check any extractor fan actually vents outside, and seal the loft hatch. If it persists, it needs looking at properly.
Can I board the loft for storage over insulation?
Yes, but not directly onto the joists if that squashes the insulation, which destroys most of its value. Use raised loft legs or a proprietary platform system so the full depth of insulation stays uncompressed underneath.
Do you fit insulation as part of a re-roof?
We deal with the roof build up, including membrane, ventilation and over rafter insulation where it forms part of the roof specification. We will set out clearly in the quotation what is included so you can see how the whole build up works together.