Marley clears up the confusion and gives some top ventilation tips for cold roofs, warm roofs and loft conversions.
Cold roofs, warm roofs, well-sealed or normal ceilings, breathable and non-breathable membranes, it’s not surprising that ventilation for pitched roofs is so confusing! There are so many different variables that affect what type and how much ventilation is used, but if it isn’t done properly it can lead to serious condensation problems, particularly in new homes.
A cold pitched roof features thermal insulation installed along the horizontal ceiling joists. The roof space is unoccupied, perhaps being used only for limited storage, and is at a similar temperature to the outside. There is little or no obstruction to airflow through the roof space from eaves to eaves, so smaller openings are sufficient.
How to ventilate cold roofs
- Cold roofs should be ventilated in accordance with BS 5250, which means that ventilation should be provided at both eaves and ridge level.
- A cold roof should have a minimum of 10mm continuous ventilation at the eaves and 5mm continuous ventilation at the ridge.
- You can use either a breathable or non-breathable underlay but must also make sure there is the right amount of high and low level ventilation to prevent condensation.
- Non breathable underlays – the ventilation requirements for cold roofs with non-breathable underlays and ‘well-sealed ceilings’ are the same as for those without.
- Breathable underlays – installing a breathable membrane can reduce ventilation requirements for both well sealed and normal ceilings but to be on the safe side, it is still best to provide 10mm continuous ventilation at eaves and 5mm at the ridge. Or, you can contact our technical team to provide a free NBS specification to ensure you allow sufficient ventilation.
In a warm roof, the thermal insulation is along the slope of the roof to create a habitable loft space. Ventilation airflow is only possible above the insulation and along the slope of the roof, so larger openings at the eaves are needed to promote sufficient air movement.
How to ventilate a warm roof
- Thermally insulating a warm roof often creates a relatively air tight, sealed ceiling which restricts the passage of moisture vapour into the roof structure and increases the risk of condensation. Therefore, warm roofs actually need more ventilation than cold roofs, in accordance with BS 5250.
- While a warm roof still needs the same 5mm continuous ventilation along the ridge, it does require more ventilation, 25mm, at eaves level. This is why we sell both a 10mm and a 25mm eaves vent system.
- To prevent condensation forming, you may see architects specifying an air vapour control layer (AVCL) for warm roofs. For non-breathable membrane, the AVCL goes on the warm side of the insulation and ventilated voids should be formed between the underside of the underlay and the insulation. Each void should be at least 25mm deep and vented at both high and low level. With a breathable membrane, the AVCL is used at the ceiling line and in theory this means no additional ventilation is required, however, to be on the safe side, ventilated voids should still be provided.
When changing an empty loft space (cold roof) into a habitable living area (warm roof), it is easy to overlook the change in ventilation requirements.
- The main difference is that you will need to increase the ventilation at eaves level.
- You need to increase eaves ventilation from 10mm to 25mm continuous ventilation running along the whole length of the eaves, this can be achieved using our 25mm eaves vent system.
- The high level ridge ventilation requirements remain the same but do check that these are already in place. High level ridge ventilation should be equivalent in area to a 5mm slot for the length of the ridge.