Roof Ventilation and Interstitial Condensation

Roof ventilation is important to prevent interstitial condensation problems in the loft space of your home.

Roof ventilation is important to prevent interstitial condensation problems in the loft space of your home. Interstitial condensation is a form of structural damp that occurs when warm, moist air penetrates inside a wall, roof or floor structure. The standard of roof ventilation varies widely but the current Building Regulations specify that 270mm mineral wool should be used to insulate your a roof space at a minimum.

It is a symptom of modern living that our homes now contain far more moisture. We are often trying to make our homes warmer by installing double or secondary glazing, blocking up vents, blocking chimneys and changing open fires to central heating systems. Those things in addition to spending more time at home, having more baths and showers, drying wet clothes indoors and cooking more at home means that we are creating far more moisture in our homes whilst sealing shut its usual methods of escape.

Because warm air holds moisture well, as the air rises, it often causes interstitial  condensation problems in the loft, particularly if the loft insulation isn’t the most energy-efficient. The lack of ventilation increases the risk of interstitial condensation forming on the underside of the felt membrane, causing it to settle on the timber rafters.

Your roof space should let stale warm air out and fresh cool air in.


Although this image is of a pitched roof, both flat roofs and pitched roofs have warm and cold spaces. The major difference between the warm flat roof and the cold flat roof has to do with the air flow between the roofing timbers (known as joists) that help to hold up the roof. In the warm flat roof construction, there is insulation packed into every possible space where air can flow. In creating the cold flat roof, air is allowed to circulate freely between the joists above the insulation. In order to achieve this air vents at either side must be used to ensure cross flow ventilation.

Roofing felt, especially the non-breathable kind used a lot in the 1960s-1970s, significantly reduces air movement in the roof space and holds the moisture that would otherwise evaporate out of the roof. As a result of this, timbers in the roof space are subjected to active fungal decay, wood boring insect infestation and rot.

Before making any improvements to your loft space:

  • Don’t do anything to adversely affect the structural performance of the roof.
  • Use breathable roofing felt.
  • Never push loft insulation to the perimeter of the roof. Use an insulation tray to get a snug fit or pull it back around 200mm from the edge.
  • To eliminate the risk of interstitial condensation, you can consider the use of a vapour control layer in conjunction with the correct ventilation and roof membranes.

For other tips about loft insulation, see our dedicated blog post here or to speak to one of our experts, call us on 0121 666 7706 and we will arrange for one of our specialist surveyors to come out to your home to carry out a free, no obligation homeowner survey.