Last year we added additional insulation to our attic. The benefits of this were seen immediately. This winter, especially with low temperatures, we have noticed build-up of water on the underside of the roofing felt. We have ensured the insulation is not right up against the eves. My concern is that the roofing felt may not be breathable and may need to be replaced
I have seen such condensation in homes where I have recorded ineffective cross ventilation. Most modern homes have projecting eaves with horizontal pre-formed soffits and perforated holes. The perforations are sometimes installed at irregular spacing and do not always correspond to the opposite side. Older homes may have flush fascias and no venting.
For a typical pitched roof, the minimum standards recommend an area that is at least equal to a continuous 10mm air gap at eaves. For a “lean-to” roof, the same rule applies but with the addition of ventilation at high level at least equal to a continuous 5mm air gap. For roof pitches below 15 degrees, the eaves ventilation should be 25mm wide. The minimum standards are not always achieved.
Ensure insulation is free from the eaves. Check the amount and spacing of the soffit vents. Compare these to a continuous 10mm air gap around the building. If it’s a “lean-to” roof, make the same observations but note high-level venting is necessary. Gables allow the addition of vents and closely spaced slate vents can help. There are proprietary continuous ventilators that are placed under the front row of slates. A roofer will need to remove and refit the lower four courses. Preformed vent slots can also be placed between the insulation and felt in the attic. Also check insulation is evenly distributed and placed over and not below the water tanks.
Replacing the felt with a breathable membrane is a costly solution. It will mean removing and reinstating roof tiles and battens with wastage and/or breakage requiring new materials. The membrane allows movement of water vapour through it, which is an improvement on the impervious felts previously used. Some manufacturers claim these do not need ventilation, but they have not been tried and tested over a significant period, so for a healthy roof void you need a draughty void.
Improving the ventilation and evenly distributing the insulation should deal with your problem. The type and construction may even have a bearing on the problem. If in doubt, I advise an inspection by a registered building surveyor or registered architect.
Jim Drew is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland.