Sarking felt or underfelting through time – Part 3
The next development was in the late 1980s/ early 1990s when builders used a reinforced plastic felt called Monarflex Monofoil. Originally designed for commercial applications, its inherent strength saw it find its way onto residential developments and re-roofing projects. It is in essence a reinforced plastic type sheet. It is usually apparent as a light grey in colour in the roof void.
The composition of this layer remedied the problems of decaying found in older felts. It should have been popular for a great deal longer, however it did not deal with the ventilation issues. This didn’t allow the roof structure to breathe. It was also laid fairly taut, being stretched across the rafters, and this further limited air flow through the roof voids.
Related condensation issues
As this felt was non breathable, it suffered terribly from condensation related issues too. Water droplets formed on the underside and ran down to soak the timber elements of the roof structure. It was supposed to have a vapour layer (another polythene type sheet) installed at ceiling height. This would have reduced the moisture travelling upwards into the loft but it was rarely installed. The legacy of this underfelt in domestic housing was condensation !
Recently we inspected a property where Monarflex was present and a loft conversion had been carried out. The insulation below was wet to touch due to condensation forming in the hidden void.
Dealing with condensation in the loft void
Some actions can help reduce condensation levels in the loft void. The damp air may be removed at source with extractor fans. The often-missing vapour layer at ceiling height may be installed. This would require caution around services such as downlighters, wiring and ducting. Holes would have to be inserted for these and the loft insulation would need to be removed and refitted/replaced. External provision such as vented soffits and ridge and hip detailing could be made. But if you are going to this length, then simply re-felting with a more suitable underfelt may be a better option.