Back in the 1980s when I was working as a general builder, if we had a re-roofing job we always used a product called Zylex, a bitumen roofing felt produced by Ruberoid. It was heavy, came in 16m rolls and above all it was cheap. I can remember the first time I ever saw Tyvek which was said to be the future. Tyvek was that magic thing, a vapour permeable underlay (VPU), which would allow roofs to ventilate and do away with all the complex little fiddles we had to do to stop condensation. It sounded so cool.
Fast forward to now and Zylex has all but vanished. There are now dozens of manufacturers produced VPUs and it's what everyone now uses. VPUs used to be much more expensive but there isn't a great deal of difference in price now (they seem to cost between £1.50 and £2/m2). But what is interesting is that, according to reports coming from the NHBC, they don't seem to work any better than Zylex. The recent hard winters have led to a spate of condensation claims, as water forms on the underside of the VPUs, and then drips down through the ceilings of the new homes below. The NHBC deems that eaves-to-eaves ventilation is inadequate and that we should use eaves-to-ridge ventilation, which introduces some sort of stack effect, in order to remove the moist air form these cold lofts.
If you want to avoid having to install ridge vents, then there is another option and that is to go for an air permeable underlay (APU). A what? Well an APU promises to do what VPUs promised back in the 1980s and didn't really manage. Just like Goretex jackets don't actually stop you sweating. It's a similar idea, a woven fabric underlay, but it allows air to permeate through the fabric which basically requires bigger holes than ones that just admit vapour. The best known (only?) APU on the market at the moment is Klober's Permo Air (£93 for 50m2, so not that much more than the others).
Two thoughts occur. Will we be coming back in twenty years time saying that APUs don't work either and that we are still getting dripping condensation in lofts. We seem to have come a long way in developing superior roof underlays and got precisely nowhere. And secondly, can an APU be used as an air barrier in airtight construction systems? Or is it, by its very nature, leaky?
Good piece of further reading here on the Monier site.