10 Aug 2011

Confusion over roofing underlays

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.


  1. We just need to spec vapour control layers to top floor ceilings. That would greatly reduce the amount of moisture getting into the loft in the first place and probably help eliminate these sorts of condensation problems.

    I forget what the Americans call it now, something like atic bypass. They end up with ice formation in their roof spaces due to the much colder winters that some areas experience.


  2. interesting, I dont know if any one ever watch a Canadian program called holmes on homes.

    but this Holmes bloke talks about vapour barrier a lot. about controlling the air movement through walls.

    I guess it is simple. if you dont have warm moist air in the loft then you not going to get condensation up there...

  3. If the problem is moist air rising from the house below, I don't see air permeable being the solution. You won't stop the warm air coming through the ceiling, you may even encourage it.
    As the others say, you need a VCL for reliable moisture control - and it will also need to be airtight.

  4. wonderful .. great post .its really interesting

  5. I am a private house owner but its old and listed and got bats in the roof which we a going to retile. The bat people have discovered that Tyvek and all the other SPMs are bad for bats as they fluff up and bats can get tangled up in the fibres. Also if the bats pee on it the SPM deteriorates. So they prefer the good old Zylex. My tiler says he hasn't used it for fifteen years but if its still available he is happy to use it. What goes around, comes around eh!

  6. I only use 'Roofshield' and have done so for the last 15 years without any condensation or failures on refurbishment and new build projects, including NHBC, who approve it for use without ventilation as it is both air and vapour permeable.

    I have also never had to install a VCL at ceiling level using Roofshield; as a roofing contractor it is not my responsibility and in my experience a VCL is unlikely to be installed by the main contractor or house builder. When it is, I have yet to see one installed correctly, with sealed laps and sealed around the perimeter / around penetrations. If the ceiling is installed correctly and 'well sealed' it provides a sufficient barrier (vapour check boards can be used for kitchen and bathroom areas too).

    See BBA: http://www.proctorgroup.com/Portals/0/literature/condensation-control/Certification/Roofshield%20-%20Cold%20Roof%20BBA%20(99-3648%20i6).pdf