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  • Writer's pictureStuart Penrose

To Vent or not to Vent? Expensive underlays worth the investment..

roofing felt dripping with condensation in
Condensation can go undetected for years in a roof space

30 years ago there was only one choice when it came to undelays for pitched roofing, Type 1F heavy bitumen felt, it was heavy to work with, tore easily and stuck to your feet on a hot summer day and would start to corrode if left exposed for any length of time.

Then came the first generation breather felts or underlays as they are now known, the first one i ever used was Klober Tyvek ,and before my mailbox fills up with Roofers informing me that they are two different brands ,they were actually together back then. They were primarily used for concrete structures to allow moisture to escape and avoid "concrete Cancer"

When they started to be used as roofing underlays and claiming they could be used in cold roof applications with the need for direct ventilation it caused a heated debate between manufactures of ventilation products, and in some cases it led to the courtrooms.

BBA certification and a number of independent testing bodies certified vapour permeable underlays to be used without the need for direct ventilation under certain conditions ,what followed was an explosion onto the market of underlays under the category of "breather felts", now a breather felt is very different from a vapour permeable underlay, the inevitability of failure was quickly realised, when damp spots started to appear in newly constructed buildings around the country, Roofing contactors were being blamed for their roofs leaking and manufactures ran for cover ,

One very high profile failure was Minitex MP, roofers of a certain vintage will shudder at the very mention of it ,it was a polythene based underlay with little pin holes (the MP was for Multi Perforated) it was cheaper than all other competitors and because of the zillions of pin holes it was sold as a breather felt , technically speaking it did breathe but it wasn't vapor permeable, you could've filled a swimming pool in weeks with the amount of condensation rained down from the warm side.

So what of today, and our current range of underlays can we trust them? do they do exactly what it says on the tin ,bearing in mind some these underlays can cost as much as £300.00 ex vat per roll.

roof felt  for cold roof applications
Are they worth the money?

In the next blog Stuart Penrose walks you through the jargon of Vapor Control layers and gives his verdict on the question of direct ventilation

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