Weatherproofing – breather membrane


The weather here is going from bad to worse – earlier today I grabbed an hour to try and fit the window frame and got caught in an unbelievable hail storm – some of the hailstones were about 1cm across and they were hammering down – I really needed to stay out long enough to go through the usual faff of getting the tarp over the building , but in the end the hailstones got to be so painful I had to dive for cover under a tree until it stopped. A salutary reminder that getting things reasonably watertight is the number one priority.

breather membrane

Anyhow, back to construction – having finished the roof I wrapped the building in the breathable membrane supplied by the SIP manufacturer. I have not been able to find out much about this stuff – it is a fairly stiff fabric that seems to shed water reasonably well yet allows water vapour through.

In the US the approach seems to be to use ‘house wrap’ (e.g Tyvek) – from what I could tell from youtube, this is lighter and more papery than the material I used. One good thing about the membrane I was supplied is that it came in one big piece that I could wrap the whole building in one go. I used staples to hold it to the wall.

breather membrane



I had done a fair bit of research into how to flash the window and doors to stop water from collecting on the wooden structure, but most of the info on t’internet relates to windows with external flanges. These are (sensibly) designed to minimise the chances of water entering around the side of the window by providing an external flange you can flash over the top off.

Windows in the UK come without flanges and are designed in a way that they will shed water to the outside so long as they are properly sealed (with silicon) from the inside. No doubt this is down to the fact that most of our houses are brick built…

The lack of timber building that goes on here also limits the options for flashing materials – in the US you appear spoilt for choice with various flexible membranes, pre-formed corner sections and the like. No such luck here, and all I could find (following a rather cursory search) was a bitumen based product with a flexible plastic skin. Despite applying a primer the flashing was not very sticky – we shall see how well it holds up.

The best advice on flashing that I found on the internet was the cosmic: think like a raindrop.

… I duly did this and tried to create some redundancy in those areas where a raindrop might find its way. For instance, I i created little patches to close the gap in the corners left when you cut and fold the flashing and then covered these with further lengths of flashing running across the floor and a few inches up the posts.

corner patch

I worked from the bottom to the top, overlapping the lower pieces with the higher ones. In my raindrop inspired state of consciousness I imagined myself trickling over each layer until I reached the bottom and fell harmlessly on to the ground.

flashing being applied

Will it work? I hope so.

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