Sealed or ventilated cavity ?

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17 Feb 2011
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Hertfordshire
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United Kingdom
Hi all

I am building a SIPS extension and will be cladding it with CPB and render, the cavity will be 120mm wide above which will sit the guttering as this is the best way to maximise floorspace.

Here is the section drawing for how I intend to construct the wall

The main question is really wether to do away with the gravel backfilled soakaway at the bottom of the cavity as this may allow vapour and rising ground water levels to enter the cavity but will also allow water from the gutter (if fault occurs) to escape and also any sudden downpours or surface flooding of neighbours paved patio (if it finds a hole in cladding) to escape.

Alternatively I could continue the slab right up to the boundary with maybe 1 or 2 pipe connections to drainage cut flush with slab level and seal the whole thing so no water may enter but this wouldnt allow any vapour that gets in to evaporate and exit the cavity and may lead to small pools where the slab is uneven.

In either case should I have grills in external wall to allow ventilation or just allow it to vent the moisture the same way it entered ?

I want to maximise the life of my building which I intend to make airtight as possible but OSB is susceptable to long term moisture ingress so this is a worry for me. I am happy to spend as long as is neccessary on the detailing as needed to achieve this.

Any tips, thoughts, opinions or information are VERY welcome

Thanks
 
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1st question, why is your gutter inside the wall, when it leaks the inside will be drenched before you even have a clue it is happening!
 
The gutter is on the inside so I don't have to take away floorspace for a wall cavity.

If I put it external what would be the minimum sufficient cavity width ?
 
osb = glorified chipboard :evil: WBP is the one for me ;)
 
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