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Discussion Starter · #1 · (Edited)
Greetings to you all and all the best for 2010!

I'm glad I finally found a group of like-minded individuals with whom experiences can be shared.

I stumbled across this forum by accident after researching the pros and cons of vapour barriers in combination with rigid closed-cell PU. Some of you recommend them (even when using closed-cell foam), others don't.

I'd like to share our experience, this particular incident taking place during the initial stages of our Compu-Homes™ product development in 1986: One of our modular peaked roofs had collapsed after strong winds and heavy rains. The roof consisted of a 40-50mm two-component rigid PU core on top of a polyethylene substrate. Almost 3,000l of water filled the collapsed peak and it remained so for over three months. There were no cracks and NOT ONE DROP of water permeated the closed-cell PU foam (see attached pics).

This type of unplanned 'test' had not even been conducted by the suppliers, not to mention the authorities or contractors who specify a vapour barrier irrespective of what is used to fill the cavity.

Based on our experience, we feel there's no need for a vapour barrier when using closed-cell rigid PU. The rigid PU IS the vapour barrier. Maybe this 'test' will convince those of you who are currently unsure.

Looking forward to hearing from you all.
 

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Discussion Starter · #3 ·
Initially we thought the foam would be rigid enough to maintain the dome shape on its own. However, not with the winds we have down here. We subsequently introduced a crossbeam and upright pole.

PU stands for 'polyurethane'. In our case it's a two-component closed-cell rigid polyurethane spray foam. Each component is stored in its own drum, from where it is pumped to the spray gun. Here the two components mix, expanding 30-40 times once they hit the substrate. It's an amazing chemical process to witness. YouTube have quite a few videos on the topic.
 

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i don't know about vapor barriers but isn't water vapor molecularly different from standing liquid water?

isn't that how tyvek works?seems to me that ''test'' just proves pu is water proof,not nessecarilly vapor proof

but it may infact be
 

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Discussion Starter · #5 ·
Good point, Tom. Tyvek is spun from high-density polyethylene fibers, vapour can pass through but not water. On the other hand, closed-cell polyurethane spray foam is applied as a liquid, expanding 30-40% and filling every cavity. It then acts as a monolithic (continuous) moisture-proof vapour barrier, primarily due to its closed-cell structure.

The same cannot be said for open-cell PU spray foam which absorbs water (and water vapour) like a sponge.
 
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