Role of breathable membrane under floors

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Looking to insulate under a suspended wooden floor downstairs. Access from the top, as I will have taken all floorboards up. Am considering using rolls of loft lagging (got plenty left over) laid on a membrane draped over the joists and hanging down enough tosit the lagging into. This seems the easiest way of doing it, to my mind.

I can't get my head around breathable membranes at all - is it that they allow air movement but not water? In my c ase would it be OK for the membrane to lie over the joists, or should I be pinning it beneath the joists?
 
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I once suggested using BM as a convenient way of providing 'netting' to hold the insulation.

However, the properties which make breathable membrane useful in roof and timber frame sheathing locations are not needed on suspended timber floors as the space below is well vented.
 
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Try some chicken wire mesh.
If you have the boards up it will be no hard job to nail into place to the undersides of the joist. Use broad head nails close to a weave joint.
 
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Chicken wire would help spread the load, light though it is, whereas staples will only support it over a very small fixing point.

Sorry, do you mean staples for the chicken wire?
No reason at all, in fact probably a better method than broad head nails. Well spotted, lol!
 
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Try some chicken wire mesh.
If you have the boards up it will be no hard job to nail into place to the undersides of the joist. Use broad head nails close to a weave joint.

Cutting Celotex to fit and fixing with expanding foam is so much tidier.

Cheers
Richard
 
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but more expensive, and I am on a budget!

Thanks again to everyone.

I am still unclear as to what role breathable membranes play in life, though!
 
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I am still unclear as to what role breathable membranes play in life, though!
They do as the name suggests - they "breath". In practice this means that they allow little air through, but they will allow moisture to permeate through.
So when used under the roof tiles.slates they will prevent draughts and water coming in - but they will allow moisture to permeate through and avoid condensation. If the relative humidity in the loft did get cold enough for condensation then it would form on the BM, permeate through, and evaporate away into the air that will be moving around above it due to the gaps between the tiles/slates. Other than that, any time the humidity is higher in the loft than above the BM, moisture will still permeate through and be carried away.
 

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