Underfloor Insulation - To Vapour barrier or not...

[RushupEdge]

Hi all, have all my floorboards up at the moment from replacing a few rotten joists and while we're here thinking of insulating the underfloor

I've read a bunch of different things regarding insulation and causing joist rot.

What I was planning on doing is using a breathable insulation suspended between the joists using a breathable membrane and that be that.

However, I've come across a number of articles/reddit posts etc. that say a vapor barrier should then be placed above the joists and below the floorboards. To me this seems like it's going to trap any condensation and cause it to sit in the insulation rather than giving it a path to evaporate away, causing us all kinds of issues that we can't see.

Basically, just wanted to see if anyone has done this kind of thing and can advise if a vapor barrier is needed or not.

 

[jamest]

I haven't done it as I don't have suspended floors, but given that the floorboards are the warm side of the insulation/joists, that won't be where the condensation forms. It will be on the cold surfaces under the floor. Provided your underfloor void is properly ventilated (and you're not relying on vapour getting out through the gaps in your floorboards), then putting a vapour barrier under the floorboards is unlikely to be a source of problems. If you put it under the floorboards without insulation, then the barrier would be a relatively cold surface and perhaps condensation would form on it and eventually rot your boards. This won't happen when there's insulation below and it will stop vapour migrating from above (where it's warm and vapour laden) to underneath where it's colder with less vapour and that could be an advantage. Hopefully others with practical experience will be able to offer comments based on experience!

 

[fernicarry]

Modern practice is to put a vapour barrier on the warm side of the insulation as jamest explains. The entire habitable space is intended to be vapour tight with planned means for removing water vapour generated inside e.g mechanical ventilation, trickle vents etc.

Personally I would try avoid anything that makes a period building less vapour open especially if you’re doing piecemeal interventions. You just don’t know how the behaviour of the whole system is going to change.

When I insulated under the floor I put a breather membrane both sides of the insulation rather than a VCL on the warm side. I probably won’t be around long enough to find out if that was a good decision.

If you were to use a vapour barrier then a possible unintended consequence could be driving hot wet air into the joist ends. You should ensure the insulation is continuous up to the external walls and that any exposed parts of the joists that warm air could come into contact with are parged with lime mortar.

 

[RushupEdge]

Modern practice is to put a vapour barrier on the warm side of the insulation as jamest explains. The entire habitable space is intended to be vapour tight with planned means for removing water vapour generated inside e.g mechanical ventilation, trickle vents etc.

Personally I would try avoid anything that makes a period building less vapour open especially if you’re doing piecemeal interventions. You just don’t know how the behaviour of the whole system is going to change.

When I insulated under the floor I put a breather membrane both sides of the insulation rather than a VCL on the warm side. I probably won’t be around long enough to find out if that was a good decision.

If you were to use a vapour barrier then a possible unintended consequence could be driving hot wet air into the joist ends. You should ensure the insulation is continuous up to the external walls and that any exposed parts of the joists that warm air could come into contact with are parged with lime mortar.

Yes this was what I was a little concerned about. My initial plan was just to create a 'hammock' of breathable membrane and then fill this with a breathable insulation as deep as the joists.

There's plenty of ventilation in the sub floor, I just want to make sure I'm not creating damp issues in the void as the previous owner did (though he had a patio running rain water directly into the air bricks and sub floor, so I think I'm already winning by not doing that!)