suspended timber floor insulation

su22

Hi all, I know this is a regular topic but looking for reliable advice and finding lots of conflicting info so would appreciate any simply explained advice (I'm not very 'techy')

I'm planning to insulate under the floors of my Victorian terrace. They are suspended timbers with a void about 30cm below the floor. I 've looked into a number of different options.

Builder has suggested Celotex under floors between joists.

Q1- does it need a vapour barrier above?

Q2- his original suggestion was to attach it with polyurethane adhesive. i haven'y been able to find anything on this, only info I can find recommends using battens or clips. I was a bit concerned about breathability/ air circulation but I guess the place that air needs to circulate is below the joists? Any thoughts?

Thanks, all

FreeBear

A1 - No. The Celotex will be a sufficient vapour barrier, and the joists will benefit from having a path for moisture to migrate to.

Q2 - Battens, definitely battens. They will stop the insulation boards from dropping in to the void as they are fitted. Gaps down the sides can (should) be filled with a little squirt of expanding foam.

Be very careful not to compromise ventilation under the floor, especially around any sleeper walls. There must be a reasonable space between the sleeper walls and any insulation to allow air to circulate. Block the air flow, and you'll end up with damp, rotting joists before too long.

Davesnave

^ What he said.