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Insulating Under a Suspended Timber Floor

Postby su2111 » Wed Nov 20, 2019 10:33 pm

I know this is a regular topic but looking for reliable advice and finding lots of conflicting info so would appreciate any straightforward 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?
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Simply Build It

Postby B4Less » Wed Dec 18, 2019 12:17 pm

The best option here would be to use Celotex or similar. However it would also be a good idea to do the following

1. Fix a timber batten to the side of the floor joists at the bottom to stop the insulation falling down over time. Alternative to this is also use netting underneath the insulation. As it is only 300mm underneath I dont think you will be able to get below it to fix the netting to the joists so you will need to let it down in between the joists and then up and over the joists all the way along the room. The netting will then prevent the insulation from falling.

2. Ensure the Celotex or similar is cut in tight between the joists and there are no air gaps. Good practice here would be to seal the joints with an airtight mastic as you go.

3. On the floor side of the joists I would insert a breather membrane. Laying it flat over the insulation and turned up at the walls and sealed using an airtight mastic or similar. If you think you can do this in place of the netting it might be worth giving it a go. So instead of inserting your netting then your breather membrane you maybe able to do the job in 1 with the membrane.
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Postby AlwaysLearning » Sun May 17, 2020 10:40 pm

Ok, someone kick me if I'm barking up the wrong tree here.

I'd sit down with the builder over a cuppa and see what they have done that's similar and dig into their methods.

I'm going to assume you've removing all of the existing floorboards for this job. (If they're in good condition, perhaps a salvage yard will give you a few quid.)

The method sounds ok, but here's the challenge that comes to mind. For best effect, the top of the Celotex has to be in contact with the underside of the flooring when it's put down so air can't circulate. If the Celotex is going to be kept in place with battens, they would need to be precisely positioned to eliminate that gap. Ok, so the builder would need to knock up a jig out of scrap wood to set the height correctly. Then cut a Celotex slab to suit bearing in mind those joists are unlikely to be parallel. Then seal the edges to stop air flow.

An alternative would be to lower the Celotex perhaps 75mm, battens, slabs, sealing all the same. However, in that 75mm gap between the top of the Celotex and the underside of the flooring, fill it with 100mm rockwool or equivalent. That will compress down easily and pretty much stop any airflow. It reduces the builder's need for precision so they will be a bit quicker, and ups the overall insulation.

Vapour barrier? Honestly don't know. Question is what is the source of the vapour given the Celotex was sealed to the joists?
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