Insulation between suspended timber floor renovation.

[Oz07]

As per title I know i've seen this covered on here before. Whats the best way to detail the mineral wool between suspended timber floor joists. Looking to improve u value to .25. 

 

Can roofing felt be used, stapled between joists to form a sort of cradle flush with bottom of joists then insert mineral wool then re deck floor? This way bottom of joists still receiving ventilation.

[Jeremy Harris]

I did this in our house in Scotland.  I had the advantage (only slight) of being able to crawl into the undercroft, as the house had been built above a concrete slab, with a 3ft space underneath, accessed via two floor hatches.  I used fishing net, stapled to the underside of the joists, and packed rockwool in to fill the space between the floor boards and the netting, as tight as I could get it.  Bloody awful job, but I was installing central heating at the same time, so ran all the pipes in under the floors whilst I was doing it.  Made a heck of a difference to the oil heating bill.

[Ed Davies]

Quite a few people have done it using roofing felt as you suggest. Just to be clear, though,  it needs to be a vapour permeable membrane type rather than fully watertight stuff used on some roofs. Obviously you need to make sure that you don't block any ventilation. The usual thing is to put a taped polythene sheet over the top of the joists to make the whole thing airtight and further reduce the water vapour migrating downwards before putting the floor deck back.

[ProDave]

Our previous house was done using a netting made for the job, I believe it was called Netlon or something similar.

[Oz07]

I guess most modern roofing felts are vapour permeable? If marketed as breathable that is?

 

Don't like the idea of a dpm/vcl over top of joists. Seems overkill and makes life more difficult. Is a u value of .25 really likely to pose a condensation risk with a mineral wool type insulation?

[Ed Davies]

Yes, but there are some which are marketed as completely waterproof (e.g., Protect A1) which it would be a bad move to use and it was worth clarifying as the OP didn't specify which sort of felt they had in mind.

 

The polythene over the top is mostly for airtightness though as a VCL it'll be a small help rather than a hindrance. Remember that the bottom surface may not be that well ventilated.

[MikeGrahamT21]

We used an AVCL over the top of the joists, and its surprising before you seal it to the walls, just how much airflow there is, even when you have fitted the insulation very tightly, and sealed everything with expanding foam! Bottom of the joists is of course open, and the benefit of the AVCL is reduced moisture flow downwards, and the airtightness actually gives an increased airflow between air bricks under the floor, since less of the airflow is breaking into the living space.

[daiking]

There's no further requirements on this is there?

 

Insulation support netting and loft insulation for my mix of 7 inch and 4 inch ground floor, floor joists. There's not going to be a any airtightness or VCL provision. Is it worth running a foam gun around the underside of room perimeters? I know its not air tight but must be an improvement worth the small extra effort?

 

The finished flooring is a complete mix: tiles over plywood + marmox type stuff, tiles over floorboard and marmox type, 14mm engineered wood over underlay over 9mm ply over floor boards, 14mm eng wood over underlay over floorboards. So there's no draft through the floor itself but the edges are leaky and the small understairs cupboard areas are more or less open to the void.