Suspended floor insulation and damp proofing questions

[Sismithin]

Hi Folks

I've done lots of research into this for many months now but I'm just after some advice really on exactly what approach I should take.

I am insulating my suspended wooden floor in my 1910 built house, I have gained access to the sub floor which has about 5 ft of space at its deepest.

The sub floor soil and rubble is covered in white efflorescence/salts and is rather damp about 37% on the moisture meter.
The walls about 12% and 17% moisture with no signs of any rot or decay at all. I have semi adequate ventilation bricks at 90degrees as its only sub floor in the house.
The room itself in winter is very cold and damp and my suspicions is this is rising from the sub floor soil.

Question 1 - Should I to leave the subfloor/rubble as is or should I place a large plastic damp proof sheet down and seal this around the edges the best I can?

Question 2 -
I have joists 43cm apart and 30cm deep (yes they are huge) do I use a 46cm insulation slap and squeeze it in?

Should I double the mineral wool insulation to have 200mm (2 x 100mm doubled up) and just staple my membrane/support 10cm up from the bottom of the joist? Or fill the entire 30cm joist up?

Question 3 - I have lots of breathable roof membrane left over which I was planning on using to support the insulation, will this be ok?

I have attached some photos if any one is interested.

Thanks all, any opinions welcome.

 

[Notch7]

I wouldnt put down DPM -you risk forcing damp into the walls

I would leave it open and make sure you have enough air flow through the void -adding more vents if needed.

I think you want to fill up between joists or keep insulation tight to the underside of floorboards
you will need vapour barrier on top of floor boards.

Im not sure about breathable membrane -most people use netting, maybe the membrane will be ok

 

[Sismithin]

Hey, thanks for the reply and advice.
I think I may install another air brick on
the side wall. I have three on the front wall and only two on the side wall and neither really get the wind to aid in ventilation.
Apart from the sub soil it’s not too damp down there, even off cuts of floor board on top of the rubble that have been there for 100years are not rotten in the slightest.

I am thinking of adding 200mm of mineral wool underneath held up by nets then in this case.
Do you advise putting down a vapour barrier directly on top of the floorboards. I’ll then lay my underlay and new carpet directly above this.
I’m sure this in itself will help the warmth and insulation compares to my old laminate.
Thanks again.
Simon

 

[Sismithin]

I went exploring, the amount of efflorescence on the oversite was amazing.
I dug down the the soil does not seem excessively wet. I have a very good bituminous DPC which appears to be working well there is no moisture at all above this.
I am contemplating laying down a plastic DPC across the entire oversite to reduce the moisture escaping up. I think the smelly stale air is because of lack of ventilation and increasing the size of the two side vents will do little to really address this I think as it’s so sheltered anyway.
This is the exterior of the corner for perspective, all vents can be pictured.

Regards
Simon

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