Insulating suspended timber floor - minimise damp issues

[djjohnnyb]

Hi, I am hoping for some quick advice please. I am looking to insulate my suspended timber floor. I have done quite a bit of research and have a good idea what needs to be done, but I wanted to run it past some experts on here please before I take the plunge.

I was previously concerned about damp under my floor, my previous post on this site goes into detail (generally-damp-ground-under-the-house-t81326.html). I am now confident those issues have been rectified: we have had a chemical DPM installed, a few joists have been replaced, new air vents have been installed so airflow is good and I have laid a Visqueen sheet across the whole underside of the house on top of the bare soil, as this was creating a rather damp feeling environment. That last job was horrible but well worth it! The atmosphere under the house now feels and smells much better.

I now want to insulate the ground floor, but I want to do it in a way that will minimise damp problems, just to be safe. I'd rather be prudent. This is what I have decided to do:

- Use Sheep's Wool insulation between the joists as this seems to be able handle moisture better than other types
- Use some DPM around the edges of each room so the insulation never touches any brick - I don't want moisture transferred from the walls to the joists if the insulation ever touches the wall below the DPM in the wall
- Ensure no air vents are blocked
- I am not sure whether to go with some netting to support the insulation on the underside of the joists, or use a vapour permeable membrane (VPM). The netting is more open so would leave more of the joists exposed to direct air flow, but the VPM seems to be more efficient for insulating, reducing the 'air wick' effect. Will VPM still allow the joists to 'breathe' sufficiently?

Does what I plan seem correct? And which do you think would be better to use in my situation to support the insulation - netting or a VPM?

Thanks.

[toolbox]

Just a word to the wise about sheep's wool I used it and while it is non itch there are a lot of fibres released during cutting and fitting, a mask is essential. Oh and my loft smelt of sheep for about a year!

[djjohnnyb]

Thanks. Yes I've heard about the smell, in hoping the fact it's under the floor and floor coverings will contain! I really don't want my ground floor to smell of sheep for a year!

The main thing I want to understand is whether I should use netting or a vapour permeable membrane. The netting appears it wouldn't give as good performance as a VPM, but I'm really nervous about completely covering up he joists, even if the membrane is vapour permeable. Not sure if my concerns are valid, or if the performance difference matters much.

[toolbox]

I will be doing my downstairs bathroom soon and having the same thoughts. I am wondering about treated battens screwed across the underside of the joists about a foot apart might do the trick.

[djjohnnyb]

Bloody hell just been pricing up sheep's wool vs something like Knauf Earthwool. It's like 5 times expensive or even more! Think I might have to rethink using sheep's wool.....

[toolbox]

Mmm sheepies pass my wellies boots!

[Nos]

Just finished the internal floor of my new build, I have built it out of joists 200x80 and 19mm of flooring grade chipboard, in between the joists I have put 200mm of fibre glass insulation resting on plastic netting, the red type used around building sites and for putting over pipes as a warning. I had 200m2 to lay so invested in an air nailer, some places I could stand between the joists, others I had to do it from the top, it was hard work.