Attaching DPM to underfloor void walls

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RichD1

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I have about a 1.6 metre deep void under my floors. The floor of the void is always damp and in one corner you can see where water has entered at the base of the oversize and run down to the back of the house. We understand that there are underground water courses in the area.

We do suffer with quite high levels of humidity and condensation on the windows when the weather gets colder. But we get no mould growth on walls and ceilings. I have also added 5 new air bricks to the 5 that were already fitted so it is quite well ventilated.

Having inspected the void there is no sign of any rising damp just the oversite being damp. So I intend to clean out the void and lay some DPM sheet held down with bricks and sand. The edges I was going to leave about 300mm above the oversite to make a good seal to prevent vapour rising into the void.

My question is: what will stick plastic DPM to the brick walls?

BTW: I currently have the floorboards up in one room as some of the joists have rotted over the years (house built in 1928) At the same time I will be fitting fibre insulation between the floorboards. This will be laid over a breathable membrane stapled to the bottom of the joists. A DPM will then be laid over the joists and sealed to the walls and skirting and then the floorboards relayed.

Richard
 
Having done more reading I've come to the decision that the dpm on the oversite is overkill and likely to promote other problems. With the fitting of the dpm over the joists above the insulation should minimise the moisture rising into the house.

Richard
 
You might be better ditching the breathable membrane under the insulation as well- it will allow water vapour to permeate up and then trap it in your absorbent insulation.
DPM above the insulation sounds sensible (effectively a vapour barrier)
 
I decided on the breathable membrane under after reading that the usable wire mesh will allow the movement of air (wind wash they called it) to cool the fibre insulation thereby reducing the u value.

Richard
 
Its all a balancing act...i'll take a slight energy hit over musty insulation and rotten timbers. Make sure (same as when you use the stuff on a roof) you leave the membrane baggy & consider stabbing it in between joists (just to let water drain)
 
Now I thought that using the breathable would allow the fibre and joists to breath so reducing the possibility of moisture building up.

Richard
 
No. Breathable membrane allows water vapour to pass through but prevents liquid water from passing through. In a roof this prevents condensation on the rafters(vapour goes through membrane, condenses on underside of slates, drips onto membrane & runs down to the gutter).
 
Oh no!! Going to have to have another re-think.

Originally I was going to use wire mesh but I then read on a couple of websites recommending the use of fibre board below the joists but as I had a roll of breathable membrane I decided on this approach.

As I only had 100mm joists then the fibre is already struggling with a U value but I already had several spare rolls it seemed the best solution. PIR did not seem affordable and more difficult to fit as the joist spacings are inconsistent but also are not parallel in places. Also the use of foam and foil tape added extra costs.

So you would recommend mesh under and DPM over?

Richard
 
Mesh or webbing tape or strategically placed battens. Open weave plastic mesh fencing, strawberry net, polyprop rope at 100mm intervals, anything that stops the insulation dropping out and doesn't seal the water in. Your breather membrane could work if you lay it as a roof (baggy so water will drop into the low bit then as i said earlier stab the low bits between the joists so any water can drain back into the floor void.
100mm of rockwool won't get you anywhere near required U value for refurb. Have you got enough depth in the void to get 200 in and have the 150mmclearance to subfloor?
I'm well aware that PIR is hard work & expensive but if BC are involved your choices are limited
 
I did think about dropping the netting a tad to get more Rockwool in but thought it might push its way across the under joist sealing it off as we intend to use plastic garden netting. But I guess Rockwool does breath so might not be a problem.

Don't really want to plate the bottom of the joists with further 100mm timber. Are there other methods which would suffice?

Richard
 
This work requires Building Regulation permission and probably won't comply.
 
Right, I've decided to drop battens down 100mm to the sides of the joists at regular spacings (say 600mm spacing) and then run a batten all the way across the width of the floor attached to the bottom of the batten drops. Netting will then be attached to the bottom of these battens to prevent the Rockwool from dropping out. This will give me 200mm of fibre.

If the Rockwool does cover the bottom of the joist will this be a problem? I'm thinking that it is breathable and allow any moisture to run out.

Richard
 
You should be fine with what you suggest. I presume the air vents are lower than your 100mm batten extension?

I did the 'net' method and stuffed in 150mm (joist depth) don't compress the rockwool as you lessen it's insulation properties.
Once finished, I laid a building paper (cheap from wickes) over all the top of the joists/insulation, then floorboards went down.

Only thing I will say, when you put your skirting back down, fill any gaps with silicon, it's surprising when you find a little bit of cold air coming through in winter!
 
When you say building paper do you mean breathable membrane? I have a damp void so am going to use a dpm to keep the moisture from rising into the rooms and hopefully preventing some of the condensation problem we have at the moment.
 
No, building paper is like a brown paper sandwich with bitchumen in the middle. It was recommended to me by a wooden floor company, for use on ground floors.

Like most houses with suspended floors and air vents, if the vents are working well, they will wick out the damp/condensation, so the need for a breathable membrane shouldn't be needed.
There will be some damp under all suspended floors, perhaps a little more under yours, as you stated in your original post.

If you are wanting to exclude any moisture rising to the underside of your floor, then perhaps you need to apply membrane below the joists, but I've never seen anyone do this. You could use 50mm celotex screwed to the underside of the joists then fill (from roomside) with rockwool, then floorboard.
How are the joist ends attached? hangers or sat on a perimeter wall?

Are the walls cavity/double skin?
Any insulation?
 
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