Rohfab/prefab breathing problems

tipptom · 28-02-2015 1:45pm #1

We have a rohfab(prefab) house that was built in 1973.
About 15 years ago we had an extension built on to it and in our wisdom we got the builder to put another skin of blocks around it.
A couple of months ago we had the pumped insulation put in to the walls.

We have had mould problems around the outside edges of the ceiling for years now with condensation on the windows in the mornings.
I have taken back the last two rows of slates on the roof and replaced the membrane that had degraded near the gutters.

I have just taken out a built in wardrobe that was on the gable end of the house and have discovered that behind the plaster board was lagging(or fur as my OH calls it) in among the studding and behind this is a black polythene seal on the inside of the rohfab slab which is sweating on the inside.

We obviously upset the breathing of the house by putting on the extra skin and I am going to take out the plasterboard,fibreglass lagging, studding and the polythene membrane.

Sorry for the long winded explanation but I was wondering could I just skim the inside of the rohfab slab then instead of putting back up stud and partition and would this be successful in giving the house a better chance to breathe?

Calahonda52 · 01-03-2015 10:31am

Before u do anything can u pls list, in the order in which they are, all the materials and their thickness, in the build up of the wall, starting with the plaster board and work out to the finish on the outer blocks.
It may be a ventilation issue so tell us about how all rooms are ventilated as well

tipptom · 02-03-2015 1:19pm

Thanks for the reply Cal.

Ok,starting with a approx 6mm plaster board, 2 1/2" timber studding and lagging with the black polythene membrane then behind the studding covering the inside of the rohfab slab which is 3" thick,then a 1" cavity filled with pumped insulation back to a 4" block finished with plaster.

Have 4 1/2" cored ventilation in all rooms and electrical ventilator in small bathroom.

Heating pipes running underneath the plasterboard behind skirting.

Polythene seems to be just sweating on the inside and rohfab slab behind the poythene seems to be dry.Fibreglass is wet as well but on the concrete posts that secures the slabs seems to be dry where it extends further out and is wrapped in the polythene only.

Calahonda52 · 02-03-2015 1:43pm

Thanks, so the polythene is on the cold side, i.e. outside, of the insulation as u stand in room.
eg
6mm PB ( is it foiled backed or plain
2.5 inch timber stud
insulation between studs
black polythene
3 inch rohab slab { what is this: concrete? )
1 inch cavity filled with pumped insulation ( u sure its only 1 inch" ]
4 inch solid block
cement plaster

re the piping, are the studs notched?

tipptom · 02-03-2015 2:05pm

Calahonda52 wrote: »

Thanks, so the polythene is on the cold side, i.e. outside, of the insulation as u stand in room.
eg
6mm PB ( is it foiled backed or plain
2.5 inch timber stud
insulation between studs
black polythene
3 inch rohab slab { what is this: concrete? )
1 inch cavity filled with pumped insulation ( u sure its only 1 inch" ]
4 inch solid block
cement plaster

re the piping, are the studs notched?

The polythene is on the cold side of fibreglass tight on the room side of the concrete rohfab slab.
The fibreglass is between the studding.
Plasterboard is not foilbacked been there since 73 but plasterboard does not seem to have suffered that much from the sweating except a few small patches of black mould.

I checked the slab and cavity through the corehole so it is only a 1" cavity going out to a 4" solid concrete bock.
The two qualplex pipes are under the stud work sitting in a channel under the floorboards and plasterboard skirting.

Calahonda52 · 02-03-2015 3:30pm

So the rohab concrete, which I presume is a cast concrete slab, is going to be airtight and to all intents moisture proof, this makes it a bit tricky. The polythene is on the wrong side and if u take it off the moisture will just condense on the slab. Is external wall insulation, EWI, out of the question?
There is a wood fibre insulation which is glued onto block walls, once you confirm its a cast solid concrete slab, I will check to see if it works on concrete, the theory is that it allows moisture to pass through it so am a bit concerned about the solid concrete.
If EWI worked then the temperature of the inner concrete wall, would go up and lead to less condensation.

Have u an extractor fan in kitchen?

tipptom · 02-03-2015 5:24pm

Spending on another layer of insulation is not an option at the moment as we have just spent a good bit on the pumped insulation.
We were hoping to take out the studding and fibreglass and polythene and put a skim of plaster on to the rohfab wall where it would be left open to the internal warmer temperatures.
The slab of concrete is precast so im sure it is a denser concrete than blocks.

We have an extracter for the cooker in the kitchen and a cored extractor at the other.
Getting it hard to get my head around that it is the inner wall is so cold when there is an outer wall and pumped insulation in between it.

Calahonda52 · 02-03-2015 6:49pm

I appreciate the financial aspects so lets just look at other ideas.

Re the wall being cold, 1 inch is not a lot and the wet insulation has not helped.
The other thing is to try reduce the moisture content in the air as much as possible, so use the fans and consider a humidity triggered wall unit in bed rooms where a lot of moisture is generated via human breath: the numbers vary depends whom you talk to but 3 to 4 litres a day is a good starting point.
Hope this helps

Calahonda52 · 02-03-2015 7:23pm

tipptom wrote: »

Thanks for that Cal.
Can you see any merit in just stripping everything (polythene,studding and plasterboard)back to the rohfab and just putting a plaster skin straight on and try to monitor and control it that way.
Or I could stretch to take of the the plasterboard 3-4 at a time,take of studding and put up a breathable membrane and re board it but I would prefer the top option as I can at least see what is going on and I would just have to skim.

My limited knowledge here is that normal plaster skim is not airtight and absorbs moisture to a greater degree that mass concrete so the cure might be worse.
It would also preclude you from the more optimum solution of sticking on the insulation I mentioned earlier.

I would like a second opinion on this but thats what I understand.

If you take off the studs my guess there will be a lot of filling to be done, probably steel nails...
ps

Are you happy that the rohfab walls are not getting wet from outside ?

Rohfab/prefab breathing problems

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