# Polystrene Backed Plasterboard



## Mark A (23 Mar 2011)

We have recently renovated our house and have noticed that condensation if forming in the two alcoves and fireplace in the lounge. The external walls are around 18-20 in thick stone, but the alcoves and fireplace are just brick (no cavity wall). We've bought some polystyrene backed plasterboard to line the back of the alcoves and fireplace (not in use) but don't know the best way to fix it to the wall. We aren't going to plaster the board, just paint it so we can't really screw it in place. Can we use dot and dab or will it not stick to the polystyrene?


Also, I've read somewhere that the polystyrene backed plasterboard actually worsens the damp problem? If so will we need to install a vapour barrier?


Thanks, Mark


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## Mcluma (23 Mar 2011)

Best if there is NO airgap between the polystyrene backed plasterboard and the wall, so directly fixing to the wall best with screws directly into the wall. no need for dot and dab, only there were the board is significantly out with the rest does it need packing out

trapped air without airmovement will be a potential source for condensation

are you sure your wall isn't wet from the outside??


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## billybuntus (23 Mar 2011)

Use quick fill to joint the boards and cover any screw holes then lightly sand back and paint.


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## bosshogg (26 Mar 2011)

Mark,

You have heard correctly, but it's not the polystyrene that causes the problem but the rise in temperature...warm air is capable of holding more moisture than cold air.
OK lets get pragmatic and try to analyse where the problem lies...first you say you have just renovated the property and that the main walls as I understand are traditional build _i.e._ not with a cavity. Can you tell me how old, aprox, the building is, if traditional did you by chance have a damp proof course instaled?? or has any of the external walls been pointed with a cement based mortar particularly around the damp/condensation affected alcoves...I may be able to help you, I am a Project Manager (retired) in the construction industry...bosshogg


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## Mark A (26 Mar 2011)

Hi 

The walls are 18-20 inch stone, but brick for chimney and alcoves - no cavity. 

A complete DPC was installed and a membrane was layed when the floors were renewed. 

The house was built in 1913 

I know that houses take a very long time to dry out and we totally gutted it - only 4 walls and a roof left, with a 3ft drop from the front door. 

We got the DPC installer to inspect the walls and he said that it was just condensation because we didn't open the trickle vents over the winter. We are only getting the condensation were the walls are thinner, so just the alcoves and fireplace, so we want to insulate the cold spots as the house is for sale. 

Thanks, Mark 

P.S. YeeeeeHaaaaaaaa!!!!!!!


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## bosshogg (27 Mar 2011)

Hi Mark...I'm going to ask you whether you had the chimney capped or blocked up in any way to stop draughts? and when you had the dpc installed did the brick walls in question also get a dpc from the inside of the house, you mentioned the house was just four bare walls and the roof, and these walls presumably are not part of the outer walls? 

OK a few suppositions...The dpc installer has said that that this is condensation due to the fact that you didn't open the trickle vents on your windows over the winter, but what I want to ask you now is if you know the condensation was prevalent before the renovation? I'll assume not for present. this leads to the changes brought about by the - effectively - damming of the water brought on by installing the dpc wall and floor courses, *the water has to go some where *, if there are any breaks of the continuity of the floor to wall dpc that is one area that the water will go to, and if the lower 3 feet, under the floor if I've picked you up right, of brick under build to the chimney and alcoves has become loaded with water, then this will effectively have created a cold bridge in that sector of house wall.

...applying polystyrene backed plasterboard (plusboard) most likely won't help in these circumstances as the condensation won't be eradicated just buried behind the polystyrene where it will get even less air flow to dry it off. I've picked this to try help explain - http://www.projectbook.co.uk/articl...l be willing to do the biz for you...bosshogg


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## Mark A (28 Mar 2011)

Thanks bosshogg for the advice. 

Here is an old picture of the wall after the floor was removed





The DPM and screed is above the wall plates (above airbrick at left of picture) 

Your comments would be appreciated 

Thanks, Mark


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## bosshogg (28 Mar 2011)

OK...the brickwork, was it built as part of the original shell or added at a later date? Oh! I have found a fact sheet on insulation added to traditional wall construction like yours, fact 13 is particularly relevant, you can view it here http://www.viking-house.co.uk/insul...ould also have a bearing on things...bosshogg


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## Mark A (29 Mar 2011)

I'm not sure about the alcoves being "new" additions to the existing property when it was built - I'll have to check the deeds for the house.
The chimney stack was taken down when we had the roof replaced because the pointing had gone. The chimney ventilates into the loft through openings made by removing a couple of bricks.

That link was very interesting by the way! So by laying building paper (what is it?) between layers of insulation increases the efficiency by 60%? We should have done that! Although we got a grant because we needed the whole loft insulating.

Thanks, Mark


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## bosshogg (29 Mar 2011)

OK... effectively we have a stack (the old chimney) terminating and vented into the loft space, make sure you put a fine mesh screen over the two brick opening, otherwise you could have a wasps nest or worse in there sooner or later...is the stack also vented at the bottom???...bosshogg


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## Mark A (29 Mar 2011)

Yes, the chimney's vented at the bottom and the vent cover for in the loft is now on my shopping list for tradepoint!

Thanks, Mark


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## bosshogg (30 Mar 2011)

Hi Mark...building paper is a 20%, or there about bitumen laminate waterproof paper, now sometimes replaced with breather membrane. 
No need to check the deeds, has the base of the moisture affected walls had the dpc course as has the external walls, I'm assuming the brick walls but against the inside face of the stone shell??? hence the reason for asking if it received a dpc or not...bosshogg


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## Mark A (30 Mar 2011)

All the solid walls (external and internal) have has a DPC, well 2 actually! The first one was installed by a cowboy who injected the DPC into the MIDDLE of each stone!!! Well a threat of legal action followed and we got a refund. The proper DPC was installed by a specialist company.


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## bosshogg (30 Mar 2011)

We can rule out rising damp in that case...unless we check and confirm - if you have or can borrow/hire a moisture meter, a test will confirm this. If the moisture reading at the bottom of the affected walls is much greater than the reading at the top, then something is allowing moisture to rise, also if the walls feel cold to the touch at the bottom and appreciably warmer at the top, a similar conclusion would apply. I assume the reason for leaving the brickwork exposed in the first place was as a feature? have you therefore decided that a pragmatic solution is the course of action??...bosshogg


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## Mark A (30 Mar 2011)

The brickwork in the alcoves and fireplace haven't been left bare -they're plastered just like the rest of the house. We were thinking about hacking off the rendering along the bottom of the wall on the exterior and creating another bell to help keep the rain off because there is only one further up the wall.


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## bosshogg (30 Mar 2011)

If your thinking of hacking off the base of the exterior render in view of creating a bell cast to shed off rainwater, are you thinking this could be part of the "condensation problem" or is it something new you are thinking here...bosshogg


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