Bathroom renovation advice

G

Gaz32

New Member
IMG_3237.jpeg IMG_3272.jpeg Hello all, first post so go easy on me.

I’m having my bathroom done out and I’ve stripped everything out for the plumbers to start.
I had a damp patch on the plasterboard so I’ve removed the dot and dab plaster and gone back to brick. I think the damp patch was created by bridging onto the dabs on the board joint where a lot of dab was used.

On doing this I’ve come across an old chimney, which I presume was for a an old cooker previously as the kitchen is below. Also there’s an air brick which has been uncovered. The building is 1930s with i believe a 50mm cavity and blown rockwool insulation.

So after all that I was planning on re building the wall but creating a cavity to the brick, kingspan then plasterboard.

My main question is would you leave the chimney void and air brick open to allow for ventilation behind the wall? The void doesn’t go to an actual chimney but vents straight into the loft space above. I understand ventilation is really important so I’m swayed towards leaving it open but am I creating a cold spot by doing this?

Many many thanks
Gary
 
Obviously the "damp" was caused by the chimney. Chimneys are challenging because they cause salts in plaster and bricks which attract moisture. If I was doing it I would install insulation backed plasterboard (which includes a VCL) straight to the wall with adhesive foam (which not being water based won't suck salts out the brickwork) supplemented with a couple of screws and plugs in each board. The insulated PB will isolate the wall and anything in it. I would go straight over the chimney without blocking it up.
 
Mr Rusty said:
Obviously the "damp" was caused by the chimney. Chimneys are challenging because they cause salts in plaster and bricks which attract moisture. If I was doing it I would install insulation backed plasterboard (which includes a VCL) straight to the wall with adhesive foam (which not being water based won't suck salts out the brickwork) supplemented with a couple of screws and plugs in each board. The insulated PB will isolate the wall and anything in it. I would go straight over the chimney without blocking it up.
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Thanks rusty I will take your advice. It puzzled me as I expected the damp to be on the plasterboard over the chimney void and not on the brickwork to the right of it. Someone suggested using breathable building paper on the back of a timber stud wall with kingspan backed plasterboard on the other side.
 
A stud wall won't give any benefits. What happens is the bricks are salt poisoned by the reactions with combustion gases. (basically sulphurs from coal react to form sulphuric acids which then react with the lime in mortar and cement to form hygroscopic salts.) When the wet dabs are applied the moisture in the dabs draws the salts out to the surface as that water evaporates. The salts on the surface left behind when the water evaporates then keep dragging moisture out the air (particularly in a bathroom) because the salts are hygroscopic. As they absorb moisture they grow surface crystals.

You're not stopping damp in the wall getting out, the VCL in the PB stops the room air getting to the salts in the bricks behind.
 
Mr Rusty said:
A stud wall won't give any benefits. What happens is the bricks are salt poisoned by the reactions with combustion gases. (basically sulphurs from coal react to form sulphuric acids which then react with the lime in mortar and cement to form hygroscopic salts.) When the wet dabs are applied the moisture in the dabs draws the salts out to the surface as that water evaporates. The salts on the surface left behind when the water evaporates then keep dragging moisture out the air (particularly in a bathroom) because the salts are hygroscopic. As they absorb moisture they grow surface crystals.

You're not stopping damp in the wall getting out, the VCL in the PB stops the room air getting to the salts in the bricks behind.[/QUOTE

thanks for the insight that’s very interesting and something I have not heard before. Appreciate the help. Do most insulated plasterboards have a vcl or do you recommend one specifically?
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