Keep damp moisture at bay inside cavity space?

[ashthekid]

 

A very old uneven brick wall dating back 100yrs+ with a large empty cavity space(100-200mm), then a new timber structural stud wall(sat on a new concrete slab) filled with PIR insulation in between and both sides of the studwork. To stop any kind of dampness or moisture inside this cavity space, what is the best thing to do? I say this because we haven’t installed the PIR insulation yet so I have an opportunity to do something now to prevent any build up inside. 

We aren’t airtight yet(85% tiled) and have had a lot of rain recently which may or may not be confusing the situation. The floor and lowest part of the brick wall looks to be very damp.

1) Do I just leave it knowing it may be fine and I’m just overthinking it?

2) Do I make sure the cavity space is well ventilated somehow(cannot touch the brick wall because it’s Listed)?

3) Do I fill the cavity space with something that helps prevent moisture but could be costly due to the size of the cavity space?

4) Is there an alternative solution I haven’t thought about yet? 

[MikeSharp01]

Listed walls, being quite old, may not to have ventilation blocks / slots but if there are any at the bottom then you may want to allow air out at the top somehow - how was the top finished before you built on it this may give you a hint as to what needs to be done? If no low vents then you may be able to rely on the breathability of the bricks - what is the inner side build up of this wall as this may have some impact again if changing from previous construction? 

[ashthekid]

There are no ventilation blocks/slots in the Listed part of the wall. At the top, between where the Listed wall is and the newly formed timber frame is a newly created box gutter made it of ply with a resin fibre layer on the inside of the gutter. Due the Listed wall being so old and not straight we’ve created a header course on the top of the brick wall and put a lead flashing flap down into the box gutter so not really a ventilation gap as such but because the lead isn’t sealed to the box gutter I guess there is the tiniest of ventilation somehow?

 

The inner side of the brick wall at the bottom is a mixture of exposed brick in most parts and then just old build up of plaster or cement with a small part still showing some bathroom tiling on. A very small lip of damp course plastic that flaps up from where it was laid underneath the new concrete slab. I would have ideally liked that to flap up the wall a lot higher to stop any transfer of moisture from the brick wall to the timber frame.

[Ajn]

We had a similar issue but not listed and 200 year old walls. We built a structured concrete subfloor with ring beam. The timber frame was built off this subfloor but had a "boot" of DPC fabric around the base extending up the frame. DPC was layed on the subfloor then insulation then more DPC then the UFH then screed. We used the multi foil blanket and assoiated insulation product in the timber fame.

We had various air gaps 50 - 300mm between the frame and original wall due to the wall being upto five bricks thick in places. Old buildings need to breathe as its the reason they have been around 200 years. We removed all traces of gypsom based repairs and repointed in lime. Because the insulation is in the frame the wall cavity can be vented out of the top under the tiles. We also installed some air bricks at the bottom but only one in each wall. Lime based walls are quite good at breathing on their own but the additional ventilation couldn't hurt.

 

The original walls which sits on a simple limecrete foundation act like cladding for the structual timber frame. The building doesnt't have any gutter as the corbelling is very effective. There is no DPC in the wall.