Solid wall insulation - to breathe or not

[silly_savage]

I own a 1900s Victorian end terrace. My side wall is on the boundary and adjoins 4 neighbouring plots of an adjacent road.

On clearing out the end neighbors garden yesterday, in preparation for some minor render repairs, I noticed that it's in much worse state than I first thought in places, so will probably need to be replaced. I'm not sure whether the existing is lime or concrete, will test ASAP.

I'd like to insulate this same wall internally, the fact it's a boundary wall pretty much puts insulating externally out of the question. For completeness, a previous owner has poured concrete floors at some point, which is causing damp problems on some internal walls, but not the one mentioned above as far as I can see.

So my question is whether I'd be best off trying to maintain breathability in the wall, by using lime render and mineral wool insulation (considering it has no internal sub floor ventilation now) or try and seal it off to moisture with cement render and PIR insulation (preferable due to the smaller dimension) and possibly DPC injections for good measure.

Fwiw I hope to install a whole home dehumidifier at some point to keep the air inside drier in general. At the front is the original brick facade with line mortar and at the back is a concrete block extension with concrete render on the adjacent elevation too the one mentioned above (I think).

It's also my first post, so hello and please go easy on me

 

[JohnD]

Breathability is the water being able to escape into the outside atmosphere from the external walls. As you are planning to insulate internally, you don't need permeable insulation and it will be better without. Foil-backed plasterboard, taped at the joints, will prevent water vapour from inside the house getting into the brickwork.

Concrete floors in old houses are often poured when persistent damp has caused the joists to rot. Often the underlying cause of damp is not addressed and the new concrete just makes it harder to find and repair.

A house of that age normally has leaking drains, usually where the clay elbows, traps and gullies have cracked and broken, sometimes also at the sockets where lengths are joined.

A house that age will often have a leaking mains supply pipe. This can easily be identified if you have a water meter.

The drains for terraced houses usually run along the back, and turn towards the sewer in the road at the end of terrace and pass the end wall.

The water pipe usually runs under the hall to the kitchen at the back.

Unsurprisingly these are the places where damp often occurs once the leaks start.

Chemical injections do not cure leaks. Your house was probably built with a slate DPC which, if not bridged, will last at least a thousand years.

 

[silly_savage]

Hmm, I fear getting a bit sidetracked here, but the drains run towards the back of the house and the water supply runs above the slab at the side of my hallway, so I don't think they're the reason for the dampness on the internal wall. It's a 3 way junction just off middle of the slab, so I think moisture is being forced up here where it can't get out of the ground below the concrete. If there was an original DPC below the old timer floor, it would have been breached by the concrete.

Aaanyway, re: the external wall, are you suggesting that if I maintain breathability to the outside, I don't have to worry about it on the inside? My worry is if I were to use non-breathable materials on both, then any rising moisture would have nowhere to escape to. Just want to check whether this is the case or if I'm better off trying to go all out and keep moisture out of the wall from inside, outside and below (be that via means of injected DPC or the original one)

 

[JohnD]

Aaanyway, re: the external wall, are you suggesting that if I maintain breathability to the outside, I don't have to worry about it on the inside?

yes

it would be interesting to find out where the damp is coming from though.

A sketch plan, and photos of the walls inside and out (right down to the ground) may help.

A Victorian house usually has a non-cement render, but ask on the render and plaster section.