Dry rot - again

[Woodsy]

Hi there I’m hoping I can get some level headed advice on what looks like another dry rot outbreak in my property.
I live in a Victorian semi d and am in no way a diy expert. I rely on builders/tradesmen to keep me right and as a woman I find it difficult to be heard on occasion!
This time last year I discovered dry rot following a substantial leak in the guttering which allowed water into a badly sealed window.
This resulted in the first floor bedroom wall and adjacent bathroom wall being stripped back to the brick and all affected timbers removed. A waterproof membrane was put on the wall after a week or so of having a dehumidifier in place to dry the wall out and new timbers etc put in place ( I relied on my builder to do all of this for me).
Everything sorted, or so I thought.
Over the weekend I noticed a dry rot mushroom outside the bathroom wall to the right of a lintel of a previously blocked up window. Called my builder and he said he could get a trowel well into the hole that the mushroom was coming out of and suggested blocking it off and job done. He thought there was probably some timber used to build up the lintel?
My question is - is this hole likely the source of the water to feed the dry rot -
Or do I need to rip out my new bathroom wall again to see what is really going on behind there? There are no other obvious leaks anywhere that I can see.
This part of the house is an extension from the 1950’s - my (probably misplaced) fear is that as the mushroom is so close to the main building that the it spreads to the main house which was renovated in 2012. The gable wall was wet at the time and was dried out again with dehumidifiers and vents placed at various points across it back then.
This is all causing me sleepless nights over the weekend so any advice gratefully received. Thanks

 

[Gothichome]

Woodsy,I don’t post often but this look to be a bit of a tricky spot to patch repair with success. I think I’d be leaning towards removing the white paint from this area including the wall and have a really good look at what’s going on with the rot. It might take a good rehab of the sill To fix this problem. You might get away with cutting out the rot and install a dutchma. At the same time you can actually seal the back of the sill and the wall. Most of what I propose can be done by folks with minimal skills and common sense.
Ron

 

[Woodsy]

Thanks Ron, will suggest this to the builder, I’m not comfortable just patching it up and forgetting about it.

 

[malcolm]

You appear to have a concrete pebbledash with waterproof paint which is annoying as any water that gets in won't be able to get out. these things are much easier when the external finishes are breathable.

The sill is bad detailing. There is not enough room to do anything sensible at the end of the sill. Presumably there is some wood exposed in the gap and no surprise it is soaking wet.

You could just chug the gap up with cement. The wood wouldn't be any worse off than any other wood behind the waterproof paint. You could consider adding some leadwork over the gap so the wood can breathe to the underside and dry, but that's going to be fiddly to get that perfect. Chugging the gap full of lime would let the wood dry but would let it get wet again if you have lime on top that could collect water.

It's a bad design and with the existing finishes being waterproof I think I'd go with the builder and chug the gap full of cement. It won't dry the house out (that's what breathable walls are for) but it should reduce water ingress to the bit of wood in that unfavourable position.