Cut away the plaster and found this

[Intrepid86]

This picture is of the side of one set of UPVC doors on an extension the previous owners had built in 2002. There is a void and a piece of black plastic that appears to be the DPC which is loose and terminates a few inches further down. This void is present on the other side of this set of doors and both sides of the doors adjacent.

Should there be a void? Shouldn’t the DPC be secured to the outside wall? There are cavity closing block above this void.

Could this be the source of moisture ingress? Strange thing is, nothing on the ceiling of the floor below.

 

[robinbanks]

I reckon you could probably go to every house on a new estate and find the same detail. It's not right or good from an insulation and draughtproofing perspective, but such lack of finesse is incredibly common in our housing stock

Regards the moisture ingress if you've got potential for cold world air to reach the back of a bit of room facing plasterboard it's ripe for a cold spot that will exacerbate any moisture problem by being a place for condensation to form easily. Depending on what goes on in the house regards human activity that generates moisture condensation could even be the entire source of moisture that you suspect is ingress

 

[Intrepid86]

What should be here? And can I just take a van on spray foam to it

 

[robinbanks]

That's one approach! Insulate the cavity right the way up.. if you've got room to sneak in 25mm against the bricks before you replace the reveal plasterboard it'll help reduce cold spots too

 

[Intrepid86]

Do you mean 25mm PIR? Will the DPC hanging loose mean that moisture is running to the end of it and then into the space below and plaster adjacent? Luckily, I’ve got a plaster coming next week who I’ll ask to tidy up any mess.

 

[robinbanks]

I don't see any obvious moisture on the dpc, but its purpose is to prevent capillary action lifting moisture from the ground level masonry. There isn't any harm in tucking it upwards before you insulate the cavity

Yes, any amount of insulation you can get between your internal room and the bricks of the external wall you can see there, is welcome. Is the window as far as back as possible? Sometimes you can get most of the cavity closed with window frame while keeping a decorative return on the outside brick.

 

[Intrepid86]

This is the outside