Garage insulation and damp

Hello.

I'm about to convert my garage into a habitable space (not moved in yet, but have exchanged contracts). It’s a single skin thermalite block construction, and rendered on the outside, plastered on the inside.

The main issues will be solving a small patch of damp (see photo) and sorting the insulation - would welcome some thoughts on both of these.

I think the damp it is probably condensation but I can't rule out rising damp. I was planning to knock off the plaster, inject a DPC tank with cementitious tanking slurry, then plaster with cement based render.


To insulate the walls I was planning on using a warm batten method: sticking 50mm celotex onto the walls, fixing 25mm x 50mm battens at 600mm centres over the top using concrete fixings driven through the insulation to the wall underneath, then plasterboard over the top.



So my first question is whether you think I need a DPM between the wall and the insulation. And if so, any thoughts on what to use - should I go for a dimpled cavity drain membrane, given the damp?



The second issue is about insulating the ceiling. You'll see from the picture that it’s a sloping ceiling and there are a couple of velux windows already installed. I had initially thought that I would just overboard the ceiling, adding a layer of celotex + another layer of plasterboard, but that seems like a faff because (a) it will be difficult to find the rafters and (b) I can't think of a decent way to add any additional insulation around the velux windows using that method. (We might also add a couple velux, which would be easier if I took the ceiling down.)



The trickier issue is that there is a metal thing (see other photo - not sure what it is called!) at the apex of the roof. You'll see from the pic that there is a ton of mould up there from condensation. Because its metal it is prime spot for cold bridging - does anyone have any ideas on how to insulate around there? Should I build a wooden box around it and fill it with foam? Between the two purlins near the very top of the roof I was going wedge celotex and plasterboard over the top.



Thanks in advance for any advice!



N McT

Will be penetrating damp, not rising, probably roof/gutter leak or higher ground levels or failed render.

Thanks foxhole - why do you think that?

It’s thermalite blocks and rising damp is a myth .
Any photo’s externally?

Photo below of the outside of the damp patch.

The garage is built against the party wall, but I can see from the original plans that a dpm was inserted between that and the garage.

Looks like pipe through wall is not sealed so rain can run right thru.Leaf litter will also hold moisture against wall and if thats a down pipe likely its blocked at gutter?

Hi foxhole - I've now moved in!!

The hole around the conduit has not been properly sealed, so I was planning to sort that with some silicone.

But I don't think that is the whole story.

The dpc is above floor level in the garage, but it had been plastered on the inside right down to the floor. Could the internal plaster be bridging the dpc?

It been converted without any regard floor levels, needs a suspended floor above dpc.

That seems overkill for a small patch of damp in 1 corner. It's been standing for 30 years and it'd the only evidence of damp in the building.

Can't be the only way - we had a v. Bad problem with damp in my last house, which was similar in construction: concrete floor with (injected) dpm above thr floor line.

Can't I just hack it off, use bitumen paint or tanking slurry to avoid any bridging, then insulate as above?

we had a v. Bad problem with damp in my last house, which was similar in construction: concrete floor with (injected) dpm above thr floor line.

That should tell you something.?

Should have added that the problem was completely solved, using waterproof render and injected DPC.