Moisture in newly built block shed

[Ogi]

Hi Guys,
I'm new here, although I've been visiting this place for years now as this is a gold mine of information. I can't think of better place for my problem. I read multiple threads regarding increased moisture in sheds but I still can't work out my problem.
Over a year ago I have built block shed on new concrete foundation. I placed one block on foundation for the floor. I poured first layer of concrete. Than I used damp proof membare paint (2 coats) to paint spaces marked in blue. In the middle I placed damp proof sheet (purple) but sheet didn't go over blocks. It was placed only on concrete floor overlapping dpm paint. Than i poured top level for the floor.
Now I have problem with moisture. At this time of the year it is around 90%. Last month when temperature dropped below 0 at night I had wet ceiling.
I am not sure where I made mistake and how to fix it? I was thinking to put another layer of dpm sheet with overlapping the wall as on picture no. 2 and than pour new floor on top of that.
Is this the right way to do it?
To specify, from inside I cannot see floor or walls being wet. My concern is outside, as two walls above dpm paint level turned bit "green". Walls are built from one layer of 10 cm thick concrete blocks. They have not been painted, plastered or rendered at all.

I appreciate your time reading this any answers.

Thanks

 

[freddiemercurystwin]

Needs some background ventilation, some air bricks or leaving a window ajar or similar.

 

[Ogi]

I have 4 vents just under the ceilling, two on each opposite wall. None at the bottom, though. Even if I leave window wide open for 2 days straight, it hardly decreases moisture inside.
I checked thoroughly all the walls, corners and floor inside and i can't find any damp patches, no wet walls. Everything seems pretty dry. Someone told me having onle 1 layer of concrete blocks is a problem as blocks absorbs water. I was advised to put rendering on external wall.

 

[foxhole]

A block built unheated shed will be cold and damp during winter months .

 

[freddiemercurystwin]

Agreed, my garage is block built, it's cold but no damp/condensation as such anywhere though I suspect it has high(ish) humidty, boxes against the wall low down for instance will suffer a little from getting damp, though I have a window left open permanently and the garage door itself probably has some gaps around it. The OP needs to increase ventilation.

@Ogi I think your concerns over the floor are a red herring.

 

[catlad]

Concrete blocks are like sponges without any protection ( render or cladding )

 

[Ogi]

So best solution would be rendering walls? Tbh, after recent rains I noticed that one corner outside has wet patch, as corners under the window. Also algae is at some of the blocks close to the ground. I keep checking condensation level in the shed and stays between 85-95%.
How thick rendering should be to give the best protection to the walls?

 

[Ogi]

I took some photos today. Hopefully this will paint better picture.
As you can see on photos I marked level of dpc. On both side walls damp climbed over dpc. I did have a look inside of the shed today noticed 'dark patches' on blocks at the bottom, close to the floor. Floor seems intact. It is odd, as back of the shed look clean above dpc level. I was worried this wall might cause biggest issues.
Corner of the wall on left from the door seems to have wet patch pretty much all the time. Same corners under the window.
I don't mind being slated for all the mistakes I made, I only hope you would be able to give me advice how to address this problem.

Many thanks for any answers and your time

 

[opps]

When I built my thermolite garage years ago, their tech support told me that the blocks will not allow any moisture through, but the mortar will.

If you don't want the added expense of rendering, perhaps paint the outer faces.

 

[Ogi]

opps
Tbh, I am bit worried to render the walls. I am afraid that damp/algae will come up from under the render.

 

[Andy165]

it does look ugly at present. get proper advice on what to overpaint it with. maybe wait till weather goes warmer.

 

[JohnD]

after recent rains I noticed that one corner outside has wet patch,

Why are the gutters not catching this water?