Mouldy Workshop

[diynoob]

I bought a house a few months ago that has an outbuilding. The previous owner used the building for woodworking but I will just be using it for storage. The mould is so bad though that anything I put in there is ruined in no time.

It was not mouldy when we first bought the house. The previous owner worked and stored wood in it so he kept an aircon unit running and a heater.

I've tried a dehumidifier but running it constantly for several weeks has had hardly any effect on the humidity. Should I add a few vents to the building to get some airflow?

There is a lot of insulation which looks like a professional has installed so I'm reluctant to start making holes in the building for ventilation unless that is definitely the right thing to do. The picture shows the insulation:

 

[Ian H]

Could you leave the windows open?

 

[John D v2.0]

You may need a bit of heat but I wonder where all the water is coming from?
What is the humidity and temperature? And how does that compare with outside?
Is the floor insulated to the same standards?
I wonder if the air circulating from outside is warmer and can hold more water, and when it gets inside it cools and condenses.
There could be missing dpcs or some that exacerbate.
Any building can get damp with no heat though, even your car gets soaking after a cool night and you can see the frost or condensation on it.

 

[diynoob]

Could you leave the windows open?

The windows don't open. There are doors at both ends and they are wooden so I was thinking of cutting big holes in them and covering with a vent grille but not sure if I'd end up letting more damp cold air in than out.

 

[diynoob]

You may need a bit of heat but I wonder where all the water is coming from?
What is the humidity and temperature? And how does that compare with outside?

The outside temperature is 18C and the humidity is 47%

Inside the workshop it's 14C. The humidity was 80% but after a month of running the dehumidifier is down to 70%.

 

[John D v2.0]

The outside temperature is 18C and the humidity is 47%

Inside the workshop it's 14C. The humidity was 80% but after a month of running the dehumidifier is down to 70%.

Right so there's your problem, if you let air in it'll be warmer but fundamentally you have the problem that you're losing heat inside the shed to somewhere, presumably into the ground.
Dehumidifiers don't always work well in cold conditions either. But I think the more you dehumidify the more air comes from outside.
Can you use an ir thermometer on the walls and floor?

 

[diynoob]

Can you use an ir thermometer on the walls and floor?

The room temp is showing 12C now. There's not much variation on the surfaces - the concrete floor is 9C, insulated walls are 11C and the insulated ceiling is 10C.

It's not a shed, it's a 13m x 6m workshop made out of breeze blocks. The inside is insulated and the outside is rendered. There are 2 small windows that don't open. So when it's sunny it's warmer outside than in. It's not necessarily losing heat it's just that it never gets any! It doesn't make financial sense for me to keep a heater and aircon running, it's a massive space to heat. It worked for the previous owner because he was a professional woodworker so worked in there all day and kept it heated.

I could try a trickle vent, they only use about 5w these days. Or just make big holes in the doors and cover with grilles?

 

[John D v2.0]

Not sure, you need to somehow keep the temperature up, I think a 5w heater won't really make a big difference, need a lot more. Maybe the insulation is working against you, it's stopping the warm greeting in from outside but letting the heat out the floor.
I'll let someone with more knowledge advise further

 

[IWalkLikeHimaMartian]

For ventilation to work, on reducing RH, you also need heating. (Heat the internal air which absorbs more moisture, which is then ventilated to outside and replaced with colder air with less moisture.)
As John said, when you let in air from outside that is warmer than inside, you will create conditions for RH to rise.

Mold tends to form above about 65% RH. When you achieve this level of RH, and clean down any existing mold, it should stop forming. Obviously, the further away from 65% you can achieve the better, but you will not need to go much below 60%.
Building materials, including insulation are hygroscopic, i.e. they absorb moisture.
Depending on the construction, there may be damp from the floor or walls. The floor, the walls, the materials in storage, the insulation will all have absorbed some moisture, especially when the RH was high. As you persevere with the dehumidifier, the moisture in the materials will eventually be released back into the air, and thus, will end up in the dehumidifier.

 

[diynoob]

For ventilation to work, on reducing RH, you also need heating.

I was worried that would be the case. There's no way I can heat a 13m x 6m workshop, it will cost a fortune. I think I'll be better off just making more storage in the house, using the eaves.

 

[JohnD]

A window facing the sun will gather solar gain.