Reducing Humidity in Garage

General Chat
1

Around 1.5 years ago we had a new flat roof, gutters and soakaway (as well as double glazed windows) installed in our garage. The old roof was falling down and gutters draining directly onto the gound around the foundations.

The roof is certainly not leaking but we still have very high humidity in there as I have various sheets of untreated plywood that are starting to go mouldy, and testing with my wood moisture meter, sitting around 20% at the worst mould patches.
Looking on the roof, the OSB board is fine apart from one panel near the wood has mould on it too, with a moisture content of around 20% too. No staining or wet patches on the ceiling that I can see.

I have a temperature/humidity sensor in there now and recording 85-89% humidity, obviously its been quite wet but this still seems too high.

Looking online, opinion seems to be very mixed. I have found that it either needs insulating & heat or more ventillation or a combination on both. Insulating the roof with kingspan/fibreglass could be relatively easy I guess but the walls are single skin.
The garage doors are old wooden split doors with reasonable gaps around them so plenty of air gets in but the worst ventillated spots are up against the partition wall, where the wood is stored.

I figure that starting with an extraction fan, or heat recovery unit might be the best place to start, say back left by the partition where the poor air circulation would be.

Old photo but you get the idea. There is a 2nd door at the rear section.

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Any experts suggest some pointers pease?

2

If your moisture meter is similar to mine (2 sharp prongs) then have you tried taking readings of the floor, walls etc?

Do the walls have a damp proof course and the concrete floor have a membrane underneath?

3

Yes, two sharp prongs.
There is some sort of DPM but this garage was built in the early 70s so I doubt there is anything under the slab.

4

I would try and take readings all over the place to see if you can find damp places especially low down on the walls and floor.

If everything is dry it might just be down to the weather cycling from cold weather making the structure cold followed by warmer wet weather which caused condensation on the pre-chilled surfaces.

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5

Probably unsuitable, and unwanted, but when I lived in Oregon I had wood-burning stoves for heat in my living room and out in my work shop. We had to keep a kettle filled with water on the back of the stove (both) else the air in the room becomes too dry. Just a thought.

6

I do have an old woodburner from our living room when we replaced it, I had thought about installing it in there but its inpractical as I’m not in there every day/week working.

Even in our 1930s house, the humidity never gets below ~55% so I don’t think there is a chance of it ever being too dry.

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7

Not every day, perhaps daily at first just to dry out the floor and walls, thereafter as and when you are out in the garage/shop.