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.
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.
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.
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.
If you are absolutely sure that the roof is totally water tight.
I would say that the 1st thing would be to water proof the floor slab, membrane, 50mm poly foam insulation and Wayrock or similar flooring. Similar to the half brick walls but with plaster board, insulated plaster board the ceiling. Make sure that ventilation is maintained.
I’m absolutely sure that isn’t what you wanted to hear
What about a dehumidifier with a drain to the outside, will have to run 24/7 for a few weeks or so but it may then keep the humidity down.
I recently got one of these and after running like crazy for a couple of weeks it has dropped the internal humidity from over 60% down to 45% and only gets emptied once a week.
Are there any ventilation grilles in the soffit around the new roof? Would seem ventilation/opportunity for warmer moist air to escape would be an issue. Slot vents in the window heads too. Also need incoming cooler air to create a flow.
As others have said, if there is not control of moisture coming up through the floor and walls, then a well sealed space is your problem.
An extract would help but looks like a big space, and not fixing the issue at source.
I am assuming there is no leaky garage door to assist with fresh air getting in?
Yes, pretty sure the roof is water tight, I shall look into options for the concrete slab.
I fear a dehumidifier at this point is just going to be wasted without insulation, ventillation etc first.
I’ve already had the slot vents open in the windows, tried leaving them ‘cracked’ open but no change. The garage doors are certainly not sealed, old wooden swing open doors.
Various bits of untreated plywood next to the leaky doors are fine, so I think its just a lack of air-flow around the garage itself and more damp in ‘dead’ spots.
Just seen this thread, we had a new flat roof on our garage last year and have exactly the same problem. The cold snap a few weeks ago kicked it off.
I hired an industrial dehumidifier for a couple of weeks and it pulled out the damp. I’ve just bought a couple of tube heaters with thermostats to see if a bit of heat helps.
Venting is difficult in our garage, we would have to look at mushroom vents.
