UKC

I recently moved into a house with garage. Fine in autumn but in the cold weather in January everything went sideways - all my tools rusted, all the wood and fabric went mouldy. The bar tape on my road bike grew green fur on them. Checked the humidity = 99% :| 

The garage has a metal roof and is wet to touch in cold weather. There's no obvious leaks and the floor is bone dry so I'm fairly sure the issue is solely condensation. 

Reading online the only guaranteed fix is to run and heater and dehumidifier but given the way electric prices are going it's not very sustainable. I dont want it to be toasty, I just want to be able to store things without them going rotten...humidity <60% (i think is the magic number). 

Does anyone in the UKC hive mind have any experience with condensation workshops or garages? I was thinking about bodging up and bunch of kingspan and using expanding foam, but would be expensive if it didn't work. Much more expensive but I guess definitive would be actual spray on foam insulation. Also not sure if adding loads of vents would help or if I would then just be letting in more moisture in wet/cold weather? 

would be very grateful if anyone has any ideas or can point me in the right direction! 

I'd suggest some ventilation would be cheapest and easiest.

My garage has a slate roof with breathable felt below it and gaps at the eaves for crossflow of air and I have no damp problems despite no heating or insulation.

Worth a try?

I had a similar issue with my (standalone, brick with felt roof) garage when I first moved in. Annoying as I wasn't expecting it, not being used to that sort of building. I run a heater in there now which seems to have worked well, but that's obviously becoming a more and more expensive option. A lot cheaper than knocking it down and building a new garage though (which is hopefully what I'll be doing in the long term, rebuilding it connected to the house as an extension)... 

Bit weird that the roof would be wet to the touch in cold weather and humidity at 99% - would indicate evaporation from a source slightly warmer than ambient to me? Possibly rising damp evaporating from walls and floor and then condensing on roof in the evenings?

Do you have humidity readings throughout the day? 

Without running a heater or dehumidifier, ventilation would be the best bet I'd have thought. Could try insulation on its own to try and  stop heat loss at night, not sure how will this would work though. Ventilation should hopefully keep the internal humidity similar to ambient rather than at 99% - still way above 60% for much of the time though, and daily temperature fluctuations might still result in very high humidity as the shed warms up after a cold night etc

If you could insulate the shed thoroughly (prevent too much daily temperature fluctuation and allow a small heater to make a bigger difference), run a small heater (to bring the temperature up, reducing humidity inside the shed compared to ambient air) and have a small amount of ventilation (to allow any small amount of water ingress to evaporate and leave the shed) that would be the ideal solution, and make it most similar to an indoor home environment. Comes down to cost though really. 

Ventilation .

Even with vents there will still be condensation so consider where the drips will go.

I have a steel shipping container , which has been used for storage for two winters without problem but only because I dry lined it. OSB board,  50mm cavity side walls, 50 mm gap at floor, 50 to 100 mm cavity ceiling sloping to wall cavity, cavities vented to outside, stuff stored in cardboard boxes on shelving off floor and walls. Cost me £255 for a 6m container. Basically its the same principal as a tent fly and inner.

I think you may find ventilation could work>
We moved last year, and during the winter experienced exactly what you have.
We got everything clean and then nearly every day, left the back door of the Garage open, and the problem did not re-occur.
However one single skin wall is open to the prevailing weather, and on the shelves on that wall I have made sure nothing is touching the wall, and no fabric items or the like are stored there.
I do not think heating is realistic idea.
Insulation, maybe, but have you seen the cost of Kingspan and the like, and if its insulated too much, it will cut down the ventilation
So I am back with ventilation, and I may put in a few air bricks, and possibly some kind of constant ly on fan for the winter months to draw air through, as I do not want to be leaving the door open, and hopefully I may be away for a few weeks at a time during winter/ autumn.

I will watch this thread with interest. 

Don't have to kingspan it. You could use cheap and nasty fiberglass insulation, then batten and board it.

You're not aiming for it to stay warm. Just stop condensation forming on cold spots like the metal roof.

Just make sure there's ventilation for the roof space. 

My nasty leaky garage is very very well ventilated. Stuff does rust in there but slowly in the grand scheme of things.

My plywood glider lives in a single skin draughty metal trailer year round without bad damp problems. Ventilation alone definitely can work but bare steel will rust eventually in damp air regardless.

Jk

I had a similar problem with my shed and I read somewhere storing things in cardboard boxes is a no no, so I got rid of mine and put things in plastic boxes instead 

It's definitely not as mouldy or damp but it's early days yet .

My shed was insulated .

I have had a brainwave.
Reading J Karrans response, I realise that my super draughty warehouse does not suffer with damp, it is dry as snuff.

So Ventilation is the answer.

The back door of my garage is a plastic door with a sheet of glass in it. I think I will replace the glass with some form of plastic, with a load of holes in it. Then I will be able to lock the garage up, so maintain security, and still have ventilation, as if the door was open.
I shall go and get my tape measure.

Also large heavy duty polybags with opening tied or folded under items restrict humid air access and seem to reduce rusting.

I haven’t tried this yet (I moved house and now have a dry garage) but there’s quite a lot online about simple solar air heaters. Here’s one example:

https://www.ecohome.net/guides/1075/all-about-solar-air-heaters-diy-or-purc...

but I agree with previous poster that ventilation, plus a bit of insulation, would be an essential first step.

We had a big bag of silica gel sachets and chucked bunches of them into bags/plastic boxes when we stored things in our damp cellar in Sheffield.

Is the garage attached to the house? does warm humid air from the house leak into the garage?

Regarding the tools you may have to go back to the old school way of oiling them and wrapping them in wax paper.

Metal roofs are about the worst as they conduct heat the best, so you get lots of condensation on the inside and as you’ve observed near 100%. Like a single glazed window in the moisture laden house. For sealed in buildings, as in not a two or three sided open lean to, they are best put on a roof that's at least felted and battened etc... 

Your solution will be ventilation by inserting several air bricks, vents or whatever suits the walls material, high and low to get the air circulating and drawing in fresh less moisture laden air. 5 star option is a sub ceiling, but not the easiest or cheapest, height clearances might be a factor too. 

Another vote for ventilation.  We used to have major damp problems in our house, particularly the cellar.  The following changes made a huge difference.

1. Addition of air bricks to the underfloor void.  Victorian house built without ventilation, the joists were dripping wet.  2 air bricks, problem solved.  Amazing how much difference some small holes made.
2. Forced ventilation in the cellar.  Similar to your garage, everything stored in the cellar went mouldy or rusted.  I tried dehumidifiers, which were ok but expensive to run. Knocked 2 small holes in the cellar walls (to ventilated spaces) and installed an inline extractor designed for bathrooms that pushed air out through one of them.  Problem 95% solved, I run the extractor for about 90 minutes/day
3. Positive input ventilation fan added to top of stairs - not directly applicable to your garage, but installing this has taken us from (secondary glazed) windows running with condensation and pooling on the window cills, to a nice dry feeling house, almost like magic.

If you do go the insulation route make sure you create a vapour barrier to your cold surfaces. Otherwise you will end up with wet, mouldy insulation. Simple cheap polythene liner to prevent moist air reaching cold surfaces.

Take the roof off if possible and lay a breathable felt over the rafters, put the roof back on, insulate the underside with a cheap loft insulation and board this in. Job done 

Worked on my garage a treat

Thanks everyone for all for the responses.

Definitely seems like I need to add some ventilation. Is there a point in adding insulation as well or is it ventilation Vs insulation as the strategies. No point insulating a space if it's really well ventilated? 

The walls of the garage are already lined with painted OSB boards. 

We’ve got 4 individual stables in a block, which are tinder dry even in the middle of winter. They’ve got louvred vents at the apex of the roof, and generally the top half of the doors open. Supports the ventilation theory. There are a bunch of feral cats living in there at the moment who seem to enjoy the dry straw to sleep on. As they’ve decimated the local rodent population, I’m not moving them out.😂

Same applies to the home wall in my garage. There’s no insulation, but there are vents in the ridge tiles, and lots of air bricks. No problems at all with damp, which usually manifests as black mould on wooden holds. 

I have 2 storage containers, a 40 foot and a 10 foot. Both un insulated and un ventilated for that matter and neither suffer with damp at all.

We store service items in that that come in cardboard boxes, the boxes stay dry and no runs on the items.

Re the garage, is water getting in at all? Our garage stays damp free within reason but I left a bucket of water in there and a week later everything was rusty and mouldy