Garage Ventilation

[NickMorgan]

I am lucky enough to have just had a 6 metre by 12 metre garage built for me. It has a concrete pad, three steel bays, with timber frame and shiplap walls, insulated, with a vapour barrier and lined. The roof is steel with 40mm insulation and it has an insulated sectional double door and a side door.
Other than a few gaps it wasn't ventilated at all. I have put in four six inch vents in the apex of the roof. I now realise that I should also have some sort of ventilation towards the bottom of the walls to create a bit of an air flow. I notice that the head of the strimmer has mould on the grass and if I open the doors on days when it has warmed up outside then the cars get covered in condensation.
Does anyone know how much ventilation I should put in, or is it better to have a dehumidifier running or a fan?
Thanks for any advice.

[SprintMWU773V]

I had a similar problem with a shed in my old house. It didn't leak but suffered from quite bad condensation. I cut a hole in the lower part of one wall and then cut another hole in the apex above the door. This provided better airflow, allowing warmer moist air to escape, it didn't entirely solve the problem though.

Interestingly I looked at those air flow chambers you can buy to put a car in. They've basically got a couple of small computer case fans in, drawing in air over the vehicle and then expelling it out the other end. Basically two or more fans on either end one pulling and one pushing. I've not experience of these chambers personally but if you look at the CFM offered by the fan you could easily scale it up to suit a garage to increase the airflow through. I imagine though that running costs may be fairly high as you'd need fairly large fans.

The main issue you have I suspect is the steel roof, it doesn't breathe. The warm, moist air rises and then can't escape. As the rood is insulated it will be attracted to a cool surface e.g. your vehicle to give up it's moisture and therefore you'll get condensation.

Dehumidifiers are great but you need one that works down to a low temperature. Most don't so you'd need to seek out a desiccant type. A decent one will allow you to set to auto and then it'll click in and out to keep the RH down. I've used one for years and have a nice dry garage. I've attached a continuous drain hose to mine which I then feed into a large 5 gallon container. This means it takes ages to fill up plus I get a ready supply of pure water which I use in my fish tank!

[NickMorgan]

Thank you for the response Mark. I didn't know that dehumidifiers don't work at low temperatures.
I have noticed that it is usually colder in the garage than outside. I guess that with all the insulation it doesn't warm up in the sun as my old garage did. I imagine that doesn't help with the condensation.
I was also thinking that I wouldn't want to have a fan running continuously, because of the expense.
I was thinking of adding lower vents as you did, but will need to find out how big these should be.

[xvivalve]

Nick,

To passively purge roof spaces of condensation, clear ventilation area equivalent to a continuous 10 mm wide gap around the perimeter is considered to be sufficient. Something similar would probably suffice for your garage, but I'd try to get it in several places to provide cross ventilation along its length. Work on prevailing wind coming from the south west and have it on the closest elevation to that and the opposite one.

[NickMorgan]

Thank you. That is useful. I will try to work something out on those lines.

[xvivalve]

If you use grilles, be aware the free air area is less than the area of the grille itself, you need to work on the actual area of holes within the grille.