As an update to my thread a while ago, my car port is built. 8x2 joists with spans from 4.8m to 5.8m with a metal roof. I used 200mm barge boards around the edges, and teh neighbours have been pleasantly surprised (a relief)

However, it gets terrible condensation. I spoke to a few people who all thought it would be OK as 2 sides are totally open so very well ventilated. However, not so.

If I had realised I would have used some for of insulation before screwing the roofing on. I suppose I could take the roof off and do that, but what a ballache.

Other ideas so far:
DIY spray foam insulation. I can get a kit that would give about 3/4" thickness, but it could be a steep/messy/expensive learning curve.
Buy 2"thick polystyrene boards and cut to a tight fit, stick up onto the roof boards
Use foil/bubble insulation Would need sticking to the roof again.

Anybody got any insights?

I wish I had done the behind my garage first (currently plastic corrugated roofing). That is next on the list, I already have the roofing sheets. I am wondering if a layer of 11mm OSB may be adequate to stop the issue?? Or would proper insulation be a better option?

Is your roof pitched or flat.

Almost flat (just enough to let rain flow off, a bit more than the minimum recommended fall)

That means having a membrane under the metal roof won't make any difference.
I'm afraid to say, but you have used the wrong materials for the roof.
It's good that you have plenty of ventilation but that air needs to be moving on the underside of the roof.
Is it possible to lower your fascias at each end to create an air gap under the roof.

I had the same problem with my metal roofed garage, 6m x 6 m.
I stopped the condensation, more like rain inside by sticking 25mm polystyrene insulation to the metal panels and sealing any gaps.
Now dry and toasty

Corrugated bitumen is one of the best products to use.

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Originally posted by roadrunner
Corrugated bitumen is one of the best products to use.

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sorry to state the obvious but you have to both insulate and stop all the warm wet air getting past the insulation to the roof. Spray foam works well for that (Pro might be cheaper than DIY?) or PIR (eg Celotex) push for a tight fit and sealed around the edges with expanding foam gun. You could then also cover over with waterproof membrane which might neaten the appearance.

Do you have a gas boiler flue intake/exhaust under your car port?

I had one under my car port which is closed off with house on 1 side, garage door on the front and wall 90% up the other side and only fully open at the back. The boiler flue exhaust caused huge condensation under the roof, but I cured it by fitting an extension to the exhaust flue up through the car port roof.

No boiler flue.
Steel is used for many buildings, and I understand the concept of warm air meeting cold surface etc, but I am amazed that in such an open structure there is a temp difference at all above and below the roof.
Interestingly, I have used "clear" fibreglass in one section, nd despite fibreglass being a better insulator than steel, it appears to suffer more. Not sure what I can do with that though.

And for the record, I think corrugated bitumen is sh!t. Looks terrible, no rigidity and so on. fine if it can't be seen. I used it before for a customers shed, at their

Looks like I will buy some insulation boards and glue up to the roof. I doubt I will need to foam any gaps, maybe caulk. As I said, if I had known before, I could have put the boards in a continuous layer under the roof, but now I will be sawing a dozen or so 8x4 sheets up. Ho hum. Think I will wait for warmer weather.

Seconds and co are often good prices for insulation, they are an outlet for Kingspan. I used 100mm insulation board for my extension I built recently and they were seconds because they were 105mm thick!

https://www.secondsandco.co.uk/

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Originally posted by ianhurley20
Seconds and co are often good prices for insulation, they are an outlet for Kingspan. I used 100mm insulation board for my extension I built recently and they were seconds because they were 105mm thick!

https://www.secondsandco.co.uk/

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I don't really understand why everyone says "ventilation is the solution to condensation" in garages. It's barely applicable, and a car port explains why.

You can have all the ventilation in the world (my sectional garage is like a colander) but if you have a cold surface that is colder than the dew point of the air moving through, you'll get condensation. In a house, where the house is warm and full of breathed air (or combustion byproducts) then it's more applicable - flushing the damp air out keeps it from condensing on the walls, but in a cold garage there's chuff all you can do to stop it unless you can warm the garage or make it of materials that have much lower thermal mass and better insulation.

All the heavy metal objects in my garage are drenched in condensation, and the wind whistles through that like there's no walls. The only way to fix that is to insulate and heat, or not put metal objects in it.