Insulating a metal garage roof

[Dazbo666]

Been suffering some horrendous condensation from our garage / workshop roof over the years, and it's finally getting to me that it's causing rusting and rotting to a lot of belongings in there...

Currently its a single layer metal corrugated roof which literally rains condensation whenever there's a temperature difference with the outdoors.

I've been looking at getting 100mm thick loft insulation, and either treated timber, or hardboard sheets to create a ceiling...

Bearing in mind that I'm a DIY'er, but feel a little inexperienced at this sort of job. Any constructive help or advice would appreciated.

 

[RobinBHM]

The difficulty with corrugated metal roofing is stopping warm moist air reaching the cold metal surface and forming condensation.

The best solution would probably be a spray foam insulation layer which would turn it into a warm roof, ie insulation directly below the metal roof covering so no cold side on the interior at all.

Failing that, maybe you could fit some celetex board insulation tight between each joist flush to the bottom edge, leaving an air gap above. Tape all the joints with silver tape including across each joist end. Then over board with 18mm osb3 board, or 11mm osb3 board.

If possible fit some vents to the outside to vent the gap between insulation and metal roof.

Measure insulation carefully and cut to exact size of joist gap, if gap varies in width cut insulation accordingly. If you cut a slight taper on the boards they will go in more easily. Cut with saw, or score both sides with stanley knife and snap.

Celetex board insulation is more expensive than loft insulation but has twice the thermal value. Celetex is also available as kingspan, recticel, ecotherm etc, all the same product just buy the cheapest, it is PIR foam

 

[Woody2Shoes]

One option might be to replace the tin with "onduline" - basically lino, but in the same wavy (I assume yours is the standard 3" wavy style) profile - which is less vulnerable to condensation in the first place - I suppose an alternative might be to put some onduline under the tin. Otherwise, I agree with the previous advice - you need a vapour-barrier and possibly also insulation (with the barrier on the warm side of any insulation). Cheers, W2S

There are some useful ideas, mentioning avoiding condensation, in the leaflet here: https://onduline.co.uk/products/onduline/#Downloads