I know there have been a few threads on this but I’d appreciate some input on my particular problem with my garage.

My garage is build of single skin breeze blocks, two up and overs (locked shut), a single door and a roof made mainly of corrugated steel.

It’s waterproof but in the winter, it has major condensation – presumably from the slightly warmer air in the garage condensing water onto the cold roof overnight. It pretty much rains inside when it’s bad…

This isn’t doing my bikes or anything else in there much good.

So what’s the solution? I’ve read that ventilation will help but I can’t believe that that will solve it – it’s a large volume of air that’d going to hold water and it will always condense when in contact with the cold roof.

I’ve read about people insulating with Kingspan or similar – is that a good idea? It seems like it should be fairly simple and something I can do myself and not too expensive. If I did do that, should I leave an air gap or not? If I can leave a gap, it’ll make fitting much easier as I can fit the insulation between the main beams and then seal any gaps with expanding foam.

This is how the roof’s built:

basically, metal roof attached to thin beams (approx 2″ deep) with the main beams you see going L->R in the pic doing the structural bit.

Suggestions very welcome before the “rain” starts this winter

There is a spray on coating designed for just this problem,
I’ll post up a link later when I’m in work as I haven’t got it bookmarked on the pad

leave an air gap above any insulation and make sure air is able to circulate or the timbers will get damp and rot over time.

tbh, given you have power in there it’ll be cheaper to get a decent dehumidifier (not a condensing one – they don’t work well at low temperatures) and run it so it’s on continuous drain.

I did consider that wwaswas but surely unless it completely dessicates the place there’ll still be mosture in the air which will condense when the roof’s really cold. Not to mention that it’ll be expensive to run 24/7 I’d have thought.

kingspan to do that should cost around 100 quid and actually work

if you want to cheap out – expanded polystyrene will also work much cheaper.

you dont even have to expanding foam the gaps if you dont want to – you should be able to cut it to fit snugly. – echo what they said about the air gap though – leave a gap and ive left the end over the door open to let the air circulate but i get zero damp now – from previously having rusty tools even in the tool boxes 😮

a dehumidifier will not rectify the issue merely reduce it – unless it reduces the internal temperature as quick as the external temperature. – insulating the roof will work though.

we had to run one perminantly in an old house we rented – it cost 6 pounds a quarter to run. if you plan on staying in your house for a while its a no brainer and much less of a fire hazard.

Well whatever will do the job cheaply is what I want…

So it sounds like you’ve done what I’m talking about. Did you use kingspan or expanded foam? And what thickness? If there’s an air gap that’s open, won’t that mean that water will still condense?

will condense somewhere – the temp gradiant on the roof will still mean it will form just significantly less – id rather it was able to get out than be trapped up there rotting shit away. – when i say ive left a strip open – i mean 3 inches. just to let it breath.

i used 50mm kingspan sheets cut to size and wedged between the rafters with expanding foam in the gaps as you describe as its less delicate than expanded polystyrene – i store stuff up in my roof so often gets hit.

Cool – so basically exactly what I’m looking at doing and we have bikes/etc hanging from the roof so some durability would be good.

OK, so it sounds like I should go and look for the best place to buy a load of kingspan then…

unless you have a big vehicle or want to cover a carpark in snow – consider getting it delivered. it comes in BFO sheets.

mines came from buildbase in the end but the builders merchants are your friend when it comes to this stuff – its ludicrously expensive at bnq

Yeah, I was looking at the 2.4×1.2m stuff so I don’t think I’d be fitting that in the car 🙂

cheap used kingspan

I like that shop Nobeer 🙂

Could do my garage for ‘not as much as I thought’.

Ooh! That’s a good idea! £7 for 40mm 2.4×1.2 sheets (plus £30 delivery though).

EDIT – that’s flooring insulation…

Best to phone them, as they don’t have everything on their site.

flooring – roofing

it wont matter one iota.

i didnt know about that site nobeer – bookmarked for future projects.

and at that price – forget about the delivery. i got charged 15 quid for delivery from merchants – to mid nowheresville mind you.

If you are going to leave an air gap it’s likely you’re just transferring a moisture and condensation problem to a position you can’t see it happening and where it’s going to take a lot longer to dry out. If you go down this route it needs to be ventilated well, but this is actually quite difficult with what is, in effect, a flat roof as there’s no benefit of the thermal stack effect you get with pitched roofs.

If you really want to do it properly, full fill the space with a vapour permeable insulation like rockwool or wood fibre board or similar (not kingspan as this holds on to the moisture keeping your timber structure nice and damp for as long as possible). Then underdraw that with an intelligent vapour barrier membrane (one that is more vapour open in the summer than winter) like Pro-Clima Intello and make sure this is as well sealed as possible. If any moisture gets into the construction (whatever you do you can’t eliminate this) it is released as soon as humidity levels drop and the temperature increases.

Just found the link I alluded to earlier:

http://www.grafoproducts.co.uk/

Ideal if you don’t want/need to insulate as well

my dads garage has been done the same way i suggested for over 20 years…. went from being a cold shower if the door blew shut to nothing just by sticking up the kingspan.

doesnt seem to be rotting in anyway.

i guess its the old french quote that was on here last week – works in practice but not in theory 😉

I’ve tried added the planks of pink kingspan stuff and the condensation still happens in certain weather and drips through the gaps.waste of time!

I think the only way would be to go with the foam spray

or a small oil burner that could heat the air temp up a little.

I’ve added loads of ventilation without any luck too.

unless you have a big vehicle or want to cover a carpark in snow – consider getting it delivered. it comes in BFO sheets.

Yep, this is 1/3 of the stuff in my Workshop!

Celotex GA4100 arriving by brf, on Flickr

if you fit Kingspan between the rafters you should really maintain a 50mm airgap over the top, and ventilate at eaves & ridge.

Packing it full of kingpsan (no air gap over) and fitting a vapour barrier below the insulation would be a reasonable (slightly bodged) method. Should prevent the warm, moist air meeting the cold, and vastly reduce the space available for it to condense in……

I wouldn’t recommend that for anything habitable, but for a garage it wouldn’t be too bad……..

how thick is that stuff sefton because its looks mighty thin

its not thin – about 3″ I think

what materials the roof then , you sure its not just leaking ?

asbestos roofs commonly go porous in old age- mines have on the wood sheds.

[edit] answered the wrong question…

Well that’s kind of my concern – I fit the kingspan costing a couple of hundred quid and it still ‘rains’….

first things first then for you

looks like you have a plastic roof – as do i – hose – test for leakage

check all the roof fixings have their sealing rings – 2 of mine were missing and the roof leaked. If in place then look for holes

didnt solve the issue but it was one less issue – still had condensation.

stuck the kingspan in – no more issue.

raising the temperature with an heater of anykind(other than a wood burner) will compound the issue

Is that aimed at me? My roof definitely doesn’t leak.

everyone i guess this question comes up all the time.

my roof is bitchumen – is not leaky – its condensation for a few weeks of the year in winter.

fair enough no idea why that is , never done a bitumen covered roof though

ive got 2 garages – 1 with a plastic corrugated roof , 1 with a compressed fibre roof (not asbestos) both single pitch

both suffered insane condensation issues.

did what i describe above to both.

no condensation issues in my garages.

my parents is a felt covered ply roof – same story. stupidly damp, they fitted expanded polystryene to the roof 20 years ago . condensation gone.

I got a job lot of 50mm polystyrene and stuck it to the underside of my garage roof (painted, corrugated metal).

Condensation problem went away…

Slight hijack….
For the soon to be pent roof type shed with a ply roof and shiplap walls could I get away with less than 50mm kingspan type insulation between the roof supports and should I use some sort of lining paper or membrane?

Thanks for the link nobeer

presumably from the slightly warmer air in the garage condensing water onto the cold roof overnight

Correct – Night Sky Radiation.

Not an easy thing to cure. Ventilate or insulate. Replacing the roof sheets with a composite would be the best solution.

The principle of spray foam insulation is better then cut sheets of a rigid insulation due to the profile of the roof sheet.

Ventilation – a bit suck it and see.