Metal roof insulation to fix condensation

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Picalilli

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Hi all
I’m planning on turning the garage into a workshop and have noticed this winter (we only moved in last year) that the corrugated metal roof seems to gather a lot of condensation. It drips down, not torrentially, but I’d want to sort it out before I get the electricity put in obviously. Having it a bit warmer would be a bonus too, so I was thinking of putting in some framing and insulation board beneath the metal.
Just looking for some advice on what thickness of board is minimum and what space I need to leave between the roof and the board. Then, do I need anything else? Does the whole thing need to be sealed with a vapour barrier? I also wonder whether I need to make sure that the cavity between roof and insulation needs to be totally sealed or if it needs ventilation?
Lots of questions here but I’m sure people have had similar issues so just looking for general advice. I’m trying to keep the job as cheap as I can - and it’s mainly about keeping it dry rather than toasty warm.

thanks for any advice in advance! I’ll try to attach a couple of photos to this post as well
 

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artie

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I think the condensation and heat retention are two different problems.
A fix for the condensation is water absorbent paint. It doesn't stop the condensation, it just soaks it up and lets it evaporate slowly instead of dripping over everything.
Heat retention would be a more involved job which if done right might remove the need for the paint.
 

Picalilli

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Thanks - Do you have a link to any products? I’ve searched online but all I can see is ronseal ‘anti condensation paint’ which seems to suggest it works by forming a thermal barrier rather than absorbing moisture.
 

Sachakins

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You could consider putting insulation on the outside top of roof, then board and felt over that.
If the garage is cold and damp, I would ļeave any inside work on it till warmer weather, and/or provide some form of heat, with a dehumidifier to extract as much moisture from the walls as possible, before adding vapour barrier, battoning and insulating walls.
 

thetyreman

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I'd get some rockwool in there and board it all up with thin plywood, making sure the air can still flow where possible.
 

Picalilli

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I'd get some rockwool in there and board it all up with thin plywood, making sure the air can still flow where possible.
This is more or less my plan, but I was thinking of polystyrene insulation boards or maybe even the kingspan ones if I can stretch to that. Mainly because I hate working with the fibre insulation! Just no idea what the gap should be between roof and the boards etc and whether I need to put plastic sheeting up aswell.

You could consider putting insulation on the outside top of roof, then board and felt over that.
If the garage is cold and damp, I would ļeave any inside work on it till warmer weather, and/or provide some form of heat, with a dehumidifier to extract as much moisture from the walls as possible, before adding vapour barrier, battoning and insulating walls.
Hadn’t considered putting it on the outside of the roof... Why would you recommend insulting the walls too? That sounds like quite a lot of work and I’d also lose space in an already small garage
 

Jelly

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Hadn’t considered putting it on the outside of the roof... Why would you recommend insulting the walls too? That sounds like quite a lot of work and I’d also lose space in an already small garage

You can only really control condensation by controlling the relative humidity of the air, and it's very difficult to do that without having at least some control over the air temperature.

Good insulation and a small heat source to keep the air temp slightly above ambient temp outside are usually the overall cheapest and most effective way to control condensation and humidity.
 

Sachakins

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Condensation and airborne moisture need to be kept down to control rust and to maintain a stable level of moisture in your timber.

Insulation on outside of walls too can be done too, its the same way you see them insulating older houses nowadays.
Any single skin wall will always be a source of moisture, as this is were inner and outer temperature meet and any variation between the two gives rise to condensation.
 
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Warby1

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I've suffered from condensation from a recently fitted Comptons metal roof that is supposedly fitted with the highest rated felt which is designed to soak up condensation then let it evaporate naturally. What actually happens is it saturates and dumps, providing a rainforest at times. A dehumidifier definitely fixes it, but is expensive to run. Make sure you get a dessicant version that is rated down to 0 Deg. Mine is a meaco an is absolutely brilliant for my 40sqm workshop. I run it for a couple of days a week when the conditions are such that my felt coating starts to soak up water, it dries it out completely. I've also found creating better airflow reduces the problem but not as effectively as the dehumidifier. I have a low watt fan running all the time at one end of the workshop and I've made sure there's ventilation for air to get out. What this does is reduce the chance for air to settle, one of the causes of condensation. It also reduces the need to have the expensive dehumidifier on quite as much.
 

Robbo60

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Hi all
I’m planning on turning the garage into a workshop and have noticed this winter (we only moved in last year) that the corrugated metal roof seems to gather a lot of condensation. It drips down, not torrentially, but I’d want to sort it out before I get the electricity put in obviously. Having it a bit warmer would be a bonus too, so I was thinking of putting in some framing and insulation board beneath the metal.
Just looking for some advice on what thickness of board is minimum and what space I need to leave between the roof and the board. Then, do I need anything else? Does the whole thing need to be sealed with a vapour barrier? I also wonder whether I need to make sure that the cavity between roof and insulation needs to be totally sealed or if it needs ventilation?
Lots of questions here but I’m sure people have had similar issues so just looking for general advice. I’m trying to keep the job as cheap as I can - and it’s mainly about keeping it dry rather than toasty warm.

thanks for any advice in advance! I’ll try to attach a couple of photos to this post as well
https://www.cladco.co.uk/insulated-panels . Friend of mine used these on a barn. Said very good
 

OBr

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Hi all
I’m planning on turning the garage into a workshop and have noticed this winter (we only moved in last year) that the corrugated metal roof seems to gather a lot of condensation. It drips down, not torrentially, but I’d want to sort it out before I get the electricity put in obviously. Having it a bit warmer would be a bonus too, so I was thinking of putting in some framing and insulation board beneath the metal.
Just looking for some advice on what thickness of board is minimum and what space I need to leave between the roof and the board. Then, do I need anything else? Does the whole thing need to be sealed with a vapour barrier? I also wonder whether I need to make sure that the cavity between roof and insulation needs to be totally sealed or if it needs ventilation?
Lots of questions here but I’m sure people have had similar issues so just looking for general advice. I’m trying to keep the job as cheap as I can - and it’s mainly about keeping it dry rather than toasty warm.

thanks for any advice in advance! I’ll try to attach a couple of photos to this post as well
I have a concrete section garage with corrugated metal roof. Condensation would drip onto my car causing marks I couldn’t remove. I solved this by using OSB sheets from Wickes. I was able to lift the roofing and put the OSB sheets down and the reattach the metal. I used no insulation. Keeps the garage dry now. It’s still cold but unless you have some form of heating it will always be.
 

Picalilli

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I’m certainly able to do that. Which was more or less what I was thinking but maybe putting some insulation in too just on the off chance it made it slightly warmer.
it really needs a new door putting on too as the old wooden ones are starting to rot a bit. Whole thing sounds like it’s becoming a bigger job than I thought, although I think perhaps just solving the dripping problem and then maybe having a dehumidifier setup in the winter Months maybe be a good start.
 

OldWood

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This is not a problem I have any personal; knowledge of but my brother is a cabinet maker, electrician, do-it-all and I've heard him talk about this problem. He mentioned spaying the underside of the roof with foam - a commercail task. If you think about it that metal is always going to be colder - or in the summer hotter - than any materials around it, and the only realistic way of dealing with that is to make it a composite material as industry uses. This basically what Roobo60 recommends as a roofing material replacement.

Apart from the suggestion of re-roofing over your current roof - and I would strongly recommend the Onduline material and avoid felt at all costs - I feel that the other suggestions here are sticking plaster and won't really work. Certainly you need to insulate the walls to ensure that the working space has a reduced possibility of dropping below the dew-point.
Rob
 

OBr

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I’m certainly able to do that. Which was more or less what I was thinking but maybe putting some insulation in too just on the off chance it made it slightly warmer.
it really needs a new door putting on too as the old wooden ones are starting to rot a bit. Whole thing sounds like it’s becoming a bigger job than I thought, although I think perhaps just solving the dripping problem and then maybe having a dehumidifier setup in the winter Months maybe be a good start.
I found my garage too hot in summer and too cold in winter. The OSB boards have done the job for me as I said. The garage roofing sits directly on the boards. Maybe I should have used a waterproof membrane but I didn’t and it’s been 20/ 25 years now and still fine.
 

Spectric

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I’m planning on turning the garage into a workshop and have noticed this winter (we only moved in last year) that the corrugated metal roof seems to gather a lot of condensation. It drips down, not torrentially, but I’d want to sort it out before I get the electricity put in obviously.
Been there in the past, it used to literally rain when the humidity & temperature were at the right point. One cure is to keep the workshop warm, raise the temperature and keep it there 24/7 but that could be expensive. Depending on the construction you could use PIR boards on the inside and seal all gaps with tape and foam, if the cold metal is shielded then the condensation cannot form.
 

Woody2Shoes

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The fundamental problem is that the metal sheet conducts heat very well, and being on the roof quickly radiates heat upwards. The underside of the sheet is likely to spend a significant amount of time cooler than the dewpoint of any air below it (even in the absence of additional heat/humidity created by human activity underneath).
The only way to stop it is to spray on insulation as mentioned or replace the sheets with pre-coated ones e.g.
Any solution where air is able to touch the underside of the metal (even indirectly via vapour permeable element) will fail to solve the problem.
 

recipio

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Thinking of building a steel workshop. The various companies seem to offer insulated walls and roofs but I haven't got any prices yet. Would it be much cheaper to build in bare steel and get a commercial spray on foam insulation. ? I could put in OSB lining on the walls but would the roof not be a dust trap and feel like you were inside of a duvet ! :oops:
 

phil p

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I'm interested in this thread as I intend, when the weather improves, to renew my garage roof.

It currently has cement fibreboards which are the original ones from when I got the garage installed back in 2000 so they've lasted 20 years or so however water is starting to come though quite bad now.

I do have boards on the underside with some Celotex and covered with plywood built into the framework so I can't really see what's happening but there is water droplets coming through the ply.

Anyway I'm currently looking at this metal sheeting panels with the anti condensation coating so I'm deflated slightly to hear they aren't that effective!

Just thought though, could you not just cut some 6 inch strips for the channel part of the sheet, maybe 25mm or 40mm thick Celotex , or similar, and some suitable adhesive to glue them to the underside?

This may help both the condensation problem and also retain a bit of heat in the garage, and probobly far cheaper than some of the insulated sheets on the market.

Regards
Phil
 

Hornbeam

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The first step is understanding the cause of condensation. Air at a given temperature can hold an amount of water vapour. If the air comes into contact with a colder surface and the temperature drops such that the air can no longer hold the water as vapour, i will condense out. So there are 2 approaches to reducing condensation risk. 1 reduce the water vapour in the air either by good ventilation or by using a dehumidifier but NOT both as you will be trying to take the water vapour out of a constantly changing air supply, If using a dehumidifier it is best to have as air tight a building as possible. Adding insulation on it own wont make the workshop any warmer. Insulation only reduces the transfer of heat, so if teh workshop isnt heated insulation wont make it warmer. It will help condensation particularly on the roof as single skin steel roofs can suffer from night time radiation making them much colder and more likely to form a surface for condensation. For insulating the building you have 3 options
1. Undercloak the existing metal roof with insulation and a board. I would always recommend an insulation quilt as you will get a better sound insulation. It can be a bit tricky to do and you are enclosing the structural members of the building so you cant see any potential failures before they occur. you want to make the undercloaking as air tight as as possible
2. Remove the existing roof, board it out with OSB with well sealed joints, then fit a spacer bar and bracket system (Accord cladding do them) and install the original sheets over the top, with an additional layer of mineral wool insulation
3. If the existing roof is in good condition. you can retain this and fit a spacer bar and bracket system, with a new outer sheeting, again install mineral wool insulation in the in the cavity
Ian
 
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