UKC

Hi all, as per title - going cross eyed looking at anti condensation paint options.
Single skin clear corrugated plastic garage roof (as expected!) creates massive amounts of condensation (we’re moping the underside of the roof every few days) we know it’s a dew point issue along with poor ventilation but is there anything on the market (designed for or otherwise) that people have used that could be painted/spread onto the sheets (either side) that would create another skin (effectively reduces the condensation) not expecting to eliminate it completely.

Roof is flat style/angled for rain to run off into a gutter (which it does effectively). The condensation issue is mostly winter time, spring/summer temps are more equalised. 

North East Scotland btw.

Cheers all! 

In reply to Tobes:

Felt (presumably synthetic) backing on sheet roof materials really helps stop condensation dripping. Don't forget with clear plastic whatever you put on is going to have to survive the fierce Scottish sun.

I wouldn't bother with anti-condensation paint, it's basically a layer of microbead insulation <<1mm thick, hard to imagine it's worthwhile in most cases.

It's ventilation you need. Vents top and bottom so convection can clear the warm wet air out.

jk

In reply to Tobes:

When your garage cools down it's got to condense somewhere.

Clearly your roof cools first.

Any attempt to insulate the roof will just mean all your walls and maybe your stuff will get wet instead.

Your options are:

• Ventilation
• Insulate it so well that the temperature doesn't fluctuate (not practical I expect)
• Take out a mortgage and heat it
• Dehumidifier (if not well ventilated)

In reply to Tobes:

Spray-foam insulation is an option. But it depends on your budget because it doesn't come cheap. Another option is a dehumidifier, if the airflow through the place is low enough for it to work effectively. Obvious the dehumidifier comes with some disadvantages too, mainly the cost and environmental implications of running it all the time.

In reply to montyjohn:

Good analysis.  It strikes me my car on the drive is dripping wet from condensation, and would be less so in a garage.  Ventilation brings you closer to outside conditions, so not necessarily the right answer. Damp air is hopefully the only way water vapour gets in there.

An extractor fan that only runs when garage humidity is greater than outside humidity.

In reply to David Riley:

In reply to David Riley:

Remember that humidity as we describe it is... relative. Warm air can cope with higher vapour pressure before reaching saturation. If the garage is warm and has an RH of say 50%, and outside it's freezing with an RH of 80% the internal vapour pressure would still be higher than the outside. Therefore replacing the warm wet air with cold outdoor air would be better, as overall there would be less moisture in the air.

In reply to Tobes:

If you take the roof off, turn it upside down and then put back on, the condensation will drip off into the outside world. 

In reply to NobleStone:

Yes I understand that.  But what is your point ?

In reply to David Riley:

I think NobleStone is saying that your statement below is wrong:

I know what you mean. But what you don't want to do is fill your garage with warm air, even if the humidity is lower than the inside air since warm air can hold so much water, it's water content may still be higher so you would be better not to run the fan. 

What you want is to only run the fan when the amount of water in a cubic foot is less outside than inside and humidity is not the way to do this, unless you factor in temperature. Which is what you meant. Just being pedantic.

A simpler approach, and probably right most of the time is just to run an extractor fan at night. Then when the garage warms up through the day, the humidity will quickly drop.

In reply to Tobes:

What is the source of all the moisture?

I have a single skin garage, full of junk, it's snuff dry.

In reply to montyjohn:

That's pretty much what I meant. And also that maybe everyone's overthinking it. The questions the OP should be asking themselves are: 1. What is the source of all this moisture, can it be reduced? 2. Do I need the garage to be warm? If the only source of moisture is a wet car and you don't need the space to be heated then providing plenty of natural ventilation should be sufficient. 

The OP has admitted that they have inadequate ventilation, that's what they need to sort. Long term having a garage that's built like a greenhouse probably isn't a great idea as everything in there will get too hot in the summer.

In reply to Tobes:

I don't know much about them but you can positive air ventilation units that should run at a cost of only a couple of pence per day.  They are supposed to be the solution to condensation...

https://www.nuaire.co.uk/product-list-page/flatmaster

In reply to montyjohn:

I did not say relative humidity,  anymore than there was reason to mention I am communicating with the humidity sensor using an I2C interface.  The twenty pence processor reads temperature and decides when to turn the fan on only when is is advantageous.   This should be a rare occurrance so the fan uses minimal power and can run from batteries.

In reply to NobleStone:

If warm air is allowed in during the day, then the water will condense in the garage at night.  More water will be transported inside the following day. That is the problem with plain ventilation.

In reply to David Riley:

You're overthinking it.

Water gets into garage (floor, wet stuff, leaks, permeation), when it's warm during the day most of it can exist in gas phase, when cool during the night and bad weather some of it condenses to liquid and drips doing damage. You can heat it to stop it condensing out or try to stop it dripping when it does or you can get rid of it either by mopping or with a dehumidifier or by letting it blow away in gas phase during the day. The latter is the easiest and free.

Ventilate, it works a treat.

jk

In reply to jkarran:

You're not providing a solution.

A dehumidifier in a ventilated garage ?

Heating a ventilated garage ?

Mopping ?

Hoping it blows away in the day ? 

If you fully ventilate a garage, then your car will be just as wet with condensation in the morning as it is standing outside, except there's no sun or wind to dry it.

In reply to David Riley:

I quite literally provided a list of them. Some more practical and affordable than others but all solutions. I've clearly stated which one is the good idea for dealing with condensation in an unheated space. To be clear, again, it's ventilation.

People have suggested dehumidifiers. Since the OP's garage is described as poorly ventilated it would work. It's a waste of money IMO but people like seeing the liquid water come out of a space and they do work.

The garage described as poorly ventilated. I don't think it's a good idea because of cost but heat is one option. Still, unless there is trickle ventilation or it's kept silly hot it will still condense on uninsulated surfaces.

Yes, mopping. What the OP is actually doing currently to remove water from the space.

Not hoping, cutting holes so it is removed by convection and or the wind. This is what we call ventilation, it keeps unheated weathertight spaces nice and dry.

Who keeps a car in a garage? Actually, maybe you should*, if it's wet inside, keep the rain out and ventilate it so it can dry.

*and me, my old convertible is growing mushrooms under a cover on the drive

jk

In reply to jkarran:

If water is going in and condensing in the garage to begin with,  then increasing the ventilation is going to make it worse.  With the ultimate open car port not helping at all.,

In reply to David Riley:

As someone who keeps my bike locked in a well ventilated, unheated structure (a shed), I can confirm I don't find dew on my bike in the morning.

In reply to NobleStone:

If you increased the ventilation, a lot, maybe leave the door open, or even remove a wall, then you probably would.  More ventilation can make things worse.

In reply to David Riley:

As someone who has a wooden glider which has lived nearly 50 years bone dry outdoors in a single skin aluminium tube I can assure you ventilation does not make unheated spaces wet.

10/10 for doubling down on the nonsense though!

FYI your car condenses not because it is outside but because there is water trapped inside it, it's an unheated, poorly ventilated space. Keep the rain off, crack the windows to let the vapour out and after a few cycles of sunshine and a bit of a breeze it'll be bone dry. Try it.

jk

In reply to David Riley:  > If you fully ventilate a garage, then your car will be just as wet with condensation in the morning as it is standing outside, except there's no sun or wind to dry it. 

Completely wrong. My garage is open to the outside air, no big doors or side door, and it's contents are bone dry, in all weather's.

In reply to Maggot:

Mine too, there's a gap I could squeeze through at the front and the back door is usually open because I'm a slob. The roof leaks, badly. The floor, and timber are dry year round unless it's actively raining and leaking or has recently rained.

This is cringeworthy.

jk

In reply to jkarran:

A dehumidifier is a box with ventilation with a cold thing in it.  It soon fills with water.

In reply to David Riley:

Stop the water by losing the cold thing, or the ventilation.

In reply to David Riley:

As you breathe in, cherish yourself. As you breathe out, cherish all beings.

In reply to Tobes:

Amazing! wasn't expecting this much interest, will have a good read and reply to some of you. Thanks again! 

In reply to Tobes:

I didn't comment on your particular case, since it's difficult to know exactly what the problem is.  It's certainly true that many people have completely dry garages, with or without ventilation.

Worst case would be a thin roof in the rain and strong wind.  That would chill the roof on the inside below ambient.  The humidity when raining would be 100%.  Under which conditions the only way to stop condensation, would be to stop any ventilation.  Which was the effect of system I proposed.  But I don't think it's commercially available yet.   So I'm sorry,  not much use to you, except as an idea.

In reply to Tobes:

To add some further info and answer a few general questions - The garage is opposite our property (other side of our single access road) there is no power to it (would need a cable run from the house but not practical). Our car is not stored in it at anytime, it's mostly full of a few bikes, horse riding gear, fishing gear and the usual rubbish. Walls are thinnish concrete type sections, floor is concrete, double door is timber, roof has timber beams that support the single corrugated clear plastic roof (rain water runs off fine into the gutter which leads to a down pipe into a large water butt (this is always kept half full to avoid it backing up) - it has a number of windows but nothing that opens. There is very little ventilation unless we open up one of the doors (which we do most days) and mop the inside of the roof (which we do every few days) - this is a seasonal issue but come spring we'd like to get it sorted so next winter we can leave it without the faff of mopping etc (and potential damage to the contents) - the source of the water is condensation so like today (2 degrees but bright sun) that condensation starts to melt - I think ventilation is the crux/solution here (and the cheapest fix!) so perhaps might knock out a couple of the windows and replace with ....?  We also have a large bag of road salt/grit - I wonder if making a basic salt box would draw in some of the moisture also? 

keep 'em coming!

In reply to jkarran:

Cheers jk, as I've mentioned above I think you've got the measure of the set up and make the 'ventilation' point well. On the flip side you're spot on about the heat in summer too - without leaving the doors open it was like a furnace this year! 

Ventilation it is then ; ) 

In reply to David Riley:

Ventilation won't make it worse.

The reason is it will get cold outside before it gets cold in the garage.

This means when ventilating when the garage is cooling you're bringing in the cold air from outside which will be dry.

Therefore no dew and no drippy roof.

In reply to Tobes:

I have a portacabin, and have decided it is less damp with the windows closed,  than open.

Rather than open the doors manually on dry days, it would be better to have something that ventilates more scientifically and automatically.

In reply to Tobes:

If you're mopping out the garage you are removing water. If you then get condensation forming *more* water / vapour has got in in the meantime. Where can this be coming from?

Our garage is similar (single wall of brick with timber / felt roof, not attached to the house) and it's fine (well it's full of crap, but it's dry)

In reply to montyjohn:

We are at cross purposes.  If the garage is full of the warm air of the day and cools at night with no ventilation, then lack of ventilation is the problem.  Although there must be some ventilation for the damp air to get in in the first place.  Hence my desire only to admit air when dry.

If metal items are left outside, dew forms on them.  This doesn't usually happen inside a garage because it is enclosed. The more you open it up, the more likely dew forms and things rust.  So too much ventilation is worse.

In reply to Tobes:

You could put some soffit vents top and bottom of the door(s). Some in the wall opposite will help wind drive a through flow but that means drilling concrete. You probably have a soft foam filler piece under the corrugated roof sheets to seal it to the walls. You could cut the odd bit out of that top and bottom of the roof. 

Jk

In reply to Andy Hardy:

is your roof flat/angled to allow run off or an A frame type? this might be the difference also our roof itself is a single skin of clear/see through corrugated plastic (no felt, treatments or other material attached) - what's your roof made up of, felt plus what else (another material would make it 'not' a single skin if that's the case) which if so could be the reason you're not suffering condensation? 

In reply to montyjohn:

"Any attempt to insulate the roof will just mean all your walls and maybe your stuff will get wet instead."

Possibly not there are many factors in play here. Likely the moisture on the roof isn't actually lowering the humidity much as its reevaporating back in to the garage daily so I wouldn't expect a big change in the walls with the roof insulated. Remember its all about dew point and relative humidity. insulating the roof may make the garage warmer but the relative humudity will lower with a rise in temp and the actual water content of the air in the garage won't change.

Probably the way to go is first try ventilation IMO.

Is the garage joined on to the house? what about entrances any in to the house? Any moisture sources in the garage like tumble driers etc?

Any good reasons why the garage shouldn't be close to ambient outdoor temps? (with vents)

In reply to David Riley:

The humidity is 100% and the roof gets chilled below the ambient temperature by evaporative cooling?  Seems legit.

In reply to David Riley:

The outdoor humidity when raining may well be close to 100%, but inside air is normally warmer. Take a volume of air at 100% relative humidity which is saturation point for air and warm it and the relative humidity lowers do the opposite and water condenses.

Commonly people make this mistake and think that venting your house when its cold and high humidity outside won't lower the indoor humidity but if its quite high humidity indoors and warm thats often wrong.

Its why his garage has condensation on the roof as at night the garage will be warmer than outside (possibly all the time if it has a heat source like a fridge or the house) but the roof is almost at ambient temperature and so is the air close to it so the water vapor in the air condenses. 

In reply to Tobes:

Our roof is flat timber (tongue and groove) with a waterproof membrane on top.

I think the reason we have no condensation is there's nothing in the garage adding water to the air in there!

In reply to Andy Hardy:

you mean you roof is wood with a felt membrane? that's quite an improvement on our CLEAR/transparent/see thru single sheet of plastic then, there's nothing in the garage that is wet (causing the condensation) it is 'most' likely the fact it's a single sheet of plastic, have I mentioned that already ; ) with no additional felt/membrane skin in a poorly ventilated space in the NE of Scotland. 

In reply to Tobes:

Had a similar situation. I pinned wall insulation boards between the wood supports immediately under the plastic corrugated sheets ie no gap. Solved my problem

In reply to Tobes:

Ventilation is your best bet,

However some of that foil insulation taped to the roof may help reflect some of the sun back (you said clear plastic roof). This will help to stop the garage getting so warm during a sunny day ( and increasing the capacity for the air to hold water). 

In reply to Tobes:

Some of these, a bit of sand, and a little bag of cement, maybe a plastic bucket while you're in the shop.

https://www.wickes.co.uk/Wickes-Square-Hole-Clay-Airbrick---215mm-x-65mm/p/...

In reply to Tobes:

An unventilated roof space in any ordinary house will suffer from damaging condensation issues, even if insulated, because warmer airborne moisture has nowhere else to go but to accumulate on cold surfaces. The solution here is to install ventilation holes at soffit height.

Similarly with your garage, except the sun will heat up the interior through the plastic roof to higher than ambient (like a greenhouse). Your garage is acting like the roof space described above. The warmer air inside holds a lot more moisture, which will then accumulate as condensation on surfaces that have cooled down at night, like the inside of your plastic roof.

Ventilation holes will make the garage colder in winter, but (because there will be fewer interior/exterior temperature variations and therefore water-holding capacity), it is much more likely to be free of condensation.

In reply to Tobes:

Some observations from my own garage which might help. It has double skin walls, single glazed windows, tiled roof with felt, unheated, unventilated but with some gaps around the up-and-over door.

• No condensation in roof.
• Condensation inside windows.
• Condensation on metal door.

Make of that what you will!