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Garage Condensation

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Howard Lewis07/01/2019 15:32:27
1924 forum posts
2 photos

Having walls and ceiling/roof (and doors) well insulated is a good starting point to minimise condensation.

A vent at floor level, will allow moist air to escape, since it heavier than dry air.

Doing these things will reduce the work that a dehumidifier has to do.


Bazyle07/01/2019 22:34:04
4533 forum posts
184 photos

Don't have a vent if you have a dehumidifier, unless you want to dehumidify the whole world. You need to work on the seals to reduce the moist air getting in.

duncan webster07/01/2019 22:41:31
2069 forum posts
30 photos

According to **LINK** humid air is lighter than dry air. Makes sense because water vapour is lighter than either oxygen or nitrogen (lower molecular weight). Whatever I think you need vents top and bottom, as the moist air flows out you need to let dry air in. Diagonally opposite would be good. Luxury model would have flaps on the vents so you can close them when you want to heat the room.

There is (or was) a ex RN frigate in a museum in Birkenhead. On one of the dorrs on the bridge it had a notice, 'you can't air condition the whole world, so close the *** door'

Joseph Noci 108/01/2019 05:26:11
476 forum posts
817 photos
Posted by not done it yet on 07/01/2019 14:08:46:

I also disregard the idiot desiccant proponents on u-toob that are not clever enough to realise that simply raising the temperature will lower the RH! That means that ......

Sine the air isn't 'holding' the water, the higher temperatures allow for a higher partial pressure of gaseous water to exist in that air. If there is then a source or supply of water, ie, damp wall, floor, roof, whatever, this then 'warmed' water source will release water molecules to suit the allowed higher partial pressure, ie, RH will increase again after a while, Then a drop in temp can result in condensation. As with most things, a single solution is often not the only one needed - get rid of the damp if you can, else make sure that you also have an effective dehumidifier running, whatever type..


Howard Lewis08/01/2019 12:13:57
1924 forum posts
2 photos

Sorry to disagree, Duncan, but moist air IS heavier, because it is carrying the water. A wet towel is heavier than a dry one. And water runs down not up! Condensation starts at the bottom of a window, not the top.

Put a little water into a glass, and stand it upright. There will be very little water vapour escaping, so the glass will remain wet.

Turn the glass upside down, with the open end over a space, and it will dry as the water vapour falls out.


Matt Harrington08/01/2019 12:48:45
102 forum posts
6 photos

Howard, are you confusing water and water vapour?

I'm no expert but I thought water vapour was lighter than air. Think of clouds and dry air under them.


Joseph Noci 108/01/2019 13:13:48
476 forum posts
817 photos

Sorry Howard, but you lose on this one - Water molecules are lighter than air - water vapour molecules are lighter than nitrogen and oxygen, which make up 99% of our air. So the more 'humid' the air, the lighter it is. Nothing to do with 'wet' air as in wet towels..

Google it if you still doubt..


Darren Shoesmith08/01/2019 15:45:17
2 forum posts

I used to have a small gas heater in my workshop in the winter but the moisture it created was very high. Just use a b&q dehumidifier now and a greenhouse heater and all is dry.

Howard Lewis08/01/2019 17:59:30
1924 forum posts
2 photos

Condensation is water. So I remain unconvinced. Clouds are actually condensed water vapour, or even ice crystals, raised to great heights by convection on currents of warm air.

You are right regarding molecular weight, of water compared to that of air, but as we all know, water falls, under the action of gravity, as it does when it is cooled.

I have yet to see a tumbler containing condensation dry out faster when in the normal vertical position, compared to when it is inverted.

I rest my case.


SillyOldDuffer08/01/2019 18:28:17
4277 forum posts
880 photos
Posted by Howard Lewis on 08/01/2019 17:59:30:

Condensation is water. So I remain unconvinced. Clouds are actually condensed water vapour, or even ice crystals, raised to great heights by convection on currents of warm air.

You are right regarding molecular weight, of water compared to that of air, but as we all know, water falls, under the action of gravity, as it does when it is cooled.

I have yet to see a tumbler containing condensation dry out faster when in the normal vertical position, compared to when it is inverted.

I rest my case.


Well I wrote a carefully researched article for MEW and made the same mistake as you Howard. Not entirely my fault because the book I used wasn't clear. Also, using an Arduino to log temperature, pressure and relative humidity I 'proved' to my own satisfaction that steamy air falls down a stairwell. Sadly, having got the result I expected, I didn't repeat the experiment. This was very careless of me and I got caught - had I tried again, I doubt the result would have been repeated. Something more complicated was going on.

I think the confusion arises because water vapour turns into droplets. Bathroom temperature steam is not pure water vapour. Clouds are light, mist is droplets floating in air, and rain is heavy! Mist can be seen to collect in hollows and valleys when the weather is suitable.

Maths challenge: what's the maximum size of a water droplet that will float in air at 0C? (I don't know the answer!)


John Paton 109/01/2019 00:09:44
153 forum posts
6 photos

s indoors.

Two points - one important and the other maybe of interest:

1.(Important) unless you heat the workshop (or it gathers warmth from the sun and retains it) the insulation will provide little or no benefit.

When we experience prolonged severe cold weather the heavy machinery in the workshop will also become very cold unless heated.

If warm wet weather then moves in swiftly from the Atlantic at the end of the cold spell there will be no mechanism to bring the temperature of your machinery above that of the dew point of the new air mass. Condensation will form on the cold surfaces and indeed ice crystals might form (frost).

Bearing in mind the high probability of a severe cold spell during the next month this is a point worth understanding. The answer is to heat your workshop and the heavier machinery in particular, before the warmer moist air arrives.

A small tubular heater in or under the machine cabinet (or perhaps a conventional high energy 100w lamp) is useful but it may be safer to heat the entire workshop if you also have smaller, precision tools of value. At such times I tend to bring my more valuable small tools indoors.

A small fan to create air movement around the back of cabinets on N or E facing external walls can also be helpful if they are prone to dampness.

If you make moisture in your workshop, have damp walls or floor it is good to ventilate a bit. If not or if using a dehumidifier of any type you are best to keep the workshop areasonably airtight.

Portable gas and paraffin heaters are a no-no as they create moisture.

2. (Unimportant) the business of density of moist air is complicated as it depends also upon the temperature of the air. When water vapour condenses is releases energy which actually warms the air, so as rain drops form the temperature of the surrounding air actually increases at that location and may cause the air to rise while the water drops fall. Whilst this is relavent to rainfall in hilly areas I doubt it has much impact in the average workshop where draughts have a more evident effect!

This is very much simplified but I hope it might help avoid a few tears if we do experience severe cold over the next few weeks.

Howard Lewis11/01/2019 13:15:10
1924 forum posts
2 photos

Don't have dehumidifier. My shop is small but well insulated. Summertime, most work is done with the door open!

Under the bench used for fitting is a small (60W) tubular heater which is switched on during cold spells.

Rust is virtually unknown (Now that IS asking for trouble).

So my solution is to keep things above the dew point.


Phil Whitley11/01/2019 17:51:48
833 forum posts
105 photos

John Paton 1, the insulation forms a barrier to the outer surfaces of the roof and walls, if it was not there, the surface in contact with the outside air would be colder than the rest of the surfaces inside, and therefore condensation would form on that surface. Insulation and positive ventilation cures condensation, because it eliminates the "cold bridge". the OP has condensation forming on the inside of fibre cement sheets which are exposed to the external temperature, and have virtually nil insulation value. insulation will stop the cold bridge, and the condensation without heating. My workshop has three roller shutter doors which are uninsulated and not at all draught proof, although they soon will be, , there is no heating, but it is well insulated. The sun shining on the roller shutter doors, which are black on the outer surface, keeps the worksho temperature slightly higher, even in winter, which would cause any moisture in the air to condense on any cold surface. Insulation means all the surfaces in the workshop, and the machines, are at the same temperature and the result is zero condensation

Mick Henshall11/01/2019 19:58:11
504 forum posts
28 photos

An update--- I ran dehumidifier for a couple lf nights for 8 hours on auto setting and a good gallon of water collected and no condensation visible, then reduced hours still on auto for 4 hours per night about 4 pints of water collected, currently still on 4 hours per night but unit set on low collecting approx a pint overnight. There has been no condensation visible on any surface and even the roof panels appear dry and are a light grey colour instead of the previous black. The garage is unheated and up and over has been insulated around its joints with polyurethane foam , the dehumidifier uses a directional fan as well to move air around, the expense of running seems to be low and currently approx 9 or 10p per day. Have yet to try 2 hours per night to see what happens but well pleased so far. The unit cost £ 177 and well worth the cost.

Thanks for some interesting comments gents


DMB11/01/2019 23:33:30
884 forum posts

My workshop runs approx North - South, so South end cops the sun during the day and West side later, causing the air to warm and rise circulating over to the N/E, replacing slightly colder air heading along lower levels to replace the warmer air which is vacating the South and West sides. That is the only way that I can account for slight rusting on my small mill in the coldest/NE corner. Just caught it in time before any serious harm done. Lathe wasin NW corner and OK, but it did have a plastic cover. The cure was to cover mill with thick plastic sheeting but I installed a small heater in the mill drip tray as a belt and braces job. All this was many years ago and no trouble since. The heater was 4 Edison light bulbs wired in series to keep wattage and cost low but retain heat output from them, gentle glow. They are bayonet bulbs in brass batten holders screwed to the bottom of a trough shaped box to protect the delicate glass bulbs. Brass holders don't 'cook's like the other type. Works a treat and costs peanuts. I have a tubular heater in the Myford drip tray but never switch it on - not needed. The very heavy Fobco Star bench drill down the SE corner enjoys a plastic hat but no heat and is OK. Plywood sheets on walls with space stuffed with fibreglass roof insulation. Roofing felt covers outside of walls for water and draught proofing. Roof inside fitted with polystyrene sheets. Temperature at the moment not bad to work in and even better when mill's motor in use.

DMB11/01/2019 23:40:50
884 forum posts

John Paton 1,

I note your comments about forthcoming cold spell but bit early for Feb - could change again by 2/2.

Let's see if Candlemas on 2/2 is dry and bright, meaning Winter coming back for another fight! Or if wet or dull, worst of winter is over? Maybe it's about as accurate as the superstition about St Swithins 40 days of rain! One thing more certain is better weather is on it's way and damp rusty conditions will soon recede.


ChrisH11/01/2019 23:45:50
816 forum posts
12 photos

John - I wish I shared your confidence!


John Paton 112/01/2019 00:37:46
153 forum posts
6 photos

John and Chris - understood, a risk not a certainty! Risk higher than normal due to recent Sudden Stratospheric Warming that the meteorological computer models have yet to get a firm handle on.

Met Office are suggesting a period of cold however and some computer models showing very cold into early Feb. We shall see.

my ooint being that these conditions would in many cases defeat the use of dust sheets and insulation alone. I keep a small tubular heater in my workshop and use it in cold weather purely for rust prevention.

not done it yet12/01/2019 07:27:32
2910 forum posts
11 photos

Prevention is far better than curing the disease - condensation/rust!

I do not operate by dates, relgious or otherwise. Actual conditions need to be observed. If it looks like being colder or wetter, increase the heating level or increase the humidity control before it occurs.

Start with excessive excessive precautions and reduce to reasonable/adequate levels is better than the other way.

Heating with properly insulated heaters is better than incandescent light bulbs. Running these at lower voltage may be preferable to full power, but remember the only insulation is the easily-broken glass envelope. They are not usually earth protected.

mechman4812/01/2019 11:28:07
2365 forum posts
373 photos

In my garage conversion I used 2 x 2 battens framework infilled with 1" Celotex ( front & back foil covered ) with 1" air gap between walls & insulation, covered with 12mm OSB. The garage door is 20mm insulated panel roll up type with rubber seal on bottom. The roof space is plaster boarded covered with a layer of 'Space blanket' insulation. all this has, so far maintained a constant temp' of + 6 - 8*C in cold weather, even when the outside temp was at - 6*C + during that last bad cold spell.

As & when I need to raise the temp' I have 2 small 3 panel oil filled radiators, thermostatically controlled to keep the temp around 15 -17*C... nice & cosy. Conversely during the heatwave we had last year I had to leave the entry door open to 'warm it up to outside temp' as it was decidedly cooler inside. I have not had any need for a de-humidifier & so far not had any rust problem.


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