Insulation and condensation.

[frugal]

There has been a lot of discussion about workshop insulation on these forums, but I do not think I have seen a discussion about how insulation affects condensation.

At the moment I have a workshop that is 20'x12' and made from concrete blocks with a roof of brick tiles. The workshop is about 10' away from the rest of the house and it is currently unheated.

One of my concerns is that when it gets cold there is a slight build up of rust on the cast iron tables . If I insulate the workshop will this help the condensation, or do I need to keep the workshop above a certain temperature to keep down the condensation and the insulation will just make that process cheaper?

Given that my back of an envelope calculations reckon it will cost about £1,000 to insulate the walls and roof (Kingspan 50mm, battens and 10m OSB to cover), am I better off spending this money to insulate, or am I better off spending the money on a table saw and trying to get some kind of anti-corrosion coating for the tools?

 

[DangerousDave]

Frugal, I think that condensation is caused when moisture laden air comes into contact with a surface (e.g. cast iron tables) that is cold enough for the moisture to condense back into water droplets. So to stop it you could either a) reduce the amount of moisture in the air (de-humidify) or b) keep the workshop (and therefore the cast iron surfaces) at a temperature above the point at which moisture will condense. The idea of insulation is to minimize heat loss so that it doesn't cost as much/waste as much energy... :? So it really only makes sense to insulate if you are going to have some form of heat in the shop. There would need to be constant low level heating to keep the cast iron surfaces above the dew point of the air and insulation would certainly help you achieve this more cost effectively. I think the idea of a vapour barrier on the warm side of the insulation is to create a cold zone and a warm zone and to prevent cold air from entering the warm zone. I think its important as well to ensure the cold zone is well ventilated to prevent condensation there. Thats my understanding of it as very much a lay-person :? I'm sure someone who knows more about it (e.g. Mike Garnham) could explain it much better
HTH
Dave

Oh, BTW I don't do any of this in my workshop; insulation and vapour barrier installed in roof, but heating only on when I'm in it. Therefore I get condensation (and rust) on iron surfaces. Clean 'em with WD40 and slap on some Liberon lubricating wax, usually does the trick

 

[MikeG.]

That is a pretty fair summary by Dave......rather than moisture laden air (all air carries moisture.......lots of it) I would say air near its dew point. The moisture carrying capability of air varies with temperature, so cold air, which can't hold so much water as when it was previously warm, needs to lose some.

In nature, you get condensation (dew) on cold still nights. You don't get dew if there is a gale blowing, nor if it is warm........all you need to do is replicate those conditions in your shed.

I have a sore eye atm so won't be typing much, except to say if you insulate properly you can easily achieve just enough warmth to keep condensation away from your tools by running a couple (max.) of 100W light bulbs to a thermostat. It really doesn't need much.......

And, ventilation is critical!

A little kitchen extract fan with a humidistat.........£15 or £20, will prevent you ever worrying about your tools again. Me.......I just have a poorly fitting door, and let the wind do it for me!

Mike

 

[adzeman]

Hi Frugal. Condensation problems is how I earn a living what you have read so far is sound advice but I just want to underline some basics.
1. Condensation is caused by temperature difference at any temperature and what is called dew point is reached.
2. You don't get dew when its windy. great clue, keep the air moving the best cure there is. Keep changing the air put in a fan.
3. Do not use a de-humidifier unless you have an air lock and plenty of money otherwise you will be attempting to de-humidify the UK.
4. Insulating your workshop will reduce condensation because it reduces the temperature difference, but you will require a vapour barrier on the external face as condensation will form on this surface.

remember KEEP THE AIR MOVING!

ONE PERSON GIVES OFF 3 PINTS OF WATER A DAY so when in the shop open a window.

Regards

 

[tnimble]

About all that there's to say has been said already.

I have a sore eye atm so won't be typing much, except to say if you insulate properly you can easily achieve just enough warmth to keep condensation away from your tools by running a couple (max.) of 100W light bulbs to a thermostat. It really doesn't need much.......

Yet another good application for incandescent bulbs.

I think the idea of a vapour barrier on the warm side of the insulation is to create a cold zone and a warm zone and to prevent cold air from entering the warm zone.

Not exactly. Air has a maximum amount of water it can contain, the amount depends on a couple of things, the most important one being the temperature of the air. As one side of the insulation is warm and the other is cold the insulation has a temperature in btween these two temperatures.

Without the vapour barrier the warm air that can contain very high amounts of water will start to condensate inside the insulation. When the insulation is wet it insulates less. The insulation will be more cold and will have the air condensate even more inside the insulation.

I think its important as well to ensure the cold zone is well ventilated to prevent condensation there. Thats my understanding of it as very much a lay-person :?

For good performing insulation there should be no space between the outer wall (cold side) and the insulation as this lowers the effectiveness of the insulation. Heat is lost through the circulating air (convection) in the space. When this space is ventilated the effectiveness of the insulation is further decreased. The insulation itself will 'blow through'. Almost all insulation works by trapping tiny pockets of air. Air will start to move inside the insulation by the differential pressure caused by the ventilation.

This said some insulation materials in combination with some wall materials do require a small gap between the outer wall and the insulation to prevent contact problems (due to cohesion properties and capillary motion). EPS, PU and PIR are examples of such materials. This space doen't have to be a couple of inches a couple of mm will suffice.

 

[Maverick.uk]

but you will require a vapour barrier on the external face as condensation will form on this surface.

Can you clarify this please, i understood the following,

Vapour barrier goes to the warm side...... inside in cold climates,
outward in warm climates.


Thanks

Mav

 

[MikeG.]

I am going to clarify this again!!

If you have a vapour barrier, it must go on the warm side of the wall.......ie the inside. The insulation must be outside the vapour barrier.

Anything else risks leaving your structure prone to condensation, rot, mould growth etc. There should be a "sticky" on the golden rules of shed building!!!! If you line the inside of your shed with OSB, ply or similar, you probably don't need a vapour barrier at all. Try not to line the outside of your shed with these materials, for fear of trapping moisture within the structure.

Mike

 

[tnimble]

As Mike sais. The outside should always to vapour open.

If you suspect the possibility of rain or winds penetrating the exterior shell use one of speciality liners that are closed for wind and water but vapour open. And make at least tripple fold sure that it is the correct material!

 

[Haldane]

I'm in the process of building a workshop and have followed most of the advise in this thread and others, but the big question I have is how much ventilation and the best place to locate it (the shop is being made from 2x4 framework on a solid concrete base with DPM, the frame raised on a dwarf concrete block wall to keep it away from the damp. The outside will be shiplap, insulation in the frame vapour barrier then 18mm ply on the internal walls. UPVC double glased window 2.4x1.2 total shed size will be 6.2x3.8x2.4 plus a bit more for the roof apex) Would a simple vent at each end of the shed cope? should they be near the top or bottom of the walls? The ventilation makes sense but in winter when you are heating worshop would not too much ventilation be a waste of expensive heat and make the insulation pointless? Is there some sort of calcultion to work out a vent area or is it just trial and error? Or am i just thinking about this to much and creating worries that don't matter that much?

Cheers
Dave

 

[MikeG.]

Dave,

Sounds like you've got the construction right..........I would vent this with a kitchen-type extractor fan. You could get one with a humidistat (I've seen them for less than £20), and mount it high up in one gable.

A low level hit-&-miss vent at the opposite end of the building would be great.

If you are working in your shed with the heating on, then simply turn the extractor off........it'll catch up with your days sweating later on.

Mike

 

[Maverick.uk]

I am going to clarify this again!!

If you have a vapour barrier, it must go on the warm side of the wall.......ie the iniside. The insulation must be outside the vapour barrier.

Oh good, cos thats the way i built mine, got worried for a minute.....

Thanks

Mav

 

[tnimble]

You can culculate the amount of ventilation needed (volume per hour per cubic metre / feet) but for a place like a shed, excuse me workshop, to much trouble. About any standard vent will do just fine. As said a unit with a humidity sensor helps to reduce both electricity usage and heat losses.

A heat reclaiming unit could be used but the savings made on heating the place will not weigh up against the investment and maintenance costs. If the house is already equipped with such a unit you could extends the tubing out to the workshop. But be aware of having good dust control in the workshop and a local filter on the inlet vent. Otherwise you'll run through the units filters in no time.

 

[johnf]

One of my concerns is that when it gets cold there is a slight build up of rust on the cast iron tables . If I insulate the workshop will this help the condensation, or do I need to keep the workshop above a certain temperature to keep down the condensation and the insulation will just make that process cheaper?

I have a insulated workshop solid brick walls insulated internally with celotex and osb the result is no rust problems

 

[MikeG.]

John,

I didn't quite follow your post.....in the first line you were saying that you had a minor rust problem, then in the last you were saying that you hadn't. I wouldn't have thought you would have problems with rust in your insulated workshop........but if you do, try the light-bulb trick as described above, try adding an electric extractor fan, and try chucking a towel over your cast iron machine beds.

Mike

 

[johnf]

Mike
the first part of my post was a quote from Frugals earlier post my workshop is fine

 

[MikeG.]

Ahh..right.......now it makes sense!

Mike

 

[adzeman]

Hi all,
Got into work this morning and thought everything had been said that required to be said but oh no!
Condensation is caused by temperature difference by installing insulation you are reducing that difference but by putting it where you have you have now created a temperature difference on the next face but it dos not matter here only to get rid of the condensation when it happens therefore put in a vapour barrier. Look at timber frame construction.

Remember keep the air moving by a fan one with an electrical motor.

Regards.

 

[Haldane]

I all makes sense now, and I will be putting an airvent fan with adjustable humidisat control the last question is what is the best humidity setting as a % to keep things rust free?

Dave