High Humidity...dehumidifier or vents??

[antihero]

Hi Guys,
I know its been done to death but...
I gotta 8 x 15 timber shed/workshop with good insulation but I got humidity readings in the high 80's.Have done for the last couple of months.
Whats the best way of dealing with this,dehumidifier?......some people say thats pointless unless you completely seal your area....not really an option as you gotta open and close your door??right??
Also been told to add vents to the shed......can't understand how that'll help as I already have a healthy 1-2 inch gap at the bottom of my door.
Surely with the damp weather we've been having all vents will do in is pull damp air in and out.
I don't heat the shed unless its Baltic even then only until I heat up then I switch it off(oil filled radiator).
I'm not getting any rust problems at moment but I do build and repair guitars in my spare time so this high humidity is of concern.
Any thoughts most welcome.
Cheers.

 

[Digit]

A dehumidifier will both reduce the humidity and add some warmth to the shop as opposed to cold air ventilation at this time of the year.

Roy.

 

[ike]

If you can afford it, get a split air conditioner. Up to 3kw heat output (or 18,000BTU in cooling mode) for 1kw power input. Also works as a dehumidifier.

I self-installed one two years ago. I spend many more hours in a toasty workshop through the winter than I ever did before.

My workshop is 3.5m x 7.5m x 3m - costs around 15p/hr to heat.

 

[agbagb]

This may sound strange but it could be all the insulation that is the problem, if as you say it hardly ever heated.

It will stay cold in morning when its warming up outside. Warmer air outside will pick up moisture and blow in under your door. Its then going to cool down in there and deposit the moisture.

Check the internal and external temperatures in the mornings.

A solution could be to give it a quick blast with the radiator on a timer in the morning. Just enough to make sure the inside isn't the first place moist air can get to it's dew point.

Could be worth a try first?

 

[MikeG.]

Time for a bit of analysis before you rush off to the solution.

Given that the air outside is rarely at 80% humidity, there must be some reason why you are getting such readings inside. There is damp getting in.

What is the construction of the shed? What are the ground levels like inside compared with outside? Does anything leak? Have you got stored water inside? What are the activitie and occupancy rates of the shed? If you aren't getting rust, are you really sure that you are actually getting 80% humidity (which I would expect to rust everything in sight)?

I hate de-humidifiers. I'll say that now for the avoidance of doubt. They simply mask a poorly designed/ built/ detailed building. Get the building right and you simply won't need one. To my mind, they are for emergencies only. Furthermore, they are hideously wasteful of energy.

If the air outside is less than 80% humidity (which it is almost all of the time), moving that air through your shed will dry your building down to the ambient humidity levels.

So, lots of questions to answer and thinking to be done before you go off and buy a fridge without a door...... the essence of what a dehumidifier is.

Mike

 

[agbagb]

I agree with Mike outside in winter the humidity levels shouldn't be 80. But take a reading at a ground level ground level in the morning (when you get dew on the grass ?). So if the outside air at say 10 degrees C with 50% moisture. Then blow it in to a cold room cooling it to 0 degrees C. The relative moisture would be about 100%.

The case in point here could be somewhere in between.

Agreed more investigation is needed.

 

[Digit]

This is a question before someone jumps all over me!
How much moisture does an adult male exhale in a given time period, would that account in part for the high readings?

Roy.

 

[RogerS]

This is a question before someone jumps all over me!
How much moisture does an adult male exhale in a given time period, would that account in part for the high readings?

Roy.

Depends what you're getting up to, Roy :wink:

Mike....what would you say was an acceptable humidity range in a workshop?

 

[Digit]

Not in my woodwork shop Rog! :lol:

Roy.

 

[agbagb]

Good point, it depends on how hard you're breathing. And in the cold you can see it so 100+ % humidity. However to raise the overall humidity the ventilation would have to be less than the amount you are breathing and not condesing on cold surfaces.

I supose you have to be careful not to breath on the meter when taking the readings.

 

[Sgian Dubh]

... the air outside is rarely at 80% humidity, Mike

Where are you getting your figures from Mike? Outside air in the UK is quite commonly at an average daily RH of 80% or more. My own readings tell me that average daily external RH values exceed 80% from mid-October to the end of February most years-- the number is actually closer to 85% than 80% in December and January. From about the end of February the values gradually lower to just over 70% RH in July and August. I am in Leeds, West Yorkshire.

Some time ago I sourced daily average external RH readings from the Met Office for London gathered over the period from 1955 to 1990 (I think without checking). Their records show a similar RH pattern to my own readings. I imagine readings from a coastal location, particularly on the west coast, will show higher numbers than either of my sets.

The accuracy of the figures I collected, sourced and referenced were important to me because they are used to illustrate points in an academic manuscript on timber technology I've nearly completed. Slainte.

 

[antihero]

Thanks for the replys guys.
I'll take a few readings in the morning and let you know what i find but I can tell you were I live (North West,N.Ireland) the humidity is bad most of the time.
Check this out for readings RH for Belfast and I live much further to the North West coast.
http://www.bbc.co.uk/weather/world/city ... t=TT003750

If it is the case that my outside RH is about the same is a dehumidifier my only option??

Thanks again.

 

[antihero]

Just checked the weather forecast for my area tomorrow,heres the RH predictions....
9.00 a.m - 84%
12.00 a.m. -78%
15.00 - 79%
18.00 - 94%
21.00 - 96%

Me thinks I live in a very damp area!!

 

[MikeG.]

... the air outside is rarely at 80% humidity, Mike

Where are you getting your figures from Mike? Outside air in the UK is quite commonly at an average daily RH of 80% or more.

This is what happens when you rely on your memory rather than checking facts. You are, of course, completely right. In my part of the UK, average relative humidity is at or just over 80% for about 4 or 5 months of the year (well below that in the middle of summer, at its highest in the middle of winter).

That doesn't change the principles of my argument, though.

I haven't time to write an essay this morning, but will simply say that if you choose to install an expensive and power hungry de-humidifier, you must seal up the shed completely, otherwise you will be trying to remove the moisture from the atmosphere of all of the UK. A cheaper and better solution could be a low level vent at one end of the workshop, and a high level bathroom-type extractor fan on a timer at the opposite end.


Or you could raise the temperature a tad by linking, say, a 100 watt light to a thermostat set to, say 8 or 10 degrees.

None of this obviates the need to check that your structure isn't damp. If you have a concrete floor, for instance, lay a sheet of plastic on it for a few days.

Mike

 

[woodbloke]

Could someone please elaborate to a numpty(me :roll: ) on this sort of stuff? I thought RH was higher in the summer rather than winter as warm air can hold more moisture :?
My 'shop is sealed and insulated and I have no problems whatsoever with rusticles on anything. It is dehumidified though (set on a low setting) 24/7 in the summer and a timer in the winter - Rob

 

[MikeG.]

Rob,

relative humidity is the amount of water vapour that the air is holding compared with how much it can (max.) hold at that temperature.

Absolute humidity better describes what you are talking about. One can easily have high absolute levels of humidity but quite low relative humidity levels, if it is very hot..........and low absolute levels yet high relative levels if it is very cold.

Relative humidity is the important measurement, because it better describes how close you are to getting condensation.

Mike

 

[Sgian Dubh]

... That doesn't change the principles of my argument, though. Mike

Mike, I wasn't questioning your other points. They are out of my area of expertise anyway; but I was curious about the RH numbers and where you got them from as they didn't match what I know. Slainte.

 

[agbagb]

I think my head has been clouded by relative and absolute as well. I know what they mean but now I'm not sure of the effects.

Am I right, Absolute Humidity is often higher in Summer (e.g. on a hot day after rain)?

Timber acclimatises to the surrounding conditions, would that be RH or absolute?

How does temperature affect the moisture level in timber?

I's it better to warm the wood and not the air? Is Infra red heating going to be more efficient than convection?

 

[MikeG.]

... That doesn't change the principles of my argument, though. Mike

Mike, I wasn't questioning your other points. Slainte.

No, no I knew that! That comment was for the original poster..

Sorry!

Mike

 

[MikeG.]

Am I right, Absolute Humidity is often higher in Summer (e.g. on a hot day after rain)?

Timber acclimatises to the surrounding conditions, would that be RH or absolute?

How does temperature affect the moisture level in timber?

I's it better to warm the wood and not the air? Is Infra red heating going to be more efficient than convection?

AH can be higher in the summer, certainly, but as I said, I think RH is the more important figure because it reflects the likelihood of condensation occuring.

We are getting to the limits of my expertise here, but my instinct is to keep the building environment as equable as possible, and to condition the wood in the normal way (in the final resting place environment for a few weeks). If you were to warm the wood in your workshop differentially to the air around it that could possibly work, but could also possibly set up all sorts of undesirable moisture movements.

If in doubt, keep things simple!

Mike