High Humidity...dehumidifier or vents??

[agbagb]

If in doubt, keep things simple!

I like simplicity as well. Conditioning in the final resting place is best.

Problem though. The wood then spends 2 months in the workshop being built in to something, (I'm slow).

Unfortunately the "simple" solution that springs to mind, for most folks, is a dehumidifier.

Maybe if we had more knowledge (Slainte. will your paper help?) we can come up with easy, practical, cheap, low energy solution.

 

[Sgian Dubh]

I think my head has been clouded by relative and absolute as well.

Atmospheric humidity refers in a non-specific way to water in the form of water vapour in air. Absolute humidity (AH) describes the actual amount of moisture vapour in the air-- another name for absolute humidity is vapour pressure. When air is fully saturated with the maximum possible water vapour it can hold at any given temperature, it is at saturation vapour pressure. Air that has a vapour pressure below the saturation vapour pressure is able to carry more moisture.

Relative humidity therefore is the ratio of the amount of moisture in the air to the maximum amount of moisture that the air can carry at a given temperature X 100, which describes RH as a percentage of AH.

Warm air can carry more water vapour than cool air. For example, 1 cubic metre of air at 35ºC can carry a maximum of 39.54 grammes of water vapour-- the air is fully saturated, ie, it is at saturation vapour pressure: the relative humidity (RH) is 100% and the vapour pressure or absolute humidity (AH) is 39.54 grammes of water vapour. For comparison one cubic metre of air at 15°C can only hold 12.82 grammes of water vapour.

Dew point is the temperature at which water vapour condenses out of air. For example, if fully saturated air cools by even 0.1ºC the gaseous water in it starts to condense into liquid water. This is essentially what happens when the bathroom mirror ‘steams up’ as you take a shower. In this case warm, saturated air hits colder glass, it cools, and this causes the water vapour in it to condense on to the glass.

In the UK house interiors experience lower RH values in the winter than they do in the summer. This is because we heat our houses in the winter. At this time of year fresh air that enters the building through vents, and natural cracks and crevices, is comparatively cold.

Cold air cannot carry as much water vapour (WV) as warm air. Air at 5ºC can carry a maximum of 6.79 grammes of WV per m³. Outside air at 5ºC fully saturated with 6.79 grammes of WV/m³ brought into a house and then heated to 20ºC without adding any more water vapour to that package of air results in an RH of 39.29%. At 20ºC air can hold a maximum of 17.28 grammes/m³ WV. To work out the RH value of the heated air calculate, Absolute Humidity of the air / Fully Saturated level of the heated air X 100. For the parcel of air just described therefore we ascertain the RH percentage as follows: 6.79/17.28 X 100 = 39.29% RH.

In British summers internal RH values are higher than winter RH values. Temperatures are warmer and the outside air is able to carry quite large amounts of water vapour (WV) leading to raised AH values. Open windows and doors let a breeze through the house which brings in water vapour and causes relative humidity within buildings to more closely match external relative humidity.

The above isn't a full explanation, but hopefully you can get the general gist of the subject. Slainte.

 

[agbagb]

Wow, thanks, proper knowledge. I'll let that info sink in for while.

Meanwhile, does anyone know how timber moisture content relates to RH and AH.

I'm thinking wood in modern house 8 - 12%? RH in the house in winter Circa 40%? or is it the AH that matters.

Cheers
Andy

 

[Sgian Dubh]

... does anyone know how timber moisture content relates to RH and AH.
Andy

Well, I do in some depth. Unfortunately there isn't the space here to describe all that you need to know in a short post. I guess you'll have to wait for the book on timber technology I'm working on. But a snapshot is as follows:

30% RH = 6% wood EMC (equilibrium moisture content)
50% RH = 9% wood EMC
70% RH = 13.5% wood EMC
90% RH = 20% wood EMC

These are typical numbers at 20ºC for most wood species. Higher temperatures lead to lower wood MC percentages and lower temperatures cause higher wood MC percentages. Wood density is also a factor that causes variations, as does the wood's structure.

Also bear in mind that to achieve EMC wood needs to stay in unvarying RH and temperature condition for considerable time. This never occurs in normal circumstances because RH varies diurnally, hourly, and even by the minute and, due to one form of hysteresis, wood does not immediately react to changes in RH. It changes slowly over time.

Further, wood is seldom the same MC all through and in normal use it has a moisture gradient within it, eg, it might be 14% MC at the core and 10% MC at the shell with an average MC therefore of about 12%. This condition might indicate the wood has fairly recently been moved from an area of higher average RH to an atmosphere that has a lower average RH condition, eg, it's fairly recently been moved from an unheated storage shed to a drier, insulated, warmer workshop.

AH is not really relevant to the situations described above-- it's only relevance is as a function for understanding the subject of atmospheric humidity as a whole, and I have missed out significant factors from this post for the sake of brevity and simplicity. Slainte.

 

[antihero]

Thanks for all your replys guys,
me head is spinning a bit with all this RH info.
Took some ground level readings this morning though......
Inside shed was 7.3c/85%
Outside shed was 6.6c/87%.

So,obviously I'd rather not go down the rather expensive way of a dehumidifier,but just can't see no other way of controlling the humidity.
Adding vents just seems to me to be pointless with outside RH being what it is(or am I missing something?)

Thanks again guys.......although still not sure what my best option would be! :?

 

[MikeG.]

Adding vents just seems to me to be pointless with outside RH being what it is(or am I missing something?)

Yep, you are missing somethinjg!

I described a system that will work.....the forced cross ventilation with a low level vent and a high level fan. Put it this way, it is about a £20 gamble if you buy new, and maybe a lot less than that with stuff off fleabay.

Think of mist. That only exists when the air is still. As soon as a breeze picks up, it disappears. It doesn't get moved somewhere else, it just disappears. I don't know the physics of it, but moving air dries stuff. You can dry clothes on a clothes line in the middle of winter, so long as there is a breeze.

This is one of the fundamentals of building design......ventilation. It works! It won't work just by having a gap under your door, because where air needs to move it needs an "in" and and "out" path......so if it is going to go out under the door it needs to come in from somewhere else.

I don't want to do the old "trust me, I'm an architect" routine.......but even without seeing your exact circumstances I would be very confident that you will improve matters by following the simple scheme I suggest.

Mike

 

[agbagb]

Thanks Slainte.
That's a combrehensive enough answer for me now. Let us know when the book is out.

Mike, I think a breeze helps drying because: If the air isn't moving around a wet object it will fill up with water vapour to saturation and no more will vaporise. If you blow the saturated air away more can vapourise in to the "fresh" air.

I don't know if you get past some kind of equalibrium of the "fresh" air RH and the dryness of the object though. If that makes sense.

Antihero, That blows my theory that it could be colder inside the shed then.

Another thought, seal the drafts and put a tray of salt out. We used to do this when we over wintered our wooden chalet in Mablethorpe. We never got any damp.

Andy

 

[jimi43]

This has been a really interesting subject for me as I made an early mistake of putting an uninsulated steel roof on my workshop the summer before last and had an actual rainfall inside in winter!

I have since cured this by insulating the cold metal and putting air vents at the top of one wall where it meets the roof and letting cracks on the otherside create the "in" bit...it works...I agree with that theory...

Incidently...I cut a tree down and left the logs in my car the other day...it took me nearly 20 minutes to get the condensation off of the screen in the morning...now it is coated with sap deposit! DON'T leave new logs in your car...especially pine ones!!

Jim

 

[aesmith]

Inside shed was 7.3c/85%
Outside shed was 6.6c/87%.

Under those conditions ventilation on its own is going to do nothing to reduce the RH inside, unless you have some heat as well. 5 deg temperature rise inside compared to out would drop that to 65% or so.

 

[frugal]

This is one of the fundamentals of building design......ventilation. It works! It won't work just by having a gap under your door, because where air needs to move it needs an "in" and and "out" path......so if it is going to go out under the door it needs to come in from somewhere else.

How does this work with insulation? You insulate the workshop to keep the heat in, and then drag cold outside air across the room to keep it dry.

I am not saying that it is not the right thing to do, they just seem contradictory for a beginner to this sort of thing.

 

[MikeG.]

Good question Frugal!

Every building needs ventilation. In a house, I always use mechanical ventilation with a heat exchanger so that the warmth isn't lost with the extracted air. A workshop is generally only in use for a few hours a week. My suggested scheme could easily be turned off for the time that it is used/ heated to overcome the problem you correctly highlighted.

For the 90% of the time that it is unheated and unused, the ventilation will be of huge benefit to the internal environment, without costing anything in terms of heat loss.

Mike

 

[promhandicam]

Although not relevant for the OP, due to the interest shown in this issue, I thought I'd relate my recent experiences with damp. We moved into our current home in July and discovered that there were some serious problems with damp which involved completely replacing the living room floor - the kitchen and bathroom will have to be done next year. The problems stemmed from inadequate ventilation under the suspended floors and water ingress through the few air bricks that had been installed due to a concrete hard standing having been laid up to the outside wall and level with the bottom of the air brick. Although remedial action has been taken, some of the downstairs walls had am MC 80% and the damp was affecting my asthma. After doing a bit of research on the internet I decided to buy thisdehumidifier and can highly recommend it. The dehumidifier has been extracting 6 - 7 litres a day everyday since starting to use it a couple of weeks ago. It has solved the condensation that we were getting on the windows and toilet cistern and my respiration has much improved. So if anyone is in need of a dehumidifier, for £150 I can certainly recommend the one I've linked to above.

Steve

 

[jimi43]

Have you worked out how much it costs to run Steve?

Jim

 

[promhandicam]

Hi Jim.

According to the technical data, the maximum power consumption of the unit is 620W. Therefore if it runs 24/7 at max power in a month it will consume 430KWh. From our supplier this equates to £43. In this case, the unit will extract approximately 200 litres of water.

The actual consumption will obviously depend on the RH of the property - the unit has a humidistat which on Auto setting turns the machine off when the RH reaches 55 - 60%. We have had it running for 2 weeks now and it has been running more or less constantly although in the last few days I have noticed it is starting to cut out as the RH obviously starts to get down to a reasonable level.

HTH,

Steve

 

[antihero]

OK guys,I seem to have really touched on apoint that interests people.
So,this is what I'm gonna do.
Install an extractor van and vent,that will hopefully keep things resonable when I'm not in there.Then possibly get a small dehumidifier and use it when working,if needed.
Would really like to keep the RH down to 50% while I'm working.
Thanks for all your help on this!
Cheers.

 

[MikeG.]

Good decision............but don't do both at the same time!

Mike

 

[antihero]

Something like this should be the ticket,what you think Mike?
http://cgi.ebay.co.uk/EXTRACTOR-FAN-KIT ... 2303fb3a5d

 

[MikeG.]

Yep, that's the sort of thing.......although I am not sure why it is low voltage.

Mike

 

[jimi43]

WOW that ain't cheap for an average few hours a day merchant! No wonder my leccie bill went up so high!

I must admit I could actually "smell" the dryness...or rather NOT smell the humidity when I did run it...but I felt (as was mentioned earlier) that I was dehumidifying Kent...and probably parts of the North Sea as well...so in a relatively sealed room like a guest room that would work but my shop was not that tight.

This is a really interesting subject though!

I had a discussion with an insulation engineer the other day about the very same thing and my steel roof and he said to put a false ceiling of silver backed plasterboard in...with air cavity openings each end in the cavity and then put a thick layer of insulation on top of the false ceiling...mmmm

More research needed I think!

Jim

 

[Digit]

That's a coventional 'cold roof' Jim. Works ok but you could lose a fair bit of head height, depending on how much insulation you install of course.

Roy.