How does humidity effect seasoning?

[Remmy122]

Its been in the 100*s here (like most every where it seems) and its way to hot to split wood, so that just leaves me with thinking about wood.

While the actual temp here is 99* there is 41% humidity. How does the high moisture in the air effect the wood seasoning process? speed it up, slow it down? Whats your thoughts?

I'm hoping this punishing heat has some sort of a silver lining and I was just curious if the sweltering heat sucks the moisture out of the wood as quickly as it does me.

 

[CTYank]

If you DAGS on [equilibrium moisture contend] without the brackets, you'll find lots of information and tools (like a calculator, for one.)

Temperature is an important factor. RH is also a factor. Of course, this is at the wood, so wind ventilation is important to optimize these factors at the wood surface.
Right now, 'round here with temps peaking around 100 F, this is prime-drying-time for wood. You can hear it lightening. Almost.

 

[WOODBUTCHER]

The wood is checking real nice here in CT.

WB

 

[jimosufan]

I would say Its a push. Say its 90 with 90% humidty. However when the hot muggy air becomes drier the wood may have enough heat to get rid of moisture....JMO.

 

[Battenkiller]

I would say Its a push. Say its 90 with 90% humidty. However when the hot muggy air becomes drier the wood may have enough heat to get rid of moisture....JMO.

Since a given mass of air always has the same amount of moisture unless you add more to it, heating that air up will lower the relative humidity and speed drying. Higher wood temps also accelerate drying rates. So, as the temps climbs, the wood gets hotter and the air gets drier and the wood dries faster.

Watch some weather sites throughout the day. If you look right now, the RH is a lot higher than it was when it was 105º this afternoon. That's not because water is being added to the air, it's because the cooler air can hold less water, so the RH is higher. When the air gets cool enough during most summer nights, the RH will rise to 100%, at which point water will condense out of the air as dew. No wood drying occurs at all at that time.

 

[Slow1]

Well, I can say that last week when the temps were lower (and relative humidity was also lower) I could hang a load of clothes outside at 8a and it would be dry by about 2pm. Today I hung a load out to dry at 7am and at 8pm it was still damp despite hitting around 95*f and having full sun and a nice breeze most of the day. The high humidity is the factor that kept them from drying of course - now I'm sure the same effect is hitting the wood stacks so my conclusion is that my wood dried more during the same time frame last week than today.

Although I'm quite capable of doing the math to calculate the actual relative humidity etc, the basic experimental evidence here proves the point well enough for me. There are better and worse days of drying - temperature is just one part of it and humidity is another. I've already seen sufficient evidence of the value of air movement and to some degree sunshine. The laundry did dry some though so it was not a fully 'wasted' day, so I'm assuming the wood dried a bit too, just not as much as other days. Likely more than the days when it was raining all day and RH was 100% keeping the surfaces of everything wet. In the long run (i.e. 2-3 years) the wood will dry enough though... at least that is what I've been told

 

[DanCorcoran]

Slow1,

If I understand your experiment, it doesn't conflict with BK's comment. I think you said your laundry dried faster when the relative humidity was lower (last week). That suggests that RH is the controlling factor and temperature merely helps to increase or decrease RH. I'm not sure, but even if it was below 32 degrees F., your laundry would eventually dry if the RH was low enough (as in freeze-drying of food, in a vacuum).

 

[Backwoods Savage]

When the air is as hot as yesterday I don't think the humidity will have much effect at all. And at the 41% humidity and 99 degrees, yesterday was a great drying day.

 

[Woody Stover]

Say its 90 with 90% humidty.

That would yield a heat index of 122*. A couple of places might have seen that briefly during a heat wave, but not most areas. Lately, we've been seeing low to mid 90s and 50-60% humidity during the hottest part of the day. I think some drying is taking place, but I agree...temps in the 90s with RH in the 40s would be kicking serious butt.

 

[spacecowboyIV]

A given amount of air DOES NOT always contain the same amount of water/moisture (if it did the dew point, temperature at which the humidity would be 100% and dew begins to form, would never change).

The two most important factors in drying anything are airflow and humidity with temperature being the third most improtant. The lower the humidity is the faster your wood will dry which is why people in Colorado report that their wood is dry after sitting outside for 4 months in the summer, anyone who has been to the mountain west in summer knows how dry the air is (low humidity). This is also why the first step when treating the air for any industrial drying operation is to cool it down as much as possible so the moisture will condense out of the air and then heat the air back up to the desired drying temperature.

 

[billb3]

I've put too many pool towels up to dry outside in the humid dog days of Summer and had them not dry.
Sometimes for days.
Humidity sucks.
Unless you are mold.

 

[Battenkiller]

Well, today would be a good day to hang my wood on the line. Temps will be in the high 90s, RH in the mid-30s with a fair breeze blowing. Those drying conditions sound almost as good as are found inside the Battenkiln in January and February.

 

[Battenkiller]

A given amount of air DOES NOT always contain the same amount of water/moisture (if it did the dew point, temperature at which the humidity would be 100% and dew begins to form, would never change).

By "given" I meant a volume of air as observed at an instant in time, like if you could take a 1 sq meter cube of air and suddenly enclose it. If you heat that enclosed volume of air up (lets neglect the increase in pressure here), it will still contain the same amount of water as it did when it was cooler, but it will be possible to add a lot more water to it at a higher temperature than you could have at the lower temp. Obviously, cooling that same air causes the RH to rise, as I mentioned later in the post.

 

[Slow1]

Slow1,

If I understand your experiment, it doesn't conflict with BK's comment. I think you said your laundry dried faster when the relative humidity was lower (last week). That suggests that RH is the controlling factor and temperature merely helps to increase or decrease RH. I'm not sure, but even if it was below 32 degrees F., your laundry would eventually dry if the RH was low enough (as in freeze-drying of food, in a vacuum).

Agreed - it is much easier to see the effect with the laundry than with wood as it dries faster overall. My point was to emphasize the point that even with a hot day the wood may not be drying nearly as much as one thinks - even the cooler (70* days) may be out performing in terms of drying days.

So many folks get hung up on the temperature. As others point out - the temperature is a factor in the calculation (the warmer the air, the more water a given volume can hold thus the relative humidity goes down), but just because a body of air is warmer doesn't mean it hasn't already found water somewhere.

I got curious to find out (based on reading other comments after mine yesterday) and found that yesterday's stats for my town were rather dismal for drying:

Temp min/max - 69/89 *f
R Humidity min/max/ave - 62/100/80

With an average RH of 80 and the minimum of 62% (assume that hit when temp was 89*f) that explains the poor drying conditions for me.

So far today doesn't look too good either... At the moment (10:45a) it is 91*f with RH of 70% - it feels very uncomfortable out there. I split up some black birch this morning (8a) and couldn't keep my glasses on due to the sweat flowing off my face. (My biological evaporative cooler was not functioning very well eh?) After 30 minutes I was done with that and ready for the shower and several large glasses of water!

 

[DanCorcoran]

Unfortuntely, when I lived in SW Colorado, I had a pellet stove, even though we had 70 acres of dead pinon pine. Those suckers would've been dry before I could get them stacked, given the high temperatures and low RH. If only I'd known of this website then, I would have found the one true way sooner.

 

[Battenkiller]

My point was to emphasize the point that even with a hot day the wood may not be drying nearly as much as one thinks - even the cooler (70* days) may be out performing in terms of drying days.

So many folks get hung up on the temperature. As others point out - the temperature is a factor in the calculation (the warmer the air, the more water a given volume can hold thus the relative humidity goes down), but just because a body of air is warmer doesn't mean it hasn't already found water somewhere.

I got curious to find out (based on reading other comments after mine yesterday) and found that yesterday's stats for my town were rather dismal for drying:

Temp min/max - 69/89 *f
R Humidity min/max/ave - 62/100/80

With an average RH of 80 and the minimum of 62% (assume that hit when temp was 89*f) that explains the poor drying conditions for me.

So far today doesn't look too good either... At the moment (10:45a) it is 91*f with RH of 70% - it feels very uncomfortable out there. I split up some black birch this morning (8a) and couldn't keep my glasses on due to the sweat flowing off my face. (My biological evaporative cooler was not functioning very well eh?) After 30 minutes I was done with that and ready for the shower and several large glasses of water!

Slow, in outside conditions you are probably right about the temp thing not being all that important. You just aren't going to get the wood hot enough for the temp to be a major driving force in wood drying, even in full sun on a hot day. For academic purposes, however, we should at least acknowledge the fact that the diffusion rate across the grain increases as the wood temp increases.

Rising temps in some areas bring about a correspondingly large increase in the absolute moisture content (how much water by weight is in a given volume of air at STP), leaving RH basically unchanged as temperatures rise. The Corn Belt is notorious for this. All that evapotranspiration of water from the corn fields at high temperatures is largely responsible for a lot of severe thunderstorm activity out that way.

http://www.weather.com/outlook/weat...potranspiration-corn-belt-humidity_2011-07-13

Still, in most areas of the country, rising daytime temps have the effect of lowering the RH somewhat. Below is a table of today's forecast in my area. RH begins at a not too shabby 80% at 78º at 6 AM, will drop to 49% as the temps climbs to 90º by noon, will get even lower at the same 90º temp later in the day (indicating that drier air is expected to move into the area), and will rise to 76% at 70º by midnight. Since the RH at a midnight temp of 70º is actually expected to be lower than the 6 AM RH at 78º, this is a further indication that drier air (i.e. lower absolute MC) is expected to move in later in the day.



When you are considering the drying rate over the long haul, you really only need to consider the average RH in your area. A lot of drying still goes on at 80% RH, especially if there is good air movement. In fact, if you had an invariant RH of 80% day and night, both your 80% MC oak and your 160% MC cottonwood will eventually get down to 16% MC dry-basis. That is less than 14% water by weight... plenty dry for any stove out there. Too dry if you split it really small.

So, although it's interesting to examine these things and to discuss them on this board, in real life there is nothing we can do about it all unless we dry in a kiln. The wood will be dry when it gets there, and it will get there in time... even at a constant 90% RH. After all, the EMC at 90% RH is 20.5% MC dry-basis, or about 16% wet-basis... the MC at the low end of the EPA testing protocol fuel requirement.



The drying experiment I did last winter was meant to try to establish a drying curve that would allow one to predict the time that the fuel would be ready based on weight data only. I discovered that, after the initial fast-drying period of free water leaving primarily from the end grain, the drying curve closely resembled an exponential decay function. Funny thing, all anybody seemed to get out of that is that I was trying to convince folks to skip outdoor seasoning entirely and dry exclusively in the basement. Whatever...

Skyline (on a couple of recent threads) has done an outstanding job of taking this idea into a real-world outdoor drying situation. His curves are very similar in shape to the ones I generated, even though they have a lesser slope because he isn't drying his wood at the 20-30% RH I had in my basement. By taking accurate weights at regular times and using these weights to determine a rough constant of proportionality, you should be able estimate within a month or two when your wood will be at any desired MC in your area simply by using an exponential decay formula.


BTW nice score with the black birch. It dries fast and burns very hot for a good long time. Your stove will think you fed it candy.

 

[skyline]

Slow1,


So many folks get hung up on the temperature. As others point out - the temperature is a factor in the calculation (the warmer the air, the more water a given volume can hold thus the relative humidity goes down), but just because a body of air is warmer doesn't mean it hasn't already found water somewhere.

I got curious to find out (based on reading other comments after mine yesterday) and found that yesterday's stats for my town were rather dismal for drying:

Temp min/max - 69/89 *f
R Humidity min/max/ave - 62/100/80

With an average RH of 80 and the minimum of 62% (assume that hit when temp was 89*f) that explains the poor drying conditions for me.

So far today doesn't look too good either... At the moment (10:45a) it is 91*f with RH of 70% - it feels very uncomfortable out there. I split up some black birch this morning (8a) and couldn't keep my glasses on due to the sweat flowing off my face. (My biological evaporative cooler was not functioning very well eh?) After 30 minutes I was done with that and ready for the shower and several large glasses of water!

Slow, Battenkiller has said it better than me, but I would add that the conditions you quoted calculate an EMC of 10.9 from this calculator (http://www.dpcalc.org/) which will give you great drying when your wood is at starts at 90% MC but a lot slower when it is down at 20%. Of course at night when you have 100% RH your conditions calculate an EMC of 28.8 which just isn't much good for drying.

What I have learned by my experiments and Battenkiller has pointed out, I could use the curves to predict when my wood will be dry. And will it seems a very gradual drying curve what I really hope to pass is:

1). That wood dries very unevenly throughout it's drying cycle. A lot of our time "seasoning" is really wasted. Nights, wet days etc. My experiments show clearly that a few hours in the right conditions can dry the same as 3 weeks of poor conditions. And since we have the ability to control for some of these factors, covering from rain, isolating from ground moisture (plastic or vapor barrier-typically ignored), setup for wind or fan for air movement, solar gain to add heat and lower RH, etc., we can improve drying times if we choose to. Until then single row, covered on top, vapor barrier on bottom, up off the ground is pretty practical storage advice.

2). With a cheap kitchen scale and a few measurements, folks can really see when their splits lose moisture in their conditions and stop guessing.

 

[Slow1]

Up until now I had missed the vapor barrier on the ground suggestion... Has anyone isolated this variable to see how much of a difference it can make? I imagine I may have a hard time doing it well so that I don't make it a small pool under the stacks, but if it made enough of a difference then perhaps it could be worth the effort to meet the challenge. In any case, what I'm doing now - using time to my advantage - seems to be working well enough.

 

[Backwoods Savage]

If it is working, why try to fix it?

 

[Battenkiller]

There really ought to be a forum just for scientific and technical stuff so some folks won't continually feel obliged to come in and tell this stuff is all irrelevant. Maybe I'll put it in the suggestion box.

Wood technology has been a passion of mine for years. I'm a pro woodworker, after all, why shouldn't it be? A lot of that science and technology carries over quite well to wood heating IMHO. Same thing with combustion technology. Why do people continue to doubt carefully controlled studies, and instead, trust there own casual observations as gospel? Face it, without the help of science and engineering, there would be no EPA stoves in the first place.

This short rant is meant for no one in particular, so I don't want any panties up in a knot over my comments. It's just that some of us like this sort of stuff and some of us like at gawk at wood stacks. That difference should be respected by all. If you think this is all meaningless and irrelevant info, then don't feel the need to read and comment about it. If you have something useful to add to the discussion, however... fire away.

 

[Slow1]

If it is working, why try to fix it?

Continuous improvement is simply a part of who I am. In anything that I do I instinctively look for ways to improve processes and outcomes - professionally and even in my hobbies.

But from a more practical point of view, I really don't have that much space to store wood. I have experienced the joy of burning very well dried wood and want to always be able to do this - but the only way to be sure of having this supply would require stacking on the order of 12-16 cords on my property (i.e. to be sure that every stick has had 4 years stacked before it is burned). I don't have much sun in the areas where I can stack and the more I stack the more my stacks will restrict airflow - a bit of a catch-22 situation due to space limitations. So, if I can identify ways to reliably improve the rate of drying such that I can be sure that say pieces of red oak are ready to burn 24 months from the time it is split, I could in theory cut my storage demands down to the 6-8 cord range. This would fit much better on my lot and reduce the risk of ever having to argue with the town due to offending someone in the neighborhood.

So that's the story here - It isn't just idle fiddling with a working system. However I'm not one to jump on any 'fad idea' assuming it is going to work. Rather I'd like to target my efforts to areas that will work. I don't have time to run my own experiments so I am happy to learn from those here who publish their methods and results.

 

[Battenkiller]

Continuous improvement is simply a part of who I am. In anything that I do I instinctively look for ways to improve processes and outcomes - professionally and even in my hobbies.

I can definitely identify with that attitude. :coolsmile:

the only way to be sure of having this supply would require stacking on the order of 12-16 cords on my property (i.e. to be sure that every stick has had 4 years stacked before it is burned)./quote] I don't have much sun in the areas where I can stack and the more I stack the more my stacks will restrict airflow - a bit of a catch-22 situation due to space limitations. So, if I can identify ways to reliably improve the rate of drying such that I can be sure that say pieces of red oak are ready to burn 24 months from the time it is split, I could in theory cut my storage demands down to the 6-8 cord range.

You probably are aware of the fact that the folks who make your stove feel that 4 years is too long to season firewood, aren't you? :cheese:

Be that as it may, I think you are really a candidate for the holzhausen method. I don't believe it will dry your wood any faster, but you will certainly get a lot more wood seasoning on the same size footprint, and it will get better air circulation than packing several rows tight together.

 

[skyline]

Up until now I had missed the vapor barrier on the ground suggestion... Has anyone isolated this variable to see how much of a difference it can make? I imagine I may have a hard time doing it well so that I don't make it a small pool under the stacks, but if it made enough of a difference then perhaps it could be worth the effort to meet the challenge. In any case, what I'm doing now - using time to my advantage - seems to be working well enough.

Slow,

I can't say that I have the any data on this, either through research or my own, however; it's funny that most folks recognize the importance of elevating your stacks off the ground but don't recommend a vapor barrier between the ground and your wood. The entire reason to elevate is to remove the wood/ground moisture contact which is a good idea but improved greatly with a vapor barrier beneath the pallets or whatever is elevating your wood. Hoping for big air flow to mix the air under a pallet of wood surrounded by grass on wet soil often next to a fence is worse than playing the lottery. Don't be worried about pools of water collecting on plastic underneath your stacks. If the plastic weren't there the moisture would still be available from the ground. If conditions don't dry those pools, they won't dry your wood either. What you don't want is plastic extending past your stacks and collecting rain water that wouldn't ordinarily be pooling water underneath them.

Climatologist require their weather stations to be placed several feet above the ground partly for consistency. The ground tends to lower temps in warmer months and raise humidity. As I said, I don't have the data yet, but I suspect that typical ground releases many times more moisture than a stack of wood does, so to not isolate it from that humidity seems counter productive. Also, many building codes require a vapor barrier in crawl spaces to help prevent moisture related rot in wood substructures, and this is under dry, roofed, and heated homes.

I just think among the list of best practices for drying wood, a vapor barrier should be listed along with the typical single row, covered on top, in the sun/wind etc.

 

[tfdchief]

There really ought to be a forum just for scientific and technical stuff so some folks won't continually feel obliged to come in and tell this stuff is all irrelevant. Maybe I'll put it in the suggestion box.

Wood technology has been a passion of mine for years. I'm a pro woodworker, after all, why shouldn't it be? A lot of that science and technology carries over quite well to wood heating IMHO. Same thing with combustion technology. Why do people continue to doubt carefully controlled studies, and instead, trust there own casual observations as gospel? Face it, without the help of science and engineering, there would be no EPA stoves in the first place.

This short rant is meant for no one in particular, so I don't want any panties up in a knot over my comments. It's just that some of us like this sort of stuff and some of us like at gawk at wood stacks. That difference should be respected by all. If you think this is all meaningless and irrelevant info, then don't feel the need to read and comment about it. If you have something useful to add to the discussion, however... fire away.

BK, I love both ;-P I am a scientist by education, but have to "fly by the seat of my pants" sometimes in the emergency response business. I always tell my firefighters, you have to study the books, but the real world seldom follows the book exactly. So for me there is room for both. Sorry, just my rambling thoughts. Don't have a lot else I can do right now. Steve

 

[jimbom]

Concerning the vapor barrier issue. I drew a free body diagram of a cord of wood stacked in a single row 24 foot long. To this diagram I added a vapor barrier at ground level. Surrounding the stack, an infinite plane with no vapor barrier. Humidity arising from moisture in the soil will reach equilibrium in the wood stack relatively quickly. I am not sure of the practical value of a vapor barrier in normal evapotranspriation conditions.

But, all soil is local as is all soil moisture:

http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Soilmst/Soilmst.shtml

So there may be circumstances I have not considered and YMMV.