Ours was.  Gets heavy use in early spring and late fall, but not much in winter (unheated) and nothing at all in summer.  In summer it's a blazing hell in there; can't even leave extra pots inside or they turn into puddles of melted plastic goo.

But then...  

We heat exclusively with wood in the winter; wood furnace in the basement.  And we cut a lot of our own wood.  This year we got behind on the stockpile, so we don't have a lot of cured stuff stacked up and ready to go like we usually do.  But we've still been cutting wood, lots of walnut, maple and locust.  That's when the light went on.  We set up a bunch of pallets in the greenhouse and have been stacking all the freshly-cut stuff in there.  It's effectively a wood kiln, and the firewood is drying out like nobody's business.  Unlike kiln-drying hardwood for furniture and whatnot, the conditions don't have to be tightly controlled... not worried about case-hardening or splitting with this stuff.  It's working like a charm.

If we weren't planning on burning it myself, I'd sell it - around here they sell shrink-wrapped bundles of kiln-dried wood (mainly for fireplaces and stuff, I guess) for $5-6 a bundle, which is generally 6-7 split pieces... not much wood at all.  Easily what you can fit in a comfortable one-arm load.  Split, dried and packaged it's really a "premium" product, but the fact that those bundles still sell well around here smack dab in the middle of hardwood country tells me it's a pretty good niche market.

Just thought I'd toss this out there for anybody else who may be in a similar situation and staring at a formerly useless greenhouse all summer.  It's working so well I'm seriously considering erecting a cheap plastic-covered hoophouse to dedicate to the task next year.

Just have fans running or how are you keeping the moisture/humidity inside the greenhouse down?

We just leave the vents (louvers) open all the time, and once in a while I go down and turn on the big vent fan for a few minutes.  Seems to be enough to keep the moisture level down.  I only run the fan once every day or two... the whole thing is kind of an afterthought project so I don't sink much time into it.  If I wanted to get serious about it I'd probably put the fan on a timer.

How big is this greenhouse?   Just wondering.
Would like to get a small one someday soon.

I have been wondering if a solar collecting box (just a box with one side plexiglass) could be used to dry wood. Now I know!

Do you have some natural convection airflow through it? Or do you get it tight as possible?

12'x24'... I had a build thread on it here in the forum a few years back, but lost all the pic links when Photobucket decided to hold them hostage and I killed that account.

This was the end result:

There's some natural convection, but not a whole lot without the fan running.  The vents are installed one high, one low, on opposite ends of the greenhouse to encourage airflow... here's the other end:



It wasn't installed when I took those pics, but on this end there's a 24" ducted fan permanently attached to the inside of this louver.  That sucker will move some air!

Greenhouses are only really humid when they're full of plants getting watered every day.  Normally in summer it's dry as a bone in there, though of course if I loaded up the whole thing at once the water coming from the firewood would do the same thing.  We've only been bringing in a little at a time, though, and really only have somewhere between a half and 3/4 of a cord in there right now.  It seemed to be drying out OK even without running the fan, so the natural convection must have been enough to keep that moist air flushed out.  More wood in there and I'd definitely have to run the fan more frequently, though.

Ingenious!

Almost makes me want to build a greenhouse just for this purpose.  

Good job OP.

Decided to try an experiment.  

This morning I split a round from a freshly-felled walnut into four sorta-equal pieces.  I weighed and tagged each, and put two in the greenhouse:



... and the other two outside, on a regular stack of firewood left to air-dry.



I'll weigh them again in a week, and maybe again in two weeks, and calculate the rough % of water loss through the difference in weight, and if and how much the ones in the greenhouse are out-drying the ones outside.

A downside to all this, though:  another poster in a different thread posted something to the effect of "every time you touch a piece of firewood, you add value/cost to it".  100% true, and the extra steps of lugging it all into the greenhouse and out again to the final stack is definitely something to think about.  Seeing firsthand how much difference there is between the two methods will hopefully convince me that I'm not just spinnin' my wheels.  

Starting weights, 10/8/20:
G (greenhouse) A:  10.6 lbs
G B: 13.6 lbs
O (outside) A: 13.6 lbs
O B: 11.6 lbs

Missed my 'one week' weigh-in by a couple of days (life got in the way), but no biggie.

Results so far:


So it's definitely having a beneficial effect, though I was personally hoping for something a little more dramatic.

One factor is that it's been unseasonably dry and hasn't rained here since I set this up, so the outside logs have actually had pretty good drying conditions too.  About an inch of rain in the forecast over the next few days, though, so that will probably slow the outside logs down a bit... I'm just going to leave them out, as would be the case with any woodpile left to cure outdoors.

I've built solar wood kilns to dry out lumber for woodworking before.  Similar concept, except they try to manage the moisture removal to prevent warping/checking/cupping as much as possible.  But the concept is sound.

Yup, I'm a woodworker myself and do a lot with locally-milled hardwoods.  Have thought more than once about building my own solar kiln; that's kinda what gave me the idea of using the greenhouse for firewood.  The beauty of it is in not having to stick to tightly-controlled kiln schedules to manage the temp, airflow and humidity.  Nobody cares if their firewood warps or checks.

I actually ended up moving the outside logs around, leaving one bark-up and the other split-up for a more accurate simulation of what goes on within a woodpile.  I thought about burying them deeper in the pile, but that would make weighing them regularly a lot more of a PITA and I'm just kinda playing around anyway.

I'm not sure bark-vs-split up or the position of the logs in the pile will make that much difference anyway; most of the moisture loss in wood as it's drying is from the end grain, not the "face" sections.  So as long as the ends are exposed, orientation probably won't be of much significance.

I will be keeping track of rainfall, though.  I have a weather station at the house so I'm always tracking it anyway.  Got .17" of badly-needed rain yesterday, a little more than 1" in the forecast over the next seven days.