# venting polebarn to reduce condensation



## saichele (Mar 8, 2010)

I have an unheated 24x40 pole barn that has had a condensation problem.  Last year I had to change the starter in my truck, and the slab was just plain wet.

I acquired and installed a wall vent fan (like an attic fan) and have a timer rigged up.  My question is when I should be drawing air through the barn?  Currently I have it set to run for 2 hrs. once around noon and once around 6pm.  However, I'm not sure I even have a good theory backing that, let alone any experimental proof.  Is there any experience or wisdom on this issue?

Thanks
Steve


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## benjamin (Mar 8, 2010)

you want the fan to run only when the dewpoint outside is lower than inside, but I'm not sure a fan is your best solution.

where is this polebarn?  state? wooded or exposed? insulated?  

If it's not heated at all, you'll have a hard time keeping it dry in the spring as the air is warm and humid and the slab is cold, therefore clammy.

It may stay dry in the fall and winter for the most part.  

You have to either open it up, or treat it like a cellar and close it up.  If it always has a breeze going through it will only be damp when it's really wet weather.  If you keep it closed tightly, it will stay more stable and hopefully never be sweating condensation.  

Ideally closed most of the time and wide open on dry windy days.

it's all about temp, dewpoint and humidity, much simpler than understanding prince of darkness engineering.


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## gzecc (Mar 8, 2010)

where is the humidity coming from?


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## begreen (Mar 8, 2010)

Agreed. I would investigate the source of the moisture first. Is this already a wet area? Are there gutters on the barn with downspouts connected to drainage to take the water away from the building?


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## saichele (Mar 8, 2010)

Barn's located in central Michigan.  Sits up a bit from the surroundings, but is shaded in the PM by some large spruce trees.  Barn has a metal roof backed by some 2" pink foam.  No drainage issues, gutter carries a lot of water over to a swale about 15' from the end of the barn.

The main source of moisture is atmospheric, especially this time of year.  We're having lows near 20, but highs near 50, with about 12" of snowpack.  So the thermal mass of the barn and everything in it is cold - we'd been below freezing for about 6 weeks before this past week.  Now we have relatively moist, 50F air floating around.  

ben's answer is the most straightforward - I need heat, and probably some better air circulation.  But I'm not going to install a furnace, or radiant floor heat, but I can work on drawing more air through, perhaps resulting in more gradual warm up and a less stark contrast (50F air and 35F barn, as opposed to 50F air and 25F barn).  

I'm guessing and experimenting, on the basis that what I was doing (nothing) wasn't really working.  The question will be whether this outcome is any better than doing nothing.

Steve


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## gzecc (Mar 8, 2010)

Do you have the ability for cross ventilation?  In the moisture extreams the only real control is heat or air conditioning. The next best is cross ventilation.  You must cross ventilate when the air is dry and shut down when it is wet outside.  If you can get sunlight in it will help you.


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## PatMcNr (Mar 8, 2010)

Hanging ceiling fans will probably move the greatest amount of air over a large area.


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## benjamin (Mar 8, 2010)

passive solar is your friend.

I have a 50's or 60's (prince of darkness again?) dairy barn turned workshop that is oriented north south, with a silo on the south side and a building on the southwest and southeast.  That floor will sweat at some point in just about every month. I also have a tiny old pig hut that is pretty darn near leed certified platinum for passive solar that never sweats, even though it is also significantly shaded.  My home/barn is about a hundred years old and also designed and redesigned for passive gain, I haven't burned in three weeks. 

At the top of my list of things to do when I have nothing better to do, is building a passive solar heat system to keep the workshop dry and slightly warmer, as I would never heat it either.  

With a pole barn this can be fairly simple.  make a plenum on the back side of the siding on the south side.  I'm thinking expanding foam or 2x4s to seal off the vertical 1 1/2" space between the purlins between the posts and siding. Add vertical furring, maybe three 2x2s one against each post and one in the middle. Then cover that whole space from ceiling to within 2 inches of the floor with drywall or your choice of fire trap foam. then rig up a furnace blower to blow the air down through the plenum and across the floor. the furnace would be on a simple thermostat or aquastat set to turn on at 80 degrees or so.  

The main downside to this plan is that it won't add much heat in May, but then it won't add as much in June or July either, and you can leave it off in August and September when it's not needed as much.


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## benjamin (Mar 8, 2010)

oops, re read the post.

If the fan you have is on the south side, or east or west, just get ahold of a few used tempered patio doors and pull them apart.  Then rig up the same thing on the outside of the wall with the fan at the top sucking air in the bottom of your collector, through the wall at the top of the collector, and into the shed.


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## Bill (Mar 10, 2010)

I have a pole barn in Wisconsin and mine doesn't do that. My roof has a ridge vent and a peak vent on two sides. My roof isn't insulated and I have a gravel floor. I get some dripping off the roof inside but it's not bad. I also have the clear plastic panels on three sides on top for extra light. Maybe my moisture leaches into the soil not sure.


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## benjamin (Mar 11, 2010)

Smokey

I don't know where all your moisture is going, maybe the gravel floor acts like a buffer or dessicant and soaks it up. I suppose the floor has averaged warmer than the air temp for several months so that gravel is essentially baked dry, and now it's colder than the air so the moisture would tend to diffuse deeper where the coldest temperature is. 

The whole concrete floor in my workshop is wet right now, I have way too much air infiltration and I can literally see the dew form as the warmer outside air blows into the colder inside. We've had several foggy days of almost 50 and plenty of snow around. It had been sunny, dry and breezy before that but not enough to warm up the inside of a sheltered barn even with all the doors wide open. 

It is possible, someday, to put in a thermosiphon collector to heat the floor, or an active collector like I described above. Even tightening up my air leaks would help a lot to control the moisture.


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