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moisture problem inside exterior shed, need advice

moisture problem inside exterior shed, need advice

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  #1  
Old 12-28-14, 10:24 AM
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moisture problem inside exterior shed, need advice

I will apologize in advance for length of this post...

we replaced a deteriorated outdoor 10x12 shed just over a year ago. It was on a slab and the plan was to extend the slad to 12x16. We had the utility companies visit to mark any lines so the digging would't damage them. The sewer utility nailed us on the location (built before we moved in) as the existing slab was too close to their lines. We wouldn't be allowed to rebuild a "permanent" structure in the same place. We were told this several days after their visit and the same day the old shed was demo'd. Our overnight rush new plan was to bust up the slab and build the new shed on concrete deck piers, so that in a worst case scenario the new shed could, in theory, be jacked up and moved temporarily in case the sewer line was ever replaced. The day of the slab demo it rained about two inches in one hour. Once the jackhammer work was done and a Bobcat had removed the slab pieces, the site was chewed to pieces. It was too muddy to even stand or walk around the site, much less to level out the ground.

Work proceeded the next day. Deck piers were set and double 2x8s were used for the base. The bottom of the 2x8s were anywhere from 2 inches to 8 inches off the ground, due to a slight slope in the yard. There was no vapor barrier put down. The shed was framed, everything turned out nicely. Put a ramp on the front and a set of double doors. All cracks and windows caulked and painted.

Even before the new shed was built, every time it rained a lot I noticed that the drainage from the neighbor's yards would flow through my back yard close to the shed location usually about 4-5 feet in front of it, due to the natural slope of the neighborhood. With the removal of the slab and all the bobcat work done, the NEW low spot in the backyard was now....under the ramp and partially under the new shed. I was concerned about that area staying damp and especially the bobcat ruts in the ground staying full of water. We have enough mosquitos here without giving them a perfect place to breed. My contractor and I discussed our options to keep runoff water away from the shed, our solution was to scrape down part of the yard further away from the shed to move the lowest elevation of the backyard, then enclose the bottom (the "crawlspace") from the siding down to below ground level with thin sheet metal nailed to the joists, then add fill dirt to all four sides to encourage drainage to the scraped part of yard and away from the shed. This seemed to do the job as all subsequent heavy rain drainage now flows about ten feet from the shed.

My problem is that I was out in the shed last weekend and had some things stored in plastic bags sitting on the floor. The bottom of all the plastic bags, plus any Tupperware containers, 5-gallon buckets and even the plastic trash can all had water pooled underneath them. There is no leak in the roof, nor was there water anywhere else on the floor, only under non-porous surfaces. The shed is fairly tight with only a few gaps around the double doors. There are two 1 ft by 1ft gable vents in each side near the roof.

So what are my options? If I cut holes to ventilate the "crawlspace" and add foundation vents, I will have to do it in the 2x8 joists, and due to the fill dirt used to improve the drainage these will now be nearly below ground level. Due to the slope of the area, on two sides the vents would be well below ground level and I don't want to give water a place to go easily. One person advised that I cut a hole in the shed floor and install a standard household register, to equalize the temperature and humidity level inside the shed with that of the ground underneath it. We live in Zone 7 so summer humidity is high.

is my problem humidity under the shed or humidity inside the shed? I'm looking for constructive solutions here rather than pointing out all the mistakes made. "Tear it down and do it right" is not an option. Both my contractor and I made some mistakes while dealing with the problems presented to us. Although it was built to theoretically be jacked up and moved in a worst-case scenario by the sewer company, neither the contractor or I are fond of this as an option as any mistake or slip could damage the shed.

Any advice you can offer for a solution would be greatly appreciated.
 
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  #2  
Old 12-28-14, 10:47 AM
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A sealed crawlspace without a moisture barrier will provide gallons of water per day through evaporation that must go somewhere. Unfortunately, small vents will not do the job (IMO). A register from crawl to inside would just invite more moisture to where you don't want it. The solution, and I know you don't want to hear it, is to install a proper vapor barrier under the shed.

TN will have two problems. Humidity in the air coming in contact with a cool surface will result in moisture. And then the moisture from the soil below passing right through the wood structure into the shed where it can find surfaces cooled by nighttime temperatures.

The shed should have been higher off the ground and included a full vapor barrier. Lifting a 12 x 16 shed (empty) should not be that difficult, some beams, some good cribbing, and a couple of good jacks would inch that up in easily. I have a 10 x 16 shed that I move around all by myself.

Moisture follows one of the old physics laws, wet moves to dry. In addition, moist air is lighter than dry air so the evaporation under the shed heads up those few inches to the dryer wood above. Take a deep breath and lift that shed, carefully.

Bud
 
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Old 12-29-14, 07:17 AM
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I would have to contract out the lifting. I don't have the equipment or know-how to even try to do it myself. Plus the shed is full of equipment and it would need to be a quick job. I might add that this is not a flimsy shed like the prefabs you see at big box stores, this is built as well as a house, with 2x8 foundation and 2x6 rafters. It is heavy.

Would the vapor barrier go on the ground under the shed or have to be affixed to the bottom structure of the shed? It seems like suicide to try and get underneath it while it's jacked, to either attach the barrier to the shed or do the dirt work and spread plastic over the ground underneath it. What keeps the barrier in place on the ground?

Lastly, do you not believe that digging out the fill dirt on two of the sides to allow 2-3 inches of space would provide enough ventilation? I could attach hardware cloth to the joists and then bury it partially in the ground to keep out pests. What about a vent inside the shed coming up from the floor that is then vented out the side of the shed, so the moisture doesn't stay in the shed? Would that allow the crawlspace to "breathe" enough to help with the problem?

thanks for your advice.
 
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Old 12-29-14, 07:32 AM
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I wouldn't attempt to raise the shed with it full of heavy equipment!
2"-3" isn't enough height to allow for good airflow.
 
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Old 12-29-14, 10:34 AM
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Directing the moisture from below into the shed would increase the humidity in there, not decrease it. The moisture below the shed is a constant source now being limited by having to sneak through cracks or diffuse through the wood. A direct vent would accelerate that process.

For a contractor who lifts buildings, this would be a very small job, so perhaps worth the time to get an estimate or two. Even a general contractor could be quite capable of doing this. I'm 67 years old and would have it done in a day without help, just need the beams, cribbing, and a couple of jacks, available at a rental store.

Being well built works to your advantage as it won't fall apart when moved. How many supports are under this shed and where are they.

The vapor barrier would go on the ground and once lifted could be installed without going under there. I do agree that if you are not confident you can do this, hire someone.

Bud
 
  #6  
Old 12-29-14, 12:00 PM
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I guess my explanation for the vent idea was to cut a hole in the floor and then have some PVC going from that to a 90 degree then to a vent on the side of the shed. the moist air would have somewhere to go but rather than vent into the shed I could direct it outside, maybe close to the gable vents.

I would empty the shed during the lift as much as possible, but my concern is what to do with all the stuff, it would have to go outside on the patio, I could tarp it I suppose in case of rain.

there are six deck piers, one at each corner and one at halfway spot on each of the 16 ft. sides.

I'll start searching for someone local to do the work. thanks!
 
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Old 12-29-14, 12:35 PM
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With all of your supports on the outside it would be fairly easy to pull a vapor barrier underneath once lifted. Only added concern would be raking a bit to make it high (not low) in the center. 12" would be nice, but just an additional 6" should allow you to keep the grading as you have it and allow some air flow.

How is your shed attached to the support posts or is it just resting on concrete blocks?
Once all attachments are removed a couple of auto floor jacks on one side would easily tip the shed up an inch or so. Then lower it and do the same plus another inch on the other side. Back and forth a few time and the shed is up where you want. Your support may be different than what I'm describing so describe again if necessary.

Bud
 
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Old 12-29-14, 02:05 PM
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it is resting on the blocks. I guess I need to check to see if taller blocks are available or if we'll have to do some sort of filling in the block holes and resetting the current ones.

another problem (one of many) is that the property line and a fence is on two sides of the shed, within five feet. not a lot of room to maneuver.
 
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Old 12-29-14, 03:13 PM
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Is there any type of footer under the blocks? normally you just add more block
 
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Old 12-29-14, 03:37 PM
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there was a round hole dug for each block and this was filled with gravel. I am not sure how deep each hole is, but they were about 18 inches across.

I would be wary of just stacking another block on top of the existing ones, seems as if it would be wobbly. the blocks are triangular with a flat top and an "X" depression where the joists sit.
 
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Old 12-30-14, 04:40 AM
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Sounds like you used Dek blocks. I don't know a lot about them but you are right that you can't stack them. You could probably set a couple of cap blocks under them but wait and see what the carpenters have to say.
 
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Old 12-30-14, 05:17 AM
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Good gracious, empty the shed, go inside, cut out the floor which is probably plywood, install the vapor barrier and re-install the flooring. All the body builders can take a break, you don't need to lift anything.
 
  #13  
Old 12-30-14, 07:18 AM
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IMO, it still needs more ventilation even with the a vapor barrier.

The other issue is the grade under there. You don't want the new vapor barrier to slope to the middle where moisture/water could collect. If the slope is good, a temporary solution would be to snake a canvas (or black plastic) under there from the perimeter and stake it down. The idea of lifting the shed is to do it right and never have to worry about it again.

Remember, your are fighting two sources of water, rain and condensation. With high humidity and cool soil there will be moisture forming that you want to drain to the perimeter and then dry with good venting.

@czizzi, "All the body builders can take a break" , you're right. I used to do a job like this with a 4x4 and a fulcrum, but I now use and recommend jacks.

Bud
 
  #14  
Old 12-31-14, 06:37 AM
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Move the Shed

neither the contractor or I are fond of this as an option as any mistake or slip could damage the shed.
A properly built shed with rim joists and joist anchors will not be damaged in a simple move. If you really want to, you can do it. Work safely.
 
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Old 01-04-15, 09:01 AM
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thanks everyone for your helpful advice. looks like I need to find someone locally to jack this shed and level the grade underneath. I hoping to find some taller blocks or as one poster said to raise the height of the current blocks with some other material. I'm assuming that the vapor barrier can be bought at any big box store (HD, Lowes, etc)?
 
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