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Why is My Shed Damp Inside?

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If your storing boxes and other items in your shed that can be damaged by moisture you want to make sure you shed is dry inside. If moisture and dampness is in the shed your going to need to take some steps to deal with it.

Moisture gets trapped inside, especially if the air outside is humid and if the shed is not used for a time. When the outside temperature drops, the warmer air inside the shed rises, carrying moisture with it. Moisture inside the shed then condenses on the first solid surface it touches and usually the highest points, such as the ceiling or inside of the roof. It can also form on the floor, especially if it is concrete.

What Causes Dampness in a Shed?

One of the main causes of dampness is the lack of a vapor barrier when the shed was built or installed. Without one, ground moisture has a chance to seep into the shed from underneath when it tries to evaporate. It gathers on the underside of the shed and eventually works its way inside. Concrete slabs can act as conduits for moisture, causing it to eventually seep upwards into the shed.

Another cause is the lack of sufficient ventilation. Moisture cannot condense easily in moving air. While ventilation permits the temperature inside the shed to be more similar to the outside air and lets in any humidity in the outside air, the benefits of moving air far outweigh letting in outside air.

Open your shed whenever possible, even in winter, at least for a couple of hours, but don’t leave it open at night when the air is more humid. While there is not an easy way to keep all moisture out of your shed, there are many ways to keep excess moisture out and to keep it from causing problems. Check out my article How to Deal with Condensation in Your Shed for dealing with condensation in your shed.

How to Stop Dampness in My Shed?

If your shed is small and movable, you might consider raising it to install a gravel base with a vapor barrier under it if one was not installed to begin with. Raising the shed on skids will also help. The airflow underneath will help moisture evaporate instead of seeping into the shed.

Another aid would be to dig out a shallow trench a foot or two wide all around the shed, maybe three or four inches deep. Fill it with crushed stone. This will help the rain pouring off the shed roof to flow away from the shed and evaporate.

Keeping this water away from the shed and out of the surrounding soil will go a long way towards keeping moisture out of your shed. If you notice standing water around your shed, you may want to invest in a French drain to help steer the water away. If you have water around your shed and under it read my article What’s The Best Way to Add Drainage Around a Shed? for tips to drain water away.

How to Damp Proof a Shed

One of the best ways is to provide adequate ventilation. Install vents in each of the gable ends near the roof. Measure the length and width of the attic and multiply to get the square footage. Your vent should provide a square foot of vent area for every 300 square feet of attic space.

These vents will allow warm, moist air to escape the shed while providing air flowing through. If your shed is electrified, you can use a dehumidifier during humid spells. A fan will keep air moving, and there are some battery-operated floor fans available if you don’t have electricity.

Open your shed on nice days, especially when it’s windy. Open the window as well if you have one. This will get some of the humidity out. You can also use drying agents, such as charcoal or non-clumping kitty litter in cloth bags hung in the shed. There are also inexpensive commercial products that will indicate when they need to be replaced and have refills available.

Damp Proofing the Walls

Caulking all the joint seams on the siding and all around window and door openings helps. Don’t forget the seam where the walls meet the roof. Check the caulk periodically and replace any that’s missing or cracked.

Even if you didn’t use treated lumber for the wall studs and wall materials, you can still treat the wood with a product made to prevent rot and moisture damage. Ask your supplier to recommend a product, as there are many available.

You may want to add insulation, but it’s actually not necessary, as long as your shed is sealed properly. One thing often overlooked is the door. Many shed doors don’t have a raised bottom sill in order to accommodate wheeled machinery. This leaves a bit of an air gap under the door. A door sweep made of rubber or similar material is easy to install and will keep air from coming in under the door.

Damp Proofing the Floor

This is the place where the best solutions are the hardest ones to accomplish, namely, digging up a concrete base to install a vapor barrier and pouring a new slab, and raising your shed on skids. Both of these are out of the question for most people. The next best solution is to install a second floor layer with insulation between. This will reduce the interior vertical space but shouldn’t make much of a difference.

If you have a concrete floor, you don’t have many options except for using a concrete moisture sealant. You’ll need to thoroughly clean and dry the floor before using it, taking special care to get in the corners.

A bleach solution will help kill any mold or mildew that has formed. The sealant is best applied with a roller a small area at a time. Keep your application thin, as bubbles tend to form if it is applied in a thick layer. Let it dry for 24 hours, then apply a second coat.

If you’re using a two-part sealant that must be mixed, you’ll need to mix a fresh batch for the second coat. If you don’t have a vapor barrier underneath the slab, you’ll still need to keep an eye on the wall sill plates.

Damp Proofing the Ceiling

You can use the same wood treatment on your plywood roof sheathing from the inside if you didn’t use treated plywood. Insulation under the roof will help, even if you don’t feel the need to insulate the whole shed. If installing fiberglass insulation over your head seems a bit much, moisture-retardant foam board insulation can be cut to fit between the studs and installed more easily.


Keeping moisture out of your shed is much better than dealing with it once it forms mold or mildew. With a little care, your shed can be protected from potential damage, even if your shed didn’t have a vapor barrier installed. Read my article How to Get Rid of Musty Odor in Shed? to remove that awful smell in your shed.

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