How to Prevent Condensation Problems in a Shed

Condensation inside your metal or plastic shed can be an issue, potentially damaging items you have stored inside. Robert Hall recently sought the advice of a major importer on how this problem can be successfully avoided.

Created by Robert Hall on Monday, 6th of January, 2014.
Updated on Thursday, 9th of February, 2017.


The interior of a metal garden shed.

Plastic and metal sheds, and garages, have gained great popularity over recent years, they are easily assembled, extremely durable and low maintenance - offering good looks and a long life.

Metal or single walled vinyl clad sheds will suffer from condensation where dampness is present. This situation is similar to condensation forming in any enclosed, unheated, and insufficiently ventilated space.  The air inside a shed is always slightly warmer than the outside. Any moisture in the foundations, or seepage from the outside, will rise up until it hits the cold roof and wall panels, where it condenses into water droplets, drips down and becomes part of the cycle again.

As these types of shed are normally supplied without a floor, it is particularly important to ensure that the shed is assembled correctly in the first place. Check that each washer under every screw (or plug) in the roof is intact and that there is no water ingress from any other screw/plug or bolt used elsewhere in the shed. Correct overlapping of all roof and wall panels is of particular importance.

Also ensure you do not store wet or damp items in the shed as this will add moisture.

The Condensation Cycle is Hard to Stop

The condensation cycle is difficult to stop. Therefore, it is vital to prevent dampness as far as possible in the first place. The following points about base construction are important – and should be followed accurately:

  1. The concrete, or slab base, should only be marginally larger than the base rails of the shed, i.e. a shed with base rail measurements of 93ins x 70ins should ideally have a foundation of 95ins x 72ins. A level base for any of these sheds is essential.
  2. The concrete, or slab base, should contain a damp-proof membrane (sized to the entire surface area the shed will occupy). This should be inserted below the flags, or 2ins below the surface of the concrete slab. Make sure that the surface of the slab, or concrete foundation, is some 2ins above adjoining soil levels.
  3. Purpose laid concrete bases should be allowed to ‘cure’ for at least seven days – longer if the atmosphere is damp. If this is not done, the water drying up from the concrete plinth will provide ample moisture to set up a condensation cycle.
  4. After securing the shed to the slab, or concrete base, apply a silicone sealant to the inside of the shed’s base rails (NOT the outside). This will preserve the drainage capability of the channels within the base rails whilst preventing water seepage under the base rails and into the shed interior.

Oversized or Pre-constructed Shed Bases

If the base has already been constructed, or is oversized and condensation is apparent, there are two solutions:

  • Remove the shed from its base and build (or buy from the retailer) a timber floor/base that is supplied with timber bearers. The entire shed should sit on the timber floor. Fix the shed to the timber floor with wood screws and washers through the shed’s base rails – then apply the silicone sealant as in 4) above. The timber floor, with its bearers below, will allow air flow under the entire shed. Ensure that water from adjacent soil levels does not pool under the timber floor.
  • Obtain the cheapest polystyrene tiles available from a DIY store with the view to applying them to the entire roof area. Then obtain 3M Scotch-Weld Super 77 Cylinder Spray at www.3mdirect.co.uk. Thoroughly clean the underside of the roof panels with methylated spirit and ensure that the panels are dry. Apply the polystyrene roof tiles following the 3M instructions supplied with this specific 3M product – this work is best carried out on a warm, dry day. Applying tiles will obviously not remove the moisture from the atmosphere inside the shed, but condensation will be prevented by the increased insulation.

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