Main Causes of Damp

Water ingress
Modern caravans and motorhomes are a marvel in terms of the facilities they offer for their size and weight. The downside of this is that that they cannot be made as rigid as, for example, a car. The welded steel construction of the car is replaced by lightweight materials bonded by special glues and mastics. These lightweight materials include aluminium, wood and various plastics. The problem with joining these materials is that they each expand and contract at different rates and so the integrity of the mastic is very important. The stresses imposed by towing, especially over bumpy roads, can  also induce joint stresses.  New mastic will not retain its elasticity forever and eventually it will become brittle and fail. The factors governing its life include weather exposure, mastic type, degree of flexing and how well it has bonded to the joint surfaces. Once the mastic has begun to fail, or indeed if it was not adequately applied in the first place, then water penetration is likely to begin.  

Prolonged hot weather is well known for inducing mastic failure, so do be mindful if taking an older van to the continent for a few weeks in the summer.

Faulty plumbing This is more common as a cause of damp than you might think and often goes unnoticed. Faulty joints, frost damaged components and cracked sinks can all produce leaks which, over time, can lead to significant damp problems. They tend to go unnoticed because damp can build up behind floor and wall coverings without any external evidence. For this reason it's always worth giving the van and its water system a good check over once or twice per year. Pay particular attention to any soft spots on or near the floor and don't forget to look underneath the van for signs of water seepage. The rotten floor seen here on the right was caused by a frost damaged shower tap that had been leaking un-noticed for some years. This shot is taken from underneath the van and shows the importance of checking underneath periodically.

Spillages
Most of us christen our vans in this way from time to time. We might think we've mopped it all up but if a significant amount has been spilled then some may have found its way underneath floor coverings and out of sight. On emptying the van one autumn I found a can of fizzy drink which had rolled into the corner of  a bed bunk and had obviously been there some time. When I came to remove it I found it was all but empty. This was strange because it had never been opened. Closer inspection revealed a minute puncture mark, presumably caused by the can rolling round a little for mile after mile in transit. The vinyl floor covering was totally dry so I was hopeful that no damp had been caused. The damp meter told a different story. Several square feet of floor covering had to be lifted and the floor took a couple of weeks to dry properly.

Condensation
Condensation forms when air is cooled to the point where it can no longer hold all the moisture trapped within it. The main problems occur during the colder months when a spell of cold weather is followed by a spell of warmer moist weather. The warmer air meets the cold surfaces inside the van and moisture is deposited. Favourite places for this are the inside corners of the van and in closed cupboards. The best way of avoiding it is to ensure that the van has adequate ventilation. This means checking that all vents are clean and unobscured and leaving cupboards and bunks open so that air can circulate. 

Over a period of time condensation can lead to the formation of unsightly black mould. This is normally harmless and can usually be removed with soapy water, perhaps with a trace of bleach added for good measure.

Avoiding Damp Problems

Inspecting and testing for damp
The best way to minimise the effects of damp are to catch it early. That means a thorough inspection and professional damp test at least once a year. The damp test is normally carried out during the annual service but even if you do your own servicing it's still vital to get a professional check done. Beware that the damp meters found in DIY stores are generally not as accurate as professional types and can never replace them.  It's also important that the person doing the damp check has been properly trained in terms of where to look and how to avoid false readings.

As well as testing for damp it's worth checking around all seams for evidence of the mastic drying out. Pay particular attention to horizontal seams such as awning rails, window hangars, roof to body seams and body to body seams. Other known trouble spots include window frame seals, awning channel rails, and grab handles (are they still solidly attached?).

External light fittings such as repeater lights and awning lights can also be a source of water ingress. These fittings are normally fed from behind via a sealed hole in the bodywork  and sit on a neoprene gasket. If the fitting or gasket appear loose or in poor condition this could be sign of trouble.

The damaged area in this photo has probably been caused by a leak involving the front offside repeater light

Once damp has been found it's important to find the source of the leak and to deal with it quickly before any more harm is done.

When washing the van look for signs of water disappearing behind horizontal trim strips and not reappearing underneath. If this happens then the water is almost certainly finding its way into the structure of the van.

Periodic resealing
It is good practice for awning rails and trim strips to be removed and resealed every at least once every 6-7 years.This is something within the scope of a competent DIY er (see below).

Storage Considerations

If you have the opportunity of storing your van in the sun or the shade, consider choosing the shade. Under cover storage is obviously best but avoid heated storage as this can accelerate the drying out of mastic.

Drying Out

Once a van is damp then the most efficient way to dry it out is with a dehumidifier. These work by taking moisture out of the air and making it very dry. This dry air is then circulated with a fan and is able to soak up more moisture. Dehumidifiers use much less power than heaters and present very little fire risk.

If a dehumidifier is not a viable option then the alternatives are ventilation and heating. One of the best times to dry  a van out using heat is during very cold weather. This is because very cold air contains hardly any moisture so when you heat it, it can absorb large amounts. The downside to this is that you need to ensure sufficient ventilation to stop the newly-absorbed moisture condensing in the cooler areas of the van.

For any course of drying out to be effective the damp area has to be exposed so that drying air can get to it. This can be difficult where moisture becomes trapped behind non-porous surfaces which cannot easily be lifted. There are special techniques which can sometimes be used but often the van will have to be partially stripped to expose the damp areas and thereby enable them to dry. One trick for drying out plastic coated wallboards which have become a bit damp is to make the surface breathable by piercing it gently with a pin to produce hundreds of very tiny (i.e. virtually invisible) holes.Once dry, any visible holes are filled using a water based sealant applied thinly with a damp cloth and then carefully wiped clean.

Repairs and Remedial Work

These days 6 year body shell warranties are commonplace but it's vital to stick to the manufacturer's service schedule for the warranty to be valid. Where problems are suspected within the guarantee period the first course of action is to get the van inspected by an approved service agent. Often this means taking it back to the supplying dealer. If he is at the other end of the country this might not be easy so the answer is to ask the manufacturer if they can arrange for you to have the work done locally.

Outside the guarantee period then any reputable caravan workshop can be used. Get them to do a thorough damp check and and to give you a quote for any remedial work. Unless you are 100% happy with what they say, take the van to another workshop for a second quote. In this way you can be more certain that you will get a fair deal.

Should you feel like tackling the problem yourself then be prepared to be both thorough and ruthless. Anything less and you'll be wasting your time! Remember that the source of a leak can be some distance from the damp area and that water can travel horizontally along internal support battens. It is quite normal for what appears to be a small problem to be just the tip of the iceberg (see pictures below). Repairs like this are beyond the scope of this article.

If you think the picture on the left looks bad enough the picture on the right shows the same area after removal of the front nearside bed box. The problem appears to stem from the window sealing and could have been present for two years or more.  Beyond the scope of DIY repair? Not so, thought the owner of this van who eventually effected a good repair!

Resealing of trims and awning rails
One of the most straightforward DIY jobs is the resealing of trims and awning rails. These are bedded onto mastic and fixed by means of stainless steel screws (normally covered by a removable plastic trim strip). The trims hide the joints between the various caravan panels e.g. sides to roof, and the mastic onto which they are bedded provides the seal to stop water getting in.

To remove them start by removing the plastic trim strip to expose the screw heads. It is normal for there to be one screw for every two holes.The screws are made of stainless steel and are relatively brittle so the additional holes give alternative fixing points if a screw should snap off in situ. When removing the screws check that they drive out easily once started. If not it could be that the wood into which they fix is soft. If this covers a large area it may be best to leave well alone as it could prove difficult to refit the rails later.

Having removed the screws carefully ease back the metal strip a little at a time until it is completely free. Next remove all traces of the old sealant, then clean up the mating surfaces and degrease them. The most suitable degreasing agent is isopropyl alcohol which may be obtained from a Maplins or eBay. White spirit should not be used as this can affect the mastic.

Once everything is spotless you can apply the new mastic. Be sure to use a  good quality non-drying bedding sealant. These can be often be obtained in either cartridge or tape form. Tape is much easier to apply, gives an even coating, and so is the preferred method. (Suitable  products include Sikaflex 512, Carafax IDL 99 and Hodgsons Seamseal CV.) Offer up the metal strip and gently push it onto the mastic so that the mastic oozes through the screw holes. Replace the screws tightening evenly until the mastic begins to be squeezed out from the sides. Finally, remove all surplus mastic and replace the plastic trim. If any screws refuse to bite properly on re-insertion it could be that the wood into which you are driving them is soft through damp. You can try longer screws but be careful they don't protrude through to the inside of the van.

Resealing/replacing window rubbers
The good news is that these are relatively easy to remove and replace. Black window rubbers normally last quite well and can often be re-used. Grey ones tend to deteriorate in sunlight and should be replaced with black.

Prop the window and remove the window stays. Next use a screwdriver or similar to remove the grey plastic trim strip from the seal. Underneath you will see the staples used to keep the rubber in place. Remove these with a screwdriver and pliers and pull away the seal noting where it is joined (normally in the middle at the bottom). Hopefully the timber frame underneath will be in good condition. If not some remedial work may be needed. 

Carefully remove any old sealant with a blunt knife or scraper. As above use isopropyl alcohol (IPA) to help clean up.

New seals usually come with mastic already in place on the side that is to fit on the outside of the window frame. If you need more use a non-drying bedding sealant such as those mentioned above and carefully bed into place before securing with a staple gun. Lastly replace the plastic trim strip. Use a hairdryer to soften if necessary.

Footnote

Until 2009 all caravans were built using the time honoured construction techniques covered in this article. Then Bailey introduced its revolutionary AluTech system. Apart from the floor no wood is used. All the panels are fabricated from plastic composites and cured in special laminating machines. They are bonded using a strong aluminium beading with internal clamps to hold everything rigid. This has resulted in a very strong method of construction, unmatched by anything previously attempted. It should be very resistant to water ingress and, even if such ingress occurs, it is likely that only the floor will suffer.

It remains to be seen if the new method of construction will stand the test of time and whether the public will be happy with the higher servicing costs (principally down to the need to periodically re-torque the 300+ bolts that hold each caravan together. If it does stand the test of time then it is likely that other manufacturers with develop similar systems.