bunk condensation

[sailorman]

i seem to get a lot of condensation between the bunk top & the u/side of the cushion, overnight. the bunk top has the aft end "with-in" the cabin the fore end has stowage under.
i know there is a propriatry product that is quite expensive & seems to be flimsey.
any thoughts?.
P.S. it seems to be more noticeable with my new sleeping bag.

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[Salty]

I've always had the same problem: I couldn't find a better solution than just turning the cushions up on their sides after a night's sleep...

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[Rowana]

A common problem which affects many people. With, I suspect, just as many "Cures".

I would suggest that your new sleeping bag is more "pourous", if thats the right word to use, and is allowing more "sweat" to penetrate through the bunk, where it condenses on the base/locker top.

2 ways to tackle the problen IMHO.
1. arrange some means of allowing air to circulate under the coushion. There are, I believe, some proprietary "mats" which are designed to achieve this. Try a caravan shop, as caravans have similar problems.
2. A barier between sleeping bag & cushion to stop "vapours" getting through to the cushion in the first place. Again, try a camping shop and get one of these cheap "groundsheet" thingies, you know, about half-inch thick foam sort of stuff, that campers use in their tents.

No doubt you will receive a few more "patent" cures from others, which I'm sure work for them. Just try a few and see what works for you.

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[sailorman]

thanks Jim
have sent u a pm relating to BP tankers

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[Rowana]

Read that, thanks.

Sent you a reply which you might like to show your brother
Jim

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[MainlySteam]

Placing a barrier between the body and the bunk cushion is one way of curing it and is what we do. We find that for spending long periods aboard it was the only cure. We use a soft waterproof plastic sheeting that one can buy cheaply from specialist baby gear retailers to go in babies cots - it is not PVC (so doesn't smell plasticy) and is soft enough so that it doesn't make crinkly sounds when you move. We put an underblanket over it for comfort.

Because the moisture is then trapped above the mattress in the bunk with one, it does tend make the bunk more humid than a shore based bed where the moisture can soak into and evaporate out of the mattress. Light, permeable bedding helps to relieve that.

Having a fully slatted bunk base for the mattress to sit on is also normally a successful solution, however, if the under bunk area is not well ventilated and if it is used for storage one just transfers the problem from one place to another.

John

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[nicho]

I cured this totally with a specialised product called 'Ventaire. It's comes in large flat, nylon 'brillo pad' type sheets, 2mx1m, covered on both top and bottom with breathing material, that are easily cut with scissors to the shape of your bunk. Not cheap, (about £35 a sheet) but it really works - the extra 1/2" or so on the depth of the mattress seems to make it more comfortable too!. It's available from chandlers, and is often advertised in the magazines.

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[sailorman]

thanks
i new about this product it just seems very expensive & doesnt appear that strong / durable

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[Miles]

Ventair hasn't really solved the problem for us. I've bought a set of beech slats from the Natural Mat Company (www.airflowsprings.com) but wll have to wait until next season to see if they are any better. Allowing a good airflow under the mattress seems to be the key to solving the problem so we tend to raise all the mattresses using small plastic flower pots when we are not on the boat. This restricts the problem to when we are on the boat for longer periods. the prblem is worse on the bunk above the cold water tank and affects the larger members of the family much more than the smaller ones.

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[MainlySteam]

After talking with many boatowners and also people in the industry, the only sure way to beat the problem (unless of course one has a boat where a full depth and breathable inner sprung land type mattress can be used) is to place a moisture barrier between the bodies in the bunk and the bunk cushion as explained in my earlier post. For example, from what I have seen, boats which have a vinyl or other essentially impervious coverings on their cushions do not have the problem from moisture migrating from bodies in the bunk through the cushion to be trapped by the bunk top.

Of course, if the boat is only used occassionally for overnight stays, then lesser solutions may work. But for live aboard or extended cruises beyond a few days then the above is the only sure fire solution that I am aware of - it also has the advantage of being virtually free. It does have the disadvantage that the bunk is slightly more humid to be in as the only passage for the water to leave is through the bed coverings.

An open slatted bunk base (ie one where there are gaps between the slats to the underbunk area) can also work, but one does run the risk of transferring the problem into the storage or other area under the bunk.

John

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[Mirelle]

Slats

Before moving aboard my boat, some years ago, I was advised, by other liveaboards, to dump all the plywood bunk bases, and replace them with hardwood slats, and so I did.

Jolly glad I did (though I moved ashore again for other reasons!). Have not had a problem with moisture collecting under the bunks, as this space is pretty open, in my boat, with a gap of at least several inches (gosh, there's a reason for not having a production cruiser, at last!) between bunk base and whatever is under it (water tank, tools and spares stowage in drawers, thin air, depending on bunk!

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[MainlySteam]

Re: Slats

Yes, I too think slats instead of a solid base are an ideal solution on a vessel, such as a classic, which lives with cosily damp bilges anyway and the natural ventilation of those spaces takes that into account.

What I have also seen, and which works well in vessels which have dust in their bilges and which also usually have poor bilge ventilation (and the under bunk spaces are often mostly air tight compartments), is slatted bunk bases with the lining up the topsides also done in closely spaced slats which allow a natural movement of air by convection up from under the bunks and bilges and out the higher lining slats thus taking the moisture away from the under bunk areas.

John

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[tcm]

Re: another solution

ventilating the condensation with slats partly removes the cold surface, and partly allows air to move and evaporate the condensation.

To stop the condensation happening, insulate between the base and the support. Get some of that blue "builders' board" (actuially, very hard polystyrene insulating foam) or nick some from a skip. Cut to shape, plonk under the matress and no more condensation.

Properly insulated, there shd be no condensation anywhere. porthole windows need just cracking open (unles they are are double glazed (?))




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[Mirelle]

Creating a Howling Draft!

That's something that I had not thought of.

On a wooden boat, one spends much of ones time creating a Howling Draft through every corner of the boat, in the fervent belief that should even the tiniest corner remain un-ventilated, Dire Consequences Will Ensue!

Hence, aboard Mirelle, an abundance of cave lockers, slats outboard of bunks, oak panelling in the saloon with about four inches between that and the planking and a sizeable gap at top and bottom, teak gratings at either end of the cabin sole, and so on. The Howling Draft is guaranteed by leaving the forehatch part open and the saloon skylight open under the dinghy, plus three big mushroom vents, when not aboard - first act on getting aboard is to diminish the gale blowing through the cabin by shutting a few things!

On a well planned wooden boat there is always ventilation from under the bunk to outboard of it, with slats, panelling, etc. on the frames.

It has just struck me that a GRP boat in a marina is a bit different, from the ventilation point of view...

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[timevans2000]

Ventair helps up to a point.

The only way to completely minimise the problem is to remove the cold surface that is creating the opportunity for condensation, ie a surface below dew point.

This is can be done by insulating the cold surface to raise the surface temperature. Closed cell foam is a good choice as it also provides a built in vapour seal to stop inside air reaching the cold surface. Open cell foam can be used but needs a non permeable cover like aluminium foil. Tape any joints with self adheasive aluminium foil to maintain the vapour seal.


With plenty of ventilation and lots of insulation you probably wont need any heat.

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[MainlySteam]

The main cause of the problem is not condensation so insulation will not provide a solution. What happens is that in the bunk the body moisture from those inhabiting it is driven from the warm bunk towards the cold both above the bed coverings and below the mattress. The moisture that migrates to the cold upwards goes into the air in the cabin. However, the moisture that goes down into the mattress meets the solid barrier of the bunk base and can go no further (well it can but only very slowly) so the top of the bunk base that the mattress sits on becomes damp and mould will grow on the bottom of the mattress.

Putting aside impractical things such as heating the underneath of the bunk so that it is warmer than the inhabited part of the bunk, the only possible solutions are to provide ventilation between the mattress and the bunk base, or down through the bunk base so that the moisture can be carried away and not be trapped, or else create an impermeable barrier to the moisture on the top of the mattress by use of a plastic or closed cell sheet so that migration down to the bunk base is prevented.

John

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[timevans2000]

Sorry but dont agree. If you remove the cold surface you get no moisture build up. Unless you are soaking wet in bed then the moisture beneath the matress is condensation, ie warm moist air coming in to contact with a surface below dew point.
If your mattress was made of closed cell foam instead of open cell foam, the moisture would not migrate through it so you would get no condensation. I have a layer of Ventiar under my mattress that does not completely remove the problem. I still get moisture and mold growth in winter. It is, however, not as bad as on my last boat as the underside of the bunk base is better insulated

Your arguement about ventilation is only part of the solution. Removing the cold surface will help conciderably as will general ventilation in the boat to lower the overall Relative Humidity.
Also lifting your mattress when you leave the boat also helps stop mold.

Your last paragragh actually agrees with what I was saying. Your are effectively insulating the bunk base with a vapour sealed insulation, your matress! I would not recommend this though as you will get very sweaty. A bit like nylon bed sheets!



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[MainlySteam]

Fine Timevans.

Still, it sounds like you get mould and I don't - even though we have lived on board for long periods of time and spend weeks away on cruises. I get paid to give advice on high quality vessels, so when I give it free I am quite happy for it to be discarded.

Lets leave it at that.

John

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