Under mattress dampness

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My boat is very dry except for under mattress condensation after a night's sleep. Drymat helped a bit but didn't really work. All the under bunk locker lids are well ventilated but there's no through flow. The best option seems to be to fit louvres in the bunk sides but these are currently watertight. Is there any real benefit in having watertight bunk sides. Other options seem to be to try IKEA type bed slats or insulate the underside of the bunks. Another alternative might be to duct hot air heating under the bunks. Anyone tried any of this.
 
Have a look at the froli spring system, expensive but worth it and improved the comfort 100%. It made it a bit fiddly to access the under bunk storage, so I finally enclosed the springs and 4 inch foam in mattress covers (3) to fit my v berth. It creates a 50mm approx. space between foam and base, no condensation, though our climate is a bit milder
 
Surprised dry mat didn’t do the trick. Another is to get a silent 12v fan on a time which circulates air beneath the drymat. I.e drill a large diameter hole under the mattress and mount the 12v sucking air downwards into the compartments. This will suck air from the gap / void created by the drymat.
 
I fitted sprung slats which gave about 1 to 2cm air gap between the mattress and locker tops worked OK. As an addition you could try insulating the locker with sheet insulation material, do sides and top.
 
Dry mat did not work for me either. If you have storage under the bunks you have to make sure that any system you use can be cut to fit the lids. Slats may not do this. I seem to recall that the Froli system consists of small sections that clip together. That may not work either, if narrow widths needed around the lids.

Having seen the comment above about insulation,(#2) I am wondering if the foil backed foam that goes under laminate flooring (get it from Wickes) would act as a thermal break. It could be stuck down with double sided tape, as an experiment, under the Dry mat. It could be cut & fitted to the lids OK. Fairly cheap so possibly worth a go on one bunk first
 
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Graham376 said:
I fitted wooden slats which have rubber end couplers which allow the slats to stand off. It came from Compass24 but the current one looks a little different. Anyway, it works a treat https://www.compass24.com/comfort/s...350/bedflex-marine-single-bed-system?c=108408
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I've got those on my full time liveaboard & they work an absolute treat but there is quite a lot of space below them, i.e. they do not sit on top of boards. If they were sat on top of boards I think you'd still get some condensation. Without fresh air being able to get under the mattress it'll get damp, if Drymat is used it should extend past the edges of the cushion/mattress to allow this, maybe it's why some people find Drymat works for them & others not so much?
 
Humblebee said:
We have drymat under our bedding. It helps but there is no substitute for raising the mattress and propping it up somehow through the day.
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As V1701 mentions above, the slats we both have lift the mattress by about 20-25mm so there's plenty of room for air movement. Looking at the prices now, it would be very easy to DIY them at a fraction of the cost. Just glue some spacers under timber slats and tie them all together with some webbing. The only drawback is they can make access to lockers a little more difficult so I don't store regularly used stuff under the bed.
 
northcave said:
Surprised dry mat didn’t do the trick. Another is to get a silent 12v fan on a time which circulates air beneath the drymat. I.e drill a large diameter hole under the mattress and mount the 12v sucking air downwards into the compartments. This will suck air from the gap / void created by the drymat.
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I have a solar powered ventilation fan in the hatch above the main bunk. Similar to this (no affiliation):

https://www.ebay.co.uk/p/Solar-Vent...pG0oTEB8q8umdfdzXFksiGq6UlMIXlZIaArqOEALw_wcB

And I stand both mattresses upright on their sides when I leave the boat.
 
Just wondering if the sea is cold will allowing cold air to circulate through vent holes in the locker hatches under the bunk be worse than sealing the hatches and insulating the surface.
 
The best thing we have found is to keep the boat dry throughout all the time you are not on board - so the mattress, whatever is stored underbunk and all the furnishings and linings in the boat are greedily soaking up the mositure of people on board - and the effect lasts for quite a few days. So a dehumidifier left connected is your friend - and the boat will seem a lot warmer too.
 
KAM said:
Just wondering if the sea is cold will allowing cold air to circulate through vent holes in the locker hatches under the bunk be worse than sealing the hatches and insulating the surface.
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You are asking the right question!

This depends on the boat. Cored hulls have a big advantage here; my last boat never sweated, my new one does, and the difference is core thickness. Condensation happens when you have more humidity (sleeping--evaporated moisture goes through the mattress) or a lower temperature.

My last boat had vinyl covers on the mattress, with a foam topper over that. As a result, moisture did NOT go through the mattress and there was zero condensation under the mattress, down to 0F (and the bunk was on a cat bridge deck, with outside air under it!). The interface between the topper and the mattress was too warm for condensation. The topper provided all the breathability I needed.

I'm more of a believer in controlling the temperature of the interface and avoiding dew point surfaces than in ventilation. You ever try to prevent a glass of ice water from sweating by blowing on it? Good luck. But an 1/8" of insulation works. So insulate the cold surface, being certain to seal the insulation so that air does not get under it.
 
We use curved Ikea bed slats cut to fit the available space. If you can arrange that the space between the battens is open to the cabin then you will achieve some air circulation between the upper surface of the lockers underneath and the slats. The last time I looked Ikea did not sell curved slats - but that is Oz some years ago.

We did both queen berths and have had no issues since.


We also perforated the upper surface of the locker top beneath, using a hole saw, and added vents to the locker below (cheap plastic vents - again using a hole saw - the vents were round) We do not have issues of condensation from extremes of cold in Oz, sea temps only drop to 16 deg and usually is around 23/25 in summer (and warmer further north). Our problems are usually too much heat - and ensuring all hatches are open (and nightlife screens inserted). Like Thinwater we are cat owners and the underside of our berths is the bridge deck and air temps in summer are 25/30 - but that does not stop condensation - humans exude moisture - it has to go somewhere - there are always temperature differences - you need to remove the moisture - somehow.


Jonathan
 
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The OP does not mention if the boat &/or cabin are themselves well ventilated. Clearly, such devices are unlikely to be a solution if the moisture has nowhere to go.
 
guernseyman said:
The OP does not mention if the boat &/or cabin are themselves well ventilated. Clearly, such devices are unlikely to be a solution if the moisture has nowhere to go.
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It's a very dry boat normally. 2 mushroom vents and two venti lites and vented wash boards. I can leave cushions and bedding aboard all winter. It's wood lined and has a grp headlining with air gap. There's sometimes a bit of condensation on hatches and frames, spring and autumn, in the morning but overall very dry.
 
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