Condensation is a serious problem for liveaboards. It will form in places you would never think of and in a short period of time, puddles will form causing water damage to everything it touches. One of the common places this issue develops is under a mattress.
When you lay in bed, your warmth comes through the mattress until it reaches the cold sides of the bed sole. At this junction of warm and cold, condensation will form; soaking the mattress and growing mildew everywhere!
To prevent mildew, you need to prevent moisture. To prevent moisture, you need to prevent condensation. To prevent condensation, you need airflow. How can you get airflow under a mattress? You need something to form an air space where it can freely ventilate the space under the mattress.
There are very expensive systems for sale that look like wafer tiles. These hollow tiles will lift the mattress up allowing airflow and provide enough strength to support the weight of people sleeping in the bed. This seems like a wonderful option until you calculate the cost. Dri-dek tiles from West Marine cost $6.29 per square foot. If your berth is 4 feet wide and 7 feet long, you have 28 square feet to cover, costing $176.12. This seems feasible, but it is still a significant cost compared to the alternative.
I went to Home Depot and purchased a few wooden battens for $0.50 a batten. These battens lift the whole mattress up to provide airflow while also providing support for two adults to sleep on this berth all night, every night, without any problems. For the forward V berth, we used 9 battens arranged in rows promote the flow of air through and under the mattress, keeping it dry and mildew free for only $4.50
Our mattress is very thick, so we don't feel the battens through the mattress, but if you find it uncomfortable to sleep on the battens under the mattress, you can always lay some wooden or PVC trellis on top of the battens to further support the mattress while keeping the airspace open.
The results were dramatic and instant. We went from a full puddle under the mattress that would drip off the foam when the mattress was lifted to bone dry in one day! If you want to spend a lot of money on this project, Dri-dek tiles will work wonderfully, but for a minimal amount of funds, the same results can be achieved using the wooden battens.