Maintaining a stable climate is one of the most important aspects in preserving archaeological materials. Organic materials such as wood are sensitive to environmental changes, especially humidity. If the humidity is too high, it favours biological activity and the conservation agent used to preserve the ship becomes sticky and attracts dust. If the humidity is too low there is risk that the wood will dry and crack and shrink.
The salt outbreaks on Vasa have most likely developed partly as a result of unstable relative humidity (RH), as the old climate plant could not cope with large numbers of visitors during wet weather. In 2004, the entire plant was substantially overhauled, and now maintains the relative humidity around the ship between 51-59 percent RH.
The temperature is deliberately maintained around 18-20°C. This is a balance between preservation requirement and visitor comfort. It would be better for the ship if temperatures were lower, as elevated temperatures can accelerate chemical reactions and biological activity.
For more information, you can read the article Vasa's New climate-control system by conservator, Emma Hocker:
Vasa's new climate-control system (pdf)
Light
The Vasa Museum is deliberately kept quite dark. This is because organic materials can be broken down by high and intense light levels, especially light containing ultra-violet radiation (UV). Therefore the light levels in the museum are kept below 100 lux, daylight is not allowed to fall on the ship, and ultra-violet filters have been placed on all the windows.
Dust
With more than a million visitors each year, it is not surprising that dust accumulates quickly in the museum. While the public areas are cleaned daily, keeping the ship free of dust is a more complicated job. Twice a year we clean all the outer surfaces of the ship and the rigging using vacuum cleaners and brushes. To reach all of the ship's surfaces, we use the work platform at the stern of the ship, a skylift and an overhead crane.