Before you decide on a system, you should make sure that the results will help your collections. For many kinds of objects, a steady relative humidity is much better than constantly changing one, and it can be difficult to provide steady levels even with a ducted system.
Another problem is that if your building envelope is not very tight, much of the effort will be in vain – the humidified air will leak out.
In addition, some of the literature is inaccurate in recommending RH levels. For example, low levels (around 35%) have now been shown to be better for archival materials.
Have you had a conservator who is expert about museum environments look at your collections?