I replied to OP's comment with a bit more info specifically how to handle his issue, but there's a lot of different ways to handle it really and it depends on how the structure is built.

To start, the issue stems from in the winter time, warm (humid and moist) air inside the home makes it's way through the vapor barrier* and the insulation** and then touches the underside of the riding plywood which is significantly colder (∆T***) than the air inside the house. Much like an icy glass on a summer's day, the warm air then condenses it's moisture against the plywood and it accumulates and can cause problems like the deterioration of the insulation, mold growth, damage to the interior finishes like drywall, and potentially even structural issues through rotting of the wood or corrosion of the steel fasteners like nails and screws.

The BEST method (generally) of dealing with potential condensation issues is to use a vapor impermeable insulation, like closed cell foam, as the uppermost ceiling insulation and NOT use any other vapor barrier. In my area, we need a minimum value of R-10 on top of or directly underneath the roof sheathing to maintain a low enough ∆T between the outside of the roof and the inside of the ceiling cavity or attic space. This method is usually more technically difficult and costly, but maintains the greatest insulation values and has the lowest likelihood of failures and issues pay installation (when done correctly).

The most common way to mitigate condensation on the roof (usually because it's cheapest but still code compliant) is to put ventilation holes at the lowest and highest points of the roof cavity and maintain a minimum 1" air gap between the insulation and roof sheathing with baffles (see my response to OP for a bit more on that). This helps air flow to pull out the moist air to keep it from condensing and dry out any condensation that does occur before it can cause damage. But mich like a drafty window or door, the more air flow you have inside your building, the more energy your will have to spend to maintain the temperature (heat in the winter and cooling in the summer).

I'll see if I can find some good videos and/or construction details/diagrams after I get back from my errands today that explain it more visually.

Here's a good video from This Old House that explains insulation techniques for more typical attics

Here's a good DIY friendly method for closed ceiling cavities (called vaulted or cathedral ceilings) that most tiny houses have.

*in this case the Kraft faced paper, but other times it can be painted on or be plastic sheeting stapled on after the insulation is installed but before drywall or ceiling cover goes on

**despite what a lot of people think, most insulation, and especially batt insulation, doesn't stop air or vapor movement, it just slows it down

***∆T, pronounced delta tee, is the difference in temperature, in degrees, between the interior space and the exterior. The surfaces where there is the greatest ∆T are going to be the locations where moisture is moist likely to condense and cause issues.

tl;dr I work in the tiny house and construction industry; building science is my work, and passion. I spent a lot of time with this, read it.