Since we have switched from plywood the composite LP Smartside people have noticed more moisture in their shelters. Plywood is prone to warping with changes in moisture and temperature. This creates a much looser fit of all  the parts of the shelter, allowing more air leakage, creating more venting of the warm, moist air that forms around the cat in the living area. The Smartside product is made from finer grains of wood in random orientations that create a wood that is much less prone to warping due to changes in temperature and moisture. This results in a much tighter shelter with far less air more warm air inside the shelter...and a warmer shelter. Warmer air, though can hold much more moisture.

The moisture people see inside the shelter is actually condensation from this warmer air...from the cats themselves! The condensation is likely to be very obvious at some times and not at others, as the temperature and humidity fluctuate. But since our shelter helps protect the cat from the cold by creating a zone of warm, moist air where the cat sleeps, condensation will always be present when the cat is using the shelter.

The cats are spending a lot of time in a small space and moisture from their fur and even their respiration condenses on the relatively cooler walls and ceiling. Because our shelters (especially the newer shelters) block the wind, the warm, more moist air stays inside the shelter. During very cold weather the moisture will freeze to exposed cold surfaces but during warmer weather will thaw and run off the insulation and be absorbed into the bedding. As long as cats use the shelters it will happen and there's no way to prevent it, but there are ways to mitigate it.

You can visualize this as a person sitting in a cold car in the winter. Spending just a few minutes in the car will cause the windows to fog/frost as the moisture from the person's breath condenses on the glass. The moisture actually condenses on other surfaces, but it is most visible on the glass. If you open the door, the windows won't fog up any more...because the cold, dry air can freely come in an mix with the warm, moist air and cool it off before it condenses so much on the glass. Many shelters with an opening directly into the living area are designed like a car with a door open...allowing cold air to flow right in...and warm air to flow right out. They don't have much problem with condensation, but the interior of the shelter is pretty much as cold as the exterior...which does little to help the cat.

To illustrate, during testing in our own colony, with an outdoor temperature of 36 degrees F, a shelter housing a medium sized cat read an indoor temperature of 56 degrees F, measured at the roof level (it's actually warmer at the bottom, but the sensor unit was too large to locate at the bottom). But more interesting, the interior humidity was a whopping 85%.

It is possible to virtually stop condensation inside the shelter. It can be accomplished by heating the interior of the shelter sufficiently that no interior surface is below the dew point temperature. In practice, though, this requires many hundreds of watts of heating. It's hard to heat it safely with that much heat, and just impractical inside a small shelter. It's like running the engine in the car and running the heater until it gets so warm that you don't see frost on the windows any more.

One solution for warmer climates is to increase convection by propping up the roof, especially at the higher peak side. Because of the overhang on the roof, you'd probably need to prop it up at least a half inch...maybe more depending on the amount of condensation. There are a number of factors that affect condensation like outside temperature, humidity, precipitation, size of the cat and even the type of fur on the cat (which affects how much moisture they bring into the shelter). So experimentation may be required.

The downside of increasing convective flow of the air by venting at the roof is that the living area will be colder as the warm air flows out, more colder air will be pulled in. This would be pretty undesirable unless you're in an area that isn't bitterly cold.  In central Indiana, we typically get low temperatures near or below zero (Fahrenheit) at least a few days during winter and have an average high in the low 30s. In this kind of climate, venting the warm air is probably a bad idea as the cat will need all the heat it can get. If you're in a warmer climate, some venting might be okay.

Practical ways to mitigate it (without degrading the shelter performance) involve absorbing the moisture. Make sure you have absorbent bedding like straw and some absorbent fabrics inside the shelter and if possible, change them periodically. Putting absorbent pads (like puppy pads, diapers, etc.) that will sequester the moisture into a gel will keep it from pooling on the floor. Put the pads under the bedding, if using straw. If using fabric, place the absorbent pads on top of the fabric. Here's an example of such a pad: