If you’ve ever told anyone that you live on a boat, you’ll have heard this question, I’m sure.
Keeping warm on a narrowboat is actually relatively easy. There are several ways to heat the inside, some of which are more expensive/simpler than others, and also several ways to keep the heat in once you’ve added it.
In my own personal opinion, insulation is by far the most important thing. There are two main kinds – polystyrene and spray foam. Polystyrene is cheap, lightweight and easy to handle, even for a layperson. It’s also easy for it to fail, creating a heat gap that can cause condensation. This is often a death knell for a wooden-cabined boat and won’t do a steel boat any favours either. Spray foam is much more difficult to work with and often more expensive, but is a far superior product. Due to the fact it expands as it cures, it will fill any small crevice it is applied to and when applied properly, will create a solid “tube” of insulation, leaving nowhere for heat to escape.
Speaking in very general terms, there are three kinds of heat: conduction, convection and radiation. In terms of physics, there are several others, but we’re only going to concern ourselves with these three. Heat is transmitted conductively by touch, such as wrapping your hands round a hot mug of cocoa on a cold day or burning your hand on a hot stove. Heat is transmitted convectively by air, such as the gust you feel wafting into your face when you open a hot oven, or conversely, the cold draught you feel on your feet when you leave the fridge door open (in terms of physics, there is no such thing as “cold”, just more or less heat energy). Heat is transmitted by radiation through infrared radiation, which is the kind of heat you feel when standing near a wood burning stove, unless you have a stove fan or are laying on top of the stove which would be convective or conductive, respectively. It’s also the kind used in this huge patio heaters you often see in pub beer gardens. In terms of transmissible distance, conduction is the shortest while convection is the longest, which is why you need to be near a stove to get the benefit from it.
Water and metal are very good conductors. That is to say, they both transfer heat energy well, while air is a poor conductor. Heat will always flow from a hot thing to a cold thing, not vice versa (see entropy, and the second law of thermodynamics). Imagine a room with a temperature of 20°C and a cup of air and a cup of water on a table. Both the air and water are at room temperature – 20°C – yet a human (with a body temperature of 37°C) will perceive the cup of water as being colder than the cup of air. This is because heat energy will be drawn much more quickly from the body by the water than by the air. Another example would be touching your bare hand to a metal car body on a frosty morning – both the car and air are the same temperature, but the metal car is much more efficient at transmitting heat energy, conductively in this instance, than the air is.
With regards to narrowboats, they are generally made of steel (a good conductor) floating in water (another good conductor). This is why it is essential to ensure that one’s insulation is up to snuff – without sufficient insulation, the entirety of a narrowboat will become the same temperature as the water it’s floating in. Without insulation, far greater quantities of heat energy would be necessary to maintain a comfortable temperature inside the boat, as anyone who has seen their heating bill when someone has left the door open will attest. Add in the relevant vents to provide fresh air for the stove, the often single glazing and the lack of thermal mass, and the answer to the question is “Well, it certainly can get cold in Winter,”.