Outdoor Wood Boiler Design: Insulation
When considering the construction of, or how to build the best outdoor wood furnace, one element that is relatively easy to understand, but not always discussed, or even thought about, is the type of insulation that is incorporated into the wood boiler design.
But first lets clarify why it is important. The first reason is rather obvious. There really is no point in heating up all this water to let the heat escape to the environment, so just like your home... the more insulation you have, the less fuel of any sort that will be required to maintain your definition of a perfect temperature.
Given that an average boiler holds somewhere around 200 gallons of water, the added fuel needed to make up for the heat lost to the great outdoors, can be significant... I know many of us would like to turn winter into summer, but this isn't the most cost effective method :)
The second key fact is that retaining as much heat as possible between burn cycles will reduce the amount of smoke that goes up the chimney. Smoke, at least the smelly kind is created every time the fire is not hot enough and sends most of the combustion gases up the stack before they have had time to completely burn.
By keeping the burn chamber as warm as possible, when the fire ignites it will not take as much time or energy to get to the temperature needed for efficient combustion.. thus less smoke!
So if I have convinced you that insulation is good, let me take that conversation one step further.
What type should you use?
I have spray on foam in my dry kilns and love it. It wraps and adheres to the wood 2x4 construction and creates a great water proof seal around all joints. It doesn't sag and never gets waterlogged.
But when you are considering how to build an outdoor furnace, or what makes the best outdoor wood furnace there is one element that is more important than any of these considerations.
Polyurethane spray-on foams do come is a number of different "varieties", but to the best of my research capabilities they do not really appreciate the types of temperatures that are common around wood boiler water jackets.
One fellow told me of a story of a furnace that he had visited where the foam had crystallized right to the edge of the boiler, and everything else disintegrated between the boiler jacket and the outside shell.
Thus the best outdoor wood furnace and approach when learning how to build an outdoor furnace is to stick to the traditional fiberglass batts.... there are a number of other distinct benefits to sticking with the traditional but this is the most important.... unless of course they can show evidence that the spray on version is temperature resistant to the boiler temperatures you expect.