posted 5 years ago
Ceramic fiber board is the latest and most efficient material for building heat risers and other parts of the hot core, but as you noted it is very expensive, not the cheapest method. Perlite-clay, if mixed to the most insulating ratio (little clay), is weak but will work fine as a heat riser if contained in a sheetmetal outer shell. It is probably too weak for use in parts of the firebox that can be reached while feeding or cleaning. Firebrick, and especially insulating firebrick, is an excellent material for building the combustion core. Use the strongest material at the feed tube where wood can scrape it, and possibly for the near parts of the burn tunnel if you may be handling it roughly. What works for a person who places wood gently in the feed may not work for someone who throws logs in.
You do not need any metal at all to build a good RMH, aside from the P-channel for a J-tube or the floor channel for a batch box. If old barrels are expensive but common bricks are cheap, you can build a brick box instead of the barrel.
What part of India do you live in? The degree of cold, and how long it lasts, makes a big difference in what will work best.
I agree with Gerry, build a J-tube outside and see it burn to get a good idea of how it works, before you try to design a system for your house. The J-tube is simple to build and has only a few important dimensions and rules, while a batch box as Peter van den Berg describes must be built precisely according to plan and is more technical. They are both around the same efficiency; a batch box holds more wood at a time and needs to be fed less often. You cannot open a batch box in the middle of a burn and toss in a few bits of fuel, you load it completely at the start and let it burn down. A J-tube can be fed with bits of varied fuel as desired.