We now have the proper insulation (see pages 24 and 28) that the furnace REALLY needs; (we and everyone else has been doing it wrong according to the insulation engineers!)

We now have insulation with R-23 in the sides and R-57 in the front against the 700 degree firebox with up to R-88 optional EXTREME Cold Weather Package!

Superwood Insulation (white) up to R-88;
with the 3 inch Extreme Cold Weather Package

Hyprotherm Forced-Air Wood Furnace shown

R-23 side insulation made by Knauf; superior to everything else because it is not always in the numbers!

Hypro Therm insulation engineering specifications. Insulation, the RIGHT Product for the application.

Furnace efficiency is achieved by preventing heat loss from the furnace through the firebox insulation. Extremely low ambient temperatures can quickly drain the efficiency from most outdoor furnaces if not properly insulated, with the correct insulation. Most outdoor furnace manufacturers use thick layers of low density, low use limit household or metal building insulations designed for use at room temperature, hoping to impress the unsuspecting consumer with the high published R values of their insulation system. Thermal resistance is expressed in R value which is the total thickness of insulation divided by the thermal
conductivity value (k value.) Typically R value is expressed at a constant temperature of 70 or 75 F. In a furnace application, R value is not applicable as the temperature of the firebox can be 10x the temperature expressed in the R value measurement. As temperatures rise, K value increases rapidly (lower insulation value per inch of insulation.) Above 75 F. these low density insulations quickly increase in conductivity due to the forces of convection and radiation as they
are not designed to be used at elevated temperatures. In fact, most of these residential and commercial insulations contain some binder system which helps them maintain mechanical integrity and repel moisture. Beginning as low as 250 F. these binders begin to break down due to the heat and by 350-375 F. most of the mechanical integrity and moisture resistance is lost.

All HyProTherm furnaces utilize Thermal Ceramics Superwool Plus® low bio-persistent high temperature insulating wool as the firebox insulation. Superwool Plus® contains no binder to burn out and is mechanically needled to maintain structural integrity at continuous temperatures of up to 1832 F. , indefinitely. The higher density and higher fiber index of Superwool Plus® yields very low thermal conductivity values at temperatures above 200 F. compared to other fibrous insulations. Superwool® fibers are used in a variety of applications from aerospace and automotive heat shields, industrial furnace linings in power generation,
metals processing and petrochemical applications, as well as many household and commercial cooking appliance applications.