A partially fire resistant cellulosic fiber thermal insulation material from a fibrous web of unrefined virgin softwood and hardwood provides fibers which provides an R-value (as measured by the ASTM C518 test) of at least about 3, and a wood ash fire retardant component present in and/or on the fibrous web in an amount of at least about 1.5% by weight of the fibrous web and sufficient to impart at least partial fire resistance (as measured by the ASTM E970-08A test) to the fibrous web. Also, a process for preparing this at least partially fire resistant thermal insulation material.