Do not confuse lightweight insulating fire bricks with heavy dense firebricks. Those are used in different applications. Dense firebricks can be cut only with diamond wheel attached to high speed handheld angle grinders, on an ordinary building brick saw or sliding drop saw for cutting bricks. Insulating bricks are refractory too and of course withstand very high temperatures range BUT for purpose they have very low thermal conductivity and don’t absorb the heat well at all. Those are intentions for heat insulation. For instance if you used them for building of the hot face in a wood fired oven (for cooking) the oven wouldn’t cook, because it wouldn’t store and hold almost any heat. You can however use these insulating fire bricks on the outer side (of the heavy firebrick walls, vault or under the floor bricks and slab) to prevent the soaked in heat in the chamber’s body mass from escaping and achieving superb results. Meaning well insulated oven which will hold the absorbed heat in its mass, where it should be to cook for long time.
How to use light weight fire bricks insulation
Use of light weight fire brick insulation is broad, mainly in industrial and hobby kilns heated up with either electric spiral elements or gas burners, furnaces, both for hot face lining or outer back-up heat insulation.
Source : http://www.traditionaloven.com/articles/81/insulating-fire-bricks