It is important to understand that the sauna heater mostly heats the air of the sauna by air passing through the heater. If the heater outer casing is closed, the hot air escapes at the top of the heater, and the stratification, thermal layer, tends to form at that level. In wood burning saunas if there is some smoke in the air this can be clearly seen. The open grid heaters are typically floor standing models. Due to the open grid heaters typically being tall, the resistors inside are also long and span from near the floor to near the top of the heater. So as there is no closed casing, hot air escapes from the sides as well, and already at very low level. Thus the thermal layering is greatly reduced.