A well-insulated softwood room and a good heater are essential for a sauna.
Literal
Softwood [genitive-の] insulated room and good heater [topic-は] sauna [for-のための] absolute necessity [copula-である].
Technical prose, slightly awkward translation-sounding. The adjectival 絶縁された ('insulated,' passive attributive) modifies 部屋 ('room'). および ('and') is the formal/written conjunction, more formal than と. のための ('for the purpose of') modifies 絶対必要 ('absolutely necessary'). The sentence reads like a product manual, which is the register it probably came from.