“严密的”的英语可以翻译为：close，rigorous，tight，narrow ...

水箱和屋顶都需要敷设严密的保温层。Water tanks should be well lagged and the roof well insulated.从防守高度严密的监狱的集体越狱a mass breakout from a top security prison这一地区周围设有严密的安全警戒圈.There is a tight security cordon around the area....