The term dry rot is a bit misleading. Rot cannot occure if the wood is dry. Dry rot is a result of wood that at some time was exposed to moisture (even high atmospheric moisture) that raised the moisture content above 25%MC. At this MC fungal spores can propogate and incipient rot begins. If this condition persists you will begin to see visible signs of deterioration. In short rot occurs only when you have the right combination for fungal spore development,and they are warm temperature, a food source(wood),moisture and air (Oxygen). Take away one of these elements and you eliminate the potential for the start of rot.