Q: What is the best weather/temperature and humidity condition to install hardwood/engineered floors?
A: That is a tricky question and I don’t think there is any one correct answer. It depends on the general climate where you live for one. If you are most comfortable at 75F with 45% RH IN YOUR HOME then acclimate the flooring to that and try to maintain that environment as closely as possible year round.
If the flooring is installed, for example in very high humidity conditions in summer (even if it is acclimated to that) when the heating season arrives, the furnace kicks in and the RH in the home now becomes 30% or less, the floor will shrink and probably significantly.
If you install the floor in winter, during heating of the house and the RH is very low, in summer if humidity is high the floor will expand and if an expansion space has not been allowed at the walls or if no effort is made to lessen the humidity, you will have a problem with cupping.