Method for providing dehumidification of a retrofitted insulation of an interior side of a building envelope, especially an external wall, comprising: - attaching an insulation material to the interior side of the building envelope such that a hollow space for conducting a fluid is provided between at least a part of the interior side of the building envelope and the insulation material; - sealing of the hollow space making it substantially fluid tight; - providing the hollow space with a fluid inlet and a fluid outlet; - providing means for entering fluid into the hollow space through the fluid inlet and for controlling one or more characteristics, such as temperature, humidity and velocity, of the fluid entering the hollow space through the fluid inlet, - providing a humidity detector for detecting the humidity at one or more points between the interior side of the building envelope and the insulation material, - providing means for controlling one or more characteristics, such as temperature, humidity and velocity, of the fluid exiting the hollow space through the fluid outlet, wherein the following steps are carried out: - entering fluid, preferably in the form of ambient air, more preferably from inside the building envelope, through the fluid inlet and into the hollow space, - controlling at least one of humidity, velocity and temperature of the fluid inflow through the fluid inlet and into the hollow space, and AWA#136518 - controlling at least one of velocity, humidity and temperature of the fluid exiting the hollow space through the fluid outlet, - regulating the velocity of the fluid both when entering the hollow space and exiting the hollow space such that a substantially atmospheric pressure is maintained inside the hollow space to create a substantially even air distribution throughout the hollow space, characterized in that, the fluid is entered into the fluid inlet and into the hollow space by means of a perforated inlet ventilation duct having a predefined inlet length and inlet configuration and wherein the fluid is exiting the hollow space through the fluid outlet by means of a perforated outlet ventilation duct having a predefined outlet length and outlet configuration. - attaching an insulation material to the interior side of the building envelope such that a hollow space for conducting a fluid is provided between at least a part of the interior side of the building envelope and the insulation material; - sealing of the hollow space making it substantially fluid tight; - providing the hollow space with a fluid inlet and a fluid outlet; - providing means for entering fluid into the hollow space through the fluid inlet and for controlling one or more characteristics, such as temperature, humidity and velocity, of the fluid entering the hollow space through the fluid inlet, - providing a humidity detector for detecting the humidity at one or more points between the interior side of the building envelope and the insulation material, - providing means for controlling one or more characteristics, such as temperature, humidity and velocity, of the fluid exiting the hollow space through the fluid outlet, wherein the following steps are carried out: - entering fluid, preferably in the form of ambient air, more preferably from inside the building envelope, through the fluid inlet and into the hollow space, - controlling at least one of humidity, velocity and temperature of the fluid inflow through the fluid inlet and into the hollow space, and AWA#136518 - controlling at least one of velocity, humidity and temperature of the fluid exiting the hollow space through the fluid outlet, - regulating the velocity of the fluid both when entering the hollow space and exiting the hollow space such that a substantially atmospheric pressure is maintained inside the hollow space to create a substantially even air distribution throughout the hollow space,