A variable U-value would be beneficial for a buildings thermal performance. One way to switch the U-value of a wall, insulated with nano-porous material, is to change the internal pressure of the insulation This paper present thermal conductivity measurements showing a possible variation around of 3 times for a fumed silica and less than 2 times for an aerogel blanket when the pressure was varied between 1 and 100 kPa. The variation factor of 3 was used in building energy simulation of a Swedish office showing that a U-value which can be varied within that range can give a significant reduction energy demand. Especially when energy used for cooling is weighted as worse than energy used for heating.