He's used a high-performance solar control glass that specializes in blocking heat. It has a more reflective appearance and the lowest solar heat gain. They aren't completely transparent - they have a greyish colour and create a mirror effect - and will protect the house from getting too hot in summer.
But all that protection and heat control comes at a price: the light transmission rate of this glass is about 30%. That means that it will take quite a lot of the light coming in through the windows, so we've finally decided to paint all the walls also in white.