Blocking glass block windows

Hi. We bought our house a year ago and it has an extension on the back of it. It's about 8 years old. Part of the extension was a our bedroom with en suite. The bedroom has two external walls. One the back wall has a French door opening out to deck the other wall which is gable wall has two glass block windows. Roughly about 1 ft in width and 4 ft height. During the winter it was cold in the room and we could feel draught coming through the joints. Plus the joints were white but now are now turning black. The room is ventilated and has a big sky light in roofI don't want to replace them with ordinary windows because you can't open them due to them been so small so I asked about blocking them up. I was told I didn't have to remove the glass blocks just use insulation board on inside and out side and plaster over it. Would this work. Any advise on this would be appreciated. Thanks.