Have got 2 of these. I do not know what they are called, Double sash windows? The inside windows were originally wooden boards but some were removed for firewood. I made new frames and UPC. works fine, but would like to D.Glaze the outside windows but it looks like it would be impossible to remove the frame. Would it be possible to rebate the window for a D.Glazing unit? How could I make the weights heavier or would there be that much difference?