A transparent thermal panel is formed by two panes of glass each curved concentrically about one axis to form a section of a cylinder, and each being sealed at their edges. The envelope thus produced is evacuated. The evacuated glass envelope is attached to the uprights of a frame using a torsioned clamp which counteracts the bending stresses created by the vacuum in the glass envelope.Two such evacuated glass envelopes may be joined together and a solar collector placed between them. Combinations of one or both of the thermal panel and the solar collector may be used to cover the outside of a building.Coating the interior of the panels with a low emissivity material provides for increased R value.