Zola's ThermoPlus Clad is Zola's best selling window, engineered for the Passive House market. Based on the Zola Thermo Clad, PLUS adds a fully integrated layer of patented German Purenit insulation in frame and sash. The result is a window that got fantastic lab test results (like an overall U-value of only 0.123 BTU/hr.ft2) while maintaining a sleek, elegant appearance and a highly competitive price in its class.