Hoklartherm Riga GreenhouseHoklartherm Riga Greenhouse The Riga is an outstanding choice for gardeners who need a year round greenhouse with a larger footprint than the Riga s. As the mid sized option in the Riga line it falls between the Riga s and Riga XL in both width and height, and is available in several different lengths. It features 8mm twin wall polycarbonate glazing panels on its side walls and slightly thicker 10mm panels on its ends, making it extremely insulative.