Best way to keep the greenhouse at a reasonable temperature (no more than 22 or 23 degrees) is to ventilate - open the door and window(s) early in the day if the forecast is good. The airflow will also help prevent diseases such as botrytis. Shading can help, Coolglass (white paint on stuff) is a bit like the oldfashioned Windowlene. Wipes off with a dry cloth at the end of the season, but is rainproof.