Greenhouse coverings, or walls, are very important, in your greenhouse, because it will effect how well your plants develop. Most greenhouses now use polycarbonate, or acrylic walls.
Polycarbonate is, generally speaking, available in 4, 6 and 10mm thickness. It is a twin wall construction with air channels in the middle, which creates the same insulating effect as a thermo window, without the high cost and maintenance of glass. (Glass can let in too much light, which can scorch plants, and does not retain heat well, at all.) The insulation promotes a more even temperature in the greenhouse which provides better growing conditions for the plants. Some polycarbonate, such as that used in Julianna Greenhouses, and some others as well, also has a UV coating to prevent wear and is virtually unbreakable – even against baseballs, rocks and hail. Polycarbonate also diffuses and disperses the light that comes in.
Acrylic is a heavier material that is usually of single walled construction, and also mostly shatterproof. It is more resistant to yellowing, over time than polycarbonate. Acrylic tends to be a little less UV prohibitive, which can be a good thing, depending on what you are growing. Cacti and succulents will thrive. Acrylic is the only solution, if you are going for a very traditional Gothic style greenhouse. It will give you the look 0f glass, with better growing support.
Glazing, or the greenhouse walls, can effect the pricing, on your greenhouse. You will need to think about the climate you live in, versus the native climate of your plants, and the amount of money you intend to spend, when looking at your options. A thicker millimeter (mm) polycarbonate, for example, can raise the bottom line, but may be well worth it, in the end. Be as knowledgeable about your plants' requirements as you can be. This will aid you in making the best decision.