When choosing your garden polytunnel remember to buy the largest structure that your space and budget will allow as it will fill up very quickly. All plants will grow much faster and produce bigger yields of crops. Depending on the size you may require extra ventilation along each or one side of the polytunnel, complete with a curtain or screen to enable you to close it off. The best way to attach your cover to the structure is with a base rail this allows you to fix the cover much easier than trenching it into the ground, it will also make it easier to replace the cover should it get damaged or come to the end of its life. The base rail will strengthen the polytunnel by locking the foundation tubes together as one. Other things to think about are ‘’how exposed is the site''? do you need extra bracing such as crop bars and or extra ridge bars, these will strengthen the polytunnel by approximately 20% in both snow and wind load. ( also handy for hanging baskets or tying crops too.)
Regarding doors: You are better to have a door whether it be double or single at each end of the polytunnel this will help ventilation for your plants or crops, If you only have one door then you will not have enough airflow going through the tunnel to keep things at a growing temperature.
Point to remember: If you only have one door and a storm brews up the wind will go into the structure but will not be able to get out, this will cause the polytunnel to start to lift and it may be damaged.