Is it a good idea to keep sheep in a polytunnel? My concern is ventilation, from what I know and have read sheep require good ventilation, which is hard to arrange in a polytunnel. I stand to be corrected, but you may find that they will develop pneumonia or other breathing related illness if there is no air movement and it is stuffy.
Further, storms can rip polytunnels apart. When I built my sheep shelter, I buried the vertical wood beams the ground, which has prevented it being pushed over by gales. My only problem is roofing felt, which sometimes needs replacing.