Do you mean a proper sheep polytunnel or a vegetable polytunnel? TBH, I haven't used either but the sheep ones have mesh sides I think for ventilation. Sheep aren't good in well ventilated places so that might be a problem in a "conversion" job. But just a thought rather than actual experience.
Do you mean "in poorly ventilated places"
I did look into getting a lambing polytunnel for lambing this year, I got the full quotes and everything in the autumn time, but it worked out expensive for what it was plus I didnt have any flat land plus as the land I have is rented I'm not sure I wanted to put one up anyway
They quoted 1 square metre per sheep, so my quote was for a small one as most would be lambing outside, I think the 50 square metre one came to around £3,000
We just built a field shelter instead, and if there are any problems we will put inside for a little