While at a home show in Calgary, Marsha asked me to save her from her husband Martin. Actually marriage counselling is as much a part of my business as "divorce dust" is a part of renovation.
Martin was doing a wonderful job of building Marsha a professional sewing centre above his new garage/workshop. What Marsha was worried about was the automobile gas fumes coming up to where she would be working 8 hours a day. So I went out had has a visit.
I helped Martin inspect his own work, looking for possible gas paths between the two spaces. Although you don't really need a vapour barrier between two heated spaces, he was putting in a sealed plastic sheet to prevent garage gasses or workshop solvents from migrating to the sewing room. Good move Martin. Then he wanted to add some insulation too, just for soundproofing. But that raised an interesting question. With both insulation and a plastic vapour barrier between these two spaces, which side should the vapour barrier be put on? The "warm in winter" side rule still applies. The sewing space upstairs will always be heated and the garage will sometimes be heated, so the barrier goes on her side of the insulation.
Can one heating system work for the two spaces? No, because of the potential for automobile gasses to use ductwork to get to the occupied space above. Since both spaces are rather small, electrical heaters was the best bet, even if electricity is expensive in Alberta. Hot water heating wasn't practical because the garage was not always to be heated and the pipes would freeze.