Need some advice on how to prep my garage floor prior to carpeting. I'm converting it to a small pool den. I live in Virginia; garage is dry.
1. Should I seal the concrete first?
If you've been there for a while and haven't noticed any moisture issues, I wouldn't bother.
2. Someone told me to first lay a plastic sheet down. Won't this moisture barrier create a problem?
I've never heard of anyone doing this, and I think it would create more problems than it would solve - you'd probably hear it crinkling, and it could trap moisture... one of the advantages of broadloom carpet is that it's permeable, so your concrete can breathe a little bit if necessary.
3. Can I lay the carpet directly on the concrete or must I use a pad or underlayment and if so what's the cheapest way to go?
You could-
Cut the carpet to fit, and lay it in loose on the concrete. You'll want to seal all the cut edges with seam sealer or latex;
Glue the carpet directly to the concrete;
Glue carpet pad to concrete, and glue carpet to pad;
Install tack strip, pad, carpet, and stretch it in.
The last option is best for carpet longevity and ease of future replacement with carpet, the double glue installation is probably the most expensive, and the loose lay is the cheapest and easiest.
Not to complicate your decision- making process, but if your concrete already smooth and reasonably flat you could instead install carpet tile, which would give you the ability to replace stained or damaged pieces down the road without needing to replace all the carpet or fart around seaming in patches. You need a smooth surface to install over, to read the directions on the pressure- sensitive adhesive so you end up with a releasable install, and to buy enough extra tile to have attic stock for future replacement.