We are considering having antique heart pine (from an old cotton mill) installed. We would get a better deal on 1/2" instead of the usual 3/4". I have never heard of anyone using 1/2" solid wood before and I believe the supplier has not used it before either. The planks will be nailed together. There are no tongue and grooves. It will be installed over a slab foundation with vapor barrier and 3/4" plywood. The planks will be random width from 4-8 inches wide. I live in humid coastal South Carolina. I am also wondering how much gapping I should expect during the dryer months. The contracter said the wood is at 15% moisture. While I do not mind the look of the gaps (it goes along with the antique look IMO) I worry about keeping the gaps free of dirt and dust for health reasons.
Thanks in advance for any advice.

To review:
1. 1/2" will by top nailed. This is ok. You may see the putty filled holes.
2. Nailed to Plywood. This is good too. Hopefully they will put the moisture barrier, as outlined by NWFA and NOFMA, under the plywood.
3. Humid area? This is ok, but if you use heater due to cold weather, put humidifier in the room or put water on the stove... Try to maintain Humidity levels.
You'll have gapping between the planks if the moisture content of the wood is excessive. Try to install around 10%, or 9%..
You should have a beautiful floor. Enjoy.