hi. i am building a 20'x20' timber frame tea house and want to install a hardwood floor in it but the space will not be continuously heated (i live in the mild west coast). I have pressure treated joists with crawlspace and plywood over that. I think what i need to do is lay plastic down on the ground in the crawl space and put a vapor barrier on top of the plywood (roofing felt) and then put the hardwood floor down. Since the space will be unheated and temperatures will fluctuate, not sure if i should insulated under the plywood or if hardwood in this setting will totally fail. Any advice ?

I think the best floor for your tea house would b e vertical grain Douglas fir. It is commonly used for porches and since it is milled with some quarter sawn characteristics it will hold up to different moisture and temperature swings. I would staple it down after it has acclimated on site so it matches your framing lumber moisture content. Seal with a penetrating oil or a hard surface film. Insulating under the floor would be only for comfort when you are heating the room.

thanks for the posts