If you're not doing any work in the crawl space, I don't see why you'd have to redo anything. If you do an addition next to the crawl space, there will have to be some care in design and installation of drainage. There's probably an existing footing drain that either drains away from the house or directs in to a sump pump that will need to be tied into or replaced. Old footing drains can get clogged and non-functional.

It depends on the work being performed, where, and how. If your addition extends the crawlspace, you'll probably want to open an access through to the new one so you don't have to provide another separate one. If your encapsulation involves spray foam, that may cause installation headaches routing plumbing or wiring through the rim joist area or elsewhere. If it involves a ground vapor barrier, that will be harder to remove debris (like shards of plywood) falling through from above or installing underground plumbing. If you are going to encapsulate a new crawlspace, it may be dollar-efficient to do it all the same time with 1 same contractor. Just a few things I thought of, probably more.

Hi, James,

If there's not an emergency need to encapsulate your existing crawl space, I suggest you wait and do both the existing space and new addition at the same time.

Thank you all for your comments

3onthetree - I was just thinking of putting down the vapor barrier film only covering the floor and the side of the conrete footing while leaving wooden beam exposed. crawlspace has to be extended when i do the addition unless it's possible to have concrete slab foundation. i am not sure if it's a good idea to have a part of the house in pier-and-beam foundation and another part in a concrete slab foudation. assuming the addition still has a crawlspace, encapsulation has to be extended and added later on. but i am hoping that the encapsulation on the most of the area can remain.

Chrles Ross homes - I want to improve the indoor air quality. sometimes when the air pressure is low inside the house, i think the bad air from the crawl space is sucking through the gaps in the floor into the house. also radon level goes up as a result.

Seabornman - since our area is pretty much in drought (San Jose, CA), there's no drainage or sump pump in my crawlspace. just a pile of dirt and construction debris from previous owners.

Charles Ross Homes - absolutely. i will do those together.