The quick answer is you don't encapsulate the crawlspace before you take care of thewater issues first or you are just asking for some major issues.
Source of the water
One of the first things is you need to determine where the water is coming in.
I would start with the outside making sure the ground is slopingaway from the house, gutters are cleared and draining at least 5 feet away.If that is the issue then you will have to run dehumidifiers to help bringthe moisture levels down.
Now if that isn't the issue or proves not to be all of it, then you willhave to work on a way to drain the crawlspace either to daylight or to asump pump especially if you have a high water table. After this is done,then you need to go back to a dehumidifier.
Common moisture sources
Some common issues I see:
- leaky & uninsulated duct work,
- condensate lines not draining outside,
- uninsulated or leaky water lines,
- dryer venting underneath instead of outside, and/or
- other leaks (toilets, drain lines).
All of those would need to be dealt with.
Whew, now that you have taken care of the water intrusion and everything has dried out, nowyou can really start discussing how to encapsulate or seal the crawl space.
There are numerous options and they are all good ones dependent on your budget and how you use it the space.
The walls. If you think water might still try to seep through,you could go with Closed Cell Foam directly sprayed to them. Igenerally would recommend using a product like Delta Dry that leads directlyto a French drain so if any water manages to seep inside it drains away.
- Make sure that the top is sealed so vapor does not have a pathway directlyupwards.
- As for foam boards, while they can work it is can be difficult toget them installed properly in a confined space.
Plastic on the floor. Plastic is almost a given but no matter which way you go itneeds to be cleaned out and the smoother the better. Six mil is the thinnest you can use, but I would recommend going thicker.
- Makesure they follow the manufacturer's directions for sealing the seams andmounting it to the walls (it should wrap up the walls by at least 6 inches).
- It should be under the foam but over the Delta Dry if you go that route.
Rat slab (concrete). This is a good alternative system especially if you store or need to beunder there frequently. Trying to do it now that the house has been builtis generally not a good idea due to the amount of moisture it will release. It is hard to do in many cases. With that I have heard of some companiesthat have managed to get around those issues, but I haven't seen or heardenough about them.
Foam on the ground. The final option is to spray closed cell directly on the ground. Thatcan be a great solution but finding a company that can do it and do itproperly though... I would highly recommend you find either an energyauditor that knows their stuff to verify work done / spot other issues ormake sure the company(ies) you do hire, know their building science and can doit properly.
For more information:
Read "What can we do about water build up on our basement floor? We had a french drain put in and it worked for the first 3-4 years." a Q&A answered by Edward Wright.