I just read greentree's wood foundation post, got me thinking.  What are thoughts on a wood basement floor?  We are converting a crawlspace (hunchspace, originally post and pier) to a full basement.  We are pouring concrete foundation now, but I don't like concrete floors (for personal and environmental reasons).  I was thinking 6- 12 inches of gravel, EPS insulation, vapor barrier, air space, truss or joists attached to foundation wall via ledger board, cellulose in joist bay as sound deadener and insulation (so floor doesn't sound hollow.  Would support joists in middle with concrete footing.   Would have gravel layer drain to a sump (2 actually, one as back up).  We are in Pacific NW, so water is a worry. 

You might check Polycore's site at http://www.polycorecanada.com/products/residential-foundation-floors.  They show a wood floor below grade sitting on sand.  I'm interested to see what our building department thinks of the system.  I wish Polycore's site had typical type construction drawings.

Good luck,
Bruce

What you have described is basically a sealed crawlspace. Just below grade. There is no reason to believe that it would not perform just as well as an above grade crawlspace system would. Double layer the vapor barrier, seal the perimeter, and around piers, and tape the seams. With the gravel, drain tile, and sumps to keep down the water and moisture, you should have no problems. BTW, we installed a wooden floor system in an ICF basement several years ago, and, to my knowledge, it is working fine.

Thanks for the polycore link, but I think I would rather just build it myself.
Wes, what did you use to seal the perimeter? Did you use pressure treated wood? If the joists don't contact the ground or concrete, and are in conditioned space, seems like wouldn't need treated wood.

well ok;

when living in NW Pennsylvania , I finish my basement floor like this.......
2x2PT @ 12" oc fastened with const. advesive & Hilti pins
1" Dow EXPS insulation in between
1 layer 6mil visqueen
1/2" cdx plywood
finished floor

That was over 30 years ago and is still good

well,
Typically I would seal the vb with caulking to seal it to the perimeter wall and then screw a small lathe over the top edge to hold it in place. The only place that would require PT is if that wood was touching concrete.
Chris's system is similar to one I saw used in conjunction with PWF walls about 25 years ago. I think it is still in good condition. That system was designed to set directly on the gravel backfill using PT 'sleepers' directly on the gravel, then vb, then PT 2x floor joists, and PT plywood decking. However, if you raise your floor system off the gravel, as you have described, you would not need the PT wood. Especially, if you introduce some conditiioned air into this space.

Wes;

I think the IRC requires 18" min space for non-treated lumber

Should work fine as long as you keep any vapor barriers under the lumber and make sure that the sheathing above the wood joists & cellulose insulation is permeable enough so that any moistuire that gets into the joist bay can dry to the room side.

The longer the span the more height you lose as the joist size increases, something to think about.

Southerpine.com has the permanent wood foundation manual and inside that manual are the details for a treated floor system in lieu of concrete. I'm not sure cellulose would be the insulation you would want for a horizontal basement pack situation under the floor even over a xps and vapor barrier because of possible wetting but that's my instinct not based on any experience.

IRC requires 18" from 6 mil vapor barrier

I don't understand why non PT has to be 18 " from the vapor barrier, but so be it. I think it makes more sense to have the vapor barrier warm on top of the insulation, and that puts it a few inches from the joist.
Thanks for all of the information, makes me optimistic to see this has been done before.

well;

this link is from San Diego's bldg department  (using UBC) demonstrates various wood floors close to ground, it is very similar to the IRC

http://www.sandiego.gov/development-services/industry/pdf/bnl23-04.pdf

Chris - the IRC R319.1 calls for the wood to be PT when it is closer than 18" from the exposed ground. I don't see any requirement about the distance to a vapor barrier, but maybe I'm not looking at the section you refer to?

Well-ok "makes more sense to have the vapor barrier warm on top of the insulation" putting the VB above the insulation in your floor assembly may cause moisture from the ground to condense in the joist bay. I've seen that happen & rot out untreated joist lumber.

Note that in the the San Diego attachment all drawings show that the area under the floor needs to be vented. I think the original discussion was about an unvented basement floor. Regardless, an overlapped and sealed vapor barrier on top of the stone floor is necessary to keep out as much ground moisture as possible.

here is the actual diagram from the IRC , it denotes 18" and 6 mil polyethelene film for wood foundation with crawl space, a clearer but upside down PDF is attached

Bob,
I don't understand, how will placing the vapor barrier on top of the EPS layer increase odds of condensation in the joist bay (I don't want to put it on top of joist bay insulation)? My rational is that the VB will be protected from the gravel layer during floor construction, and that by keeping it warm there will be very low chance of condensation on either side of the VB.
Thanks

I read as vented or unvented?

Chris: I would read that dwg as showing they want a minimum of 18" for the crawl space. What it does not show and should is that the poly should be brought up & sealed against the foundation. These code guys are at least a few years behind Dr Lstiburek, so if there is a conflict I would go with Dr. L.

Well - ok: The purpose for the ground poly - or on top of the foam in this case is to keep moisture from the soil from escaping into the crawl space - decreasing the chance of the moisture condensing anywhere. I just don't think you can be assured that it will stop all the moisture, and if I understand this correctly , this is not a spot which you can check periodically & change any of the features.

In your average house floor, say over a basement or above a garage the main direction of vapor travel (cold cllimate - in winter) is from the heated space to the unheated space. That may happen here, but you also have vapor coming up out of the gravel, so the trick is not to trap it anywhere where it could damage wood or insulation. Maybe the best bet is to leave the VB out of the floor assembly so it can dry to either direction. Since water is a real issue, plan on a worse case situation.
Bob

Bob;  18" is not the minimum crawlspace, you can get smaller with PT

In my community 90% of homes are on pwf foundations. We live in a cold climate to -40 in winter with high water table in summer. PWF foundations have proven durable and warm, as long as the drainage system works properly. In the past 20 years we have had to repair about 5% of our foundations due to drainage problems which could have been avoided with proper design. We prefer wood floors to concrete because they are resilient and warmer. We don't insulate our basement floors except if they are used for radiant heating. We build on a 6-inch gravel pad with a sump close to the centre. We have tried various floor designs but pwf 2x4 sleepers laid flat over the gravel at 2-foot centres has proven just fine, then 2x4 joists, vapor barrier and subfloor material. If you live in an area with radon gas issues, you can install a ventilation pipe under the vapor barrier to allow ground gases out. Your truss floor sounds like an expensive solution and I'm not sure it will be an improvement. I recommend the pwf builders manual that can be found at http://www.woodfoundations.com/shop/ . This manual covers a wide range of floor options and it clearly explains the theory of all aspects of pwf foundations. It's the bible of pwf foundations.

do you know of a contractor in the edmonton area that gould help me with my wood basement that has a small leek?