We are currently in the process of leveling out our basement floor (new construction) with gravel and then sand.  We are below grade and the earth underneath is hard pan (a glacier passed through the valley a few million years ago).  The gravel has been down a while and we get a fair amount of rain in our location during the winter months, so there is a considerable amount of water trapped.  Getting the water out would be next to impossible, short of waiting until August which is not an option.

We will obviously put in a moisture barrier (6 mil. poly) and then will be placing 2.5" foam down and then 3" of concrete (mesh and pex in the concrete).  My concern is with the water that will be "trapped" under the moisture barrier.  Will it be there for eternity?  If so, will it create a problem, assuming we are careful and keep the moisture barrier in tact?

Also, we've never gone through this procedure before.  We are trying to level and compact the sand as carefully as we can.  How much variance can there be in level (ie 1/8" or 1/4" over 10' under the foam)?  Just not sure how much the foam will "bend" or flex during the pour.

Thank you.

sounds like a perfect time to use the Crete-Heat or the BARRIER as they are flexible  but more important will not allow moisture to come thru or break. let me know if you want more info

Posted By radiantbarrier on 17 Feb 2010 11:54 AM
sounds like a perfect time to use the Crete-Heat or the BARRIER as they are flexible  but more important will not allow moisture to come thru or break. let me know if you want more info

Our building code calls for R14 insulation under the slab.  The barrier is R4, if I'm not mistaken.

6 mil poly is pretty flexible as well so I'm not sure what the Barrier could really do for us.

Hi,
Don't know about the ultimate dispersal of the moisture but the 6 mill plastic works well and the 2 inch foam will provide the best insulation of all the products out there.
The 2 inch foam will span an un level slab but try to avoid big dips, it bends as concrete is placed, and as you walk on it over hollow areas under it will crack, with out harm to the grade.
Have fun with your project,
Dan

Crete heat is 2" foam.

however no 2" foam product will hit an R14 code requirement. that's 3" or more.

NRT.Rob, we will be using 2.4" high density foam with an R14 rating.

spray foam?

that has to be quite a bit more expensive than 3" of rigid, doesn't it?

2.5lb or 3lb XPS would run about R14 at 2.4" thickness, but I'm not sure where to get it. (2.4" sounds a lot like 60mm sheet goods.)

2lb polyurethane or sprayed iso would be about R15-R16 at that thickness

Sorry, my mistake.  I don't know why I had R14 in my head.  The code here calls for R12 and we will be using 2.4" Styrospan by Dow with an R12 value.

Are you putting a drain in the gravel, underneath the slab? If you have water trapped below your slab, having a moisture barrier is going to be the last thing on your mind... Tell me that you have stamped plans from an engineer showing a drain...

Jeff

Posted By jbaron on 17 Feb 2010 03:27 PM
Are you putting a drain in the gravel, underneath the slab? If you have water trapped below your slab, having a moisture barrier is going to be the last thing on your mind... Tell me that you have stamped plans from an engineer showing a drain...

Jeff

We consulted with a geo tech engineer before starting construction and he indicated wuith the type of soil, we shouldn't have problems as long as care is taken with extertior waterproofing and perimeter drainage, which we have been quite diligent about.  I specifically asked him about hydrostatic pressure, as I have seen first hand what problems that can create.

But no, there are no drains for the water to escape that is already there.  This must be a fairly common occurence when building in areas with decent amount of rainfalls; that water collects within foundations before the floor is poured.