[GBlist] Re: Big Green: Radiant Floor Heating

[Sarah Holland/David Foley <hollandfoley@acadia.net>]

Title: Re: Big Green: Radiant Floor Heating

on 10/26/2001 3:23 PM, Frazer Dougherty at fdougher@optonline.net wrote:

> Frazer says -
>  
> It was me that said NO poly, based upon Lstiburek's argument that the slab should be able to dry to both the inside and to the outside. It is the radiant installer who wants poly & TekFoil. SO I think he is confused - 1) TekFoil is a vapor retarder, 2) it's function is radiant barrier and 3) for the same cost, he ought to be recommending another 2" of rigid foam.

Dear Frazier,

Be careful.  What Joe Lstiburek says, in Builders Guide to Cold Climates is:

“Polyethylene Under Slabs: A sand layer is sometimes installed over a polyethylene vapor diffusion retarder located under  a concrete slab.  It is thought by some that the sand layer will protect the polyethylene from damage and act as a receptor for excess mix water in the concrete slab when the concrete is cast.  This is an extremely bad idea because a reservoir of standing water can be created.  If groundwater rises sufficiently to contact the underside of the polyethylene, water will enter the sand layer and be held in the sand layer by capillary forces, even after the groundwater level drops.  Since the polyethylene is between the water soaked sand and the ground, the only way for water to get out is up into the building throught the concrete slab by diffusion.  The wetting of the sand by groundwater can take only minutes, but the drying out may take a decade.  The polyethylene under a concrete slab can function as an effective vapor diffusion retarder even if it has holes.  It does not need to be protected.  The best way to deal with excess mix water is not to have any.  Use low water-to-cement ratio concrete with an accelerator or super-plasticizer.  It is faster and easier and, therefore, less expensive.  The concrete costs a little more but the labor is much less.”

Hoo, boy, how well we know that Joe is right!  We’ve made the sand-layer mistake, and regretted it.  We also always put rigid insulation under our slabs, since, uninsulated, they can reach the dew-point temperature at times in our climate.  This is bad because then water vapor condenses on the slabs.  We always put at least 12 inches of crushed stone beneath our slabs as a capillary break and drainage bed.  If you look carefully at Joe’s book, you’ll see that you can skip the polyethylene IF you have a capillary break (stone or gravel) AND Expanded Polystyrene insulation – NOT Blue Board, or Extruded Polystyrene.  IF you use Extruded Polystyrene, there isn’t drying potential toward the ground, and you need the polyethylene.  Joe appears to like a system with capillary break drainage and Expanded Polystyrene, or Rigid Mineral Wool, and no polyethylene.  He says it has the least likelihood of mold.  I’m a bit skeptical, because I wonder whether, over time, the crushed stone or gravel layer may suffer from water intrusion (if say, a drain pipe fails) and act not unlike the sand, although less severely.

This is probably overly complicated, but I hope it helps.

Best wishes,

David Foley
-- 
Holland & Foley Building Design L.L.C.
232 Beech Hill Rd.
Northport, Maine 04849 USA
p: (207) 338-9869 f: (207) 338-9859 e: hollandfoley@acadia.net