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Radiant Floor Heating/radiant slab insulation slab on grade


What is the best way to insulate a concrete slab for radiant heat in new construction?  I have read where the inside of the stem wall, under the slab and around the perimeter of the slab are all essential.  Stem wall will have a ledge for the perimeter of the slab.  I have seen homes built with insulation
just under the slab and on the outside of the stem walls.  Is
this adequate?  Also how far from the surface of the slab should
the top of the pex tubing be?


Badger Radiant Designs
Badger Radiant Designs  
Insulation for any structure is first determined by your climate (outdoor design temperature) and your desired indoor conditions (indoor design temperature. Typically when designing radiant slab heating systems here in Minneapolis we insulate below and at the outside exposed edges of the concrete slab with a minimum R-10, which can be reached with, readily available, 2" thick, 4'x 8' sheets of extruded polystyrene (XPS).

This technique is common and more than acceptable in most cold climates for basements and buildings with independent wall foundations such as pole or steel framed buildings.  

We also design radiant floor heating systems for frost protected shallow foundations. In designing and specifying such systems, frost or frost heave to be exact, is the main concern. The Swedes have concluded, after decades of research and much experience that the insulation for such foundations has to be closely matched to the climate considering the insulation effect of snow cover among other things. The most interesting observation I have gleaned is the minimal effect a moderately elevated slab temperature has on the effective frost penetration under and to the interior of a radiant slab.

If you must have a block or poured foundation wall you have to think of thermally isolating the radiant slab from the exterior. We often find concrete people insisting on "pouring the slab directly on the interior foot of the foundation wall. This common mistake adds nothing to the structure, since the slab couldn't possible be suspended between the exposed interior foundation edges, but pouring the slab on the foundation wall will transfer heat directly from the heated slab to the exterior foundation wall or at best follow the wall down to the relatively cold soil below. If there is no snow around your foundation, you are paying to melt it from the inside.

We have consulted on many such system that used more fuel than made sense for the size of the building.

Whatever you do, do not waste good money on "reflective insulation" such as "bubble foil" or suppose any sort of "blanket insulation" is a substitute for rigid board insulation such as XPS or EPS in the suitable weight (psi) for your particular application.

Don't forget to control and allow for drainage, both from the gutters and in the immediate area around your slab. Heaving will occur with moisture and freezing temperatures. We can only control one of these.

Finally, see the attached document for HUD recommendations for FPSF. It is quite illustrative and thorough.

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Morgan M. Audetat


Radiant panel heating design, including floors and ceilings, European wall panels and snow melting for residential and light commercial buildings..............................................................-Master of Steam and Hot Water,City of Minneapolis....................................................--- Master Plumber, State of Minnesota...........


30 years..... Charter Board Member - Radiant Panel Association, former manufacturer of condensing boilers, former national distributor of radiant floor products, son and apprentice of mechanical & plumbing contractor. Current mechanical/plumbing contractor specializing in hydronic based, integrated HVAC systems. Radiant floor heating/cooling. Snow Melting. Condensing Boilers. Indirect water heaters. System design, consulting and technical training world-wide. Licensed contractor, designer, installer and consultant.

B.S. University WI 1981 CONTINUING EDUCATION: Viessmann Condensing boiler and Solar water heating certification 2010, N.D.S.U. Lead Worker RRP certification 2010, Knight condensing boiler certification 2009, Wrightsoft Manual J CAD certification 2009, RPA Designer & Installer Certification 2008, Nate Hydronic/Forced Air Certification 2008, Uponor/Wirsbo advanced design school certification 2007, Buderus Wall-hung certification 2007, Power Limited License (low voltage controls) 40 CE credits 2005, Basic Hydronic Certification IPEX & Northern Alberta Institute of Technology 1998, Charter Board Member - Radiant Panel Association 1994, Residential Off-Electric to Hydronic Conversion Heating School Canadian Hydronics Council 1994, B&G Little Red School House 1993, Rood Utilities (now Auburn Technical Institute) Oil Burner School 1993 ,Tekmar Controls residential and commercial 1993, Division Manager and Advanced Hydronic Seminar Instructor for the first Exclusively Hydronic Radiant Floor Distributor in the USA 1990, Hydronic Radiant Heating Association Workshop participant with Richard c. Bourne, PE spring 1988, Master Plumbing, Hydronic, Solar Course Red Rocks Community College 1987.

Awards and Honors
2009 System Showcase Award - Radiant Panel Association

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