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Radiant Floor Heating/Between joist radiant & concerns with floor assembly


I'm a homeowner in the Chicago suburbs. We're replacing a very old heating system and I'm strongly considering a hydronic radiant system (professionally designed, DIY installed).

I'm getting conflicting answers regarding whether a between joist radiant system will be effective given our home's floor assembly. I don't want to spend tens of thousands on a system and have it fail.

Excluding bathrooms (which are tile), the flooring assembly in the house is as follows (top to bottom):
1) Standard oak 3/4" hardwood
2) furring strips 24" OC. This creates a small 3/4" airgap. I'm not sure the purpose of the airgap but I know some electrical conduit runs within.
3) 3/4" 1x4" subfloor

In other words, the assembly is an oreo cookie. Two 3/4" pieces of wood with an airgap in between.

I've had some people tell me this assembly won't work for between joist radiant. I've had others tell me that if I double-up the PEX tubing (i.e. two runs of PEX per joist bay) that I'll be ok.

The house is a 1950s ranch but I've worked extensively to upgrade airtightness and R-Value. 90% of walls have been ripped to studs and had R-10 to R-15 of XPS added (had been uninsulated brick & block). Attic has been airsealed and upgraded to ~R-50 insulation. Despite its age, house is pretty airtight and insulated.

Cost of system is not my concern, just performance and comfort. I'm surrounded by $1.5M McMansions in one of the city's nicest suburbs. I'm not concerned about cost because my little ranch is already worth 3x what I paid. I just want to know if in floor radiant will work given that darn air gap.

Thank you,

Badger Radiant Designs Viessmann 222F
Badger Radiant Designs  

Badger Radiant sub-floor heating
Badger Radiant sub-flo  
My experience tells me this is certainly possible if not ideal. It also tells me that proper modelling using radiant specific ACCA Manual 'J' software is the only way to know for sure.

The air gap between the sub-floor and the finished will act as an insulator, but the over-code insulation you've installed in walls and ceiling also shortens response time and lowers the supply water temperature required.

I have designed many "thick" wood sub-floor systems using heavy extruded aluminum plates with great success.

A properly sized condensing boiler, or combi-water heater, with the added advantage of a properly programmed outdoor reset feature should do the trick.  

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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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