On the Hydronic Based Biomass Heating Systems Discussion Board, John Siegenthaler answers student questions…

How do you deal with cold floors over unheated or outdoor spaces? If radiant heat (typically used for walls/ceilings) for joisted floors is properly designed and installed, can it be efficient, effective and comfortable?

Student 1: I would like you to elaborate on the floor joist heating.

I have two scenarios:
1) Dealing with the cold floor over unheated basements or crawl spaces.
2) Second floor bedroom dormers that extend beyond the first floor walls and half of the floor is supported on piers. I am in Michigan’s Upper Peninsula and this type of bottom outside exposure presents a challenge to keep the exposed floor comfortable in our cold environment.
Insulation alone is not the answer; some source of heat is needed in the floor. Rooms can be adequately warm at thermostat level but very chilling to bare feet. Your suggestions and thoughts are appreciated.

John Siegenthaler: Floor heating using some type of aluminum plate system can be done in floors over unheated spaces. I suggest a minimum of R-30 underside insulation, and, wherever possible, a system the puts the aluminum plates on top of the subfloor. You might also want to consider continuous circulation of the floor circuits, and close (6″) tubing spacing in the floor areas with exposed undersides.


Student 2: [In one of the course lectures] You took site-built radiant ceiling heating panels, turned them 90 degrees, used the same construction buildup and built them into walls with also the same “q”, heat output constant. Why can’t the same configuration be used for floors, with a consideration for loading (no crushing) for a radiant floor application? This would be for a non-slab floor. I have no reservation for sufficiently insulating the floor joist and installing radiant floor heating. I would not expect “bare foot” comfort but I would expect an efficient hydronic heating system. Any comments?

If radiant floor heat for joisted floors is properly designed and installed, can’t that be efficient, effective and comfortable?


John Siegenthaler: The output of the radiant ceiling is roughly 0.7 (Twater-Tair), and for the same construction on a wall it is roughly 0.8 (Twater-Tair). The wall panel has a slightly greater output because of vertical convection effect.

This assembly could potential be modified for a floor installation, but the foam would have to have sufficient bearing strength, and there would obviously have to be something other than drywall as the finish.

You might want to check out the Roth panel system. It’s a premolded 2×4 foot panel system with higher density foam and an aluminum upper cladding with preformed channels for the tubing. It could be used on floor, walls, or ceilings.

Radiant floor panels with aluminum heat diffuser plates can work well. We’ve designed many systems with them over the years. The problems arise when someone doesn’t understand how vital the plates are when it comes to diffusing heat, and thus ends up with a “plateless stapleup” to either underside of subfloor or to the joists. It’s a shame there is such a lack of understanding of basic heat transfer (as you point out). You won’t see such nonsense outside of North America.

John Siegenthaler’s Hydronic Based Biomass Heating Systems begins 2/22- claim your discounted seat today!

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John Siegenthaler – Principal, Appropriate Designs

John Siegenthaler, P.E., is a mechanical engineer and graduate of Rensselaer Polytechnic Institute, a licensed professional engineer, and Professor Emeritus of Engineering Technology at Mohawk Valley Community College. “Siggy” has over 32 years of experience in designing modern hydronic heating systems. He is a hall-of-fame member of the Radiant Professionals Alliance and a presenter at national and international conference on hydronic and radiant heating. John is principal of Appropriate Designs – a consulting engineering firm in Holland Patent, NY. The 3rd edition of his textbook – Modern Hydronic Heating – was released in January 2011. John currently writes about hydronic heating and solar thermal system design for several trade publications.

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