Please bear with me on this.  I've been going over this forum for a few days now and the depth of knowledge and advise has been astounding.  but I have a very basic question concerning floor coverings and radiant floor heating.

I am planning on building an ICF home in So. Kansas.  1600-1700 ft/sq.  with full basement.  Want to install radiant floor heating and a pump and dump heap pump for A/C in the summer.  Here is my question.  Some of the posts state that the pad used for carpet acts as a very good insulator and that is almost nullifies any advantage of a radiant floor heating system. 

Am I understanding this correctly?  Would I be canceling any benefit of the radiant floor heating by turning it up so high that it runs more than a forced air furnace?  I know about the add on heating elements for the geo. thermal  heat pump, and I will be exploring all options.  One thing that is stated over and over is to plan, plan and go back and plan again.  I just don't want to spend a cold winter or see the same energy bills if I build the house I want and install the wrong heating system.

I've seen where hardwood is acceptable and ceramic tile is better.  But I don't want an entire house of either.


Thanks in advance for all the great information presented here and thanks for any insight you all may be able to offer.

Terry Smith

With padded wall to wall carpeting will put about R2-R3 of insulation between you and the warmed surface, which means you'll have to run higher temps to deliver that heat to the room. (You'll also have to insulate to R20+ underneath, or the fraction of the heat that gets dumped in the basement will be large.) If you're running gas/propane/oil-fired boiler for the radiant it could still be cost-effective though. Forced air is inherently less efficient than hydronic due to the pressure differences created between rooms when running. A significant portion of the "return" from those differentials occurs as air flow into/out-of the house (figure 15-25% typical, but as high as 40% for not-so-tight houses with poorly designed ducts without mastic-sealed joints throughout.)

If the under-carpet radiant works for most of the season but can't quite deliver "design day" heating loads, you could have a separate hydronic ("force hot water" heating coil (also running off the boiler) in your AC air-handler as a separate zone to run when the radiant doesn't quite keep up. (Slightly lower efficiency for 10% of the heating season isn't such a big performance hit.) I know of a few systems with radiant floors controlled by floor thermostats to keep it a constant toasty ~73-75F under foot, but the main room temperature is controlled by a coil in an air handler with a standard programmable set-back thermostat. Much of the season the air handler doesn't run, or runs for a half-hour or so for the morning warm-up after a cold-ish night.

Alternatively, one could run hydronic baseboards for the carpeted zone (or as a redundant zone for the coldest days). Specify to the designer/heating-contractor to design the baseboards for 140F water. It'll mean ~50% more baseboard, but the system efficiency will improve dramatically over the typical old-skool 180-200F design temps, and you can run the same water temps as in the radiant zones. It amounts to about a 3% fuel savings for every 10F you can peel off the operating temp. The additional cost of the longer baseboards is paid back very quickly.

Make sure whomever designs the system for your house does a full Manual-J or similar heat load analysis. Key to getting any efficiency out of a hydronic boiler is getting the right size. Even Manual-J overestimates by 25-30% for really tight houses, but it's typical to see systems designed with 100-150% , even 200% oversizing. When this happens it'll heat up quicker, but will never achieve anything CLOSE to it's AFUE efficiency rating. Efficiency accrues with lower operating temperatures and longer-but-fewer burn cycles. Radiant floors are ideal for keeping temps low, but be sure to get the smallest possible boiler that stills delivers sufficient heat to lengthen & limit the number of burn cycles. A 200% oversized boiler with an 85% AFUE rating will short-cycle and you'll be lucky to achieve even 70-75% real-world efficiency.

Once again, great, in depth information that is useful. Thank you so much for your reply.

Terry Smith

Greetings to all from Tajikistan in Central Asia. I am looking for a definitive opinion on the following dilemma. We installed warm flloor technology (electical not water based) in a day care center for disabled children. As I understand it, the base is concrete, then covered with indoor/outdoor carpet. Some officials from the Dept of Social Services came to check on progress of the construction, and when they learned about the warm floors they went ballistic. They claim that it is very dangerous for children to sit on such floors, that they will absorb pulses and suffer nerve damage, that the risk of the carpet catching fire is very high, etc. I contend this is a remnant of Soviet dark ages thinking but I have to vet the question. Can anyone give me some information for or against their argument? Thank you!

Jealousy is a terrible thing. To my knowledge there is no confirmed evidence that electomagnetic fields are detrimental to human health (Though many have tried to find it).

Given that the floors will never excede 30°C, it is unlikely that the children will suffer (this is the same temperature as a warm hand shake). See guys, I will defend resistance heat when pressed.

However, the Swiss have certain concerns specific to certain electric floor heating products, if you follow this link:
http://www.bag.admin.ch/themen/strahlung/00053/00673/05139/index.html?lang=en

If the floor was warm enough to catch the carpet on fire the room would be 175 C, The max you’re going to see the cement (as MA suggested) is about 30 C, even that would really warm up the room. I grew up (1980's) in a house heated primarily with off peak electric in floor radiant heating. I was totally amazed when I finally moved out how cold my feet were all the time.

The only downside I see are the warm happy kids willing to play on the floor all day long :)

Sponge Cushion, Inc., http://www.sponge-cushion.com/, claim they have a carpet pad compatible with radiant heat. Launstein Hardwood recommends a pad SCI makes for use under floating hardwood over radiant so I see no reason to doubt the claim for carpet pad. Most any carpet dealer can probably obtain the pad from their wholesale supplier.

Any 1/4" solid - sometimes called "commercial" - pad will work and present little resistance to a properly designed radiant panel.