Replies

1. 
   Michael Chandler | Nov 05, 2009 01:59am | #1

   We do a lot of radiant concrete floors and they are such great emitters of heat that you can get away with a rug with no worry. In North Carolina where we live we can heat a house with a radiant slab that takes up just 25 to 30% of the floor area. Because it emits enough heat to warm the house in a smaller area the slab is noticably warmer than if there were more exposed.the more you insulate the under rug area the warmer the rest of the slab will have to run to maintain the set temperature. Under some conditions with large temperature fluctuations this can create problems with over heating as the day warms up and a lag in the evening. If you experience these problems just reduce the covered area.

2. 
   Andrew Henry | Nov 05, 2009 12:00pm | #2

   Hi Jerome,

   Anything that increases the R-value over your radiant panel will slow the heat transfer from the radiant panel's circuits to the room. This will have an efficiency impact because to get enough heat into the space to make up for the increased R value above the radiant panel, the controller will need to increase the water temperature in that spaces circuits, assuming you have outdoor reset. Increased water temperatures will require more energy!

   All that said, I have an 8' x 10' rug on a concrete radiant floor and a king size bed on an engineered hardwood / warmboard radiant floor (buying bed frame has been lower down on our list of priorities) and the house is still comfortable, and I'm blissfully unaware of the cost this reduced efficiency is costing me.

   With your small rug you probably won't notice any difference. The only caveat is if the water temperature required on the coldest day of the year for your house puts the radiant floor temperature near the uncomfortable point, which if I recall is 85 F. But seeing as you are reading GBA, chances are you have insulated your house well enough that the circuit water temperatures for you house won't get near the point that it will make the floor temp uncomfortable.

   If you want further reading John Siegenthaler writes a very good column on radiant hydronics for pmengineer.com and he does focus on overall system efficiency. Just do a search on his name on pmengineer.com and you'll come up with lots of stuff to read.

   Andrew

3. 
   Riversong | Nov 05, 2009 03:50pm | #3

   One rule of thumb is to avoid any radiant floor covering greater than R-0.5. The Carpet and Rug Institute estimates that carpets and pads offer approximately R-2.6/inch.

   Generally, as others have indicated, a small throw rug will make little difference. Avoid carpet pads, particularly foam pads.

   If the concrete floor is also used for passive solar thermal mass, then a rug will reduce the radiant absorption of the mass.