I am currently house shopping here in Alaska, and radiant heat is on my wish list of new house features.  I just read the "Building Green" article Umbra referred to, and have the following observations:

The authors work in Loveland Colorado, which at 40 N latitude is a lot closer to the equator than where I live (61 N).  As they say, their review of radiant heat is relevant to houses in "moderate-to-cold" climates.  In places like AK where there is virtually no solar gain during the coldest winter months (our sun is only a few degrees above the horizon for 3-5 hours/day), passive solar is not an option for replacing or supplementing active heating systems.

They say that much heat is lost to the ground with slab-on-grade radiant-floor heating systems. I'm sure that's true.  But you generally don't see slab-on-grade in areas where frost penetrates deep into the ground, because such foundations would soon crack with frost heaving.  I've never seen a slab-on-grade house here in AK or in New England.  We have to have full, deep, well-insulated basements or crawl spaces to keep our houses on the level.

They note that most radiant-floor heating systems cannot provide cooling.  Not an issue here where the average maximum temperature during our warmest week of the year is 66 F.  We have air conditioning -- it's called an open door or window!

I currently live in a house with hot water baseboard heat, which isn't as bad as forced air, but nowhere near as nice as the radiant heat systems some of my friends have.  Something the "Green Building" authors do not talk about is thermal mass: any liquid-based heating system has more mass than does hot air.  Yes, this means there can be a lag between turning up the thermostat and starting to feel warm.  But it also means the house maintains a comfortable temperature more easily, with fewer drafts and cold spots.  You can set the thermostat and leave it (or better still, use a programmable one) instead of having to toggle it up and down as the blower kicks on and off on a really cold day.

Having experienced all three kinds of conventional heating systems, plus passive solar, wood, and wood pellet, in AK, northern New England, and the Maritime provinces, I still choose radiant floor heat (maybe with wood backup/area heat) in a location where solar can't do the job.

Radiant floor heat is exceptionally good with geothermal heat pumps, especially if the floor is uncarpetted.  Carpetting acts as an insulator.  I heat my Minnesota home with geothermal and forced air.  I calculated how much it cost to heat my house a few winters ago, how much it would have cost with a 95% efficient natural gas furnace, and how much it would have cost with radiant floor heat and uncarpetted floors.  The costs were $410 for my present system, $855 for a gas furnace with gas at $9.50 per million btu, and $290 if I had a geothermal heat pump and radiant floor heat.  The heat pump is much more efficient if it can output water at a lower temperature.  With an uncarpetted floor, it outputs the water at 85 and has a COP of 4.7.  My forced air system has a COP of 3.3.  If the floor were carpetted, the water would have to be output at 100 F, which would result in a COP of about 4.

I really like the people at Building Green but they're dead wrong on this one.  They're just not builders.

As the other comments definitely point out well, there are different strategies that make radiant by far the best option, even when combined with passive solar and super-tight, energy efficient building envelopes and design.

And I agree about radiant and geothermal.  There's an innovative geothermal installer in our area who designed a solar-assist component to the geothermal system (recharging the source side if I remember right).

This is one issue where Building Green just gets it wrong.

Great information, but, seriously, we are having a heat advisory today where I live. I'm finding it really hard to get into the spirit =)

Donnakay, please beware: electric resistance heating is very inefficient and can be extremely costly to run, even as underfloor mats. We use it occasionally, as a luxury, in small areas like tile bathroom floors with a timer set for an hour in the morning, but not for general heating purposes.

JoshS - have to disagree with your blanket disagreement with Building Green. Radiant floors have their place but they're not for everyone or every climate. The U.S. is a big country with many climate and microclimate variations. Slow response, high thermal mass radiant systems are ideal for areas with long predictable heating seasons but are problematic in places with humidity issues and fast unpredictable weather changes. And some folks (I'm one) find radiant floors just uncomfortable - makes our feet sweat.

It's horses for courses. Costs are always a factor. If you need to air-condition, you'll be putting in two parallel climate control systems, with two separate kinds of maintenance issues.

Spaceshaper, I disagree with your blanket disagreement of my blanket disagreement.  :)

I definitely can see how my post left certain impressions open to interpretation, but I'm not arguing that radiant floors are for everybody or every climate.

I disagree with the essential reasoning behind Wilson's disagreement about radiant heating in certain climates, namely colder climates with greater numbers of heating degree days.  It's written with the beauty of a true non-builder's thinking.  :)

That said, other excellent strategies exist for such climates as well, not involving radiant.

I live in South Korea where underfloor heating (called ondol) is the standard. Koreans are really proud of the ondol system, which dates back to somewhere between 37 B.C- A.D. 668. Underfloor heating is the reason Koreans traditionally had tables and beds close to the floor.
It's so nice to come home to my apartment in the winter and lie on the toasty floors. Some of my friends will throw their blankets on the floor before work and then come home and curl up in the warmed blankets. At restaurants, it's so nice to sit on the heated floor, too. It's amazing. After living here, I can't believe how long it's taking the rest of the world to catch on.

I work at BuildingGreen, and worked with Alex on that article. (We're based in Vermont, by the way, not Colorado.) We're certainly guilty of spending too much time at our computers, and it's been a while since we've done any substantial building, so I'd like to know what specifically you have in mind when you say that it comes from a "non-builder's thinking."

But I have built a house with a radiant slab-on-grade. It works fine, but it's not what I would do today, given what I've learned in the last 15 years about energy efficiency and climate change.

I think you're missing the key point here. Radiant floors are a wonderful way to distribute heat when you need a lot of it (so in Alaska--perhaps). They are overkill to distribute heat when you need very little, and in fact they don't work well in that setting, because they will either deliver too much heat, or they'll feel cold.

If you have drafty, leaky home, sure it will be more comfortable with radiant floors. But if you have the option to spend $8,000 on radiant floors, or the same on upgrading windows and air sealing, how could you possibly choose to invest in the floors? Either way, you get a similar level of comfort, and by upgrading the thermal performance of the home you reduce your heating bills dramatically. Even if you're rich enough that you don't care about the costs--how can you justify that choice today, in 2007, with the ice caps melting faster than anyone predicted?

Although it is true, that you will feel more comfortable with radiant heat, it might lead to problems with your veins if it comes from the floor, especially in case of varicosis.
Here in Central Europe, where I live, the waterpipes are placed in the walls underneath the plaster. The larger the heated surface- the lower the heat required in the pipes. If your house is well insulated ( also the floor ) this will keep your rooms constantly at the same temperature.
Traditionally we use tiled stoves. We have one in our house and I don't want to miss it anymore.
If it is well constructed it can heat your whole house, it needs little maintenance (cleaning every six years), it works for decades and doesn't depend on electricity.