as houses become better built, from one decade to the next, they become more and more airtight so people rely less and less on air leaking in from the outside
to give them their fresh air. If all doors and windows got taped / sealed shut tight, most houses would leak
a lot of air through the door frames, moldings, switchboxes and floor boards. When a little wind blows, huge pressure differences force air in wall cavities to come through those opening as outside air is forced in through brick or siding and sucked out on the other side of the building. In winter you want to keep subfreezing air from coming in, and in summer you want to keep moist hot air from coming in. It's worse when the air has
high humidity, relative (RH) to its carrying capacity. It's still bad when hot desert air comes in or cold dry air.
Once you get an airtight envelope, you now need a plan to manage your indoor air.
Indoor Air Quality (IAQ) is key to comfort; a million things you don't want to breathe so you won't give your body extra work eliminating what you have inhaled. Dust mite excrement, offgases and more.
Heat stored in "mass" is felt as comfortable, up to a point (let's call that
warmth); cold objects are felt as uncomfortable; a cold structure is felt as uncomfortable. The ASHRAE has defined heat "comfort" in terms of the temperature felt at ankle height and at head height, and the difference in the two readings. Cold floors and walls make less comfort.
When air alone is heated, its heat is absorbed by the structure, but slowly and never as much as the air temperature. In winter, walls and floors are still cool when air is warm. The sensation it gives is that winter is coming in through the walls. Go to the big picture window or bow window in wintertime, sit there for an hour, and you will feel cold all over. When objects are warm and the warmth is spread out uniformly everwhere, nobody feels the sensation that winter is "right there" right in the structure. Warmth spread out radiant heat. Electric heat cables, or hot water pipes. Old fashioned radiators can be nice, but they can be uncomfortable too because the heat they give off is concentrated so high in one object that that object is too hot to touch and everybody "knows" it, they avoid it. (BTW, millions of hot water radiators in a million houses, don't require Dad to work all the time adjusting or fixing things.)
Heating air forced to circulate distributes dust and indoor contaminants; it also does not ensure heat comfort as defined by the ASHRAE. Air warmed up will rise above other air not warmed, so heat rises when it is "
in air". Heat does not rise in any other circumstance. Heat does not rise. It radiates, in all directions. The sun sends its heat in all directions, not "up". A warm object sends its heat in all directions. Only hot or warm
air rises; this causes the top half of a room to be warmer than the bottom half and it does not warm the floor so your feet and ankles always feel cold. Not comfort.
Heating applied internally to the structure at the coldest points (junction of floor and wall) does not force dust and indoor contaminants to become airborne. It provides warmth (not too high for comfort) at the places where cold is greatest in winter.
-David
p.s. ASHRAE is American Society of Heating, Refrigerating and Air-Conditioning Engineers
www.ashrae.org/
http://en.wikipedia.org/wiki/ASHRAE