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Wright designed the house on a tight budget. The exterior walls are a single wythe of 8" CMU with some vermiculite in the cells which has sunk to the bottom (Wright noted an option for two wythes of 4" CMU with an insulated cavity between, which was beyond the Sweeton's means); the windows are all 1/4" plate glass in pine sashes (no weather stripping-I'd have to rout the sashes), the only weather stripping is the copper strips in the main entry door and what I installed at the kitchen jalousie door (what the weather stripping saves the jalousies giveth away).
As heat rises, I addressed the roof insulation when we replaced ceilings. The roof is framed with 2x6's, between which I inserted 4" of foil faced polyiso board and tacked sill sealer foam on the rafters for a thermal break with the gypsum board (yes, Wright used a gypsum board ceiling on this one, albeit with sand finish paint). This replaced 2 inches of rockwool batting. The new roof insulation reduced our heating fuel consumption by a third! In fact, after insulating the roof, I discovered that the partitions and walls heated by the floor act as "thermal bridges" which project the house's floor plan onto the roof in a light snow or heavy frost...I'll post a picture. This is heat loss, but the prospect of cutting redwood plywood walls that provide lateral bracing for the bedroom wing, and working thermal breaks between the masonry wall tops and existing roof sheathing was beyond my means. It should be noted, without the radiant floor, the workshop's exterior walls are not warmed and their "ghosts" do not show on the roof snow or frost.
The radiant heat truly is necessary in a scantily insulated house like this. It is true the radiant floor heats the elements of the room and its occupants not just the air. Because all is warm, my body is not radiating its heat to cold surfaces unless I stand or sit close to a window. In the below grade workshop, we had a failure of the radiant floor and chose not to replace it. Instead we ran a radiant baseboard off the main which keeps the room 60ish in the winter...I use a small electric radiator to get the room temperature up to 70, but I'm still chilly because the floor and wall are still cold. The difference in the sense of warmth between the workshop and the adjacent radiant floor heated bedroom is striking.
I've never noticed much heat loss from the fireplace though I suspect the masonry heated by the floor slab is radiating quite well above the roof. The 24"tall plate steel raised grate I had made to hold the fire up off the hearth to improve draw heats up and retains the fire's heat well into the next morning after the fire has died down....this may off set some of the loss, but I doubt it.
DRN's nifty fireplace equipment:
One wonders, if building anew, what additional measures would be necessary to achieve a more acceptable efficiency in the Usonian concept...? Seems to me that the nature of radiant floor heating could open to creative ideas of how to best utilize it––not by taking the attitude of a traditional hermetic approach––but of a more elaborate thermal mass scheme.
The other upside of a radiant floor heating system is that, theoretically, it could easily be run on electricity. And if this country gets its act together and provides adequate green electricity, there's no reason why the Usonian couldn't run entirely on green energy, minus the wood for the fire.
Thermal mass is a significant factor in the situations I described.
A frame construction mounted atop a basement (particularly one with a boiler or ductwork running through it) will stay warmer in the winter than a slab on grade room having an uninsulated, unheated slab. The frame floor has a 50ish-60ish degree environment beneath it to keep it from getting too cold, and it has little if any thermal mass exposed to the exterior cold to overcome. Shut the heat off and the frame house will cool down at a rate dependent upon the amount of insulation and effectiveness of the weatherstripping. Most frame houses are heated via the air they contain, and that warmed air typically doesn't have the energy to overcome the thermal resistance of a massive element like a floor slab. Water has a greater mass than air and is better able to carry and transfer heat to a massive element such as a concrete floor slab than warmed air. The heated mass of a floor slab retains its heat longer than a room full of air...it takes longer to initially heat, and longer to cool down.