The surface temps of R40 walls/ceilings and
Marc Rosenbaum has been designing high-efficiency and net-zero houses for decades now, and has several climate zone-5 houses (net-zero and other) heated with ductless under his belt (some of them in some pretty fancy neighborhoods).  Since  the turn of the millenium he seems to have shifted from away from active solar thermal w/storage and in favor of ductless (sometimes geo) + PV (grid-tied).  As the efficiencies of ductless have gone up and the cost of PV have come down it's turning out to be a simpler & cheaper solution than active solar space heating (although solar hot water is still in his repertoire).   Under his guidance, a whole development in Concord MA (a few miles away from where I'm currently sitting) has most of homes heated & cooled solely with multi-splits. (Most are new construction, but one single-family and one 2-fer were rehabbed older housing.)  Most of the homes at Eliakim's Way on Martha's Vineyard are heated solely with a single-head Daikin. 

OK, so Marc is a pretty smart guy with a lot of experience, but how many existence proofs does it take?  People dropping $500-750K for a cushy house in Concord aren't usually hippies willing to put up with comfort issues. I'm still not convinced that it's a hard design problem in a high-R building to get point-source or multi-point source solutions to provide comfort in a zone 5 climate.  If  uncle's & mother's places are any indication, it's dead easy to design for in zone-4 climates without going crazy adjusting the floor plans, U-factors or R-values, given that they're both retrofits on homes previously heated with ducted systems. That's not to say that radiant floor might not be a comfort enhancement in those zone-4 retrofits (neither is at current-code U/R, or super tight), but as a matter of cost/comfort it's not always warranted in higher-R new construction with miniscule heat loads the way any solar-heated home NEEDs to be.

If an R4 glass wall doesn't affect comfort I guess an R4 insulated wall would be comfy too... it's an improvement, but it's not the end of the story. depends on glass area, construction method, proximity to regular occupation patterns... I'm sure it can be nailed down to some guiding principles, but even then, it's basic training your building team needs to execute.

agreed pv/heatpump is the way to heat with solar... that's why we're doing it

it's not hard to do your kind of approach if that's what you do and you have a client happy with Ecobox design. it's hard if you are a regular architect or builder and you have a client with a view... I work with marc on projects occasionally that blow my ecobox characterization out the window and still hit very low heat loads, it can be done, but takes more than just R value to make it happen, it takes someone like Marc, a well defined building process that includes verification steps, builders that know how to execute to a high degree of detail, and architects that know how to design.

it's that or you build a simple ecobox. me, I like ecoboxes. built one for my shop. but then, I'm an energy wonk of sorts, and definitely in the "unconcerned with aesthetics" camp. My wife would not be pleased with my building design decisions...

again, I'm not saying radiant is always the way to go, that's in danger of becoming a straw man in this discussion, no one here has ever even tried to insinuate that. I AM saying distribution can be improved beyond single-point-source... i.e. more heads, ducts, or hydronics (radiant or otherwise)... in most homes, including an awful lot being built by pretty green people these days. the homes being built that are really suitable for single point heating... or monitor heaters before them... are fairly small, at least if a consistent comfort level is actually valued.

there are also people who will happily keep their money in the bank and deal with less comfort, and plenty who don't notice discomfort because they are used to it. they have monitors in their homes now and don't care that the bedrooms and bathrooms are 60 degrees. probably use less energy as a result too.

The idea that people are spending too much on their HVAC systems -geothermal notwithstanding- is a little far-fetched. I have to talk some out of a zone for each room, but more often it is adding a zone in a room with loads so lopsided from the rest of the house as to make it nearly unusable under design conditions.

In metro areas where transportation noise is an issue, HVAC is the answer.

Clearly radiant is a component and especially suited to retrofits where ability to improve the envelope is limited. Overcoming low MRT and lowering fuel bills is the result of most of our designs. This same strategy is no less valid in new construction especially where the bulk of the load is for heating. In my own recent remodel a 1.5T mini is holding at 75°F as we approach design conditions in 2100sq.ft. on three levels. But when it gets cold the walk-out would hardly get to 68 if it weren't for the radiant slab. I am a minimalist as long as you get what you want.

Comfort is a relative thing

Quite right and statements along the lines of "so and so is comfortable in their house even with solar gain on the south side, closed doors and a single point heat source" are meaningless. Show me some real data. Like actual graphs showing all rooms within a few degrees of each other under all conditions.

Most of the homes at Eliakim's Way on Martha's Vineyard are heated solely with a single-head Daikin

Even the marketing materials don't say that:

Supplemental heating is provided by ceiling-mounted Enerjoy electric radiant ceiling panels. "The houses are designed such that the single point source of heat – the heat pump – should be able to provide all the required heating, as long as the doors to the bedrooms are left open to allow heating by natural convection. The radiant panels allow heating in the event of a doors-closed operation, or to provide supplemental heating in extreme cold conditions."

No closed doors here. Naturally a little common sense goes along way. Experience is better. Suffice it to say; we have many tools at our disposal and the real challenge is in customer preferences for architecture and comfort as they are often antithetical i.e. Frank Lloyd Wright.

The real challenges come from climate, insulation, windows and homeowner expectations. Budgets can be split down those lines as well.

I think l have made Rob's argument again.