Hello
I am building a new 3350sq foot house in Detorit Michgan area.
My wifes folks have electric in floor heating in their home in Minneapolis, and the wife loves it.
I want to add it to our kitchen, nook, and master bathroom.

Should I be considering electric or liquid heating?
I am on natural gas, and will be using 4" spray foam in the walls

I'm leaning towards a 95% effic furnace, BUT beothermal is also an option right now with the 30% tax credit


ANY help would be GREATLY appreciated
thanks in advance
Ray

Per delivered BTU geothermal might be cheaper to operate, but much more expensive to install (even with the 30% tax credit) than a condensing gas furnace (for hot air), or a condensing gas boiler (for hydronic radiant floors.)

Resistance electric would only be a reasonable option if you were looking at a true high performance building envelope (R40+ walls, etc) or have the worlds lowest power rates. It's cheap to install, but expensive to operate in most markets. Before plunking down an extra $15-20K for geo, consider how much it would take to go much higher-performance on the building envelope (high-R on foundation, walls & ceilings + better windows + verified air sealing to a very tight level, etc.)

BTW: Closed or open cell on that 4" of foam? Any exterior rigid foam going under the siding (or behind the masonry)? If open cell it's not enough R, if closed cell in a studwall it's still lower performance than what COULD be done- there are cheaper combination stackups to consider that would deliver a higher performance wall for similar or less money. See:

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

For new construction in your climate 2x6" 24" on center construction with flash & fill (1-2" of closed cell sprayed on to the sheathing from the interior as part of the air-sealing, followed by spray cellulose or sprayed/blown fiberglass), and 3-4" of rigid iso on the exterior would deliver R40+ whole-wall R values. Combined with R24+ insulated concrete form foundation and R16 EPS under the slab and R55+ of cellulose in the attic you may still be looking a less of a cost differential to your current plan than adding geo, with a bigger boost in performance. 4" of closed cell in a 2x6 stud wall is only delivering a ~ R20 whole wall R value due to all of the thermal shorts of the framing. (And with open cell it's substantially less.) Foam-clad 2x6 framing is a relatively easy build. Even if you split the difference in performance with only an inch or two of XPS on the exterior + flash & fill you're looking at substantially higher performance comparable cost. (A variation on 2b in the high-R walls document.)

No matter what your insulation package is, building for air-tightness and verifying it with blower door testing, and getting it down to < 1 air change per hour @ 50 pascals is a cost effective efficiency enhancement. Defining the air-barrier in the plan and implementing it in construction this isn't too tough a requirement, but retrofitting it to that level can be a bit much.

By going with a moderately high-performance envelope you can cut the heating/cooling requirements by 50-75%, at which point the relative efficiency of the heating systems starts to matter less. In Detroit even a smallest-of-the-line modulating condensing boiler might be overkill in such a house, but a condensing heating + hot-water combi might not be (or even a non-condensing combi.) If you can get it down to where design condition heat-load is under 25KBTU/hr (likely, with a high-peformance envelope), something like the Daikin Altherma air-source hydronic heat pump could supply both radiant floor heating and hot water at near-geo efficiencies for less money (more up front than a combi or gas-fired mod-con boiler, but with a lower operating cost.)

IMHO high-efficiency heating systems make more sense for very large loads, or low-performance building envelopes. For new construction it's an iterative process, but erring toward the higher-performance building when all else seems equal usually results in higher comfort, even when the operating costs are equal. Radiant floors can achieve the same comfort at a lower room temp though, but even that becomes moot at R60 walls and PassiveHouse heat load levels, at which point you don't really need a heating system, but putting in some electric radiant in just the spots where you might sit/lie can probably improve the comfort & efficiency of even one of those (compared to the usual resistance-elements in the ventilation air schemes.)

"My wifes folks have electric in floor heating in their home in Minneapolis, and the wife loves it."

Radiant floors are naturally comfortable, but if your in-laws are using electricity for space heating here in Minneapolis, they are paying 3 times too much to operate it! Further, they are burning coal to heat the floors...definitely not GREEN. Worse yet, finding the error of their ways, can't even change there minds and use clean burning domestic natural gas.

Geo-thermal, more properly ground source heat pumps, don't make sense in most cold climates where natural gas is available, as the gas is usually cheaper than COP3 electricity and much less expensive to install using condensing boiler and indirect for domestic hot water.

Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....

Posted By minkia38 on 23 Feb 2011 07:43 PM
Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....

And most of that cost uptick is 5.5" of foam vs. 3.5" of cellulose.  Every inch of depth of closed cell foam adds the better fraction of a buck per square foot over what an inch of spray cellulose costs. It's more R (R6/inch vs. R3.6/inch), but from a cost-effectiveness point of view...   Once you have an air-tight assembly you're primarily comparing R/$, and closed cell spray foam is more than a 2x multiplier on that basis (more than 3x sometimes.)

Closed cell foam can be used to adjust the vapor permeance==drying capacity of the assembly, and limits the wicking of wintertime condensation into the studs & sheathing, but the right amount of exterior foam can raise the average temp of all the structural wood above the dew point of interior air, rendering that aspect somewhat moot. (And cellulose can safely buffer a significant amount of moisture even if you skimp a bit on the exterior foam.)

At 3-4" closed cell foam can be structural- it adds rigidity to the assembly and "glues" the studs/plates/sheathing together, which can be third level of benefit in hurricane zones (but not so much in MI.)

The number of board-feet of lumber for 2x6, 24" on center framing is nearly identical to 2x4 16" on inch, with similar structural capacity, but fewer boards to cut==lower labor cost.  The cost the builder is usually a net savings, not a cost.   Going to 24" on center results in a lower framing-factor on thermal shorts through the insulation. It usually results in comparable or cheaper construction costs, with a measurable boost in thermal performance.

The installed cost of rigid foam per square foof is typically less per unit R than closed cell spray foam for larger flat surfaces, but can be comparable if there are lots of angles, joints, window-openings, etc.  But putting it on the exterior of the sheathing A: protects the structural wood far better than when applied to the interior and B: Boosts the whole-wall R more than an cavity-spray job by providing a thermal break over the framing. If you detail each cavity and stud plate with acoustic sealant or simliar caulking as an air barrier, doing a cavity fill with cellulose and adding 2" of XPS or iso sheathing (2-layers, joints overlapping by a foot or so, joints taped/sealed both layers)  to the exterior puts ~30-35% of the R on the exterior of the sheathing, which would be enough to allow you can use only standard latex paint as the interior vapor retarder in your climate (which isn't nearly as severe as Minneapolis' averages, which is where the thermo-hygric analyses for High-R wall studies are simulated.)   That would put you over R30 center cavity, and over R27 for whole-wall R.  That's quite a bit more R for less money than 4" of closed cell foam, and the sheathing & studs are better protected from moisture issues. 

If you design the foundation with ICF such that the plane of the exterior foam of the studwall aligns with the exterior EPS (even if you have to cantilever it an inch or two off the concrete, which may take an engineering signoff, but it's doable) you also get a huge thermal break on rim joist & foundation.  In termite zones you'd want to extend metal flashing out and over that seam (foam seal it) but that's nothing compared to the thermal shorts that floor & rim joists/foundation sills represent- a very cost-effective detail for enhancing the thermal performance of the structure.  (Some buildes use an exterior inset or step to extend the exterior foam sheathing a foot or so below the foundation sill on poured concrete foundations when the foundation sill are set back from the exterior edge of the concrete, but ICF + aligning the planes is usually simpler/cheaper to build.)

This is so NOT a radiant-heating discussion, eh?

Not a rad heat discussion, but very important stuff I need answered none the less. VERY GOOD STUFF.
Maybe I can just hire you to come out!

Excellent analysis on the building envelope Dana,
Thanks,
Dan

So, I might be retarded, but according to Dana, where does the 3-4" rigid ISO go? between the studs and the bricks? so you would need an extra 3-4" of foundation/footings as well?

The rigid foam is applied to the studs, and leave at LEAST 3/4" of cavity space between the iso & brick.

Be sure to vent the top of the cavity well to the exterior under the eaves (or in some instances, to a well-ventilated attic), and allow ample weep-hole area to the bottom to course to aid condenstion/penetration drainage, and some amount of convective purging of the cavity air.

With brick veneer construction there is an inherent thermal bridge at the top of the foundation wall bypassing the exterior foam of an ICF foundation, unless you beef up the insulation on the interior side of the foundation. Insulated concrete forms are still the way to go, but assuming you went with R20-ish symmetric ICFs, spray foam the foundation sill & band joist to at least R15, and add 1" of XPS (R5) sheathing to the interior side of the foundation at least at the top half.

Review that high-R foundations document for other options, but also pay special attention the locations of capillary breaks- the breaks between footing & foundation wall, and between the slab & foundation wall, as well as under the foundation sill are as important with ICF & brick veneer as with any other type of construction.

To save on foundation wall thickness you might hang the foundation sill off the foundation a couple inches to the interior with only 3-3.5" of sill bearing on the foundation, as well as using a narrower brick or stone for the facing . Hanging the sill off the foundation may require engineering review the extra cost of the design detailing to save the yardage of concrete won't necessarily pay off in reduced foundation cost. (And just like having more R value than spelled out by code, nobody ever complains after the fact about having too sturdy a foundation.)

just an update, I decided to go with 3.5" of spray foam, and fill the rets fof the 2x6 cavity with blown in cellulose.
thanks for all your help!

Excellent choice and the one I made. Though often dismissed for its high cost, 2# high density foam is impossible to beat in some applications such as my 1912 balloon frame which is now air-tight, structurally sound and nearly water proof from the hot roof to the previously porous rim joist.

After foaming my own old home (and soon the steel building next door) and doing the heat loads on my Wrightsoft heat load program I couldn't find a better application and since have specified 2 pound foam in nearly every retrofit heating job I design or install. I just can't get enough of it!

One of my subs does a blower door on his foamed jobs and the proof is in the door.

I think the 2# cc in the band joist on my house was a key component to our great blower door result. Also the infrared camera could barely see the roof rafters and other roof framing components through the 7 to 10 inch 2#cc foam on the cathedral ceiling, a testament to how well it seals and insulates. But it IS expensive. Hopefully as it becomes more ubiquitous the price will decrease.

-Rosalinda

Posted By minkia38 on 23 Feb 2011 07:43 PM
Wow, that is awesome! I read those 2 studys, and they were very informative.

My builder said to upgrade from 2x4 construction to 2x6 construction, and upgrading also from blown in cellulose to 5.5" of spray foam, its an extra $7500

Since I am going to be living there for a long time, I want to make the most economical choice....

You might be able to cut your costs if you blow in your own spray foam. That's my current project and it's working out pretty well so far. Better read the instructions if you decide to use a diy kit though.

If you do consider geothermal, then take a look at DTE's offpeak geothermal electric rate which supposedly cuts the cost about in half. Even better if you have some thermal mass (concrete or water tank) to avoid use during the peak period - at least in the common case of non peak load weather (where the geo unit has excess nighttime capacity to be stored).

By thermal mass, do you mean just a large tank of water or slab of cement?
I guess that I have not ever heard of this option.
I do have a 40x80 insulated barn, with a full 4" cement slab about to be poured next week, about 75 feet from the house.....

Either one could work.

I guess I do not know what you mean at all by thermal mass. Care to expalin what you mean? or how I would go about doing/implementing it? thanks

I would hire a pro to go through all the options with you. But thermal mass can take the form of concrete floors or walls. Or it can be a buffer tank that stores water (possibly a lot of it). In either case, the goal is to use as little of the on-peak electricity as possible. The thermal mass allows you to coast through that period.

More info at:

http://en.wikipedia.org/wiki/Thermal_mass

2x4 with foam and an 1 1/2" thermax skin for the dreaded "thermal bridging" R31 exceedes the US standards for sidewall insulation and infiltration is next to zero if windows are properly installed. I still prefer this in most of my retrofit jobs and stick-built homes as well.

I would typically prefer cellulose if you're doing a well sealed external foam "skin" already. way more cost effective, typically. Greener too.

Yup- the primary benefit of the foam cavity fill is the air-sealing, but that's fairly easily achieved by detailing the Thermax as an air barrier. At K values of ~R4/ inch the low-performance R1/inch framing begins to dominate the heat transfer of the whole-wall (sub)assembly of a typical residential wall assembly, robbing the performance of the center-cavity R considerably.

The R18-R20 center-cavity value of a full cavity fill of 2lb foam is pretty much wasted by the R3-R4 thermal bridging of the framing. With 3-3.5" of 2lb foam and no exterior Thermax you're looking at a typical whole-wall R (that includes the thermal bridging framing factors of headers/plates/windows doors etc) of only ~R12-R13 in a 2x4 16" on center cavity,compared to R10.5 whole-wall for the same structure with cellulose, a less than R2.5 improvement, for a whole lot more money.

With 1.5" of exterior Thermax it rises to ~R23 for the 2lb foam cavity fill, or ~R20 for cellulose. Increasing it 2" of Thermax instead of 1.5" with cellulose fill makes them equivalent-performers, as long as the seams of the Thermax are taped, and the edges foam-sealed.

"Flash and fill" using 1" of foam to seal the cavity and filling the rest with spray/blown fiber is another approach that could be taken for air-sealing. The difference whole-wall R with 1.5" of exterior Thermax with flash'n'fill 2lb foam + spray cellulose and an all-foam show works out to be ~R1, about a 5% performance delta. Increasing the exterior foam to 2" in a flash'n'fill is cheaper & higher performance than 1.5" of Thermax and a full cavity fill of 2lb foam. But it adds another step in the schedule, which has to be played off on the "time is money" equation. In general you get more bang per foam-buck putting the foam outside the framing, where you reap the full performance of the foam.

FWIW: From a design point of view, in climates where it stays below 25F for weeks/months at a time, the K value of exterior Thermax needs to be derated to ~ R5.6/inch, whereas polystyrene K-values can be up-rated about 10%. At 0F they're more equivalent than the ASTM C 518 ratings at 75F might indicate. Given that foil-faced Thermax is an exterior vapor barrier and Styrofoam (tm) XPS is not at thicknesses of 2" or less (just like 2lb foam), if using 2lb foam at 2"+ as cavity fill, consider switching to XPS on the exterior, as it is more protective of the structural wood, since it provides a drying path toward the exterior. An inch of 2lb foam is vapor-retardent enough to protect the wood from inteior vapor drives, and with well over half the total R at center-cavity exterior to the cellulose in a 1.5-2" XPS + 1" SPF + 2.5" cellulose stackup there won't be wintertime moisture accumulation in either the cellulose or the sheathing/framing, and it can dry in both directions.

Naturally your numbers are sound but in residential renovation, infiltration is THE factor and nearly impossible to address without foam. The skin is often off limits e.g. Queen Ann downtown St.Paul and the stucture is often nearly impossible to access with other types of insulation. The difference in comfort and performance is often quite dramatic inspite of the often overstated "bridging" affect. From a creature comfort standpoint, even here in Minnesota, thermal bridging is a non-issue. For the purist foam is not GREEN enough, but for a pratical matter what we save in fuel is green in itself.

I have a hot roof in my own 1921 balloon frame and don't have to worry, given the weighted load for roof insulation, but would not trade the performance of foam in terms of overall performance and labor savings in the wall either. When I think of "super" insulated in a cold climate foam always sticks out if labor is considered.

When you speak of double-walls, hay bails etc. I think DIY retired, laided off etc. Maybe good for a few, but will not help the vast majority living in pre-1978 homes. The same is true of passive-solar designs in cold or cloudy climates. If you have enough money to invest and/or will suffer large ambient temperature swings, you,re all set.

There is much talk about performance, but as you point out, the performance is based upon the quality of installation. It is hard to cheat on 2# foam.

If you're not allowed to touch the siding as in historical districts, etc, yes, 2lb foam is the way to max out the R-value of what is the possible, but even in those cases the performance difference between a flash'n'fill and a full 2lb foam fill just isn't very much- far less than the center-cavity R value differences might imply.

It's the applications where you were talking about using exterior Thermax where a full cavity fill of 2lb foam is likely to be mis-spent. At a buck a board foot or more it's hard to rationalize just for the air sealing, over a flash'n'fill approach or detailing the air barrier at the rigid-foam layer. And any more than 2" of standard 2lb foam with exterior foil-faced insulation creates a moisture trap at the sheathing layer, and thus not best-practice.

It was the only way to retrofit the 6" raftered cathedral-ceiling attic rooms of my house up to any reasonable R without re-roofing with exterior foam, which I intend to do when the time comes. I used 2lb Icynene taking a ~10% hit in the whole-assembly R value under other 2lb polyurethane foam, but it allows the roof deck to dry toward the interior due to it's higher perm rating. And when I put foil-faced iso above the roof deck in short years it won't create a moisture trap the way most 2lb foams would. I considered doing it flash'n'fill, but it was a pretty small job and the insulation cost was subsidized by the state & local utility, so the cost difference was less of an issue than the time-factor of 2-steps vs. 1.

I've never been in the "no foam" camp- it's more a matter of how MUCH foam to achieve both the air-tightness and vapor retardency that makes sense for that layer & climate. Spray cellulose is cheap stuff, with a very reasonable K-factor for use as cavity fill. Dense-packed cellulose in wall cavities also air-retardent enough for retrofit air-sealing, from a comfort point of view, but requires the interior vapor retardency to be adjusted with paints (and or ventilated siding) to avoid excessive moisture cycling of the exterior sheathing in places as cool as MN.

I never talk about hay bales, and rarely about double-walls/larsen trusses etc. except in the context of PassiveHouse type super-insulation, never as retrofit. I DO talk about air-sealing though- and advocate air-sealing the sheathing and verifying/remediating with blower doors PRIOR to the cavity insulation.

I appreciate your grasp of vapor migration, something still lost in many building codes. I don't believe plastic vapor barriers are effective enough for most application except in basements where they shouldn't be used at all. I am glad to flesh out the complexities of insulation with an real expert on the subject.

Were it that everyone did a blower door test before during or after. This fall we will provide a blower door test with every new high efficiency boiler installation (with foam if applicable).

I would not dispute the value of celluose but am concerned with settling in paper products, a factor of no concern with foam.

Morgan, I applaud your blower-door testing service!- It's not a usual service for a heating contractor. But air-sealing is often/usually the lowest of the low-hanging fruit on building efficiency, and knowing the infiltration rates up front the heat-loss calc is more precise.

Wet sprayed (aka "stabilized") cellulose is dimensionally stable over decades due to the water-activated adhesives used.

The stability of dense packed cellulose is also very good, if installed at the density appropriate for the climate. Annual humidity cycling in dense-pack introduces some creepage/shrinkage over time if it's density is too low. This has been studied fairly carefully for calculating minimum cellulose densities for Scandanavian climates:

http://www.nordicinnovation.net/nordtestfiler/rep565.pdf

Basically at 3.5lbs density or higher even the coldest parts of the lower 48 will experience no shrinkage over time. Since the density increases with settling the process is inherently self-limiting, so even significant under-density cavity blows will stabilize to within 99% of it's 200 year number in 20-25 years, and it's typically in the low single-digit percentage in 2-2.5lb density "two hole method" low density blows. At only ~3.2lbs density dry blown in US climate zone-5 is good forever, and you'd have to look at a century-long time-frame to find settling worth remediating in the colder parts of zone 6. Dense-packing stabilized cellulose at even 3lbs is also good pretty much forever- it eventually glues itself in place, even when dry-blown.

Foam (particularly closed cell foam, which is less flexible than open cell) also has shrinkage & bonding issues over long time frames (or even in the short term in the hands of an incompetent installer. There is no free lunch, and all things-construction are transitory, grasshopper.

Plastic vapor barriers cause as many problems as they solve, IMHO. Almost all moisture problems in wall structures are ascribable to either bulk-leakage (poor flashing), or air leakage from the interior side in cool climates. A plastic vapor barrier on the interior in a cold climate is typically full of holes from plumbing & electrical penetrations, or even hanging pictures on the walls with nails, and relies on vapor diffusion & air leakage from the exterior side to remove air-transported moisture from the cavity. In a foam-sandwich that's perfectly air-tight it's an all vapor-diffusion show, but nothing in the real world is actually perfect. It's possible to build assemblies to be resilient to both interior air leakage and vapor diffusion by adjusting R-values and the vapor permeance of both the interior and exterior sides so that A: the average winter temperature of the wood stays above the average dew point of the interior air (37-40F for a 30-35% relative humidiy 68-70F conditioned interior) and B: is permeable enough to dry seasonally should some moisture find it's way in (by whatever path).

Using somewhat permeable exterior sheathing foam and providing a 3/8" (10mm) vented cavity between the foam and the siding (aka "rainscreen") to dry into improves the year round exterior drying capacity to the point where you can get best results using only standard latex paints (2-3 perms) as the interior vapor retarder with relatively modest sheathing to cavity R ratios. Without the cavity there will be many annual hours where the micro-space
between the foam & siding is at or near saturation, with zero drying going on. With a 2-3perm interior the vapor diffusion into the wall is slow enough to limit moisture accumulation by diffusion alone, and has substantial spring/summer/fall drying capacity toward the interior.

Cellulose cavity fill can buffer a LOT of moisture (up to something like 20% by weight) without losing R-value, and actively wicks condensation away from the sheathing during the coldest hours when condensing might occur. A flash coat of 1" of closed cell foam on the wood makes the foam (and not the wood) the condensing surface, but a condensing surface that can't wick moisture toward the wood. Yet at 1" it remains vapor permeable enough for the wood to dry toward the interior. The inch of foam only adds ~R2 to the center-cavity R (and ~R1 to the whole-wall R) over an all cellulose fill, but it puts a modest vapor-retarder at a near-optimal point within the stackup, while providing at least half the detailing necessary to make the sheathing an air-barrier. This makes flash'n'fill using 1" of 2lb foam + wet-spray cellulose a real value-proposition, using the properties of both the foam & cellulose to best price/performance advantage.

Vapor barriers do involve a lot of debate. For example, here is an actual test in Minnesota where it looks to me like every test wall with plastic on the interior worked well, even the one with plastic on the interior and the exterior. Most of the other walls have mold.

http://www.forestprod.org/woodprotection06huelman.pdf


I would pay more attention to infiltration, either non-absorbent sidings or a rain screen, overhangs (keeps siding dryer), interior humidity and pressure differentials (which will push moisture into a wall).

I looked at the slides and didn't get that message. I got bulk water problems and a common synthetic stucco failure with poor workmanship thrown in. I work on them all the time and get calls to fix plumbing leaks that are really poor site prep and rain run-off problems. This was an uncharacteristically poor presentation raising more questions than answers.

My first rule is to do more work than you make, I think they broke the rule.

Posted By BadgerBoilerMN on 13 Apr 2011 04:20 PM
Your conclusion makes sense and I have seen it done, though selling it to the leyman is a bit of a chore. I have lost many a job to a monolithic salesman taughting open, closed, cellulose or even glass batts. And then you have to argue with the local code official about what constitutes a vapor barrier and where it should be. In the end it is as much about perception as reality and why I have to get involved with fields outside my immediate expertise.

I appreciate the technical nature and astute analysis, as it will help with the more technical of my client base.

There is still a gray area for me when sorting out new construction - an area to which I am increasingly called- and renovation of older houses - which I was born in.

In renovation - as in your ceiling - space and structural limitations seem to dictate 2# foam (knee walls, finished attics and sill plates full holes, cracks and mechanicals come to mind). Open foam may suffice with lower R value per inch but is not water-proof and the same can be said of blown cellulose.

The blower before - to help with confirming the heat load - and after to prove our work.

Open cell foam is too vapor permeable to use in unvented cathedralized ceilings in MN-  moisture diffused in and accumulated in the roof deck too quickly in winter.  (0.7lb Demilec could work if you go thick enough, but not half pound polyurethane or half-pound Icynene.)  The 2lb Icycnene  MD-R-200 (and not MD-C-200) is something in-between- it's K value is ~R5/inch (compared to ~R6+/inch for polyurethane), but is 1.3 perms @ 3" thickness,- a thickness at which 2lb polyurethane is under 0.5 perms.  Even though I object to some of that company's marketing strategy (selling people on insulating to sub-code R values, in particular) their 2lb product is useful in applications where you want to keep the vapor permeance higher at R values that would be more vapor-tight than best-practices using polyurethane (or Icynene MD-C-200) 

At 5-5.5" in a 2x6 cavity it'll be about 0.7-0.8 perms, which is about ideal for Zones 5-7 in most stackups.  It's only R26 @ 5" as opposed to R30+ for most 2lb polyurethane, but with the fraction of R5 thermal bridging at the rafters/studs the difference it makes in whole-assembly R is quite small.   At under 4" it would probably be useful to use vapor retardent latex on the interior to reduce the interior vapor permeance for zone-7 unless there is sufficient exterior foam to keep boost the average winter temp of the sheathing above the interior air dew point.

But as I stated before, despite all the discussion about vapor barriers/retarders, the primary causes of moisture related rot or mold in buildings is still bulk water incursions and air leakage, both of which are mitigated considerably with even an inch of closed cell foam. In air-tight vapor-tight assemblies (even double-vapor barrier situations with extremely slow drying rates) the risks are low. It's the non-so-air-tight assemblies with vapor barriers where the vapor barrier becomes a "solution-problem", causing as many issues as it solves.

Note, regarding the Minnesota study, I wasn't clear that I was referring to the test walls they built, not the houses (which had a wide variety of problems). Fixed.

That study pretty clearly debunked the notion pushed by some cellulose installers/manufacturers that you shouldn't use interior vapor retarders. (See bays 5 & 6) But it's pretty clear that cellulose would do OK with only vapor retardent latex, about the same ~0.5 perm rating as kraft facers (see bays 4 & 9 with the kraft faced fiberglass).

The vinyl siding used in the test bays are a rough approximation of a rainscreen gap, but not an exact equivalent.

Apparently no exterior foam was done in the test bays, which would have increased the temp of the OSB, reducing the number of condensing hours at that layer in any stackup.

Sorry jonr I overlooked it...should be focused on my current drawing anyway.

What does this have to do with radiant heating.

I generally say go with hydronic heating but it seems that all you want is floor warming so the advantage of electric would be low installed costs and low maintenance may out weigh the lower operating cost and flexibility of a hydronic system. If you wanted to heat a garage or basement slab to go along with the rest of it then i would use a gas hydronic system. You could use some type of boiler water heater combo.