So I am planning my first and hopefully last home and have a few questions. I would love my house to be as close to net zero as realistically possible here in northern Utah. Because of the covenants my house must be at least 2000 sq ft minimum. My design has a 2000 sq. ft. main floor and the same 2000 sq. ft below grade. As I have plenty of land I will be installing a geothermal heat pump (J load yet to be calculated to determine sizing). I plan to have radiant heating in the basement. 10kW of solar panels. My main question is that of my wall assembly. Wall assembly: exterior cladding (rock and fiber board), OSB, double wall framing on 20" centers to mostly eliminate thermal bridging and allow for 12" of densely packed cellulose, OSB (to keep walls flat from bulging dense cellulose), gypsum board. Also, I plan on installing the radiant heating, electrical, plumbing, solar, and ground loop for GSHP myself so labor isn't an issue in these regards. Thoughts? Suggestions? Do I need to add a vapor/air barrier? Do I need Furring strips underneath cladding for ventilation? Will 12" of densely packed cellulose be enough of an air barrier? Will this wall assembly work or are there better performing options for the money? Any advice is welcomed and very much appreciated.

I'd either run BEopt (to optimize cost vs energy) or find someone who can. Consider using a rainscreen gap, a smart vapor retarder, excellent interior side air sealing and exterior plywood (more resistant to moisture damage) to make a moisture safe double wall.

What is your soil like? Do you have good drainage"? Is your lot on a hill"? What frost depth does your code specify? What kind of foundation are you planning? When you say " below grade" what do you mean? How high above the basement floor's top will the highest grade around the basement be? Will the below grade area be finished as living space? A PWF, permanent wood foundation, can save initial money and much energy later. 20" framing is non standard and problematic for most sheathing. better to choose 24" or 16" which evenly divide 48" the common material dimension. Geothermal heat pumps are no "better" than and far more costly than low ambient mini splits! Do you have a contractor that can properly dense pack cellulose in 12" cavities, pretty deep?
I couldn't find such a contractor and opted for mineral wool instead. What is your wall r value goal? The OSB is plenty of air barrier if protected and taped. What snow load do you need to design for? Typically only one of the double stud walls is load bearing, this may need to be 2x4 16" OC or heavier because of the roof load. FWIW I'm building a "pretty good house with a PWF walkout basement under a double stud main floor and have done all the design. I have 9 feet of back fill on the uphill side, r10 foam under the basement slab, r20 basement walls using mineral wool bats on the inside. My double stud main floor walls are 2x4 24" OC inner and 16" OC outer insulated with 3 layers of mineral wool. the inner wall is load bearing and uses insulated box headers, it is sheathed on the exterior side with OSB taped and caulked. The outer wall is framed @ 16" to allow structural fiberboard exterior sheathing which I well may omit. I plan on fiber cement board siding over house wrap. With the fiberboard my main floor walls calculate as r40+with all thermal bridging accounted for. My house will be about 2500 sq ft.

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If you don't mind me asking Liebler, how thick will your double wall assembly be? After some recommended research on mineral wool (thanks!) I am thinking about a double wall assembly that is 14.5" thick to allow for two layers of R-30 mineral wool batts (they are cheaper than the R-23 for some reason). That will leave me with R-60 for the main level and R-30 (just one layer thick) in the basement. The exterior wall being 2x4 16" OC and the interior wall 2x4" 24" OC with a 7.5" gap between the two. I don't know of any contractors with real experience in doing a permanent wood foundation. I think I will stick with the slab with 2" of XPS and radiant heating in the basement. The basement will be almost entirely below grade except for the North side of the home will be 3/4 below grade due to the slight slope. The basement will eventually completed as living space, with 2 bedrooms, recreation area, utilities, storage, and home gym. The snow load to design for here is 30psf. So far based on Home Depot's prices for mineral wool I will be out $10,500 for insulation (excluding the attic). Lastly, a 5 ton geothermal will cost me less than $5,000 since I am excavating and installing the horizontal ground loop myself. Going with the mini splits I will need at least 8 separate heads and several compressor units on the exterior that weather and wear out much quicker than my geothermal which will have no elements exposed to the weather. I can't find that many mini splits for anywhere near the low cost of my geothermal. Just my point of view at least. Feel free to share thoughts and criticism freely! And thanks again for previous responses and ideas.

I just found an option for "bulk pricing" at the big box store which makes it cheaper to do 3 layers of R-15 mineral wool. For roughly $6,600 I can achieve R-45 on my double wall and R-30 for the basement by using 3 and 2 layers respectively. My double wall assembly will only be 10.5" thick (plus gypsum and OSB). covered by gypsum board on the inside. The exterior will be covered in OSB, house wrap for air barrier and vapor permeability, 1" furring strips and then the fiber cement board cladding. Does this assembly seam logical to anyone but me? Are there any caveats that I am not seeing here?

Not a expert at all and have never done this but it appears you will have vertical voids in your wall that will have convection currents because of using bats. This is usually not a good thing at all. You will also have to close the gap at the top between the inner and outer top plates(for fire blocking). I think this is why people put the inner and outer wall right next to each other and do loose fill.

Most of northern UT is US climate zone 6B, which is cold enough that you can't get away with exterior side OSB without an interior side vapor retarder, even with a rainscreen, even at half the wall thickness you're talking about. OSB isn't really all that permeable- it's even less permeable than CDX plywood (but not by enough to matter.) The IRC prescribes a Class-II vapor retarder or tighter on the interior side of wood sheathed buildings, in zone 6:

https://up.codes/viewer/utah/int_residential_code_2015/chapter/7/wall-covering#R702.7.1

It'll probably work just fine with 2 mil nylon (Certainteed MemBrain) or Intello Plus or some other "smart" vapor retarder behind your wallboard, or with half-perm "vapor barrier latex" primer on the wallboard. But there isn't yet a 100 year history on any of those materials (OSB, 2-mil nylon, or vapor barrier latex), so it's not clear what the full lifecycle would be.

Cellulose would give you better margin over rock wool due to it's moisture buffering capacity, but has to be installed at 3.5 lbs + density when installed in a double-studwall to not have long term creepage/settling issues. Achieving constistently high density is tough when there are no partitions to segment up the wall into narrower chunks, but blowing mesh partitions at the stud spacing would make that easier.

I'm not sure what makes you think you need 5 tons of heat pump or 8 separate for a 2000' high-R house with 2000' of mostly below-grade basement. (Even my 2x4 framed antique with sub-code windows wouldn't need 5 tons of geothermal to keep it warm even at -10F outside.) With U0.25-ish double low-E double panes and at least as much insulation in the attic as in the walls you're probably looking at less than 25,000 BTU/hr @ 0F, maybe even less than 20,000 BTU/hr. (Got a ZIP code, or a 99% outside design temp?) You can get almost that much heat out of a 1.5 ton mini-ducted Fujitsu at 0F. High R houses don't need anything like "a head in every room" if going ductless, and at UT style wintertime dew points you won't be giving up a lot of capacity to defrost cycles with an air source heat pump solution either.

Even if all the digging is "free", and all the design and installation is DIY (not recommended) I don't see how you can get your total hardware costs for (the unnecessarily large) 5 tons of geo under $5K.

If the basement is heated with a radiant slab, you may want more than R10 under the slab, and R30 on the foundation walls is approaching extreme overkill, given your ~R30-ish "whole wall" double studwall for the above grade section.

Bottom line, don't get too far ahead of yourself on the mechanicals systems until you have at least a first rough cut of the load number. jonr's recommendation for modeling it with BeOpt then tweaking it in is a good one.

Sanity checking, the "whole assembly R" numbers listed in Table 2 p.10 of this document still work-mostly. With higher-efficiency mini splits or a geothermal heat pump you probably won't need 10kw of PV on the roof to hit Net Zero, even using the recommendations for zone 5:

https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

Thanks for the very educational and eye opening information. I have yet to research "BeOpt" but will definitely look into it as it sounds very useful. I meant to have said that the house is 2,000 sq. ft. up and 2,000 sq. ft. down for a total of 4,000 sq. ft. I think it would be very hard to achieve even temperatures in all of the different living locations without each of them having their own separate head units. If any bedroom doors were to get closed temperatures would not be consistent. Here is a link for a 5 ton 2 stage geothermal. I would also need to pay for ductwork and the horizontal ground loop piping.

http://ingramswaterandair.com/stage-tranquility-digital-geothermal-heat-pump-p-25998.html

So I will likely need an air barrier/vapor permeable rain screen and also a vapor barrier on the interior side of the interior wall (just below the gypsum) if I'm understanding correctly?

As far as the insulation goes, it will all depend on if I can find a reputable company that has experience installing dense pack cellulose in cavities 12" thick. I am also concerned about the "bulging" from the dense pack that will make gypsum board difficult to instal. Will I need a separate layer of OSB or will dense pack with 24" OC framing be able to be rolled flat? Any issues with drywall screws popping loose after the fact? Mineral wool would be able to be installed DIY without issue, although very time consuming.

That table recommends 10-15-R for below grade slab from what I can understand. So I should plan on 2 layers of 2" XPS giving me an R-20 sub foundation insulation. Will this be sufficient for radiant heating? There is some great information on that building science article - Thanks Dana! - oh and as far as my zip, the home will be in 84037

You really don't want OSB as your exterior sheathing, that is courting disaster mold, rot etc. My double stud walls are stud face to stud face 10 15/16" deep with 3 layers of r15 mineral wool and one thickness of 7/16" OSB on the outside of the INNER wall. That puts at least r19 (r30 between studs) outside of the OSB. With OSB in this location it serves 3 purposes, it is the structural "racking" strength of the wall it is a fine air barrier when caulked and taped and it is a "smart" vapor retarder, allowing more moisture through when it' wetter. If you must have exterior sheathing consider structural fiberboard is pretty low cost and ads about r1 on the outside and is still vapor open allowing good drying to the outside . My whole wall r value is between 39 and 40 with the fiberboard, or DC14, and I have a lot off windows and doors, a total of 300+ sq ft. I doubt that much more insulation is economically justified, adding a few more solar panels is a much better investment. Going to 5 1/2" between stud lines or 2" thicker would cleanly add r7 but I think it's overkill even for your climate. I'm going to use a drainage mat (Greenguard DC14) outside the WRB and fiber cement siding over that, a lot less labor than strapping. I also have no contractors familiar with PWF, I had to become the "expert" and will supervise the hell out of that portion of the build along with air sealing details! Fortunately I do have an enthusiastic framing crew. If you insist on a concrete tub You should still follow the drainage and water management of a PWF, like 4" of gravel under the foam, gravel under footings and a complete drainage plan with a sump or trench to daylight. Otherwise it'll no doubt hold you house but it won't be "livable" space. And don't forget a radon vent.

moderator please remove duplicates!

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