So it's looking more & more likely that we will be building another house in the next year or so.  On the current house, I tried to incorporate the best insulation/air sealing/shell combination that fit within our budget and timeline, but we had a lot of limitations.  The biggest was that we were on a very tight timeline to get in the house before the homebuyer credit expired.  We had a bad spring, weather-wise, so from the time we started, we only had about 3 months. 

We did pretty typical construction - 2x6 exterior walls, 16" OC - insulated with "flash & batt" - inch of spray foam in the cavities + R19 fiberglass batts, and then 1" of rigid foam on the exterior.  All the studs were caulked/sealed prior to the spray foam.  The attic was blown R50 with what they called "attic seal package", where they spray foam the top plates, around can lights, etc., to seal everything up.  Rim joists in the basement were all spray foamed, and there was 2" of rigid foam on the exterior of the foundation but nothing under the slab.

I don't have all of the numbers in front of me, but last winter, our highest utility bill last winter was about $210 - that's in Green Bay, WI (around 8000 HDD), and is gas & electric, so not just heating, for an approx 2000 sq ft house.  Average bill over the winter was probably closer to $175/month.

So that said, if/when we build a new house, we won't have the time restrictions & should be able to put more planning into it.  The purpose of selling our house & building another is to downsize a bit & build something cheaper.  We'll probably only downsize from 2000 to around 1700, but plan to spend less on the interior amenties, etc.  With the goal of building something very energy efficient but at the same time doing something that does break the bank, I'm leaning toward a double-stud or Larsen truss wall.  I'd like to do dense-packed cellulose or possibly blown fiberglass & use poyiso foam board on the exterior as the vapor barrier (drying to the inside). 

Ultimately, the question is - if we're not doing a super-thick wall, like 12" (more likely shooting for 8" walls & around R40), is one better than the other (Larsen truss vs double-stud) in terms of cost to build & "bang for the buck"?

If you're going with exterior foam anyway, the additional complexity of a double-studwall or Larsen Truss approach probably isn't going to be worth it. To hit R40 with exterior iso, start with a 2x6 studwall & cellulose, but caulk the stud bottom plates to the subfloor, and sheathing & top plates to the studs for enhanced air tightness. If at 16" on center that would deliver ~ R14. Adding 2 layers of 2" iso (seams staggered & taped) you'd be adding about R22-24 to the stackup for a total of ~ R36-38.

That's a very simple thing to frame, no builder is going to be confused by it, and the thermal bridging will be less than a double-studwall, and comparable to a Larsen Truss.

Dana - thanks for the reply!  A couple thoughts/questions for you..  

1.  When we built the house that we're currently in, I wanted to go thicker than 1" with the exterior foam, but the builder was concerned about attaching siding, installing windows, etc. with 3-4" of foam.  Do you think that's a real concern, or is it just a matter of the builder not being familiar with the proper techniques?

2.  I know that foam board isn't the cheapest thing in the world.  The idea of a double-stud or larsen truss appealed to me for the thermal break that it provides without having to use foam board on the exterior.  My thinking with the 1" of poly-iso board was strictly to use that as a vapor barrier & seal it up tight.  Maybe that's not the most cost-effective method.

3.  In terms of best "bang for your buck", what wall/stackup/system would YOU use to provide the tightest house for the lowest cost?


Last thing - I don't know if it matters, but we're actually considering building a 2 story this time rather than a ranch - jury is still out on that one, though..

The vapor barrier would have to be inside the exterior sheathing to avoid condensation & rot issues to work in a double-studwall or Larsen Truss assembly. If you're going to place the vapor barrier between the studwalls or inside the Larsen Truss it complicates the application of the cellulose, since you need to keep the vapor barrier as perfect as possible.

While a Larsen Truss does a pretty good job of thermally breaking the framing, double studwalls have significant issues at the subfloor & band joist.

A comparison of these assemblies lives here:

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

What I've proposed is essentially Case 2b (but substituting iso for XPS, gaining an additional R3-ish), the Larsen Truss is Case 5, and the double studwall is case 10. Note the difference in R value at the rim joist. If you go 16" o.c. instead of 24" AF framing peel about R1 or 1.5 off their numbers since it'll be more like a 20% framing fraction as opposed to 16%.

Rigid iso isn't cheap, but it's cheaper than 2lb SPF. The additional complexity of the framing for double studwalls or Larsen Trusses isn't free either, and can get pretty complicated at bump-outs & corners.

Thicker exterior foam is primarily a contractor-education issue. At 4" rigid foam mechanically strapped to the framing with furring that's through-screwed to the studs is pretty straightforward. At 4" with most siding types you'll be fine with 24" spacing (the screws can get pretty expensive if you need to go tighter, and tighter space== more thermal bridging too.) Over 4" it gets more awkward, but people are succesfully adding as much a 6" as retrofits to existing buildings. A decent primer lives here:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

It's worth following the links on that page for some of the details too. You may want to print them out to hand out to potential contractors.

Bang for buck, a rough stab at getting to a true R40 at lowest possible cost: A 2x6" with 4" of exterior Type-I EPS foam, then adding in lateral 2x4son the interior for thicker cellulose "Mooney Wall" style also works (similar to Case 9, but with 2x4 instead of 2x3, and with exterior foam.) The extra 3.5" of cellulose buys you another R10 after framing, but the foam is then only ~R16, but less money than 4" of iso, and at 4" it's sufficiently permeable to allow for exterior drying. With only R16 foam on the exterior you'd also need to use vapor-retardent latex on the interior. Alternatively you could put up varibable-perm vapor retarder such as CertainTeed MemBrain. If instead of 4" EPS you used 3" of iso (nominally R18) you'd have sufficient exterior R to skip the interior vapor retarder altogether, but 4" of iso would be even better (more protective of the sheathing from a condensing-hours per year point of view than lower-R foam), and you could thin out the lateral strapping to 2x3 and still hit R40.

Detail the structural sheathing as the primary air-barrier, but still tape & edge-seal the exterior foam, as well as the interior wallboard. The structural sheathing is protected from accidental penetration during construction & thereafter, and is the most-reliable on a century-long time scale.

There are some thermal advantages to 2-story structures over the sprawling ranch house approach- they tend to have a smaller exterior surface area to interior floor-space ratio, but that gets cut down a bit by the hit in usable floor area from stairwells & stairs.

In any of these approaches, designing the foundation to accommodate the insulating structures of the wall above is an important detail to get right. A 2x6 foundation sill or worse, R0.5 concrete can become a major thermal leak in an otherwise R40 wall. Also, at R20 windows that are only U0.30-U0.34 start to dominate the heat loss numbers, and at R40 you might want to be looking a U0.20-ish. At your latitude & climate south facing glass will still be net-positive from a solar-gain/nighttime loss point of view, and using high-SHGC variants on the south side can make up for quite a bit of loss. But if you're going over 5% of the total floor area with south facing glass it's worth doing the math (or simumlation) on how much interior thermal mass has to be designed in to keep it from overheating. A lot of passive-solar homes have ~8-12% of floor area glazing factors, but with truly high mass such as concrete slab floors you can go higher.

Air leakage at windows & doors can also outstrip the conducted losses trough R40 walls by quite a bit. Avoid sliders & single/double-hungs, since they inherently have more leakable seam and less reliable weatherstripping. Stick to fixed windows where you don't need them to open, and for operable windows push-out casements & awning/hopper windows work pretty well, and provide more egress/ventilation area per square foot of glass. Similarly, swinging patio doprs seal better and provide better access area than sliders. (You can bring a grand piano in through a pair of 34" patio doors without breaking it down, whereas even an 8' slider would only give you ~ 45" of lateral space to work with.)

Great informative thread. Thanks for the info.