Writing from Climate Zone 3, Farm House seems to have worked out many of the details for the dream house he plans to start building in a few months.
“Plan to live in it for 30+ years,” he writes in a post at the Q&A forum at Lakesideca Advisor. “The house will have Zip System sheathing and will be well insulated on the inside. I will just leave it at that. Not interested in installing rigid foam on the outside of the roof sheathing. (I have my reasons, so please don’t try to convince me otherwise.)
“So, house fully exposed to sunlight. One inch rigid foam with foil facing. Strapped with fiber-cement siding.”
That’s the plan: no rigid foam on the roof, but 1 inch of rigid foam on the exterior walls, plus a vented rainscreen and siding.
The Farm House challenge? “Sell me on all the reasons this [rigid foam on the walls] is worth the time and cost.”
Not our job to sell anything
“First of all,” says GBA senior editor Martin Holladay, “it’s your house. You can build it any way you want, as long as your local code enforcement official is happy. I don’t see any reason to convince you of anything.”
That said, if Farm House decides that it’s a good idea to increase the R-value of the wall assembly or reduce thermal bridging between the studs, he could take the advice of Steve Knapp and use , which combines structural sheathing with foam insulation.
R-sheathing can take the place of conventional OSB or plywood sheathing. It comes in various thicknesses, with R-values ranging from 3 to 12.
“But I see no reason to try to convince you.”
Skip the exterior foam and invest in PV
Dana Dorsett won’t try to convince Farm House, either. He suggests using , free energy-modeling software available from the National Renewable Energy Laboratory. Then, given the good solar exposure that’s apparently available on the house site, Farm House could compare the return on investment for spending the same amount of money he’d need for insulated sheathing for a rooftop solar array, calculated at $3 a watt before any subsidies.
“In U.S. Climate Zone 3, it’s possible (with a decent design) to hit net-zero energy with 2×6 (R-20) walls, no exterior insulating sheathing, and with a PV array that fits comfortably on a sun-drenched house, using better-class heat pump technology properly sized for the modest loads,” Dorsett says.
He refers Farm House to a for more information. That study suggests that an R-20 wall in Climate Zone 3 — but times have changed.
“That document was created in 2009/2010, when PV cost $7-8/watt and was 15% efficient, whereas now it’s $3/watt and ~20% efficient, requiring less rooftop real-estate to deliver the same amount of energy,” Dorsett says. “And, better-class heat pumps at that time were running HSPF 10-12 with SEER in the teens, whereas current better-class ductless minisplits are in the HSPF 12.5-14 range, with SEER well north of 20.”
With those two advances in technology, Farm House could aim for the R-value recommendations that Building America suggests for a milder climate zone (Zone 2) and still hit net-zero performance. A 2×6 R-20 wall framed at 16 inches on center with a rainscreen gap between the sheathing and the siding would work; so would a 2×6 R-23 wall insulated with rockwool batts (but with no rainscreen).
“So, skip the exterior insulation and concentrate the investment where it pays more,” Dorsett says.
Farm House might still consider the more rigorous Climate Zone 3 recommendations for other building assemblies, while aiming for a building envelope testing at less than 1 air change per hour at 50 pascals in a blower-door test, plus a heat-recovery ventilator. A simple house shape minimizing corners would also help.
Consider fossil fuel use
Brendan Albano asks Farm House to consider the benefits of reducing the use of fossil fuels: a better insulated building will need less heat, and less heat means a lower carbon impact.
“Reducing your use of fossil fuels (and other non-renewable resources) is an ethical choice from an intergenerational equity (a.k.a., ‘think of the children’) point of view,” Albana writes. “If exterior insulation is an effective way for you to do this, then it would be ethically responsible for you to add exterior insulation.
“Whether or not that line of reasoning is important enough to you to justify the expense is a question only you can answer.”
But the introduction of fossil fuels into the discussion raises additional questions, replies Dorsett.
“The ‘future generations/fossil fuel’ discussion can go a number of different ways,” he says. “Higher wall performance can be had with lower initial and long-term carbon emissions by staying away from foam altogether (which has a very real carbon footprint) and using cellulose as the cavity fill (which is net-carbon-negative and sequesters carbon), or rock wool (low carbon footprint relative to any foam) and/or using only electric HVAC equipment combined with renewables-only electricity (site sourced or purchased).”
A question of diminishing returns
Farm House has given some thought to the environmental impact of his new home and the need to reduce fossil fuel emissions.
“That’s a given for me,” he says, “but I don’t have an unlimited budget and I am certain that at some point during an envelope design, one starts spending money for very incremental improvements in performance.”
So, while recognizing that exterior foam insulation increases the whole-house R-value, the question is whether it makes “good sense.” Farm House wants to know whether a radiant barrier he’s apparently planning on including will offer the advantages he’s hoping for.
Further, he asks, “Is increasing the stud-to-siding R-value from 6.5 to 13 going to have a significant impact? For example: What percentage of heat loss or gain is expected through the 25% of the exterior walls that’s made up of 2×6 studs? (If it impacts the answer, my goal is to achieve between 1 and 2 ach50.)
“We all know that an internal combustion engine is an glorified air pump,” Farm House continues. “The faster you can get the air to go from intake to exhaust, the better the performance. At some point, though, you’re spending silly money to increase performance that can only be justified if you race NASCAR every weekend. Catch my drift?”
Questions about plans for a heat pump
Farm House says that he’s planning on “spending some serious $$$” for a variable-speed heat pump. Good idea? Maybe not, says Dorsett.
The smallest model of that air-source heat pump has a capacity of 2 tons, he says, and although it’s able to modulate its output, it has a of about 2.5 to 1, Dorsett says, “which could be sub-optimally oversized” for a higher performance house of about 2,500 square feet in Climate Zone 3.
“In a new house design, it’s not super hard to hit the 2,500-square-feet-per-ton of cooling range, and even a code-minimum house can hit the ton per 2,000 [square foot] range, even with full sun exposure,” Dorsett says. “The minimum modulated output of the 2-ton Trane XV20i is about 10,000 BTU/h at 95°F outside and even higher at lower outdoor temps. That means it can’t really modulate at high efficiency if your load at your 1% design condition is only 12,000-15,000 BTU/h.”
He adds that the “framing fraction” of 25% in exterior walls would account for roughly half the total heat transfer in walls.
“But the walls are only a fraction of the total heat gain and heat loss, and with 2×6/R-20 construction staying at code-minimum, … the total window area can account for as much heat transfer as the total wall area (or more),” Dorsett says. “These sorts of issues are why modeling the house with BEopt can be useful, since you can then determine the bang-for-your-buck of different component upgrades.”
Another approach with Zip-R sheathing
Dorsett has one more idea for reducing thermal bridging without the use of a continuous layer of foam. Farm House could start with conventional framing using 2x4s. Then, rip sheets of 2-inch Zip R to widths of 1 1/2 inch and 3 inches, and glue them to the inside of the framing to create a total stud wall depth of 5 1/2 inches. (The 3-inch strips are for doubled-top plates and other areas where studs are doubled.) Use conventional insulation designed for a 2×6 wall.
“That brings the framing fraction up from about R-4.2 to about R-14, cutting the heat transfer through the 2×4 framing by more than 2/3, and the whole-wall performance improves by about 15% compared to a 2×6/R-20 wall at the same wall thickness,” Dorsett says.
The idea is derived from .
Following up on Dorsett’s suggestion, Gordon Franke writes that he ran some calculations on heat flow comparing the Bonfiglioli wall to several other options (see image #2, below).
“If my calculations are correct, the Bonfiglioli [wall] insulates better than a 2×4 wall with 1 1/4 inch of exterior insulation,” he says. “One might say, ‘Well, 1 1/4 inch is too thin.’ However, a 2-inch layer only buys you a 3% improvement compared [to] the Bonfiglioli wall.
“In my own renovation, I can implement the Bonfiglioli wall myself over a couple of weekends for the cost of some EPS strips, 1x3s, and the upgrade from 2×4 to 2×6 batts. That seems like nothing compared to the additional complication and expense of including an inches deep layer of insulation on all exterior walls.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost thinks:
There’s lots of good analysis and advice built into this Spotlight. I agree with Dana Dorsett that BEopt modeling can provide good guidance and useful sensitivity analysis around the key elements that Farm House is considering.
For my money — and based in no small part on the work that Building Science Corporation has done for high-performance design in Climate Zone 3 — here are the three most important design considerations for homes aiming for energy efficiency, indoor air quality, and thermal comfort:
Airtightness. Even without much information on the actual design, in my experience the continuity of the building enclosure air control layer is easiest to do on the exterior. You certainly can get this in many ways, but the Zip wall system is a good option. Another advantage of this approach is that a continuous exterior air barrier protects from wind-washing at corners.
Better windows and solar shading. BEopt would certainly show this, but windows tuned by location and aspect and properly shaded give a big performance boost in Climate Zone 3. In a recent analysis I did using , exterior shading of windows and doors alone pulled around 25% of annual hours into the .
HVAC. All ducts and equipment should be inside the conditioned space, and the systems should be designed using ACCA Manuals J, S, and D. This was not mentioned in the Q&A exchanges, and it’s critical to high performance (energy efficiency, IAQ, and thermal comfort). Achieving a high-performance HVAC system is easy if it is identified as a driver of design and if you have an HVAC contractor who can deliver the design.