in pursuit of energy efficient minimalism
Martin Holladay likes to harp on Passivhaus designers for not evaluating the ‘last inches of insulation’ in pursuit of Passivhaus. And while I think there is merit to this issue, we still contend that the reason for some pretty ridiculous quantities of sub-slab foam is a combination of poor design, failure to optimize and shoehorning. As we’ve said in the past, absurdly excessive assemblies don’t help the movement – it leaves a bad taste in our mouths and leads folks to [incorrectly] believe Passivhaus is overkill/ridiculous.
There are a number of issues at play here… One, nearly all PH projects in North America have been single family houses – notably, detached houses have the hardest envelope requirements for meeting Passivhaus. Generally, commercial/institutional/multi-family projects won’t require the levels of insulation of detached housing (and can result in significant mechanical savings!). Two, a number of the more published projects have fairly large surface area/volume ratios, further exasperating the issue. Three, I don’t think getting bogged down in this minutia, based on a handful of bad examples, is productive. And four, as has been claimed by PH designers outside of the U.S., insane levels of sub-slab foam are not typical in the EU, where Passivhaus tends to be more difficult (in terms of envelope requirements) than the US, comparatively.
However, we wanted to perform that exercise Passivhaus designers seemingly never do, comparing that ‘last bit of sub-slab foam’ to PV… So, we took a designed/PHPP’d project (ShigaShack!) and looked at the savings in switching from 4” of sub-slab EPS, to 6”. We then compared that with approximate PV production (via PV watts). Some guidelines: PV installed cost of $4,500/kWp, we’ve a fairly light building envelope for a PH (we’re trying to disprove the nabobs here, so R-31 slab/R-35 walls/R-55 roof), when migrating to other sites – assuming solar access and orientation match existing conditions.
Due to the site, we’re utilizing a slab on grade, with 6” of EPS. The slab has an area of 401 ft². The TFA is an awesomely low 577 ft² (uh, yes you read that correctly – there ain’t no small house penalty, just a bad design penalty!), and a Specific Space Heating Demand (SSHD) of 4.40kBTU/ft²a. This results in total space heating consumption of 2,540 kBTU/a or 744 kWh/a. If we reduce the sub slab foam by 2”, the resulting SSHD is 4.88kBTU/ft²a, with an annual demand of 2816 kBTU/a (825kWh/a). Thus, 2” of insulation saves 81 kWh/a. The additional 2″ of sub-slab foam is 67 ft³ of EPS, or (13) 2”x4’x8’ boards, which cost ~$325. At $0.09/kWh, the extra two inches of insulation save $7.29/year. That’s a payback period of 45 years. Granted, a long time – but then again, we don’t plan on the slab being ripped up for several decades.
Now how many kWhs would that $325 produce if invested in PV instead? At $4,500/kWh installed, about 70kWh/a for Seattle. So the 2” of insulation actually saves more energy (though only slightly) than the PV would produce. Alright, that holds true for here in the solar deficient NW, what happens when we migrate the project to other regions?
6” EPS: SSHD = 4.40 kBTU/ft²a x 577 sf (TFA) = 2540 kBTU/a (744 kWh/a)
4” EPS: SSHD = 4.88 kBTU/ft²a x 577 sf (TFA) = 2816 kBTU/a (825 kWh/a)
kWh saved: 81 kWh/a
Electricity cost/kWh: $0.09
Annual savings $7.29/a
Equivalent PV production: 70 kWh/a
6” EPS: SSHD = 3.85 kBTU/ft²a x 577 sf (TFA) = 2221.5 kBTU/a (651 kWh/a)
4” EPS: SSHD = 4.51 kBTU/ft²a x 577 sf (TFA) = 2602 kBTU/a (763 kWh/a)
kWh saved: 112 kWh/a
Electricity cost/kWh: $0.143
Annual savings $16.02/a
Equivalent PV production: 88 kWh/a
6” EPS: SSHD = 4.33 kBTU/ft²a x 577 sf (TFA) = 2498 kBTU/a (732 kWh/a)
4” EPS: SSHD = 5.06 kBTU/ft²a x 577 sf (TFA) = 2920 kBTU/a (856 kWh/a)
kWh saved: 24 kWh/a
Electricity cost/kWh: $0.094
Annual savings $11.67/year
Equivalent PV production: 96 kWh/a
Minneapolis (design doesn’t meet PH, but incorporated for comparison)
6” EPS: SSHD = 9.86 kBTU/ft²a x 577 sf (TFA) = 5689 kBTU/a (1,667 kWh/a)
4” EPS: SSHD = 11.00 kBTU/ft²a x 577 sf (TFA) = 6347 kBTU/a (1,860 kWh/a)
kWh saved: 193 kWh/a
Electricity cost/kWh: $0.065
Annual savings $12.55/a
Equivalent PV production: 92 kWh/a
But what about Portland, OR? This is an interesting case study… the building performs much better in PDX due to the milder climate – so 6” is actually too much. We can scale back to 4”, and compare to code min (R-10)
4” EPS: SSHD = 3.81 kBTU/ft²a x 577 sf (TFA) = 2198 kBTU/a (644 kWh/a)
2” XPS: SSHD = 4.83 kBTU/ft²a x 577 sf (TFA) = 2787 kBTU/a (817 kWh/a)
kWh saved:173 kWh/a
Electricity cost/kWh: $0.129
Annual savings $22.32/a
Equivalent PV savings: 74 kWh/a
Granted, the most significant savings are going to be realized with the first inch of insulation, but that’s always the case. There is a point of diminishing returns, and smart PH designers should be able to find that sweet spot without much effort.
Now recently there’s been some flack regarding the ‘tons’ of embodied energy and carbon of the additional few inches of EPS needed to meet PH that would take decades to recoup, but we’re just not finding that to be remotely true. The payback periods always seem to be relatively quick. For Shigashack…
Embodied carbon of add’l 2″ EPS: 30.4kg x 2.5 kgCO2/kg = 76 kg CO2
Embodied energy of add’l 2″ EPS: 30.4kg x 89 MJ/kg = 2,706 MJ
simple payback of embodied carbon: 81kWh/a x 0.68 kgCO2/kWh = 55 kg CO2 avoided per year, payback = 1 year, 4.6 months (75 kg CO2/55 kg CO2)
embodied energy payback: 2,706 MJ avoided per year, payback = 9.27 years (2,706 MJ / 292 MJ/a)
In colder climates, where that 2″ of add’l sub-slab foam saves more (e.g. Boston), the payback periods are even shorter. Heck, we could use natural materials (cork, cellulose) and the payback could be immediate. And, as we briefly showed on our retrofit post, the embodied carbon and energy payback can be ridiculously short as well – and economic could be under a decade, especially with electrical prices north of $0.20/kWh.
As Passivhaus designers and consultants, it’s incumbent upon us to not only keep our pencils sharpened, but to ensure we’re not wasting resources. This is why we geek out on THERM, this is why we champion compactness, this is why we believe buildings in ubercold climates should maybe huddle together to maintain a little warmth (with the added bennies of lower construction costs and increased density!).
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