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Are “Energy Nerds” in agreement on this?

tech1234 | Posted in General Questions on

“PV’s are cheap enough now to remove the need to super insulated houses”

I have been reading tons of information both online and in books nod* to “Musing is of energy nerd”. And it seems to me that a lot of the experts especially in the Northeast are switching from passive house or nearly passive house building envelopes to a more moderate 2 x 6 wall basically code built with an extra attention to air sealing and then just offsetting insulation energy efficiency with the installation of PV panels on the roof running mini-split heat pumps for heating and cooling there by achieving a more reasonable cost, wall assembly risk, difficult build detail house as well as less need for foam board and fancy materials.

This question if it matters to anybody is related to a house build I am planning for this summer. My original intention was to build a super insulated house but after hearing some of the top building science guy’s talking about this angle I’m wondering if I should just build a well air sealed 2×6 house (dense pack cellulose) and throw PVs on the roof because I was planning on running air source heat pumps anyways. HOuse site is in southern NH

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    GBA has a lot of information on building a pretty good house. I think that is the type of construction you are referring to.

  2. tech1234 | | #2

    Thank you for the response steve. I guess I should clarify my question: How do "energy nerds" feel about trading insulation for PV panels? The math seems to work out but there is still something about going backwards in wall enclosure science...

  3. Calum Wilde | | #3

    I agree, though I don't think I qualify as an journeyman level energy nerd, maybe a first or second year apprentice level energy nerd.

    My house in Nova Scotia is ~2400sqft on the outside, east-west facing with literally no south facing windows :(, 2x6 walls with 24" centers, well placed batt insulation (grade A I'd say as rare as that is), R60 in the roof, R20 on the basement walls, nothing under the slab :(, pretty good 2 pane windows, ductless mini splits for heat, and the last blower door test was 1.0ach50. (I've sealed off the dryer duct since that last test, and sealed the electrical conduit going to my meter, so it should be a little bit lower now. Nothing to write home about, though.) Essentially, local code built in 2004 with the low hanging fruit upgraded as my budget allows.

    I don't have a proper energy model or usage data, but my power bill showed ~18kWh per day* during the no heat or AC time of the year last year. Using that to make a VERY rough guess, and making no attempt to account for how much my dryer uses in the winter when we don't use the line, we use about 4000kWh/year for heat. If I add another R20 to the walls and under the slab how much is that going to bring that number down? Lets say it reduces the heat load by a very liberal 2/3, now we're using about 1333kWh/year for heat or saving 2666kWh. Just the cost of the insulation alone at consumer prices it would likely be over $20,000 CAD. For that I could put up enough solar to cover almost all of my homes usage, not just offset that 2666kWh.

    So yeah, I think even superinsulation, especially in the US where there's plenty of rebates for solar, is dead. My house is comfortable with little to no discernible temperature difference from room to room, even with point source heating. I would like to see more efficient windows come down in price as there is some very minor discomfort when very close to the windows, and I would have put 4 inches of rigid under the slab. But in my opinion, for what that's worth, I don't think it's worth building beyond that level of sophistication at this point.

    *I'm trying to get a handle on that 18kWh/day. I'm not sure where it's all going but I've since bought a heat pump dryer and I'm presently monitoring our water heaters power and fridge's consumption. Next up will be the dishwasher. After that I'm stumped, though. I've measured everything else and nothing seems to be adding up to anywhere near that.

  4. tech1234 | | #4

    Calum, Thank you for that well written and well thought out response!

    3 questions:
    Is your 2400sf on one floor or two?
    How many people live there?
    Do you find it too big or too small?

  5. Expert Member
    Malcolm Taylor | | #5

    Calum's response seems very compelling to me. When this has been debated here in the past, the advocates of more efficient envelopes have argued that it makes more sense to try and limit our energy use and then generate what consumption remains. I guess a case could also be made that efficient envelopes are more resilient than relying on PV, and less subject to policy changes, like possible repealing of net-metering agreements. But right now I find it hard to argue with Calum's analysis that super-insulation doesn't make much sense.

    Like a lot of choices, how things unfold may have a lot less to do with building science, or even the economic arguments in favour of one or the other approach, and a lot more with policy and who advocates the most successfully. Passive House seems to have had some success lobbying municipal councils to include their standards in local building codes. targeted actions like that can be very effective.

  6. tech1234 | | #6

    Malcolm some interesting points that I had not considered

  7. GBA Editor
    Martin Holladay | | #7

    Tech,
    Perhaps the strongest argument against a strict economic analysis (when considering trade-offs between envelope improvements and PV investments) is the one Malcolm brought up: the uncertainty over current net-metering arrangements. If your local utility now credits PV production at the full retail rate for electricity, PV is a good deal. But if that arrangement changes in 5 years, you may have an expensive PV array on your roof that isn't credited very much for its PV production.

    I agree that the argument in favor of Passivhaus levels of insulation is weak. But there is a big gap between Passivhaus and code-minimum. In between is something called the Pretty Good House.

    The current building code is more stringent than in the past, but it is rarely enforced. Batt insulation is usually installed in a sloppy manner, thermal envelopes leak a lot of air, and ducts are installed outdoors.

    If you want a pretty good house:

    1. Pay attention to airtightness. Insist on a blower-door test that achieves 1.5 ach50 or 1.0 ach50.

    2. Pay attention to insulation installation quality. Fiberglass batts, casually installed, are no longer acceptable.

    3. Make sure all ducts are inside the thermal envelope.

    4. Strive to install a bit more insulation than minimum code requirements.

    For a thorough discussion of the evolution of superinsulation vs. more PV, see this article: Deep Energy Retrofits Are Often Misguided.

  8. Calum Wilde | | #8

    Tech,

    The house is a split entry, the 2400sqft includes the finished basement and the main floor. There's four of us here, two kids and two adults. As for do I find it too big or too small, my answer is neither. I find it very comfortable. We could have lived in just the upstairs without finishing the basement at all, but this is more comfortable. Our basement includes a home theater room, a small storage area under the stairs, a large laundry room that will eventually include a 3/4 bathroom, a play room for the kids, an office, a mechanical room, a mud room, and a workshop that will be a spare bedroom when the garage is built. That whole basement is luxury that isn't needed, and I can see us downsizing in the future when the kids are grown and moved out, but that's a long way away right now.

    Martin is, obviously, correct about the possible policy changes. Another concern with my argument is the lifespan of the panels. If I remember correctly Martin has reported that he's still getting good output from his ~30 year old panels. I'm curious if there's been any sign of panel life expectancy changing and if it's getting better or worse.

    Another part of this argument is what might change about the policy. Right now I pay $10.83/month for a meter fee and everything else on my power bill is usage. There's no grid access fee or anything of the sort. The usage costs are $0.15331/kWh plus 5% federal tax on the whole bill. If the policy is going to change I would bet that a fee structure change to include much lower per kWh rates and a much higher grid access fee is just as likely as removing the net metering arrangement. The fee structure change would remove the savings of either approach.

    At my present power rates, and using the very loose estimates of savings I made above, the increased insulation would save me about $36/month. I'm presently leaning toward PV to meet my own net zero goals, but I'd love to hear more on the topic.

    Martin,

    My understanding of a PGH was that it still required ~R40 walls with double studs or Larsen trusses or something along those lines. Your list of <1.5ach50, well installed insulation, ducts in the proper location, and possibly a little more than code minimum insulation is different from your article on pretty good houses that suggests superinsulation. https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house Have you changed your thinking on that? Does tightly packed cellulose insulation in the walls solve the issue of poorly installed batt insulation adequately for pretty good house?

    Thanks everyone.

  9. GBA Editor
    Martin Holladay | | #9

    Calum,
    The builders and designers in Maine who coined the term "pretty good house" are anarchists, in the traditional (philosophical) meaning of the term. Your understanding is wrong. You wrote, "My understanding of a PGH was that it still required ~R40 walls with double studs or Larsen trusses or something along those lines."

    The wonderful thing about a pretty good house is that there are no requirements. It is not a standard.

    The article you refer to as "your article on pretty good houses" was in fact authored by Michael Maines, not me. I wrote a different article on the topic; here is a link to the article I wrote: Martin’s Pretty Good House Manifesto.

    Moreover, a key part of any discussion of envelope specifications is that climate matters. Decisions made by builders in Maine will differ from decisions made by builders in New Jersey.

    A builder or designer who adopts the "pretty good house" philosophy takes current prices for energy and building materials into account when making decisions, and tries to build a comfortable house that performs better, and uses less energy, than the typical home produced by conventional builders in the area.

  10. Calum Wilde | | #10

    Interesting. Thank you Martin.

    Would you mind chiming in on my questions of PV panel lifespan and the effectiveness of cellulose wall insulation?

  11. GBA Editor
    Martin Holladay | | #11

    Calum,
    My oldest PV module is now 38 years old. I haven't removed it from the roof rack for individual testing since my 2010 test, but there are no signs that the PV array that the module is part of has diminished output. All signs are that it is still going strong.

    Concerning the current generation of PV modules from China, we don't really know if they will last as long as my Arco module. But they have aluminum frames and glass protection, just like my Arco module, and there isn't any evidence that they will have a short lifespan.

    Cellulose insulation from the 1970s is aging just fine, although early installations of blown-in cellulose in walls sometimes settle. Ever since dense packing was perfected in the 1980s, though, the settling problem has more or less disappeared.

  12. Calum Wilde | | #12

    Thanks!

  13. tech1234 | | #13

    Such great info here! This discussion has slightly turned my boat towards a slightly more insulated house

  14. John Clark | | #14

    My two cents.

    Builders do not want to take on additional risk of installing exterior rigid foam and consequently have successfully managed to lobby that reductions in energy use should instead come from solar. Case in point is new code coming down the pipe in California.

    Yes builders still need to build tighter homes, and may/may not be forced to add mechanical ventilation (although they fight that as well by limiting the degree of air tightness) because this relies on properly sized HVAC systems which don't rely on "rule of thumb/this is how we always do it" design methods and instead require the correct implementation of Manual S, J, T etc.

  15. Expert Member
    Michael Maines | | #15

    Martin's right about the Pretty Good House concept--the only rule is that there are no rules. It's more of a philosophy, and encompasses a wide range of performance. I am a designer and Passive House consultant, and encourage everyone to go as far as they can with the more-or-less permanent aspects of their homes, but when you do an economic analysis--never perfect, but still useful--I find that the cost of Passive House construction does not usually make financial sense for the small, single-family homes on imperfect lots that I am often involved with. Even the prescriptive PGH-in-Maine numbers of R-40 walls and R-60 roof doesn't always have a very good return on investment, depending on the situation; they are more of the average upper threshold of a good ROI while still being environmentally responsible and providing excellent comfort.

  16. Lance Peters | | #16

    I've read several places that PV and Net Metering are working OK for now, but that's based on the relative scarcity of PV. If everyone put 10 kW of solar on their roofs tomorrow there would be issues. I don't have links to articles handy.

    What I do know for sure is, I was told by Ontario Hydro that they will only allow 8% of local demand to come from PV (in my area anyway). I was told this when inquiring about Net Metering for our future build. I did not clarify if that meant 8% of peak summertime demand, overall generation, or what (if someone has more insight, please share). But reading between the lines, once that 8% threshold is met on a majority of grid divisions the Net Metering game will be over for new applicants, the rules will change for existing Net Metering agreements, or some combination of the two.

    The conspiracy theorist in me says Big Hydro is in serious trouble and it will do whatever it can to protect itself, which means dipping into our pockets. Trends are towards cheaper more efficient solar and cheaper more effective batteries as electricity rates are trending higher with no end in sight. In my opinion, at some point we will reach critical mass and people will start dumping their grid connections, at which point things are going to get ugly for Hydro and its remaining customers.

    How far away that is, I have no idea. But when it happens the people in houses that use less energy will be in a much better position than those living in energy hogs. My build is R50+ walls, R80+ attic, U.20 or lower windows, R24+ Basement walls with exterior above grade insulation, and R12 sub slab. Above code below grade, way above code everywhere else. With today's relatively cheap energy costs I have not bothered to do an exhaustive payback analysis. My gut tells me that 20 years from now I will be extremely glad I took this route, not unlike paying a little more for a good tool; buy once, cry once. Time will tell.

  17. Expert Member
    Rick Evans | | #17

    Superinsulation or a larger PV array?

    I think the answer is you need both and in that order. If the goal is to get to Net Zero- then I think it is more economical to add insulation before adding solar.

    I tested this using the heat loss calculations for my own home that I am building in NH (zone 6a, 1400’ elevation). The house is a double stud wall ‘raised ranch’. Whole wall R-values are: Attic: R-85, walls: R-46, windows: R-6ish, basement walls: R-38, slab: R-25. According to my calculations, I can heat this house with a single ductless mini split. Total cost for mini-split/ install labor: $4,000. Total cost of extra insulation, lumber, and framing labor to achieve higher R-values: About $9,000.

    I estimate that our total electric usage for the house will average 7411 KWH for the year (this is for everything). How much would a PV array cost to take this house net zero? My guess is that I would need a 5 kwh array. Cost after rebates (assuming $3.15/ watt): $11,000

    So, the total for superinsulation ($9,000) + 5kwh PV ($11,000) = $20,000

    Now, what if I went with a code built house and just addedsolar?
    If I keep everything the same (including exfiltration figures), and simply decreased the whole wall R-values to that of a code built house (R38 roof*, R-22 Walls, R-3 windows, R-9 basement walls**, R-9 slab insulation***) then the average annual usage will increase to 14,117KWH. My design load for my HVAC sizing also increases from about 11,000 btus/ hr to nearly 30,000 btus/hr. Hence I need another mini-split or a larger one (These tend to have lower COPs- but I’ll leave this aside for now).

    In this case, I would probably need an 11 kwh PV array to achieve net zero. At $3.15/watt, minus 30% federal rebate, that would cost me about $24,000. However, I need to add the cost of the extra mini split: We’ll go ahead and say that’s an extra $2,000.

    So, the total for insulation improvements ($0) + 11kwh PV array ($24,000) + larger or multiple mini splits ($2,000) = $26,000

    That is a difference of $6,000.

    *a whole wall R-value of R-38 is generous given the poorly insulated garbage that passes for cathedral ceilings in new homes that I see around here.
    **I’ve never seen a ‘normal home’ built with anything other than 2”of XPS on the outside of basement wall. Aged-adjusted value given here.
    *** same with sub-slab insulation. Always 2 inches of XPS even though it doesn’t meet code. Aged adjusted R-value used here.

  18. Jon R | | #18

    I suggest that in the future, homes that can store energy will also be in a better position. Either because rates will vary depending on wind/solar/demand conditions or because the home itself will have PV solar (without attractive net metering rates).

    The discussion should be less about PV vs insulation and more about storage vs insulation. Also, less about kWh and more about environmental impact (IMO, net zero kWh is a silly goal).

  19. tech1234 | | #19

    Rick- Great post! this is exactly the breakdown I was trying to do but got mathematically burnt out. I follow everything there and it all seems to make sense to me other than the $9000 "extra insulation" figure.. I'm not saying that's incorrect but can you explain how you got to that figure?

  20. Expert Member
    Dana Dorsett | | #20

    The starting point guidelines in John Straube's 2009 analysis is still pretty valid, despite the drop in PV price and the rise in heat pump efficiency since then. See Table 2, p.10:

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

    Some of those whole-assembly R values are now currently IRC 2015 code-minimum (eg, a continuous R15 for basement walls in zone 5), but others still vary by a bit. https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

    That table is really just a starting point. Where it's cheap to improve upon that, say, using reclaimed Type VIII roofing EPS under basement slabs or as slab-edge insulation instead of XPS, even doubling it can still cost-rational on a lifecycle basis, and cheaper than heat-pump leveraged PV (net metered or not.)

    At most the technology and price improvements in heat pumps & PV since 2009 would move the financial rationality point by one climate zone in Table 2, all else being equal. In high energy cost locations such as NH it might not move at all. So for zone 5A southern NH (or the warm edge of zone 6), see what it takes to build the zone 5 row in that table:

    R30 whole-wall walls

    R65 whole assembly attic (or 50 cathedralized ceiling

    R15 basement (currently IRC code min for both zones 5 & 6). R30 floors over pier foundations or ventilated crawlspaces R10 slab edge if slab on grade

    U0.24 or lower windows, with SHGC greater than 0.50

    R7.5 under slabs (both basement or slab on grade)

    So, how do you get there within budget?

    A 2x6/R20 wall with 3" of continuous reclaimed roofing polyiso sheathing is a bit more than R30 whole-wall. So is a 2x4 double-studwall with 9.5" of cellulose.

    A 20" deep energy heel truss filled with cellulose would hit R65 in the attic, for not much more money than a code-min R49 in a 14" deep energy heel truss.

    A 2" + 2" EPS minimalist insulated concrete form would beat R15 for the basement. So would 3" of reclaimed roofing polyiso strapped to the interior side of the wall with 1x4s through-screwed to the foundation with masonry screws, hanging the wallboard on the furring.

    A dual low-E double pane with low-E coatings on surfaces #2 and #4 will duck under U0.24 for less money than a triple pane. Care has to be taken in specifying the glass to ensure sufficient SHGC, but there are multiple glass vendors with products that hit those marks (eg Cardinal's LoE-180 + i89.) With some modeling you may want to tune the SHGC by orienation, keeping the high SHGC glass on both the south and north facing windows , but something lower gain for the east & west facing glass to limit peak cooling loads. In places a lot colder than southern NH the coating on surface #4 makes it cold enough for copious window condensation ,making a triple-pane a better (if more expensive) choice, but it's fine in that location.

    A couple inches of 1.25lbs density (Type VIII) or higher EPS under the slab gets you to the R8+ range. If using recliamed foam (typically less than 1/3 the cost of virgin stock), double that. The additional cost of another 2" of excavation isn't much, and 4" of reclaimed EPS is still considerably cheaper than 1.5" (R7.5) EPS.

    Build that, and the amount of PV necessary to cover your entire annual energy use (including 1-2 electric vehicles) can still fit on the house, if you don't screw it up by cutting up the roof into a dozen mis-oriented planes with dormers hips and valleys. Net-metered, subsidized or not, the levelized cost of energy for PV + storage is expected to become cheaper than current retail price electricity in NH by 2030, using only modestly aggressive assumptions.

  21. Expert Member
    Rick Evans | | #21

    Hey Tech1234,

    Glad it was helpful!

    The $9,000 was the extra cost for me bring the home up to PGH 'standards'. My house will use cellulose for the attic and dense pack fiberglass for the walls. total cost for insulation was only about 8500. My insulation contractor told me that it wasn't that much more than a code built house as the labor is quick and the material is so cheap. He estimated that it was an extra $3,000 for the insulation/labor for my house compared to a code built house.

    The framing cost was an extra $5,000 to build the double stud wall. We used 2x4x10 J-grade lumber for the interior non-load bearing wall. This was an extra 500 bucks.

    We have 6 inches of Type II Eps for below slab insulation. It was much cheaper per sheet than the commonly used XPS. We spent an extra $1,000 or so for this compared to 2 inches of XPS.

    The triple pane windows (Logic) cost less than the higher end Anderson windows at Home Depot. So no premium to pay there...

    So all in was actually around $9,500.

  22. Expert Member
    Malcolm Taylor | | #22

    I don't doubt Ricks figures. I suspect he made some intelligent choices that kept the increases so low, that perhaps wouldn't be reflected in everyone else who made the same decision to upgrade.

    The real jump comes not from going from code-compliant to PGH, but beyond that to super-insulation and adherence to one of those standards. You don't get to Passive House for 9k.

  23. thrifttrust | | #23

    Insulation provides reliable benefit 24/7/365 from the moment it's installed till the day the house is abandoned. Solar is capricious. In its current state solar users depend on their local utility to provide electricity when the sun isn't shining through net metering. The utilities can't be trusted.

    Due to clouds kicked up by the great lakes Michigan has marginal solar potential. Residents have been slow to adopt solar. There have been only a few thousand solar installations to date. This has not stopped the utilities from getting ahead of the issue. Today the Michigan Public Service Commission abandoned net metering, with no indication of the new reimbursement rates. Higher monthly grid hook-up fees for solar customers are sure to follow.

    It's been a constant lament here at GBA that the value of solar installations are not reflected in real estate appraisals or resale value. This has not stopped localities from increasing property taxes on people who have installed solar. This week a home in Ann Arbor saw it's taxable value increase 5500 USD. This will result in an annual tax increase around 250 USD. Good luck getting your tax assessment lowered if the array is removed.

    The war on solar is real, and solar adopters are the victims.

    Douglas Higden

  24. Calum Wilde | | #24

    I'm going to have to dig into local prices. Rick's analysis/prices seem drastically lower than what I expected to find on the local market.

    Edit:not that I doubt Rick's numbers. I'm in a different country so the prices for labour and materials could easily be very different.

  25. Calum Wilde | | #25

    Malcolm,

    I don't doubt Rick's numbers either. I should have phrased my last post better.

  26. T Carlson | | #26

    Both scenarios with a well built house will produce comfort to the occupant. Its an operating cost/ROI question which would require modeling if you really had to know.

    There is a consultant out there who runs all these models and financials on scenarios and his results would surprise you.

    The "pretty good house" you hear about is just a moniker for common sense.

  27. Expert Member
    Malcolm Taylor | | #27

    Calum,

    There was nothing in you post that implied you were casting aspersions on Rick's numbers. What I'm suggesting is that Rick is probably pretty smart, and most of us would have a really hard time matching his results with that low a budget.

  28. Trevor Lambert | | #28

    Net metering is not available where I am. Ballpark price for an off grid system was in the vicinity of $40-50k. That was based on the energy usage at super insulated levels, if reducing to "pretty good", I can only assume the PV cost would go up. Then you have to replace batteries at some point, power converters etc have life spans as well. From my perspective, super insulated is far from dead.

  29. GBA Editor
    Martin Holladay | | #29

    Trevor,
    You are certainly correct that going off-grid is not cost-effective. No one at GBA has ever advocated "cutting the cord" as a way to save money. For more information on this issue, see Batteries for Off-Grid Homes.

  30. GBA Editor
    Martin Holladay | | #30

    Trevor,
    One more point: Even when a PV system is out of the picture, it still makes sense to perform a few calculations to determine whether the energy savings associated with insulation upgrades are large enough to justify the incremental cost.

    In other words, just because PV is unavailable, doesn't mean that "superinsulation" (depending, of course, on how you define that word) makes sense.

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