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Musings of an Energy Nerd

R-Value Advice from Building Science Corporation

Are the recommendations from a paper published in 2010 still valid?

Image Credit: Alex Wilson

To reduce energy use, green builders often install above-code levels of insulation. Thick insulation is expensive, however, so it’s sometimes hard to know how much insulation is optimal.

To help guide builders wrestling with R-value questions, I wrote an article in May 2016 (“How Much Insulation Is Too Much?”) reporting on R-value recommendations from three energy experts: David White, Marc Rosenbaum, and Rachel Wagner.

A few years earlier, building scientists John Straube and Joseph Lstiburek (along with several other co-authors) addressed the same questions in a paper called “High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones.” Produced by the Building Science Corporation (BSC) and published by the Building America program, the 2010 paper included an oft-reproduced table with R-value recommendations for all climate zones in North America. As might be expected, the colder the climate zone, the higher the recommended R-values (see image below).


Although these BSC recommendations were interpreted by some readers as appropriate for net-zero-energy homes, the authors of the paper did not make that claim. Straube and Lstiburek didn’t connect the recommendations in the table to net-zero-energy home design (although they did write that their recommendations were appropriate for those with a “desire to provide new homes that will be ready to be powered by renewable energy sources immediately or in the future”).

The dropping price of PV

Some net-zero builders who read the 2010 paper nevertheless used the R-value recommendations in the BSC table as a starting point for envelope design. A few experts have recently questioned these R-value recommendations, however, in light of the steeply falling price of photovoltaic (PV) modules. In the seven years since the paper was written, the installed price of a residential PV system in the U.S. has dropped from about $4.50 or $5.00 per watt…

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  1. nick_vk | | #1

    My favorite morning read
    Martin, you consistently take up important questions in a manner that's both educational and entertaining. I look forward to each new blog entry, and tide myself over by reading your responses to reader queries in between. Coming to GBA is always a bright spot in my day (although occasionally a impediment to my productivity!). Many thanks.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Nick Van Kleeck
    I enjoy researching and reporting my blogs. I'm glad to know that some people enjoy reading them. Thanks.

    And, hey -- don't forget to buy a copy of my book!

  3. Expert Member
    Dana Dorsett | | #3

    Conscious or otherwise...
    ...those were pretty much the numbers it took to make Net Zero fairly straightforward with an array that still fit on the house (looking at numerous Net Zero houses of the era), and it's not extreme overkill for Net Zero even today, as the crayon-on-napkin analysis in response 20 of this thread shows:

    Shortly after BA-1005 was published I compared the specs to several existing Net Zero houses, and made the association that it was a pretty good rough guide to Net Zero in a number of climates, and have encouraged people to use that as a starting point when Net Zero was an explicit design goal. (I may be guilty of having oversold it as a Net Zero guide, but it's still more true than not.) But as PV has gotten cheaper and higher efficiency (= less rooftop real estate necessary) it's probably going to be possible to build to a somewhat lower spec, and still get there, provided reasonable energy use simulations are used to optimize it for low energy use. (BeOpt is your friend! )

    In places where Net Zero is being incorporated into code (all new residential construction in California starting 2020) we'll soon find out what's really cost effective over a wider range of climates. In Austin TX (zone 2) where Net Zero Ready is already code minimum they've been doing it with 2x6/R20 walls, no insulating sheathing, and the other components pretty much fall in line the recommendations for zone 2 in that table. But I'm aware of at least one Net Zero house in zone 3B that made it with just dense-packed cellulose 2x6 walls as well. The statewide mandate in California will force the architects & builders of tract homes to sharpen their pencils to find the true economic balance lies. It will vary a bit with location even within climate zones, due to differences in total insolation, but even if PV were free it would still have to fit on the available rooftop area for photon-farming.

    Curiously, the recommendations in the table for basement wall/crawlspace wall insulation has become code minimum under IRC 2015 for zones 4C & 5, but there's still a way to go for most other components.

    The impact of distributed renewables on utilities and utility business models is somewhat orthogonal to Net Zero Energy, and is going to have to be solved whether Net Zero becomes the new code-min or not. The model of central generator feeding transmission & distribution grids with one way power flows is already way out of date. Germany's problems with deep renewables penetration have more to do with the inflexibility of their slow-ramping coal & nuclear fleets than the renewables themselves. While some regional grid in the US have that problem too, most have considerably more flexible combined cycle gas & large scale hydro in the mix, as well as better transmission grid capacity for importing & exporting power over a wider regional area than Germany does. NREL's 2016 Rooftop Solar Photovoltaic Technical Potential survey shows that in some states (even some New England states) the existing suitable rooftop area for PV could generate more than 50% of all power consumed in those states annually. (see: ) If all of that got built it wouldn't be Net Zero by any means, but the hours where peak PV output would far exceed the real-time load would be many!

    When widely distributed small scale renewables reach the level of becoming a problem for the distribution grid operator, storage and self-consumption still works, and is becoming more economic every day. On some over-subscribed feeders on Oahu they're already at saturation, and back feeding to grid when then real time production exceeds the real time load in the neighborhood. There are now different rates for new PV for those willing to not export power vs. simple grid-tied, and in neighborhoods with backfeeding issues only self-consumption is allowed for new PV. But the cost of PV + battery + smart hot water heater + smart inverter necessary to optimize a self-consumption grid tied solution is already cheaper than the average PV installation was in 2010, and has a cheaper levelized cost than power purchased from the grid (even without subsidies.) Before 2025 that will be true in New England / NY and California too.

    Even at much lower deployment levels the effects of PV on the grid are a problem, but its a soluble problem. The independent grid operator in California identifed the problem several years ago, and called the minute by minute daily graph of the net load the "duck curve", based on the shape. (In Hawaii they called it the "Nessie curve"). But this is a soluble problem that can be cost effectively managed using existing technology (technology already beginning to be deployed in affected areas.) A decent readable white paper on the topic can be found here:

    New York state is completely re-writing the electricity market rules to make the transition cheaper & easier and equitable, and it may become the model for the rest of the US. At the moment there is no way for most PV or battery owners to get paid for providing peak power capacity or frequency & voltage control ancillary services that their distributed resources COULD provide, but when market rules allow it, the economics of small scale PV and storage will get a lot better. In the PJM grid region those with smart hot water heaters can now get paid for demand response and ancillary services through aggregators such as Mosiac Power. The remuneration is modest, but as more PV & storage goes on to the grid, the sheer size of the distributed resource will put most grid-stability problems to rest. Distributed demand response using smart Wi-Fi thermostats and smart water heaters is becoming common, and is cheap enough to grow rapidly if the market is opened up. FERC Order 745 essentially mandated that demand response be paid the same rate as generators for the services provided. It was delayed for ~18 months in the courts, but the Supreme Court made it the law of the land for any power grids crossing state boundaries, and it will soon become the norm even in locations that had previously didn't have those markets. (The ISO New England demand response program rules are still being written, but will go into effect in June 2018: )

    The states where the transition to widely distributed renewable is going to be hardest where they have regulated utility markets and large statewide monopoly utilities owning the majority of the generation, since the sunk costs of the central generators are built into the utility rates. This is distinct from the so called "deregulated" markets, where the power is provided by independent "merchant" power generators, and the utility only owns a fraction (if any) of the generating assets, and aren't allowed mark up the cost of the power. Those utilities aren't on the hook for the capital cost of the generating equipment. There is a coming tsunami of stranded power generation assets coming, as distributed PV, then distributed PV + storage, drops below the grid retail rates. With vertically integrated utilities that's a far more serious problem than for "poles & wires" utilities, since the shareholders have effectively been promised a return on those investments, but dwindling kwh sales will force the rates to rise, making it even more economic to self-generate and self-consume. Dubbed the "utility death spiral", this has already done-in some utilities in Germany, and some in Australia are on the verge. But with those coal-mine canaries as examples, there is still time for the regulators and utilities to respond. Simply stiff-arming distributed PV off the grid won't work- it's a delaying tactic at best, and they have to come to terms with it. This train has already left the station, Net Zero houses or no.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Dana Dorsett
    I appreciate your optimistic predictions concerning whether the grid can absorb more PV, and I tend to agree with you more than with Lstiburek on this issue. (Joe Lstiburek implied that it's nuts for homeowners to expect the utility to accept their PV power, because the excess power is just too hard for utilities to handle).

    I suspect that forward-looking utilities like Green Mountain Power in Vermont will be better prepared for the coming distributed power revolution than heads-in-the-sand utilities in Georgia and Florida. We'll see.

  5. Expert Member
    Dana Dorsett | | #5

    Different folks, different...
    ...grids, sizes, needs, neighbors, sunk costs....

    Georgia Power and their parent, Southern Company appear to be at greater stranded asset risk than Florida Power & Light, but both are vulnerable. The failure to execute the new Vogtle nuclear power plants on-budget & on-schedule makes GP/Southern more vulnerable, even though the GA regulators had allowed them to rate-base some of the Vogtle cost years ahead of the scheduled completion. It now appears the the GA regulators may not even allow them to finish the project, with the contractor Westinghouse in bankruptcy, with no clear cost or schedule to finish it, even at billions of dollars over the initial contract. GA power is looking a flat to falling electricity demand growth environment, and new renewables are now at a lower levelized cost than that of Vogtle's output even if the costs were capped at the money spent do date. Southern Company is also looking at enormous cost overruns for their Kemper "clean coal" (now natural gas only) carbon sequestration project, which has set a record as most expensive generation source EVER in the US. It's unlikely that Mississippi regulators will allow those enormous costs to be pushed onto the ratepayers. And this is all before the distributed solar party really gets going in those fairly sunny states! (FPL was perhaps saved from a similar new nuclear construction folly only by the regulators not allowing them to go forward on planned projects.)

    Green Mountain Power the largest utility in VT, but it's still quite small, serving a population only about 10% that of metropolitan Atlanta, or about a fifth the population of just Miami-Dade County. That diminutive size gives them a leg up on flexibility, and they don't have huge sunk costs in large existing (or under construction) power plants to pay off. Better yet, they are addressing the distributed resources issue head-on, and is even participating in the deployment of distributed assets. Their CEO, Mary Powell is one of the most progressive in the industry, and by making deals early in the transition with storage technology company Tesla they have a better shot at staying ahead of the tsunami than most. This recent podcast interview with Powell demonstrates just how flexible and forward looking that (comparatively) tiny utility really is:

    Being reasonably well connected to the Quebec, and NYISO grids as well as the larger ISO-NE grid (their regional grid operator under which they operator ) and being fairly small, there is no question about adequate grid infrastructure for being able to import & export power as needed, even if the distributed resources within their service area don't balance well. But to serve their customer base well it's better if they stay on top of it, and they seem to be up to the task. The grid storage mandate in place in neighboring Massachusetts (the details of the first stage announced just today: ) will also help keep balancing costs lower than they would be otherwise. As MA rolls out the mandated off shore wind power over the next decade it will put downward pressure on imported power costs to VT as well.

    So yes, Green Mountain Power is in a GREAT position to be able to surf the renewables tsunami without wiping out! Florida Power & Light and Georgia Power will have a much tougher ride.

    Lstiburek would do well to study up on how the Sacramento Municipal Utility District is dealing with distributed behind-the-meter PV & storage, turning it from a cost/liablity into an asset to save their ratepayers money. (Start here: ) The regulatory environment in California could still be updated to make utilization of behind the meter resources even better (and probably will), but the notion that the utility or grid operator is nuts to accept residential PV is true only in and old-school vertically integrated monopoly model, with not-so-smart grid operator rules. The New York REV (Reforming Energy Vision : ) is similar. It's a huge can 'o worms to sort out shifting from the old-school hub & spoke to a distributed resources model, but this CAN be sorted out to everyone's benefit if the regulatory & business models keep pace. The grid won't go dark, and the utilities don't have to collapse financially. If the utilities dig in their heels on rebuffing distributed power, it's only a matter of time before they collapse. By 2030 outright grid defection or independent micro-grids will become an economically viable solution for larger better funded power users, as in this current Australian business expansion:

  6. Jon_R | | #6

    Forget tables and run the
    Forget tables and run the numbers for your situation. Don't count on the utilities paying much for unreliable PV power. Look at thermal storage beyond DWH (far cheaper than batteries).

  7. Expert Member
    ARMANDO COBO | | #7

    Our ZERH in Dallas
    I know here in Dallas, to achieve ZERHs in our mostly 5K+ sf houses, we are installing 2x6 @24" o.c. walls with DP Cellulose and 1/2"-1" outsulation, either conditioned or unconditioned attics with full R38, and a really good job in sealing and taping the envelope at 1ACH50. We add all LEDs and efficient appliances to achieve HERS 42-43 without PV. We can achieve this for about 3-5% higher costs.
    When the Owner installs PV, we go with SunPower327 at 2kW/sf. Usually that gets us around 85-90% of the energy needed for the house (often includes a pool & teenagers). We achieve this for a total of 8-10% extra cost.
    If you ask me, Joe's guesses were pretty good and I think we make a good case of easily attainable ROI.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Jon R (Comment #6)
    I agree with you that batteries are expensive; to read my article on the topic, see Batteries for Off-Grid Homes.

    But I am baffled by your advice to "look at thermal storage beyond domestic hot water (far cheaper than batteries)."

    Here's my reaction: (a) Comparing stored heat at 140 degrees F to stored electrical energy is like comparing lobsters to spiders. When you need electricity, warm water doesn't help. (b) I've looked at a lot of schemes involving large insulated water tanks, and I've never seen a single one that made economic sense.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Armando Cobo (Comment #7)
    Thanks for sharing your real-world experience with net-zero homes in Dallas. And congratulations on being able to provide net-zero for an upcharge of only 8% to 10%.

    As John Straube said, “The price of PV is shocking ... These days, when we talk about PV, it’s all about the size of the array and aesthetics, not price."

  10. Jon_R | | #10

    And when you want heat or
    And when you want heat or cooling from PV, they are comparable. I recently took a brief look at water thermal storage vs batteries and the former was 1/3 the cost, even accounting for extra to-water heat pumps so that all of the energy could be utilized while the sun shines. If you only need thermal storage for AC, then (as Dana pointed out), ice makes more sense (and is being sold). I'd like to see other's analysis of similar systems - say AC in the case of off-grid or $.45/kwh.

    Seeing this and understanding that to a large extent, utilities have to build their grids for the worse case (hot day, sun just went behind clouds, etc), I conclude that in most cases, it doesn't make sense for utilities to buy residential solar at more than ~$.07/kwh. Politics will distort the market for some, but I wouldn't count on a net zero house with PV being net zero in $ for electricity.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Happy Canada Day
    To Joe Lstiburek and John Straube. They have done a lot more for the country than most of us.

  12. charlie_sullivan | | #12

    Canada Day and water tanks
    Happy Canada Day to Malcolm too.

    I haven't seen a hot or cold water storage tank that makes economic sense for grid or PV buffering, but the ones I see are usually rated for much higher pressure and temperature than what would be needed in a heat-pump based system, so I remain optimistic that at some point there will be a combination of inexpensive tanks and ways for homeowners to easily capture the value of thermal storage with appropriate controls that talk to the grid.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Jon R (Comment #10)
    I appreciate your suggested price for electricity fed into the grid by PV-equipped homeowners (electricity purchased by the local utility) -- 7 cents per kWh. That may be about right, on average. But there is no "average" need for electricity. The price always depends on (a) the time of day and (b) the load, so time-of-use pricing structures appear inevitable to me.

    Several studies have shown that time-of-use pricing may change the way we install PV systems. In many locations, a west-facing array will result in more revenue for the owner of the PV system than a south-facing array, even when the west-facing array produces less electricity on an annual basis.

    Like you, I welcome cost figures from homeowners or contractors who have installed a large water tank or ice-making system in a single-family home. I've never seen cost figures that make such an installation appear reasonable. Such systems can make sense, however, for large institutional or commercial buildings -- especially if local electricity prices are high.

  14. Jon_R | | #14

    It's not hard to come up with
    It's not hard to come up with scenarios providing good returns on water thermal storage using time of use/interruptible electricity pricing (or solar) and large unpressurized plastic tanks (up to 120F). On the other hand, modern energy efficient building trends (super insulation, non-hydronic systems, better heat pumps, no basements) may make it increasingly non cost effective for typical situations.

  15. user-1143864 | | #15

    The Case for Occupant Health and Comfort
    The building is a system, and because of issues with climate change, and costs of energy (lowest life-cycle costs done by Building Science Corp), the relationship between the Building Envelope and the Mechanical Systems are driving most of our decisions, often times to minimize overall costs and/or emissions. How does this impact occupant health and comfort? Generally we just presume that these super-insulated, airtight, heat recovery ventilated buildings exceed any minimum requirements. I don't fully disagree with that, although I do feel that Occupant Health and Comfort seems to have a lower priority than it should, and sometimes it takes time for these issues to arise.

    I want to move the clock well forward, to a time that may present itself to us, where energy sources for buildings are emission-free, and the life-cycle cost of this energy is relatively low when compared to the life-cycle cost of the building construction and it's materials. In this scenario, even walls with relatively low levels of insulation compared to today's standards would not result in a much higher life cycle cost, due to the low cost of energy. With abundant, low-cost clean energy, what do we use to determine the "right" amount of insulation in our walls? What does the HERS rating of a building really mean? In this case the Occupant Health and Comfort your Building Envelope/Mechanical System package delivers will become a better measure of how successful a building is. In preparation for this, Building Science should do a better job of connecting Occupant Health and Comfort to a Building Envelope/Mechanical System package. Possibly in the future an Occupant Health and Comfort rating similar to HERS. Better yet, an overall building rating that includes Occupant Health and Comfort factors.

  16. arioda | | #16

    Wow, our utility in Washington State gladly pays us for our PV power. I expect utilities will figure out storage or maybe we'll abolish nations and establish a world wide grid at which point storage will become a moot point.

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