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Musings of an Energy Nerd

Martin’s Energy Quiz

Test your knowledge of energy efficiency, building physics, and building science trivia

Sharpen your virtual pencil. It’s multiple choice, so you can always guess.

This week’s blog is an energy efficiency and building science quiz. Remember, using Google for research is cheating. Answers are at the bottom of the page.

1. During the winter, a home inspector climbs into an attic and notices that the underside of the roof sheathing is covered with frost and the rafters have stains indicating mold. The most likely cause of these problems is:

a. A roof leak

b. Insufficient insulation on the attic floor

c. Air leakage paths between the wet basement and the attic.

d. Bats or squirrels.

2. Which siding type is associated with more cases of wall rot: vinyl siding or stucco?

a. Vinyl siding

b. Stucco.

3. To calculate air changes per hour at 50 Pascals:

a. Divide CFM50 by the building volume.

b. Multiply CFM50 by 60 and divide by the building volume.

c. Multiply CFM50 by 50 and divide by the building volume.

d. Call up a HERS rater and ask.

4. Who first suggested that the solution to energy shortages is “Drill, baby, drill”?

a. RNC chairman Michael Steele

b. Senator John McCain

c. Former Alaska governor Sarah Palin

d. Joe the Plumber.

5. Icynene spray polyurethane foam:

a. Is a good air barrier but not a vapor barrier.

b. Is a good vapor barrier but not an air barrier.

c. Is a good vapor barrier and an air barrier.

d. Can be used to repair holes in sofas and mattresses.

6. Of the following building scientists working in the U.S., who is the only one who was born in this country?

a. Wagdy Anis

b. Mark Bomberg

c. Eric Burnett

d. Andre Desjarlais

e. Achilles Karagiozis

f. Joseph Lstiburek

g. Iain Walker

h. Anton TenWolde

7. The blower door was independently and simultaneously invented by:

a. The “Princeton House Doctors” and Harold Orr

b. Max Sherman and Jim…

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  1. Doug McEvers | | #1

    15# asphalt felt
    15# felt is quite a meager product these days, not much asphalt used. I started using 30# felt as a substitute about 10 years ago, much more tear and wind resistant.

  2. GBA Editor
    Martin Holladay | | #2

    No. 15 asphalt felt
    Regardless of your preference, this is the wording in the 2006 International Residential Code, section R703.2:
    "Water-resistive barrier. One layer of No 15 asphalt felt, free from holes and breaks, complying with ASTM D 226 for Type 1 felt, or other approved water-resistive barrier, shall be applied over studs or sheathing of all exterior walls."

    This requirement is problematic for several reasons. Here's what I wrote about the requirement in the January 2006 issue of Energy Design Update:

    “The most puzzling aspect of paragraph R703.2 is that it specifies using a roofing product on walls. Manufacturers of ASTM D226 asphalt felt are unanimous on one point: the product is intended for roofs. For example, the Web site of one asphalt felt manufacturer, Emco Building Products, describes its Esgard ASTM asphalt felt as ‘a roofing felt used as underlay beneath asphalt shingles, wood shakes, tiles and other sloped roof coverings.’ Similarly, the Atlas Roofing Web site describes its ASTM D226 asphalt felt as ‘a specification-grade felt used as a waterproof barrier between the roof deck and the shingles.’ Builders who ask a manufacturer of asphalt felt for installation advice are usually told, ‘If you’re using it on a wall, you’re on your own.’

    “Even without support from asphalt felt manufacturers, builders in the eastern US have been installing asphalt felt over wall sheathing for over fifty years, and there is no evidence of any problems arising from the use of asphalt felt in this application.”

    Here's the final twist: builders who prefer the more rugged option — #30 asphalt felt — are in violation of the code. Since every asphalt felt manufacturer agrees that their products are intended for roofs, not walls, #30 felt is not an approved substitute for the code-required product, #15 felt. Unlike manufacturers of plastic housewrap, no manufacturer of #30 felt has had their product tested or submitted their product for approval as a WRB for use on walls.

  3. | | #3

    # 19 Radiant Barrier
    Nice quiz. Unfortunately, I did not do so well. As pertains to #19, there is in fact an under slab radiant barrier product available. Thanks.

  4. GBA Editor
    Martin Holladay | | #4

    No, there isn't
    "There is in fact an under-slab radiant barrier product available" — sorry, but there isn't. The link you provided goes to a distributor of foil-faced bubble-wrap manufactured by TVM Building Porducts.

    Here's what I wrote about these products in the September 2003 issue of Energy Design Update:

    “Claims that underslab foil can reflect radiant heat are misleading. In order to be effective, a radiant barrier requires an air space adjacent to the reflective material; if aluminum foil is in direct contact with other materials, it acts as a conductor, not a reflector. According to David Yarbrough, a research engineer and insulation expert at R&D Services in Cookeville, Tennessee, when foil-laminated bubble pack is installed under a slab, it is not a reflective insulation. ‘The material as it is used under concrete doesn’t meet the definition of a reflective insulation,’ says Yarbrough. ‘It contains a reflective material, that’s true, but it is not really being used in a reflective insulation application. It is not performing like a reflective insulation because there are no significant reflective air spaces.’

    “Mike Boulding is the president of TVM Building Products, the manufacturer of rFoil. ‘In our case we claim 57% reflectivity, which means that the product will reflect 57% of the radiant component of heat transfer,’ says Boulding. But in the transfer of heat from a slab to the ground below, radiant energy is almost irrelevant. Any radiant heat source in the room above, or any heat source in the concrete itself, transfers its heat to the concrete, at which point the heat transfer mechanism becomes conduction, not radiation. ‘Under concrete pads there aren’t any air spaces,’ says Yarbrough. ‘The foil or bubble pack is in direct contact with the ground or fill material, so radiant transport is not a big issue. The big issue is the other heat transport mechanism, conduction. And there is no radiation going directly from the warm room through the concrete and hitting the aluminum foil.’

    “Some manufacturers of foil-laminated bubble pack assert that the air bubbles trapped in the bubble pack adjacent to the foil permit the foil to perform as a reflective insulation. The problem with this claim is that any performance enhancement contributed by the air bubbles adjacent to the foil is already included in the measured R-value of the material. ‘The aluminum foil could contribute a little bit to the R-value of the material, because it is changing the radiation transport across the air cells,’ says Yarbrough. ‘But that effect is still part of the measured material R-value. Unless there are reflective air spaces adjacent to the material, you won’t get any additional effect from the aluminum foil.’

    “For underslab applications of these products, the much-vaunted reflective ability of aluminum foil is irrelevant. Even the Reflective Insulation Manufacturers Association (RIMA) concedes as much in their technical bulletin, ‘Reflective Insulation Materials Used Under Concrete Slabs,’ which is posted on the Web at This bulletin describes the performance of an R-1.1 ‘reflective insulation material’ installed in a concrete floor system. By including the R-values of 5 inches of gravel and 2 inches of concrete, the floor assembly is assigned a total R-value of 1.95. According to the bulletin, ‘It is important to note that the calculation used to generate the example described above does not include any additional thermal benefit resulting from the aluminum surface(s) of the reflective insulation material. In other words, the reflective insulation material is performing similar to a non-reflective insulation material.’

    “Although the manufacturers of foil-faced bubble pack may not always provide accurate R-values for their products, these values are known. ‘The bubble pack products have material R-values that have been measured,’ says Yarbrough. ‘The values tend to range between 1 and 2, depending on the thickness.’ ”

    For more information on deceptive marketing by distributors of foil-faced bubble wrap, see:
    "Martin's Useless Products List."

    There's also a sidebar on the topic ("Should foil-faced bubble wrap be used under slabs?") on one of the Insulation pages in the GBA Encyclopedia.

  5. Doug McEvers | | #5

    Felt as housewrap
    I am using the 30# felt for roofs only and use Tyvek over wall sheathing. It was common practice for years in the midwest to use roofing felt on the walls. My parents house for instance built in 1957 has plaster inside, fiberglass batt insulation, 3/4" pine shiplap wall sheathing, roofing felt and redwood lap siding. When I opened up part of the house for an addition in 1997 the walls looked like new, the heads on the common nails used for the shiplap wall sheathing were still shiny!

  6. homedesign | | #6

    Wall R-value
    Martin, Here is one for you....

    Consider a well designed house (similar to Passivhaus) in a mixed climate (2400 HDD 2400 CDD)
    During which season will the enclosure benefit more from higher Wall R-value?
    A. The heating season
    B. The cooling season

  7. homedesign | | #7

    Should have said occupants
    Which season will the occupants benefit more?

  8. GBA Editor
    Martin Holladay | | #8

    OK, John, I'll bite
    I don't really know — other than to suspect that your quiz, like mine, is full of trick questions and traps.

    I assume that a higher wall R-value saves more energy during the heating season than it saves during the cooling season. That would appear me to be a "benefit" to the occupants. But now I'll let you spring your trap.

  9. homedesign | | #9

    not a trap.....just my opinion
    I believe that conventional wisdom says Wall R-value is not a big factor during the cooling season.
    I might agree with this when talking about existing or conventional homes.
    I think however that there may be a "shift" as enclosures move towards Passivhaus standards.

    A local builder is looking into building some small homes to Passivhaus standards in North Texas.(mixed humid almost equal HDD & CDD)
    Preliminary feedback from a consultant using PHPP with North Texas climate data indicates that Wall R-value will need to be considerably higher to cover the Cooling side compared to the Heating side.

  10. David C. James | | #10

    Question #15
    Hi Martin,

    Great test, covered a very wide spectrum.

    Although I don't agree with your answer on #15. Spray Polyuretane is used for insulation not roofing, it's not a roofing product by it's self and needs to be covered by an approved "roofing" material.

    David C. James

  11. GBA Editor
    Martin Holladay | | #11

    Spray polyurethane foam roofing
    David James,
    On a low-slope roof, spray polyurethane foam roofing can be covered by a layer of ballast (stones). The purpose of the stones is to reduce the amount of ultraviolet light striking the SPF. However, the stones do not contribute to waterproofing in any way.

    It is the spray polyurethane foam that constitutes the actual roofing. It waterproofs the roof deck, as well as providing additional R-value.

    To learn more, see my article in Energy Design Update, Roofing With Foam.

  12. Doug | | #12

    Icynene as air barrier
    Great quiz!
    Our local spray foam guys told us open-cell foams are an air barrier only at 2.5" thick or more. I went to the Icynene site just now and the only air-barrier info they have is a "test" where they tested icynene for air tightness--with a layer of felt paper over it! I'd love to hear what the real story is on icynene alone without a layer of felt.

  13. GBA Editor
    Martin Holladay | | #13

    Air leakage specs for Icynene
    I don't see any reference on the Icynene Web site to the use of felt paper in an Icynene air leakage test. Where did you see it?

    Here's what I found: in their Icynene specs, the company claims:
    "Air permeability of core foam: ASTM E283 data:
    0.009 L/S-m2 @ 75 Pa for 3.5 inches."

    That level of airtightness meets the commonly accepted definition of an air barrier material (0.02 l/sec-m2 @ 75 Pa).

  14. Ryan | | #14

    Air Barriers Requirements
    The correct test method for air permeance is ASTM 2178 and not ASTM E283. Information on air barrier testing can be found at ABAA's website: I have yet to see an open cell foam product tested to ASTM 2178. The test method is set-up to test a single material and not an assembly of materials, so applying the foam to the substrate is giving an assembly air permeance and not a material air permeance. ABAA's requirements for an air barrier is based on testing of the material only to meet 0.02 l/sec-m2 @75 Pa.

  15. GBA Editor
    Martin Holladay | | #15

    Thanks for the information
    I appreciate the clarification concerning the ASTM tests.

  16. David C. James | | #16

    Thanks for clarifing (low slope or flat roofs) Martin.


  17. Doug | | #17

    Icynene air barrier test link

    Though I think Ryan probably already explained the issue.

  18. GBA Editor
    Martin Holladay | | #18

    Steep-sloped roofs, too
    David James,
    Although I used the example of stone-covered low-slope roofs to make a point, I didn't mean to imply that you can't use spray polyurethane roofing on steep-sloped roofs. You can. Here's what I wrote in 2005:

    Many Florida homeowners are looking for more hurricane-resistant building materials, and a few brave souls, unconcerned about the opinions of their neighbors, have re-roofed their houses by installing SPF roofing directly over concrete-tile roofs. Although less attractive than tiles, SPF roofing contributes to cooler attics and is better able to resist high winds.

    “It looks a little like snow,” says Mason Knowles, the executive director of the Spray Polyurethane Foam Alliance (SPFA). “Some homeowners’ associations didn’t like the look of it.” Indeed, even SPF advocates don’t pretend that sloped foam roofs are attractive. “Aesthetically, a foam roof doesn’t add much to the looks of the building,” admits Rick Radoboenko. But Jim Andersen, vice president of technical services at Foam Enterprises in Minneapolis, Minnesota, takes a pragmatic approach, advising homeowners in hurricane-prone areas: “It’s kind of like, do you want to get wet, or do you want to build for robustness and watertightness?”

  19. Riversong | | #19

    Icynene Air Resistance
    Sprayed foam cannot be independently tested as an air barrier material since it is not a stand-alone membrane. ASTM E-283 is the test for air barrier assemblies, such as windows, doors and curtain walls and is the appropriate test for an assembly of framing, sheathing and sprayed foam.

    Of course a single test under highly controlled conditions is not a fair assesment of foam's performance when applied by various installers, on various substrates, at various environmental conditions. Nor does it indicate what the long-term performance might be after wood shrinkage or extreme temperature/humidity cycling and aging of the plastic.

    It's interesting that the test revealed that decreasing the foam thickness 38% (5.25" to 3.25") increased the permeance by 63% (though it's still well within industry standards).

  20. Riversong | | #20

    Foam on Steep Roofs
    By the way, a friend of mine built a geodesic dome back in the 70's that was covered with spray foam. It looked like a huge mushroom.

  21. SEI - Rachel | | #21

    Thank you Martin!
    This was a great distraction for work score indicates I'm not up on energy history and I'm still a wanna be.
    I want another quiz...

  22. Charlie Brown (another one) | | #22

    Nerd or Groupie?
    Good stuff - did okay except for the Energy Nerd Groupie questions. Sorry guys, no matter how much we love and appreciate your work, nobody in the paparazzi cares where you were born. TMZ won't be covering your love life either, unless you are dating Sarah Palin. Guess its just the curse of the Nerd!

  23. GBA Editor
    Martin Holladay | | #23

    An homage to foreign-born scientists
    Yes, Charlie, I'm a nerd and a building science groupie. When I realized how many of the top building scientists working here were foreign-born, I reached two conclusions:
    1. Building science education in the U.S. lags building science education in other countries.
    2. Our school systems aren't producing enough engineers and scientists to meet this nation's needs.

    Moreover, top-notch students from other countries, including engineers and building scientists, often find it difficult to get U.S. visas when applying to our graduate schools and academic jobs. That's unfortunate. So, hats off to the foreign-born scientists who have contributed so much to building science knowledge. I salute you.

  24. Ben | | #24

    RE: Homage
    Well I didn't get a perfect score like I should have... but I could tell you how many LEED points it takes to get Platinum...

    Maybe case & point...I'm US born & educated, experienced mep engineer, energy auditor, & LEED AP. They definitely do NOT teach you this level of practical building science in engineering school. This is knowledge fought hard for in the field over many years. My hat's off to those that push the known limits and help our "technical" folks get out in the field and back to ..... solving real world problems & working with the scientific community to improve our building systems.

    Has America moved too far from it's practical, efficient, & scientific building roots?

  25. Tom Wehner | | #25

    #17 - on comparing similar houses with different HERS Indices
    It is NOT always true that the house, of two similar houses, with the lower HERS Index is more energy efficient. Because the HERS Index includes select energy generation and occupant behaviors, a less "energy efficient" house but with lots of solar PV can unfortunately have the lower HERS Index. The shell may leak like a sieve, but solar PV is allowed to make up for the energy leaks. This is not reasonable. Adding solar PV does not change the energy efficiency of the house. In addition, two houses with identical HERS Indices can be VERY different in terms of their energy efficiency. The HERS Index is NOT a reliable measure of a home's energy efficiency. However, the HHI, the Home Heating Index, is. The HHI is the number of BTUs required to be added to the shell per square foot per heating degree day to keep the temperature inside comfortable. With HHI you can directly compare the energy efficiencies of houses anywhere in the country.

  26. GBA Editor
    Martin Holladay | | #26

    Thanks for your comments
    I'm sure you realize that the point of the quiz question was to determine whether test-takers understood that a low HERS Index was better than a high HERS Index.

    That said, I disagree that the HHI, unlike the HERS Index, is "a reliable measure of a home's energy efficiency." The fact of the matter is, no system, including HHI or the HERS Index, is perfect; each has strengths and weaknesses.

    What really matters? Some would argue energy use. If so, just compare energy bills. But then a miserly homeowner who keeps the thermostat at 55 degrees might "win."

    So, others argue — including you, evidently — just ignore occupant behavior and rate the building. That's fair, perhaps. But why ignore PV? A PV array lowers energy bills, just like an efficient furnace (compared to an inefficient furnace). But (if I understand your description) HHI evidently chooses to ignore equipment efficiency and PV contributions. But what you see as a strength of the measurement system, others will perceive as a weakness.

    I'm not arguing in favor of one system over the other. I'm just being humble. All of these rating systems rate different aspects of a building. Choose your metric and understand what it measures. Include asterisks, if necessary — because no metric is perfect.

  27. Tom Wehner | | #27

    response to Martin Hollaway on subject of HERS and HHI
    I couldn't agree with you more, Martin. No metric is perfect, and I appreciate that you’re trying to educate a diverse audience with your wonderfully crafted test (its great!), but you’re implicitly buying in to a system that lacks common sense.

    As a HERS rater, I know how to play the HERS points game to make almost any house a HERS winner and get tax credits, even when the home is not very energy EFFICIENT. I have no problem with governments giving people tax incentives to people who want to operate their own intermittent electrical power plants and put solar PV on their roofs, but let’s not call that energy EFFICIENCY. Efficient use of energy essentially means CONSUMING LESS ENERGY.

    I suggest that you take another look at the HHI, the Home Heating Index. It’s the closest I’ve found to an EPA-MPG equivalent for a house with respect to its thermal performance. It rates energy use for the house itself, and the various types of HHIs allow you to include how well the occupants are driving the house.

    HHI-SHELL is the calculated number of BTUs/sqft/HDD that the SHELL or structure must have pumped into it to maintain comfortable temperatures in the winter heating season. Passive solar reduces HHI-SHELL, so a passive solar house comes out shining. So do Passive Haus's. HHI-SHELL is based primarily on conduction heat losses through the shell and convective heat losses from air leakage. The air leakage rate is required, usually necessitating a blower door test. HHI-SHELL for houses is the equivalent of EPA dynamometer MPG for vehicles.

    HHI-MECH is the calculated number of BTUs/sqft/HDD that the MECHANICAL equipment must have pumped into it to supply the structure with what it needs. This number is higher than HHI-SHELL for a given house because some of the heat is lost up the chimney. Passive solar and Passive Haus still shine through HHI-MECH because so much less natural gas, propane, wood, electricity, etc. is needed in the first place. Active solar thermal and high COP heat pumps can also reduce HHI-MECH, which for houses is the equivalent of EPA road-simulation MPG for vehicles.

    HHI-OCC is the calculated number of BTUs/sqft/HDD that the OCCUPANTS are using. It is calculated from gas and electric bills after BTUs for cooking, water heating, grilling, lighting, etc. are subtracted out. HHI-OCC may be higher or lower than HHI-MECH depending on how the occupants "drive" the house, as in the phrase, “your mileage may vary.” HHI-OCC, when compared to HHI-MECH, is a perfect ingredient to a Personal Energy Rating System (PERS) measure.

    Builders can be rewarded (rebates, tax credits, tax deductions, etc.) based on HHI-SHELL or HHI-MECH. Occupants can be rewarded based on HHI-OCC. Yes, occupants who keep their house at 55 degrees will have a lower HHI-OCC, and if government wants to reward that, the HHI-OCC measure could be that metric.

    More importantly, future home buyers should be able to compare the energy efficiencies of houses they are considering. HERS cannot do this. HHI-MECH lets them do that very simply.

    Adding electrical energy generation equipment to the house, such as solar panels, micro-hydro, wind generation, etc., doesn't affect the HHIs. This is reasonable as this equipment does not change the underlying basic thermal energy efficiency of the house. Energy generation equipment should be rewarded separately, and is already.

    See the article in Home Energy magazine on the HHI at

    I believe it’s misleading to add energy generation into an energy efficiency score as HERS does. If you disagree, then why does HERS not give credit to wind-generated electricity or microhydro or fuel cells as well as to solar PV? This makes no sense.

    We need a mid-course correction to put the real focus on home energy efficiency where the biggest payoff is, and not on home energy generation. I agree that no metric is perfect, but HERS is misguided, and the HHIs are much better and simpler.

    P.S. In New Mexico, we have an additional issue. The software most HERS raters use apparently does not model Trombe walls and other passive solar features very well. This puts very green, very energy efficient, passive solar adobe homes at a serious disadvantage in the HERS points game.

  28. Steve | | #28

    condensending question # 20
    I felt ripped off after taking the test.. even after getting a high score. after taking out the worthless green who who history test,

    Why use an energy test to insult conservatives, and those who question the wisdom of plunging our economy into the dark ages?

    I have studied the science of Global warming, now called global climate change because the science was not enough to convince many, (that and the unseasonably cold last two winters),

    but the fact is, your question number 20 was nothing but an insult at those of us who strive for a common sense approach to solving pollution and energy problems. Problems that effect everyone today.. we do not need a distant boogieman approach to scare people into being a bit more energy conscience, the current world situation and the leaderships lack of a solid energy policy over the last 40 years should be enough..

    but feel free to attack many of us who haven't leaped off a cliff listening to a politician turned climate expert, and continue to question shoddy science all the while trying to save energy and live better with what we now have.

    BTW, if all the climate change koolade drinking believers stopped breathing for two days it would cut our green house gasses by at least 20% this year..

  29. user-1060463 | | #29

    Not 20%, at best only 8.5%
    Steve, your last statement got me interested. How much CO2 does a human exhale? A quick google came up with this statement from

    According to other Wikipedia answers, breathing by humans produces approximately 2.3 pounds (1 kg) of carbon dioxide per day per person (http:/

    If there are approximately 6.8 billion humans on the earth, collectively they would emit 6.8 billion kg per day, or 6.8 million metric tons per day. In a 365 day year, humans would produce 2.482 billion tons from breathing.

    The Energy Information Administration of the US government said in their May 27, 2009 International Energy Outlook 2009 that world carbon dioxide emissions were 29.0 billion metric tons in 2006 (http:/

    Human breathing would then be contributing approximately 8.5% of all CO2 emissions.

    So if the whole world stopped breathing it would only cut greenhouse gasses by 8.5%, not 20%. Of course this does not account for farting which spews out methane, reputed to be 25 more times potent as a greenhouse gas than CO2 so the calculation gets more complex. If we assume that one cannot fart when not breathing then it might get closer to 20%. Anyone up to the challenge of calculating the benefits to global climate change of preventing all climate change koolade believers from farting?

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