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Air Leakage Degrades the Thermal Performance of Walls

No surprises here — but testing by the Building Science Corporation begins to quantify the problem

Posted on Sep 28 2012 by Martin Holladay

For the past five years, researchers at the Building Science Corporation (BSC) in Massachusetts have been testing the thermal performance of a variety of wall assemblies as part of an ambitious project to develop a new metric to replace R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . (I last reported on the project in my August 2011 article, A Bold Attempt to Slay R-Value.)

BSC researchers recently released information on the multi-year project:

  • On August 1, 2012, BSC principal Chris Schumacher gave a presentation, “Thermal Metric Project: A year of progress,” at the Westford Building Science Symposium.
  • On August 6, 2012, BSC released a three-page report, “Thermal Metric Research Project.”

The BSC project is financed by a group of five insulation manufacturers and one trade organization — Dow, U.S. Greenfiber, Honeywell, Huntsman Polyurethanes, IcyneneOpen-cell, low-density spray foam insulation that can be used in wall, floor, and roof assemblies. It has an R-value of about 3.6 per inch and a vapor permeability of about 10 perms at 5 inches thick., and the North American Insulation Manufacturers Association — known collectively as the “Thermal Metric consortium.” While BSC hasn’t yet published a peer-reviewed paper reporting on their research project, the consortium has released enough details to draw a few conclusions.

Developing a new metric or label for wall assemblies

The BSC researchers set out to measure the rate of heat flow through a variety of wall assemblies under different conditions — for example, at different rates of air flow. The intent was to compare the performance of a wall when air was leaking through the wall with the performance of the wall without any air leakage. The researchers also hoped to test wall assemblies at different outdoor temperatures and to quantify the effect of thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. .

Eventually, the BSC researchers hope to propose a new metric for wall assemblies — something akin to the NFRC label now found on windows. “The window industry was in disarray a decade and a half ago, and it was very difficult for practitioners to select windows,” said Joseph Lstiburek, a principle of BSC. “Then the window industry came up with the NFRC ratings for windows, so that today it is fairly straightforward to get information on the three most important characteristics of windows. That labeling came from the industry, and it provides a tremendous example of how the industry can cooperate.”

Since most wall assemblies are site-built, it’s unlikely that you will ever see these new labels attached to wall assemblies in the “Walls” aisle at Home Depot. However, in the future, designers may be able to look up wall performance data on a website, just as they now look up whole-wall thermal performance values on the Oak Ridge National Laboratory website. (The thermal performance measurements made by the BSC researchers differ from the whole-wall R-value measurements made by the Oak Ridge researchers in several important ways. Unlike the Oak Ridge researchers, the BSC team measured thermal performance at a variety of outdoor temperatures. They also measured the walls’ thermal performance while air was flowing through the wall assemblies.)

“We need a big guarded hot box”

Before the BSC researchers could begin testing wall assemblies, they had to build a large guarded hot box that could accommodate wall assemblies measuring 8 ft. by 12 feet. The resulting hot box is capable of testing assemblies at pressure differences of +/-25 Pascals, at air flows of up to 30 cfm, and at a range of simulated outdoor temperatures (from -25°F to 145°F). The actual range of temperatures used to test wall assemblies was -18°F to 144°F.

Building the guarded hot box was no easy task. “We wanted to look at air flow effects, so we had to measure energy and mass flows,” said Schumacher. “The mass flow component is difficult to measure.”

Insulation performs differently at different temperatures

Researchers have long known that insulation materials behave differently at different temperatures. “An R-13 fiberglass batt performs better at low temperatures than at high temperatures,” said Schumacher. “There is an apparent improvement in thermal performance as the batt gets colder. Since there is more radiation at high temperatures, the performance is degraded at high temperatures. The same thing happens with expanded polystyrene.”

Polyisocyanurate bucks the trend: when tested at a mean temperature below 50°F, the performance of polyiso gets worse rather than better. The reason for the declining performance, Schumacher explained, is that “the trapped blowing-agent gases start to condense at cold temperatures.”

Description of the wall assemblies

During the past year, BSC researchers tested the performance of seven wall assemblies. (Confusion alert: the numbering of the wall assemblies discussed in this article begins with Wall 2, not Wall 1.)

  • Wall 2: 2x4 studs, 16 inches o.c., with inset-stapled kraft-faced fiberglass batts;
  • Wall 3: 2x4 studs, 16 inches o.c., with face-stapled kraft-faced fiberglass batts;
  • Wall 4: 2x4 studs, 16 inches o.c., with damp-sprayed cellulose;
  • Wall 6: 2x4 studs, 16 inches o.c., with R-13 open-cell spray polyurethane foam;
  • Wall 6: 2x4 studs, 16 inches o.c., with R-13 closed-cell spray polyurethane foam;
  • Wall 7: 2x4 studs, 16 inches o.c., with R-13 friction-fit fiberglass batts plus 1-inch-thick exterior XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. rigid foam;
  • Wall 8: 2x6 studs, 16 inches o.c., with R-21 friction-fit fiberglass batts.

The walls were built with a few deliberate air leaks. Each tested wall had three electrical boxes (two duplex outlet boxes and one single-gang switch box) with Romex wiring between the boxes.

Shims were used to introduce a 1/32 inch gap between the OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and the bottom plates and top plates of the walls, as well as between the gypsum wallboard and the bottom plates and top plates of the walls. While this decision may sound odd, the deliberate gaps were included so that the walls would leak about as much air as “typical” unsealed walls. “The 1/32 inch gap is based on work done by Don Onysko of Forintek, and represents the typical gap resulting from expansion and contraction of wood framing due to seasonal wood moisture content changes,” said Schumacher. “The 1/32 inch gap sounds like a lot, but it gave us leakage rates similar to what we are seeing in the field.” According to the BSC report, “All of the assemblies … were more airtight than most real wall assemblies (e.g. 0.05 to 0.20 cfm50/ft2).” The quoted range of 0.05 to 0.20 cfm50/ft2 refers to leakage rates in opaque walls, but not walls that include a band joist, door, or window, Schumacher explained.

Some readers — especially those building in cold climates — may be wondering, “Why test an R-13 wall? Who builds an R-13 wall anymore?” The answer to that question, of course, is that the R-13 wall is alive and well in Climate Zones 1 through 4 — for example, in Virginia and Kentucky and states further south.

Each of the seven wall assemblies was tested at five outdoor temperatures (-18°F, 0°F, 35°F, 108°F, and 144°F) without any air leakage through the walls. Then they were re-tested at two outdoor temperatures (0°F and 108°F) with two types of air leakage at each temperature (first with 10 Pascals of infiltration, and then with 10 Pascals of exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration.). “Air leakage increases when it is cold,” said Schumacher. “When cold, all the walls get leakier.”

Testing a wall assembly at multiple outdoor temperatures and pressure regimes takes a lot of time. According to Schumacher, “It took us a month to test one wall assembly.”

Heat recovery during infiltration and exfiltration

When infiltration and exfiltration occur at multiple leak sites in a wall — imagine a colander — the heat flow through the wall is less than when the infiltration occurs at a single hole and the exfiltration occurs at another single hole. This is because a leaky wall acts like a heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank..

Here’s how the phenomenon works in a wall with infiltrating air. First, imagine a perfectly tight wall with a single hole in the center of the wall. All infiltration occurs through this one hole; let’s say the hole admits 20 cfm. This type of infiltration does not benefit from the type of heat exchange under discussion.

If the same 20 cfm of infiltration occurs through one hundred tiny colander-sized holes, the energy penalty attributable to the infiltrating air will be less. That’s because the infiltrating air that passes through all those little holes gains heat as it moves through the wall; in fact, the air gains more heat than it would if it passed through the single hole.

Here’s how the phenomenon works in a wall with exfiltrating air. If all of the exfiltration happens at a single hole, you don’t get any benefit from the heat-exchange phenomenon. But if the infiltration occurs through one hundred tiny colander-sized holes, the warm exfiltrating air will raise the temperature of the wall assembly as it exits, reducing the conductive heat flow through the wall.

“Exfiltration reduces heat flow through the wall,” explained Dave Ober, one of the presenters at the Westford Building Science Symposium on August 1, 2012. “There is a performance benefit. The wall acts like a parallel-flow heat exchanger, so you are getting a little heat recovery.”

Ober continued, “The effect of discrete of holes” — that is, holes that do not resemble a colander — “is easy to calculate because there is no interaction. But uniform air infiltration” — that is, air infiltration through colander-like holes — “recaptures some of the heat loss through the wall by heating some of the air flowing through the wall. Uniform air exfiltration also recaptures a small amount of heat — actually it changes the temperature gradient and reduces the heat flow. You are not recapturing all of the heat; it is a net loss. You are reducing the conductive losses somewhat.”

Because of the heat-recovery effect, Schumacher noted, “Walls with fibrous insulations have reduced heat flows compared to sealed walls with an equal airflow through a discrete hole.”

It’s important to remember the heat carried away by escaping air in the winter incurs a greater thermal penalty than any benefit accrued by the heat-exchange that occurs as air flows in or out of a wall. “There are a lot better ways to achieve heat recovery,” said Schumacher. “It’s better to seal the walls.”

Here’s how the BSC researchers summarized the heat-recovery phenomenon:

  • Infiltrating air may recover some heat flow through the windward wall, thus reducing the energy impact of the air leakage;
  • Exfiltrating air can reduce the heat flow through walls on the leeward side of the building;
  • Any air leakage through a wall assembly will result in increased heat flow.

As part of their thermal metric project, the BSC researchers developed a data-analysis methodology that accounted for the heat-exchange process that accompanies infiltration and exfiltration.

Some of the researchers’ findings

First of all, if the walls were sealed and there was no air flow through the walls, all of the R-13 walls behaved the same. As Gertrude Stein might have put it, an R-13 wall is an R-13 wall is an R-13 wall. “When the nominal R-13 walls are sealed and tested, they have the same heat flow, plus or minus 4%,” said Schumacher. “They all perform roughly the same.” (The results represent averages from tests conducted at five outdoor temperatures: -18°F, 0°F, 35°F, 108°F, and 144°F).

These findings are shown in the bar graph reproduced below.

When the walls were tested at a 10 Pascal pressure difference, however — in other words, when there was air flow through the walls — they no longer behaved the same. The walls insulated with fiberglass allowed more cfm of air flow through the walls than the walls insulated with spray polyurethane foam. The walls insulated with cellulose had air leakage levels that fell between those of the walls insulated with fiberglass batts and those of the walls insulated with spray foam. “There is not much difference between inset-stapled fiberglass and and face-stapled fiberglass,” said Schumacher. “But there is much less air leakage through the spray foam walls.”

The findings are shown in the bar graph reproduced below. (The results shown in this graph and the results in all of the subsequent graphs on this page represent averages from tests conducted at only at two outdoor temperatures: 0°F and 108°F).

Of course, infiltration and exfiltration through a wall assembly degrade the thermal performance of the wall. The leakiest walls (those insulated with fiberglass batts) saw a greater degradation in thermal performance than the tighter walls (those insulated with spray foam). As one would expect, the wall insulated with cellulose has thermal performance results that fall between the performance of fiberglass batts and spray foam.

These findings are shown in the bar graph reproduced below.

When tested under a 10 Pascal pressure difference, the heat flow through the fiberglass insulated walls was about 35% higher than when it was tested without any air flow. The spray foam wall performed better. When tested under a 10 Pascal pressure difference, the heat flow through the open-cell spray foam wall was only 16% higher, and the heat flow through the closed-cell spray foam wall was only 23% higher, than when the same walls were tested without any air flow.

These findings are shown in the bar graph reproduced below.

The percentage degradation in thermal performance was higher for the 2x6 walls insulated with fiberglass batts than it was for the 2x4 walls insulated with fiberglass batts. While the increase in heat flow attributable to air leakage was between 34% and 35% for the 2x4 walls, the increase in heat flow attributable to air leakage was between 45% and 48% for the 2x6 walls. “The percentage effect is much larger on high R-value walls because the heat flows were lower to begin with,” said Schumacher. “So with higher R-value walls, it’s more important to take care of air flow.”

These findings are shown in the bar graph reproduced below.

Note that air leakage degraded the thermal performance of every single wall — even the walls insulated with spray polyurethane foam. According to a summary released by BSC, “All wall assemblies experience a loss in thermal performance due to air movement through the assembly. This is true for all of the assemblies tested regardless of the type of insulation material used (e.g. cellulose, fiberglass, ocSPF, ccSPF, XPS). The energy impact of airflow depends on the flow path, the interaction between the air and the solid materials in the assembly, and the installed R-value of the assembly.”

Address the big holes

In a discussion of advice for builders, Lstiburek said, “In houses, the big holes matter. Plain walls don't contain the big holes. I can make a house insulated with fiberglass just as tight as a spray-foam home or a cellulose home if I seal the big holes. Most folks who use cellulose also get the draft-stopping big-hole thing; the same thing for spray polyurethane foam. Most typical builders who do fiberglass do the bare minimum, and typically don't get it. ... Consider that there is not a lot of leakage in a stud cavity, even with the shims, compared to the big holes everywhere else in a typical building.”

When asked to comment on the results of the thermal metric consortium’s work, Lstiburek said, “Air tightness is important to all insulation systems. I know this should be obvious, but it is clearly not, based on what we see being built out in the field. Having good numbers helps make the case for air tightness unassailable.”

Lstiburek is very proud of the work performed by the consortium. “This is a huge first step,” he said. “I don’t know if anyone appreciates how difficult it was to get to here. We’re not done, but we are a long way down the road to giving good advice.”

The way I see it

The findings of the BSC researchers seem to me to be consistent with long-standing advice from insulation experts, including advice that GBA has provided for many years. Here's a sample of some advice from the GBA website that remains valid:

  • “Stopping air leaks is just as important as — maybe more important than — adding insulation. Unless builders prevent air from leaking through walls and ceilings, insulation alone won't do much good.”
  • “If you choose an insulation that doesn't stop air flow, it's important to install an adjacent air barrier material.”
  • “Fiberglass batts are air-permeable — they do a poor job of resisting airflow — so it is essential that they be installed in continuous contact with an impeccable barrier to air movement. For the very best performance, fiberglass batts should be installed in a sealed cavity (for example, a stud or joist bay) with an air barrier on all six sides.”
  • “Dense-packed cellulose resists air infiltration better than fiberglass batts, but it's not an air barrier.”
  • “Spray polyurethane foam is better than any other type of insulation at reducing air leakage.”
  • “Fiberglass-insulated homes are the leakiest. … Bruce Harley, the Conservation Services Group’s technical director for residential energy services, … assembled airtightness data on Energy Star homesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. (including single-family and multifamily homes) completed in 2004 in Massachusetts and Rhode Island. All of the homes were blower-door tested after completion. … Harley found that houses with walls insulated with spray polyurethane foam were significantly tighter than those houses with walls insulated with cellulose, and that houses with walls insulated with cellulose were significantly tighter than those insulated with fiberglass.”

Martin Holladay’s previous blog: “Study Shows That Expensive Windows Yield Meager Energy Returns.”

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Sep 28, 2012 8:45 AM ET

Good blog... but I'm curious
by Armando Cobo

Great summary Martin. I can't wait till BSC comes up with the new metric for replacing the R-value system. I’m wondering why BSC is using 144°F outside temperature for testing… anyone building in Death Valley? Or is it the reflective sun light of a Low-e window heating on the neighbor’s walls?

Sep 28, 2012 9:05 AM ET

Edited Sep 28, 2012 9:47 AM ET.

Response to Armando Cobo
by Martin Holladay

When Joe Lstiburek originally announced his bold plan back in 2007, he said that he intended to test wall assemblies at temperatures ranging from -18°F to 144°F. Although these tests were eventually performed on sealed walls, the researchers scaled back the number of temperatures at which they tested the walls with air flow through the walls -- in part because the testing is expensive and time-consuming. At a 10 pascal pressure difference, the walls were tested at only two outdoor temperatures: 0°F and 108°F. I'll leave it to the insulation experts to judge whether the results of tests at these two temperatures are more or less meaningful than tests performed at more moderate temperatures.

Concerning your question about testing at 144°F: I think that it is certainly possible for an attic to get that hot.

When it comes to the upper temperature limits for walls, your reference to window reflections is appropriate. Researchers have measured siding temperatures as high as 219°F due to sunlight reflecting off a nearby window.

Sep 28, 2012 3:11 PM ET

GBA doing what yaa all do
by aj builder, Upstate NY Zone 6a

GBA doing what yaa all do best, AAA blog.

Sep 28, 2012 6:01 PM ET

Ah, glorious data!
by Kyle Chase

Great stuff, I know these tests take a lot to perform but I sure would like to see another assembly or two tested, particularly with a Blown in Blanket type application with a product like CertainTeed's Optima or other short fiber wall insulation dense packed and/or cellulose. Is this on ongoing study?

Sep 29, 2012 6:12 AM ET

Response to Kyle Chase
by Martin Holladay

Q. "Is this on ongoing study?"

A. Yes. The hope is to test more wall assembly types in the future.

Sep 30, 2012 1:26 PM ET

Kyle, read again, damp spray
by aj builder, Upstate NY Zone 6a

Kyle, read again, damp spray cellulose is wall 4, and basically is the test you are looking for.

Oct 3, 2012 10:20 AM ET

Edited Oct 3, 2012 10:37 AM ET.

As Earth quickly warms
by Doug McEvers


BSC is looking ahead, the 144 F is the cold climate version.

Martin makes a good point about high attic temperatures, I have recorded 130 F in Minneapolis. I would be interested in the dynamic between a 75 F conditioned space and a 130 F or more attic.

Oct 3, 2012 7:29 PM ET

Edited Oct 3, 2012 7:34 PM ET.

Counterintuitive, for me
by Derek Roff

Very interesting information. Thanks for this article. I wonder if anyone can help me understand a few items that are counterintuitive for me. For example, why is open-cell foam less affected by moving air than closed-cell foam? I would expect the opposite.

I also cannot visualize the "heat recovery" effect when infiltration or exfiltration happens through colander-sized holes. My thinking is that the delta-T across the wall assembly doesn't vary between the two scenarios, in that the indoor and outdoor temperatures are the same in both cases. If the same mass of air at the same temperature is moving from inside to outside, from where do we get the described energy usage differential, when we compare more small holes to fewer bigger ones? Heating the same mass of air by the same number of degrees from outside to inside should require the same number of BTUs.

The analogy to a heat exchanger seems off the mark. In a heat exchanger, intake air passes exhaust air in close proximity, so that heat can be exchanged between the counter-flowing streams. It sounds like in the test apparatus, and certainly in a home on a windy day, the flows are separated. Infiltration tends to occur on the windward side and the lower story, while exfiltration is greater on the leeward side and the upper story. There is no path for heat exchange between the incoming and exiting air streams, so far as I can see. What am I missing?

Oct 3, 2012 7:56 PM ET

Edited Oct 4, 2012 7:50 AM ET.

Response to Derek Roff
by Martin Holladay

Concerning the differences in performance between the walls with open-cell spray foam and closed-cell spray foam: I agree that some of the findings are hard to explain. It's hard to know, at this point, whether these differences will prove to be consistent (with other wall assemblies) or simply represent the "noise" in data with which every researcher is familiar.

The second bar graph ("As-Built with Imposed Air Flow") shows that the walls with open-cell foam had more air flow at high temperatures, but less air flow at low temperatures, than the walls with closed-cell foam.

The next graph ("Q Sealed vs. Q Imposed") showed that air leakage degraded the thermal performance of the closed-cell foam wall more than the open-cell foam wall -- a result that is, indeed, counterintuitive.

Here's an attempt to address your question about heat exchange.

First, let's talk about infiltration. If all the air leaks through one big hole, it basically enters the room at the outdoor temperature. In winter, the infiltrating air is cold.

If the infiltrating air comes through colander-sized holes, it filters through the insulation on the way in, and the air picks up some heat from the insulation fibers in the wall. By the time the air reaches the room, it has warmed up a little.

Now let's talk about the exfiltration scenario. The wall is losing heat by conduction. The outer layers of the insulation are cold; the inner layers of the insulation are warm. If the exfiltrating air leaves by one big hole, the exfiltrating air doesn't warm up the fibers of insulation in the wall.

However, if the exfiltrating air leaves through colander-sized holes, it has a chance to warm up the fibers of the insulation in the wall on the way out. This reduces the conductive heat loss through the wall.

Oct 3, 2012 8:47 PM ET

ICF walls
by Curt Kinder

I hope a future test includes an ICF wall panel

Oct 4, 2012 12:54 AM ET

Spray foam counter-intuivity . . .
by Alistair Jackson

Excellent blog - thanks to both BSC and Martin/GBA for sharing and posters for your comments.

The spray foam performance issue is very topical for us right now. I wonder if the "counterintuitive' result for closed cell foam might have anything to do with the physical realities of the wall assembly. An R-13 layer of closed cell foam does not fill a 2x4 cavity (I'm assuming this part-filled cavity was the condition for the tested assembly??). Is it possible that the lack of alignment between insulation and air barrier on one large face of the stud bay resulted in increased heat loss, perhaps due to convection in the void between insulation and interior drywall? Accepting that even ccSPF allows some infiltration opens up further discussion about whether it is OK to only part fill a wall cavity with ccSPF? I was taught that insulation had to be in contact with air barrier on all 6 sides. Given how labor-intensive and wasteful it is to achieve this in a ccSPF install, I've been tempted to say OK to leaving the ccSPF fill just shy of the drywall plane of the cavity. But I'm wondering if this "counterintuitive" result hints to the contrary. Any thoughts? Alistair

Oct 4, 2012 6:22 AM ET

Air tight?
by Roger Anthony

Perhaps, someone should mention that brick built walls, with internal wet plaster finish.
Are airtight!
A possible move to more robust building?

Oct 4, 2012 7:35 AM ET

Response to Alistair Jackson
by Martin Holladay

You have proposed a possible explanation for the data reported by BSC researchers. I think, however, that we need more data before we can draw conclusions.

Certainly, R-13 closed-cell spray foam is a meager amount of insulation, and the effects seen by the researchers may be connected to the fact that they were testing low-R walls.

I don't think the effect has anything to do with whether or not the air barrier and the thermal barrier are properly aligned, however. As long as there are no air-leakage cracks between the spray foam and the framing members, this problem of alignment does not occur with spray foam -- because the foam is both an air barrier and a thermal barrier.

Oct 4, 2012 7:39 AM ET

Response to Roger Anthony
by Martin Holladay

You're right that some wall assembly types (including load-bearing brick walls finished with plaster) have lower rates of air leakage than others (for example, a wood-framed wall insulated with fiberglass batts).

However, creating a good air barrier involves more than just choosing a method of wall construction. Most air leaks do not occur in the field of the wall -- they occur at seams and penetrations. That's why a house with brick walls can still be very leaky.

Oct 4, 2012 9:46 AM ET

Any ideas about how dense
by Rob Silbajoris

Any ideas about how dense pack fiberglass (blown in Spider) would compare to the tested assemblies?

Oct 4, 2012 9:58 AM ET

Response to Rob Silbajoris
by Martin Holladay

There is no indication from BSC researchers that they have measured the performance of a wall insulated with blown-in fiberglass.

Perhaps that is one type of wall assembly that will be tested in the future.

Oct 4, 2012 10:19 AM ET

Edited Oct 4, 2012 10:21 AM ET.

Why no modeling?
by Robert Beach

This is a great project and a necessary step in the right direction. I agree that quantifying the performance of wall assemblies' acting factors, such as materials and geometry, is very important.

But I disagree with the process for making those determinations.

Why are they not taking this amazing data and calibrating CFD models? They are only testing 8 wall assemblies. With CFD the number of unique wall assemblies (and cost of performing the tests) would be vastly improved. Furthermore, the types of measurements would no longer be limited. Relying solely on physical testing is going to shut out possible innovation because of the huge cost involved in physically creating the data to generate the performance stickers. Invention will be hampered because when a sticker is required for validation, those assemblies not tested get left out. In order to reduce the cost of testing and to allow for more creativity in design, we have to start thinking about how software can enhance our scientific processes. Indeed it can! BSC needs to admit that modeling in software is the future! Physical testing and digital modeling can work together for deeper understanding.

Oct 4, 2012 10:27 AM ET

Edited Oct 4, 2012 10:28 AM ET.

Response to Robert Beach
by Martin Holladay

[Note to GBA readers: "CFD models" = Computational Fluid Dynamics models]

I don't think there is any evidence that BSC researchers would be averse to having their data used to improve existing computer models.

The first step is to make hot box measurements. The second step is to use the hot box data to improve existing models.

That is the path taken by those who developed the NFRC labels, and the BSC researchers appear to be using the NFRC path as a guide to their work. Virtually all NFRC values are now calculated with software, not direct measurements.

Oct 4, 2012 11:13 AM ET

by Robert Beach

The point about how NFRC primarily uses software for generating labels is good to know. But I counter your point that their data will improve models; it will calibrate/validate the models. I would say that the models improve/expand upon the tests, not the other way around. The two processes should be happening in parallel, with the primary focus on creating modeling processes. I believe that the Thermal Metric will be hampered by relying solely on physical testing to determine the label's logic. I admit I am making many assumptions about BSC's process and hopefully the peer-reviewed paper will shed light on how they plan on, or have been modeling these suckers.

Oct 4, 2012 11:48 AM ET

Where are the SIPs?
by Thomas Moore

I am curious why SIPs were not tested? Perhaps the comparison would not be apples to apples. Hopefully SIPs and ICFs are tested in the future - and maybe even compared to each other. I would like to see those results. Not sure how they would compare to the stud walls though - Prius vs. Durango???

Oct 4, 2012 12:05 PM ET

Response to Thomas Moore
by Martin Holladay

As far as I know, the reasons that BSC has only tested a limited number of wall assemblies, and has not yet tested a SIP wall, are the following:

1. It takes a long time to test a single wall assembly.

2. The testing is expensive, so researchers have to prioritize which assemblies to test.

3. The first wall assemblies tested were those that were of interest to the sponsors of the project (all insulation manufacturers).

Oct 4, 2012 6:09 PM ET

Do you know if reflective
by Brandi Borkgren

Do you know if reflective insulation is on the list to test? I would be interested to see where this product falls.

Oct 4, 2012 9:19 PM ET

Response to Brandi Borkgren
by Martin Holladay

There's no need to test foil-faced bubble wrap more than it's already been tested.

I've written many articles on the topic; one of them was called Martin’s Useless Products List.

Here's what I wrote in that article: "Distributors of foil-faced bubble wrap 'insulation' have a rich history of exaggeration and fraud. A September 2003 exposé in Energy Design Update documented several wild exaggerations by manufacturers. Although foil-faced bubble wrap has an R-value of about 1 or perhaps 2, several manufacturers have falsely claimed R-values ranging from 5 to 10. In hopes of avoiding FTC enforcement action, the manufacturers, caught red-handed, sent EDU a comical cavalcade of apology letters. The bottom line: foil-faced bubble wrap costs just as much as — and in some cases much more than — 1-inch-thick rigid foam. As building scientist John Straube pointed out, 'I might recommend it if it were half the price of R-5 rigid foam, but if it costs more than R-5 foam then you have to be crazy or stupid to use it.' ”

Oct 8, 2012 8:14 PM ET

Edited Oct 8, 2012 8:17 PM ET.

RE: Robert Beach
by Sam Jensen Augustine


CFD and energy simulations are still not at the point where they can model everything we need to model. They have a hard time coupling mass flows with energy as well as difficultly with some forms of radiant transfer. Lawerence Berkeley National Labs (writers of EnergyPlus, eQuest, THERM) has a relatively new hot box project working on resolving some of these issues. The fact is that if you really want to get down to the exacting level described in the article the best way to do it currently is via physical modeling. That doesn't mean energy models are useless, nor that they will not be made better in the future. It does mean that for this pioneering type of work it seems the team is going for the hard slog leaving nothing to assumptions (or at least clear limitations) path.

I would love to see this with variable humidity as an added complexity.

Is the exterior XPS taped or sealed to the framing? If so I am blown away by the spray foam performance improvement over it. I would not haveexpected that.

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