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Energy Solutions

Storing Heat in Walls with Phase-Change Materials

National Gypsum introduced ThermalCORE at this year's GreenBuild conference, though the product is not yet commercially available.
Image Credit: National Gypsum

I just returned from the Greenbuild conference in Phoenix. This annual event, now in its eighth year, has become the leading locus for exchange of information about the rapidly growing green building movement. This year’s event drew some 22,000 attendees, including architects, builders, engineers, developers, and manufacturers, from the U.S., Canada, and dozens of other countries.

As is typical at Greenbuild, I spent a lot of my time in the trade show. While there were only a few hundred exhibitors at the first Greenbuild in Austin, Texas, this year’s conference drew 1,800 exhibitors–so I had acres to cover. Indeed, there was lots of new stuff–despite the state of the economy and building industry. This week I’ll focus on one of them: a phase-change wallboard, ThermalCORE, just announced by National Gypsum.

You might remember from high school chemistry that when materials change phase (from solid to liquid or liquid to gas) they absorb a lot of energy, and that energy is released when they revert to the lower energy state. If you add heat to a bucket of ice cubes at 0°F, for example, the temperature will rise steadily until it gets to 32°F; then it will remain at that temperature until all of the ice has melted, at which point the temperature of the water will rise again. (This is why we put ice in our drinks; it keeps the drink cool even as we add heat to it–from the room.)

This principle can be utilized with heat storage in a house if the melting point of the phase-change material is about room temperature.

Phase-change materials (PCMs) were first introduced in the 1970s. Most common were “eutectic salts” and specialized paraffins with melting points between 70° and 80°F. Some were housed in coffee-can-sized metal containers, others in plastic bags. None of these products stayed on the market long; after a few hundred cycles, they no longer melted or thawed completely over the narrow temperature range needed to make them effective for heat storage.

National Gypsum’s ThermalCORE that was introduced at Greenbuild (but is not yet on the market) is a micro-encapsulated paraffin PCM. Tiny spheres of paraffin, just 5-10 microns in diameter (less than half the size of Portland cement particles), are encapsulated in acrylic shells, and these are mixed with the gypsum in drywall. The paraffin melts at 73°F, plus-or-minus 2°F. The PCM used in ThermalCORE is Micronal, made by the German chemical giant BASF. Micronal was introduced about five years ago, and BASF has been looking for a U.S. drywall manufacturer to partner with on a PCM wallboard.

BASF’s Micronal PCM is available in commercial products in Europe, and it has gone through extensive testing. According to a BASF scientist I spoke with at Greenbuild, the material has been tested through 10,000 phase-change cycles (equivalent to 30 years of use) with no loss in performance.

National Gypsum’s ThermalCORE has fiberglass skins (instead of paper), but otherwise looks no different than standard drywall; the micro-encapsulated PCM spheres are far too small to see.

Studies show that the ThermalCORE wallboard stores about 22 BTUs of thermal energy per square foot. The idea is that warmth from the sun during the day will be stored in the wallboard, and then released at night to keep the space warm. It will both help prevent overheating during the day and help reduce heating costs during the evening hours. In essence, it’s a high-tech form of thermal-mass materials that are typically used in passive solar design (brick facing walls, tile floors, etc.).

Following the roll-out at Greenbuild, National Gypsum will begin field trials to determine how this product can most effectively be used in home building. Field trial sites are currently being sought through the California Emerging Technologies Coordinating Council and the U.S. Department of Energy National Renewable Energy Laboratory in Golden, Colorado; most will be in California.

National Gypsum has not said what the cost of ThermalCORE wallboard will be, or when it will be commercially available–except to say that it will not be introduced until after the field trials are complete. For more information, visit ThermalCORE’s website. I’ve been following PCMs for thirty years now, and I think this is the first such product that has a chance of really succeeding–if the cost isn’t too high. I look forward to tracking ThermalCORE’s roll-out.

I invite you to share your comments on this blog. You can also follow my musings on Twitter.


  1. Beideck | | #1

    Micronal availabilty
    Can micronal be purchased directly from BASF. I could imagine it being possible to try using the product in a number of different applications. Mixing it directly into plaster that then goes on the walls comes immediately to mind. Any reason this couldn't be attempted?

  2. Expert Member
    CARL SEVILLE | | #2

    What are the downsides?
    I spoke to the National Gypsum reps at the booth and they had no answers to questions regarding health impacts of the phase change material or their effect on recycling of scrap drywall. Thermal mass is all well and good, but if you end up with health and waste impacts they may well more than offset the benefits of this cool, space age product. This product, while interesting, appears to me to be another high tech solution to a problem that may not really be there.

  3. Alex Wilson | | #3

    Micronal availability and downsides
    I have a query into BASF about selling Micronal directly. My guess is that they will sell it only to OEMs (manufacturers) so that they maintain control over how it's used. I'll report back if I learn anything different.

    Carl raises good questions about potential health concerns. The Micronal product isn't nano-scale (where there's concern that it might pass through cell membranes) and both acrylic and paraffin are "relatively" safe--but these are questions we need to be asking.

  4. Dave | | #4

    BASF has a good approach but there are other PCM applications for walls already available. There is even a wall matt and ceiling that doesn't use the petroleum based paraffin and instead is bio -based PCM. This smart wall solution cam be found at

  5. Robert Riversong | | #5

    More questions than answers
    In addition to issues of cost, recycling, health and environmental impacts, the product is also combustible, eliminating one of the prime benefits of gypsum drywall.

    ThermalCore would seem to be most beneficial in non-solar or indirect-gain applications where it can undergo phase change at temperatures slightly elevated above normal. In solar direct-gain locations, ordinary ½" gypsum drywall has a specifc heat of about 0.5 BTU/sf per °F, so a 10° temperature rise would store 5 BTU/sf, nearly ¼ of what the ThermalCore offers, and a double layer of ½" DW would store 10 BTU/sf.

  6. Robert Riversong | | #6

    BIO PCM?
    Dave, do you know anything about the materials used to fabricate BioPcm? There's nothing on their website to indicate ingredients and a MSDS sheet is available only on request.

    Thier "technical data" are confusing and don't offer a latent heat capacity per square foot for comparison to other options (I've sent an email request to them for this information).

  7. Robert Riversong | | #7

    BIO PCM data
    I found more information on their FAQ page, and they seem to be claiming a latent heat capacity of between 50 and 180 BTU/sf.

  8. Robert Riversong | | #8

    More BIO PCM Data
    I got a quick response from and the latent heat capacity of their honeycomb roll material varies between 27 and 180 BTU/sf.

    It does seem to be an environmentally benign material, made from soil and palm oil and soon from algae, but the pockets of PCM will leak if cut or punctured. The product is a roll of 14 mil film with ¼" to 1" thick pockets of PCM that is applied between cavity insulation and drywall, with the potential for poor conductivity of heat into the PCM unlike the ThermalCore which embeds the PCM into the drywall.

    So my jury is still out on this. I would love to know of any experience with either product.

  9. Kevin O'Meara | | #9

    PCM in plaster
    One of my subs has had experience mixing the microencapsulated PCM...presumably from BASF, into plaster for wall finishing with good results. I wouldn't think that approach would be as cost effective given that plastering walls in general costs more than drywall, but at least it's avaialble now. ORNL recently did study PCM walls, predicted a payback in 3-5 years, but no mention what the cost they were assuming in the calcs. A similiar product is already available in Europe called "smartboard", and comments from blogs there suggest that the cost were significantly more than standard drywall.

  10. Jay Walsh | | #10

    PCM Cellulose
    Back in 2004 Oak Ridge National Laboratory (ORNL) did some testing on PCM Cellulose (cellulose insulation containing 5% to 30% of microencapsulated PCM).
    Has anyone heard of this type of cellulose making it to the market?
    Any comments for or against this type of PCM application?

  11. GBA Editor
    Martin Holladay | | #11

    Phase-change cellulose
    The following information comes from an article on phase-change cellulose that I wrote for the July 2007 issue of Energy Design Update:

    Jan Kosny has led a group of ORNL researchers who conducted successful trials of phase-change insulation made by mixing Micronal spheres with conventional cellulose insulation. (For more information on phase-change insulation, see EDU, February 1995 and November 1995.) The ORNL researchers arranged for small batches of the experimental insulation to be mixed at an Advanced Fiber Technology plant in Cincinnati, Ohio (; several recipes were tried, including up to 30% Micronal.

    Because the ORNL researchers intended to test the performance of the phase-change cellulose in cooling-dominated climates, they chose to use Micronal with a melting point of 78.5°F. (In a heating-dominated climate, a phase change material with a lower melting point -- in the range of 65°F to 72°F -- would be more appropriate.) “Right now we have limited time and resources for testing,” said Kosny. “We decided to start with cooling applications.”

    Even recipes of phase-change cellulose with a relatively high Micronal content (30% Micronal, 70% cellulose) passed an ASTM flammability test -- specifically, the C-739 smoldering combustion test. Moreover, dynamic hot-box testing of attic and wall assemblies in the ORNL lab revealed that phase-change cellulose has the potential to improve building envelope performance (see Figure 4). According to “Science and Technology Highlights,” an ORNL publication, “Experiments using the heat-flow apparatus indicated energy savings of up to 25% can be realized through the use of phase-change material (PCM) during periods of dynamic temperature change. These tests also showed that the thermal conductivity of cellulose insulation is not compromised by adding up to 30% PCM” -- in other words, the R-value of the cellulose remains unaffected by the added Micronal.

    In Oak Ridge, Tennessee, and Charleston, South Carolina, the researchers performed two field studies of homes with cellulose-insulated 2x6 walls. The installation of the phase-change cellulose resulted in a 30% to 40% reduction of the portion of peak cooling loads attributable to heat gain through walls compared to walls insulated with conventional cellulose. (Since other factors -- including window heat gain, air leakage, and attic insulation levels -- tend to strongly affect cooling loads, the wall performance improvements noted by the ORNL researchers do not imply that phase-change cellulose in walls will reduce cooling energy bills by 30% or 40%.)

    Although initial research shows that phase-change cellulose is a promising product, the insulation is not yet available. “Nobody is selling it,” says Kosny. Kosny has calculated that the incremental cost of adding 20% Micronal spheres to cellulose insulation would be about $0.47 per square foot of 2x6 wall.

    A paper describing the ORNL research on phase-change cellulose, “New PCM-Enhanced Cellulose Insulation Developed by the ORNL Research Team,” by Jan Kosny, Dave Yarbrough, Ken Wilkes, Doug Leuthold, and Azam Syad, is posted on the Web at

  12. GBA Editor
    MIKE GUERTIN | | #12

    Another Phase-Change Wall Panel
    Perhaps we'll be seeing more phase change products such as NG's ThermalCORE. I came across DuPont's EnerGain last spring. Though not available in the US (despite promoting in via their NA website) it's available and used in Europe.

    According to their information:
    "DuPont™ Energain® comes in aluminium laminated panels, bordered at the edge with aluminium tape, which contain a copolymer and paraffin wax compound." It looks like a product applied over wall framing before attaching conventional drywall.

    It will be interesting to see how these products work and handle - once we can get our hands on some....

  13. Kevin Dickson | | #13

    Is Thermal Mass Even Important?
    An energy modeler I know has played with the thermal mass values in superinsulated homes. He uses Energy-Plus. He reports that at very low annual heat loss values, increasing the thermal mass within the building envelope provides a negligible benefit in annual heating costs.

    I think I can corroborate this with some "napkin calcs" but I was wondering if any others have seen this.

    If true, this discussion is moot for low loss homes, but these products may still be useful in retrofits.

  14. Douglas Nuttall | | #14

    Thermal mass is important
    The issue with thermal mass has much to do with where you are, and what your climate is. Near Ottawa, Canada, I just designed a massive and passive home. It requires 10x less supplemental heat as a result of the thermal mass (down to $50 of electric space heat per year) than without it. It's because the passive solar can usually provide all the heat the house needs - it's just a matter of spreading it out so there is still heat coming just before dawn.

    I'm just beginning a more massive, active and passive assembly space, that will have passive ventilation for most of the year, passive heating for 4 months, active AC for about 10 days a year, and active heating for about 2 months (overlapping the passive heat). We're using PC drywall throughout for maximal solar performance. Construction cost comparable, operating cost negligible vs conventional construction.

    If your thermal mass is sufficient, and your insulation is sufficient, then you can get STES within your building envelope. Around here, that makes your walls about 4' thick (straw bale on the outside, rammed earth or it's equal on the inside), so that your thermal time constant is into the 'many months' range. Can be done.

  15. GBA Editor
    Martin Holladay | | #15

    Count me a skeptic
    Canada is one of the least likely climatic locations where thermal mass could prove beneficial.

    Here is some information from the GBA Encyclopedia:

    ORNL research has found that ICF houses use less energy than the typical wood-framed home. But the thermal mass benefits of ICF houses depend on location: Houses in Minneapolis and Chicago showed the least savings from the thermal mass effect, while those in Phoenix and in Bakersfield, Calif., had the most. In all cases, potential whole-house energy savings were 10% or less when the R-value of the wall was 25.

    Canadian researchers who closely monitored the performance of a multiunit residential building with ICF walls reported, "The overall building is relatively airtight, due in large part to the continuity of the ICF wall assembly, as no extraordinary air leakage control measures were undertaken at the roof and foundation levels." One of the researchers, Duncan Hill, commented, "The concrete is a poured-in-place air barrier." However, the researchers concluded that an ICF wall offers no thermal mass benefits in Canada.

    According to two articles in Environmental Building News ("Thermal Mass and R-value" and "Thermal Mass"), high mass can enhance energy performance, but only when outdoor temperatures cycle above and below the indoor temperature in a 24-hour period. In parts of the country where outside temperatures remain well below the indoor set temperature for weeks at a time, the mass effect isn’t really a factor.

  16. lutro | | #16

    What's new in phase change materials?
    Have there been important new developments in phase change materials and products, since this article was posted 3.3 years ago?

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