Dreaming about cellular glass in passive housing

logo_smallCellular glass is a vapour tight, non-combustible thermal insulation with a high compressive strength. In the builing industry, cellular glass boards are mainly used in flat roofs, less as floor insulation and almost negligible as facade insulation. The rather large compressive strength is in fact only used as thermal interruption at the foundations, although this property could also be very usefull in self supporting structures. A very good introduction is given in a paper about arches and bridges. It is demonstrated that the entire arch ban be under a compressive stress when a certain curve is respected.


If we would replace the bricks by cellular glass plates, we obtain a stable structure which is vapour and water tight, non – combustible and thermally insulating. The structure can be prefabricated with cellular glass boards of 3m x 1.5m as delivered by GLAPOR. In case of 3 layers of 15cm, the passive housing norm is easily achieved. Rendering outside and inside with natural hydraulic lime is one way of finishing. The complete structure can be combined with cellular glass gravel floor insulation between cellular glass plates. I guess that we arrive at a low cost passive housing system in case of prefabrication, transport with containers and fast building at the job site.

Installation of windows should be easy because the window has almost exact the same thermal expansion coeffcient as the cellular glass. In principle, a window frame is not needed, the window pane can be directly installed in the cellular glass wall. I guess that a high air tightness, typical for passive housing is easily reached. A  building could look as follows but the inside ceiling needs extra support (or can be hanged) in the case of a cellular glass construction.


This is only one suggestion to work with cellular glass as the complete supporting outside structure, which is implicit thermally insulated. I am convinced that we have to work in this philosophy and to avoid to work with structures, which needs separate thermal insulation. In fact, this last method is patchwork on the building tradition since cement was used. Before cement, lime was used for walls about 1m thick, which was thermally equivalent with 5cm mineral wool. The introduction of cement in the home building industry was in fact an ecologic step backwards



About the philosophy of cellular glass

logo_smallThe standard thinking is to select raw materials in function of the requested cellular glass. The glass is made by the melting of sand, soda and lime. At a certain point, it became clear that the use of recycled glass was an interesting economic alternative which could be sold as an ecologic improvement. The same approach is used with towels in hotels (please drop your towel only when needed for the sake of the environment) and we all know that reducing cost is the real reason. However, the economy becomes greener which means that an ecologic and economic reasoning gives the same result.
In my opinion, we have to start from the available waste and convert this waste to a product with added value with a minimum ecologic footprint. This is a clear consequence of the future lack of fresh raw materials and energy and also of the climate change. Also transport is becoming an issue.
The best recycling of bottles is still cleaning and refill. But there will be always an important amount of broken bottles, windows and wind screens. And this waste glass is today a better investment than money on the bank thanks to the green economy. But what can we do with this glass waste?

  • The most suited glass (colour specification) goes to the bottle and window industry.
  • What is left can be converted in cellular glass but is not clear whether it has to be used for thermal insulation with the lowest thermal conductivity possible.
  • One possibility is foaming cellular glass gravel. This product is rather easy to produce and can be used as floor thermal insulation but also as landfill for streets or large gardens. Recycled glass powder is mixed with a foaming agent and foamed at 800°C. The foaming is done on a belt and free cooling after foaming generates the gravel.
  • Annealing the ribbon allows to produce cellular glass boards. These boards can have a thermal conductivity of 0.048 W/mK and sizes up to 3m.
  • Alternatively, foaming can be done in a mold of any shape. However, this method is less interesting and in fact not logic when rectangular boards with larger dimensions are requested.
  • The obtained thermal conductivity depends on the glass composition and when a value of 0.040 W/mK is needed, a special composition has to be melted. This involves extra raw materials and an extra heating up to 1400°C together with a large investment. In most cases, this is also not logic because a larger thermal conductivity can be compensated with a larger thickness.
  • As foaming agent, we know carbon, which needs a reducing atmospfere. SiC and glycerin, a side product of biodiesel can be used in an efficient neutral atmosphere. Glycerin is clearly the natural choice
  • Transport has to be eliminated as much as possible. The logic location of a cellular glass plant is the extension of a glass recycling company. I guess that glass recyclers will extent their companies with a cellular glass production line once the world realizes that cellular glass is not necessarly expensive.
  • Mineral wool and glass wool are also produced from recycled glass but both need a melting step at 1400°C. This is also not a logic choice if we compare with cellular glass foamed at 800°C. However the world still buys much more of these wools at comparable prices with cellular glass.

Walter Frank, founder of GLAPOR WERK, in the past a fair competitor and today a friend has chosen the right approach:

  • Direct foaming at 800°C of recycled glass WITHOUT remelting at 1400°C.
  • Continuous foaming in a neutral atmosphere to large dimensions and later down sizing to pallet dimensions.
  • Depending on the quality of the glass powder, thermal conductivities between 0.065 and 0.048 W/mK can be produced.

But between ready to deliver and convincing the world is a big step, called marketing. Windows still owns the PC market while LINUX is a very good free of charge alternative. The same was true for iOS of Apple but ANDROID is growing like shown hereunder.

I expect that the classic cellular glass (0.040 W/mK) will evolute like iOS and the GLAPOR method like ANDROID if enough money is available to take market share from the others (mineral and glass wool). It is the logic evolution but marketing is needed to let faster happen what is logic.

Glass foams for environmental applications

logo_smallThe University of Rennes in France is also performing research about cellular glass. Probably they were the first to use AlN as a foaming agent but moreover, they introduce now the use as cellular glass as a support of a catalyst. The use of AlN as a foaming agent was already mentioned in a previous post.

In this paper, Glass foams for environmental applications, they investigate a possible catalyst for toluene. The foaming is done with waste glass and AlN. Other additions like TiO2 and steel waste dust are studied. The foams have all a large open porosity and are probably not interesting for thermal insulation. The following table from the paper give an idea about the experiments.


From this paper, we remember the use of cellular glass as a catalyst support and the a foaming recipe for open cellular glass.

Measuring thermal conductivity with the hot wire method

logo_smallThe generally accepted methods, described in standard EN12667 to measure the thermal conductivity are:

  • The guarded hot plate method is an absolute method which does not need samples for calibration.
  • The fluxmeter method on the other side needs a calibration with samples, measured on a guarded hot plate system.

The hot wire method is not accepted as a method to measure thermal insulation, while it is in fact a two dimensional method, measuring the thermal conductivity in a cylinder around the wire. The EN12667 methods are measuring in one direction because they give the thermal conductivity through a board. On top of that, the hot wire method is a transient method, which measures the heat diffusivity and thermal conductivity.

Although less accurate, the method is fast and needs only a small sample. A cylinder of 20 cm with a certain radius is already large enough to do comparative measurement. This is very interesting when testing new formulations for a foam in a laboratory environment. The above was extensively calculated in a paper about the hot wire method for low density cellular materials. It can be concluded from the paper that the method is reliable for cellular glass, because cellular glass is opaque.

In case a unidirectional thermal conductivity is needed, we can replace the hot wire probe by a surface probe, like for example used in the ISOMET of Applied Precison Ltd. This method can be applied by for example fast non-destructive QC in the ware house by internal and external inspectors. Today, standard QC measurements according EN12667 take easily a few hours up to 24 hours  after sample preparation while the surface probe transient method is ready in less than 30 minutes without sample preparation. In a comparative mode of working, the method is reliable.

I would never trust transient methods if not tested on samples, measured with the EN12667 methods. But after testing and checking, they can be very helpful on a comparative basis.


Students at work at Rutgers

logo_smallA nice groups work about the insulation of a thermos with only recycled materials was done at Rutgers School of Engineering by Kahyee Fong, Damin Hashash – Gabe Kooreman, Crystal Mckenzieand Elaine Wang . I give the abstract hereunder:

The goal of this project was to take advantage of the capabilities of nanotechnology by using the room temperature sol-gel process to form a
silica nanofoam composite with a common household foam, in an effort to synthesize a cheap, lightweight insulator for use in a thermos. Through testing of hybrid insulators, it was determined that a silica nanofoam and packing peanut composite combines the natural insulating properties of the packing peanuts and
of the silica nanofoam to make an insulator for a thermos.

With a smart comparative and low cost thermal conductivity measurement, they were able to get the following list.


This is not direct a cellular glass subject but I simply liked the work and I was surprised by the results.

The first report about carbon as a foaming agent for cellular glass

logo_smallPatent US3666506 with inventers JH Cowan and D. Rostoker from Corning Glass Works  have developed cellular glass from rocks in 1969. For the first time, carbon is mentioned as foaming agent. The patent has further other interesting information: they mention

  • already the large production cost of cellular glass, foamed from a special glass composition in closed molds.
  • the need for a reducing atmosphere and for that reason a closed mold.
  • batch materials like clay, vulcanic ash, flux, cellulating agent and maximum 800°C
  • organic foaming agents as carbon source instead of carbon black or graphite are preferred
  • the flux added to give the proper visocity to the clay (rocks) with  sodium silicate (water glass) as the favorite one
  • a table with working and not working foaming agents
  • the need for 10% particles smaller than 5 micron in the clay
  • the use of a ball mill to mix and grind
  • producing pellets to avoids molds
  • 30 foaming recipes
  • densities between 150 and 650 kg/m³

The drive for this patent was to avoid the melting of a glass and the use of molds. Today, GLAPOR has replaced the clay by recycled bottles and the molds by a continuous foaming system. And as a consequence, very competitive prices around 200€/m³ are possible for large quantities. Cowan and Rostoker, both eminent scientists with a view should be satisfied.

The declared thermal conductivity

logo_smallToday, thermal insulation producers in Europe have to declare the thermal conductivity in a special format. This format, the declared value,  motivates the producers to

  • stabilizing their production and in that way the thermal conductivity
  • perform more measurements up to 1000 in three months

The procedure to calculate the declared value is described in the standard EN 13162 : 2008 and the measurement method in the standard EN12667, all part of the general standard EN13167. This standards system became so complicated that companies have today to hire a VP Standards. But the fact that ordinary users, like me, have to spend a lot of money to buy these standards in order to know the rules is a shame.

For the declared value calculation, I found a good public document. It explains how to calculate the declared thermal conductivity with the average value, the standard deviation and a factor depending on the number of measurements. This factor is given in the following table and is the only one thing you need from the standard.

declared value table

Master Thesis: Production and characterization of foam glass from container glass waste

logo_smallThis master thesis, written by Dina Mohamed Abdel Alim at “The American University in Cairo, The School of Sciences and Engineering” is a nice introduction to cellular glass. I copy the abstract hereunder:

Foam glass with excellent properties was produced from container glass waste. The
processing technique depended on the powder sintering approach using sodium
silicate solution as a foaming agent. The morphology, density and compressive
strength were studied in relation to different processing parameters: sintering
temperature, amount of foaming agent, soaking time, powder particle size and glass
powder color. Foam glass was sintered in the range (750-900C) for 30 minutes with
the incorporation of 12 and 19 wt. % sodium silicate solution. At lower sintering
temperature (750-800C), the foam has denser structure (bulk density ranged from
0.37-0.61 g/cm³) along with high compressive strength (ranged from 2.29-18.68
MPa). As the sintering temperature increased, higher levels of porosity were achieved.
At 850C with 12 wt. % sodium silicate solution, lightweight (bulk density = 0.25
g/cm³), highly porous (% of porosity = 90 %) foam glass was achieved. It had
relatively high compressive strength (1.62 MPa), compared to other insulating foams,
along with low thermal conductivity (0.078 W/mK) and the most homogeneous pore
morphology. Significant change in foam glass properties took place with changing the
amount of foaming agent. As the amount of foaming agent increased, the density of
the foam decreased till it reached a minimum of (0.25 g/cm³) that corresponded to 12
wt. % sodium silicate solution. Further addition of foaming agent caused the density
to re-increase and the pore morphology to coarsen. Sintering foam for different
soaking times had a slight effect on changing the foam glass properties. The
morphology of all the foam glass produced at different soaking times was
comparatively homogeneous. The compressive strength of the produced foam was
relatively high (1.6 MPa at 40 min and 3.13 MPa at 10 min). The powder particle size
had a major effect on foam glass properties. As the particle size increased, the bulk
density of the the foam increased and the morphology became less homogenuous.
Increasing the sintering temperature for the larger particle size did not succeed in
increasing the foam structure homogeneity. The glass color also had an effect on the
foam glass properties. The properties and the morphology of the green and brown
glass samples were approximately the same (bulk density = 0.38, 0.37 MPa and
compressive strength = 2.05, 1.97 MPa respectively). However, they differed from the
morphology and properties of the white glass (bulk density = 0.25 g/cm³) and
compressive strength = 1.62 MPa). The specific compressive strength of the white
glass foam (6.48*10-3 MPa m³/Kg) was higher than that of green and brown glass
(5.39*10-3, 5.32*10-3 MPa m³/Kg respectively). EDX analysis was performed for the
white, green and brown powder. It showed that they had more or less the same
compositions (except the presence of chromium element in green glass and titanium in
brown glass which are coloring additives). They had the same main elements but with
different weight percentages.
The optimum processing parameters for producing foam glass for thermal
insulation was to use sintering temperature 850 C, amount of foaming agent 12 wt.
%, soaking time 30 min and glass powder particle size 75 μm.

I don´t like to see a specific compressive strength because this property does not depend linear on the density. But further, we observe

  • a good introduction to glass
  • a nice transition to waste glass and its recycling problems and further to
  • to cellular glass: current production method and applications as thermal insulation
  • and a benchmarking between alternative thermal insulation materials with focus on life time
  • applications in building and industrial insulations and alternative applications
  • the use of foamed glass gravel
  • theoretical study of cellular structures
  • open and closed cell recipes
  • sodium silicate as a foaming agent
  • thermal analysis of this recipe
  • very interesting thermal imaging of the foaming
  • annealing of the foam (without mentioning the thickness)
  • different glass compositions to foam
  • mechanical properties
  • designed experiments to investigate the different parameters
  • the effect of the color of the cullet on the foam quality

This master thesis could also have a been a PhD thesis, it is really a nice piece of work.