The EN-standard 13167 is rather clear about cellular glass for building applications. The water vapor diffusion resistance µ should be larger than 40000. This property has to be measured according to the standard EN 12086, which has its last version in 2013. The reason for this large µ-value is rather clear. Cellular glass has the reputation to eliminate perfectly internal condensation thanks to this very large µ-value.

However, there are two reasons to doubt and review the above story in the building , namely two practical ones and a bureaucratic one.

1. The bureaucratic one is that the standard EN-12086 from 2013 contains an error on page11.  Indeed, the mentioned gas constant of water vapor is a factor 100 too small, which means that a cellular glass with a real µ-value of 400 is allowed by the (erroneous) standard. A µ-value = 400 is an open cell cellular glass. This means that today open cell cellular glass is covered by the EN13167 standard.
2. The practical one is that an open cell cellular glass has an improved acoustical absorption with thermal conductivity 0.056 W/mK. This material can be used between two thin sound reflecting materials to obtain a light sandwich with a small sound transmission. Another application is simply to use it to reduce echo (reverberation) without using combustible material.
3. Another use op open cell cellular glass can be find in the hydro-culture world, like already mentioned in a previous post. But I can imagine it makes also sense to let grow plants against this material vertically installed inside and outside. The material serves as plant growth medium, thermal insulation and acoustic absorb-er.

In our opinion, it is clear that the EN-standard 13167 should be rewritten to allow non-combustible (cellular glass) thermal insulation with a high compressive strength and interesting acoustic properties. We should simply specify a minimum µ-value and maybe even a maximum µ-value. Today, it seems hard to sell cellular glass thermal insulation with important acoustic absorption within the EN- standards system because only cellular glass with µ-value > 40000 is allowed for building applications.

For a long time, we were thinking that only closed cellular glass has a future. However, more and more, GLAPOR receives requests to deliver open celled material. In most cases, people are interested to consider this material as an acoustic absorber.

But last week, we got a request to use this material as a plant growth medium as an alternative to Growstone. But this case, there is interest for the gravel and the boards. At GLAPOR, this evolution was predicted and a one step foaming of 100% open cell cellular glass with a large water vapor transmission was developed. However, Growstone is more than only open cell cellular glass gravel like shown in their patent.

In this case, the cellular glass serves as support for the plants but is also able to keep the pH of the water stable in hydrophone applications. The patent gives such a nice introduction that I give here simply a citation:

Hydroponics is the science of growing plants in a nutrient solution with the mechanical support of an inert medium. Hydroponics is an old art, and a variety of inert
media are known as suitable for the germination, rooting and growth of plants. Such substrates include peat, vermiculite, perlite, fly ash, pumice, rock wool, glass wool,
organic and inorganic fibers, polymers such as polyurethane, polystyrene, polyethylene, and the like. These substrates have been used for true hydroponics or in quasi-hydroponic environments such as in admixtures with soil. Typically, the inert medium is either in the form of a contained loose particulate, such as sand, or as a rigid and self-supporting structure that can support growth of the plant. The rigid structure has some notable advantages over the loose particulates, in particular the ability to stand alone without a requisite container. However, the loose particulate media tend to offer better pathways for water and gasses to be delivered to and from the root systems.
One problem common to hydroponic gardening is under/overwatering. Some media offer limited porosity and/or limited means for circulating water into and out of
pores. As a result, vegetation growing hydroponically is often underwatered. Conversely, hydroponic techniques lend themselves to the provision of excessive water to the plant root system, often in response to the underwatering that is occurring. Overwatering can result in chlorosis, retarded growth, pallor, and, eventually death. In such situations, the water around the roots becomes stagnant and gasses dissolved therein are only urged to
and from the roots through diffusion. Moreover, vital gasses quickly become depleted and waste gasses saturated in the water proximate the roots, exacerbating the situation. Thus, it is desired to reduce the stagnant water around the roots by circulating the water.
Most of the substrates currently known are solids with limited porosity. Some known substrates have attempted to add or increase the porosity of the substrate
in order to better provide for gas exchange to the roots. One such substrate has been produced in the form of a sponge-like or foraminous foamed polymer body with
conduits 1-5 millimeter in nominal diameter, spaced about 1-8 mm apart and extending throughout the substrate. The conduits drain water from the substrate and provide reservoirs of oxygen for the plant roots and at the same time allow substrate to hold some water that may then be available to the roots. The porosity of this substrate ranges from between 6 and 53 percent. Soil or the like is deposited on top of the substrate and a seed, cutting or small plant is placed in the soil. With the substrate under the soil layer, over-watering induced problems are prevented, as excess water drains from the substrate, filling the conduits with air and oxygen will be readily
available to the roots. 

Another issue with known substrates is pH control. Natural substrates tend to include soluble mineral residue that dissolves at uncontrolled rates, shifting water pH. Man-made substrates likewise may include materials that dissolve over time and at nonlinear rates, shifting pH as they do. Changes in pH can have a drastic and deleterious effects on plant growth. While useful in hydroponic and soil amendment applications, the above substrates are still hampered by a lower than optimal porosity, limited wicking capacity, low capacity for water infiltration and retention, and uncontrolled pH arising from mineral dissolution. [0006] The documents US 2007/0186467, US 2009/0056221 and US 2008/0034653 describe foamed glass substrates for use as plant growth medium and methods for its production. Thus, there remains a need for a highly porous substrate for supporting plant growth. There also remains a need for improves the aeration of soil and allows for better water filtration and irrigation. The present novel technology addresses these needs.

Informally, it was communicated that cellular glass is well suited to harvest marijuana for medical purposes. On Amazon, we found that Growstone costs about 16€ for 9 l while typical gravel for building costs 50€ for 1000 l.