Fraunhofer Institute for Building Physics investigated in real life the use of foamed glass gravel as a basement wall thermal insulation and under the concrete slab of a building. More details can be found in this nice foamed glass gravel document.

The summary of the results is published in English, German and French. They conclude that the design value for the thermal resistance is not reached for the basement wall insulation, which is a less popular application. On the other hand, the design values are exceeded for the floor insulation system, where foamed glass gravel is used under the concrete slab.

In the report, the authors also explain that the calculated and measured values only match when air movement and latent heat is included in the calculation. Air movement is equivalent with convection (free or forced). Latent heat means that the evaporation of the humidity extracts heat from the gravel, which creates an extra heat loss.

Indeed, the convection spreadsheet allows to find out that indeed free convection is present in the gravel besides the basement wall. With an intrinsic  permeability of 1e-7 m² for gravel, a 2.2m thick layer of gravel and 0.1 W/mK thermal conductivity, it is clear that the free convection in winter (10°C temperature gradient) has the same magnitude as the assumed thermal conductivity.

On the other hand, the law of Darcy shows that with the same permeability and a pressure drop of 10Pa over the house, important forced convection is present in the gravel under the concrete slab. This forced convection induces an important heat loss during winter.

However, the RDS concept of GLAPOR, where cellular glass boards enclose the gravel and so avoid water ingress and forced convection, eliminates this problem. As a consequence, this RDS-system allows to exceed largely the design value for gravel thermal insulations under the concrete slab.

Standard gravel system with possible water ingress and forced convection due to wind

GLAPOR RDS-system where the gravel is protected from water ingress and forced convection due to wind.

Forced and free convection in permeable thermal insulation can be very important and is most of the time neglected. But in this case, GLAPOR improved the situation proactive. The evaporation of humidity in insulation or the heat pipe effect will be discussed in a following blog.

In a previous post, I already have shown the following graph. I got interested why the evacuation of mineral wool has a thermal conductivity decrease larger than the one of the gas.

The author of the graph has sent me two papers and a presentation.

• A presentation showing that the larger decrease of the thermal conductivity is not only the gas thermal conduction but also a coupling term.
• A paper about the evacuation of different thermal insulations.
• Another paper about the equipment to measure thermal conductivity under vacuum in a large temperature range.

It is already mentioned that the decrease can be 6 times the thermal conductivity of the gas in case a bed of glass spheres is used. This is indeed already reported in another paper where the thermal conductivity under vacuum is measured with a hot wire method.

In the last paper, convection is mentioned but a clear explanation of the large decrease is not given.

Further, in a nice master thesis of Matthias Demharter, Technical University Munchen, the evacuation of expanded perlite is described. Also in this work, the decrease is described as the sum of gas conduction and a coupling term, which is given in the following graph.

In my opinion, the large decrease is simply free convection. I have the following gedanken experiment:

• Assume the measuring system is filled with only air and that we are in a space ship around earth without any gravity and so without any free convection. We will measure 0.026 W/mK.
• We add mineral wool or expanded perlite. In that case, the solid replaces some air and contributes to an extra thermal conductivity path. The total thermal conductivity must be smaller than the sum of the one of air and the one of the solid in an evacuated state. However, in the case of mineral wool, expanded perlite or glass spheres, the sum is still smaller than the measured one on earth under gravity.
• As a consequence, free convection is present and in the case of the highly permeable bed of glass spheres, the effect is very large.

Although free convection is standard in the world of porous media, the concept is not popular in the world of mineral thermal insulation. And indeed, when the material contains free convection, it is also sensitive to forced convection like on attic in a windy environment. The only way to eliminate the free convection is to decrease the permeability by increasing the density. But in a lot of cases, it makes more sense to work with cellular glass at a lower density and closed cell structure like GLAPOR.

I guess that the above title was never used because convection (forced  and natural) in mineral wool is not clearly communicated. For example, on the Rockwool website, we read

ROCKWOOL stone wool achieves its insulating properties by ‘capturing’ the air between the fibers, so that virtually no convection takes place. Because ROCKWOOL insulation only contains natural air and no other gases such as blowing agents, the thermal performance does not change due to gases diffusing from the products – not even if longer periods like the total lifetime of a building are considered.

Virtually no convection is rather vague while we read on the Paroc website :

Heated air becomes less dense and rises and cooler air is drawn in to fill the space left by the displaced heated air. Natural convection might occur, for example, in a very low-density mineral wool insulation layer during extremely cold winter days.

Paroc writes clearly something different than Rockwool, while they are producing the same product. “Virtually not” and “during extremely cold winter days” are for me two different things.

Like already discussed, Vacuum Insulation Panels (VIP) can be produced from low density mineral wool and it is observed that the thermal conductivity decreases much more due to the vacuum than expected if we assume that the air is still (no natural convection).

The circumstances, where natural convection will be present and may even double the heat transfer as expected from the “laboratory” thermal conductivity, are more or less defined by the modified Rayleigh number. Modified means that the permeability of the insulation for air is included. This permeability is a very important parameter in this case and it is rather well known for  Rockwool thermal insulation. Like can be observed on the graph, it is not linearly related to the density. We developed a spreadsheet convection where the modified Rayleigh number is calculated and where the reader can calculate how much the heat transfer is increased compared to the assumed “virtually no convection” case. The spreadsheet is based on a paper of Paula Wahlgren of Chalmers University in Sweden. We use a measured permeability at 30 kg/m³ density and extrapolate with an air Flow Resistance graph. We also included the U-value of the thermal insulation. The calculation shows that:

• A passive roof in Denmark (U=0.06) with 30 kg/m³ mineral wool has the onset of natural convection at a temperature gradient of 40°C (outside = -20°C). Increasing the density (and so the cost) is the only way to obtain the U=0.06 with mineral wool.
• Industrial insulation of 80 kg/m³ is typcially used with a temperature gradient of 600°C and 35cm thickness in the glass industry in the assumption “virtually no convection). In reality, the heat transfer due to convection is an extra 30%. To reach the safe side, we need to increase the density to 140 kg/m³, which is almost a doubling of the cost.

In both cases, it is clear that we have to increase the density of the mineral wool up to the ones used in the cellular glass world.

Another paper, written by M. Serkitjis, Chalmers University  in Sweden, demonstrates the effect of natural AND forced convection on low density mineral wool. It is shown that a draft above mineral wool lowers the thermal resistance of low density mineral wool.  “If wind velocity above the insulation is as much as 1.5 m/s, which is reasonable in an attic space, heat losses can increase by 10 – 30% depending on the permeability of the material.”

Low density mineral wool may be a cheap non-combustible thermal insulation but can only be used in moderate circumstances. For passive housing with cold winters or under windy circumstances, an important extra thermal loss may show up. Higher densities are the only solution but in that case, cellular glass is a very interesting alternative. Cellular glass is an non-combustible convection free thermal insulation. GLAPOR cellular glass has comparable prices with mineral wool at equivalent prices. This is quite logic while GLAPOR cellular glass is foamed at 800°C and mineral wool involves a melting up to 1600°C.

We received some samples from Pinosklo, Ukraine, a company we already discussed on this site. The picture shows uncovered foamed glass, bitumen coated and mineral coated cellular glass.

We also measured the thermal conductivity with the hot wire method and found 0.052 W/mK at 20°C with this two-dimensional method. It is claimed that this cellular glass is foamed with technical carbon. Therefore, we did a comparative test  with Pinosklo and GLAPOR cellular glass at 500°C in air. The Pinosklo cellular glass became grey white while the GLAPOR cellular glass, foamed with glycerin remained black.

Cellular glass people, from the east or the west have really something in common.

This ecology document, writen by a neutral party, the IPPC (Integrated Pollution Prevention and Control) gives a lot of insight in the production of all kind of glass (but not cellular glass) and mineral wool.

It allows for example to estimate the extra production cost when cellular glass is made from a special composition instead of directly from waste glass. It gives also insight in the production of mineral wool.

This is simply another contribution of this website to be a  documention system for glass related subjects.

BELGLAS is also involved in the annealing and cooling of (cellular) glass and in that perpective, heat transfer by natural and forced convection is an important engineering issue. Recently, I found out that low density mineral wool can have a serious increase in thermal heat transfer through due to natural (on an attic) and forced convection (draft on an attic, wind on the facade).

As a very good introduction, the following pdf-book can be downloaded and consulted. It handles all these topics from the beginning.

Zhejiang Zhenshen Insulation Technology Corporation or ZES should be today the largest producer of high quality cellular glass in China. The company is driven by Mr. Chunhua Zhang, a real example of the Chinese dream. He started his career as a industrial thermal insulation worker, became contractor and immediately understood the benefits of cellular glass. He developed a process for cellular glass based on waste glass and upgraded to high  quality cellular glass with a special composition.

                  Chunhua Zhang, CEO of ZES in an unusual relaxed state

Today, ZES has a turnover of 700 million RMB or about 100 million euro, which is a very impressive realization. They are not only producing but also installing the thermal insulation. In that way, they can be very competitive. I would be pleased if I could write once Chunhua’s biography.

Chinese companies have the reputation to only copy but this is not true for ZES. They developed a nice patented  rendering system on their cellular glass to have a cellular glass concept for the building market. It looks that they are able to even render sculptured cellular glass.

Walter Frank (GLAPOR), Chunhua Zhang (ZES)  and  Andrei Zinovyev (STESS) are men with vision and a drive to ignite the cellular glass world for once and for ever.