In a benchmarking of thermal insulation materials, it is easily demonstrated that the perfect thermal insulation does not exist:

• PIR, PUR, XPS and EPS are polymers and in that way combustible.
• Mineral wool cannot carry a high compressive load, not even at high density.
• Cellular glass has not a very low thermal conductivity, which can be compensated by thickness.
• Cellular glass, directly foamed from waste glass has the best ecologic balance and the polymer foams are at the other side.

One approach is by lobbying increasing the weight of the weak point of the competition in order to hide the own weak point. This was the very costly method of the past paid by all the customers for thermal insulation. Much more constructive is a combination with another thermal insulation to eliminate the own weak point.

In some cases, a passive flat roof is requested without any risk of fire transport during reparation of the roof. A large thickness of mineral wool is a possibility but rather high densities are needed. On top of that, free natural convection is not excluded with low temperatures outside. Cellular glass may have an even larger thickness to compensate the larger thermal conductivity. Polymer foams are out of the question due to their built-in combustibility.

In Denmark, Kingspan decided to collaborate and has choosen GLAPOR cellular glass as the top layer. Indeed, Kingspan PIR thermal insulation is giving the largest thermal resistance while GLAPOR cellular glass takes care about fire resistance and also delivers a hard protective surface under the water proofing membrane. The system assumes  a high quality vapour screen on the concrete to avoid humidity problems in the PIR. 

The combination guarantees the requested U-value, fire resistance and walkability of the flat roof. The thickness and weight of the roof remains below an acceptable limit. The solution will have a large life time and a nice (but not the best) ecology balance. At the end, the solution has an interesting price because GLAPOR cellular glass, directly foamed from waste glass is used. More information can be obtained by Celleglasplader in Denmark.

Glass is used to produce wool (glass wool) and foam (cellular glass). Mineral wool can also be produced from basalt. The following picture shows both with a large magnification.

wool                                                                           foam

The principle of both thermal insulations is to hold a (still) gas and to block radiation. In the left one, the solid fibers have only point contact and are contributing only slightly to the thermal conductivity. The thermal gas conduction is the main contributor in the heat transport. In the right one, the foam holds the gas and in case of closed cells, we can choose a low conductivity gas like CO2. But the solid structure contributes much more to the thermal conductivity. It is clear that the thermal conductivity may be slightly lower for the wool structure, even under vacuum (VIP). But we all know that a higher thermal conductivity can be compensated by a larger thickness.

The fibers are kept together with a binder but nevertheless, both mineral thermal insulations can be considered as non-combustible.

The fiber structure allows the transport of gas, liquid and vapours and even the heat pipe effect is possible. The closed cellular structure does not allow transport of liquids, gas or vapour. That is clearly a bonus for the cellular structure. Secondary heat transport effects are excluded for the closed cellular structure.

But it must be clear without any calculation that the cellular structure should have a much better mechanical stability for the same density. Point contacts are highly mechanically unstable, even with a binder. For example, Rockwool CRS has a density of 150 kg/m³ and a compressive stress of 50 kPa at 10% creep. A cellular structure like GLAPOR PG900.2 can sustain 300 kPa ( 6 times !!!!) forever with a negligible creep at the same density. Nevertheless, fiber structures are much more sold for flat roofs than cellular structures, although the roof is loaded in tension by wind and in compression by walking maintenance people.

The logic explanation should be the old story that cellular glass is much more expensive but that is even hard to understand. Mineral fibers are formed after heating raw materials (basalt, waste) at 1600°C and another 200°C step for the binder. The above GLAPOR PG900.2 is manufactured by foaming ground waste glass with glycerin at 800°C. After correction for the thermal conductivity (thicker layer), both should have a comparable price.

GLAPOR cellular glass has indeed today a comparable price thanks to the introduction of new technologies in the old world of cellular glass. And indeed, the thermal insulation market becomes aware of the new opportunities resulting in a booming market.

The thermal conductivity of thermal insulation should be the sum of the following transport mechanisms:

• conduction through the solid
• radiation
• conduction through the gas (collisons between molecules in a brownian transport)
• a coupling term between the solid and gas conduction

This mysterious coupling term was needed to “explain” why the thermal conductivity drops more than the thermal conductivity of the gas after evacuation to a good vacuum (10e-1torr)

The coupling term was observed in four cases:

• evacuation of glass wool
• evacuation of perlite
• evacuation of a bed of glass beads
• evacuation of Aerogel

In all these cases, the coupling term was needed to explain the decrease of thermal conductivity due to evacuation. In the case of glass beads, the decrease is about 6 times the thermal conductivity of air. A bed of glass beads can be very permeable and natural convection is not excluded as explanation. However, in the other cases, natural convection is not obvious.

The coupling term is explained as a kind of short circuit like shown in the following graph.

There should be an extra heat transport system in the gas (on top of the normal gas conduction) in the neighbourhoud of the point contact, shown by the blue arrows. Today, nobody seems to be able to explain the nature of this transport. But I know a bright physicist at the KU Leuven in Belgium and I surely will consult him about this.

According to Richard Feynman, turbulence is the most important unsolved problem of classical physics and maybe the coupling term will be the second most.

In a previous post, I was already dreaming about pontons made of cellular glass. But dreams can become  true in Aarhus, which will be the European capital of culture in 2017.

In this city, a project Cirkelø is planned. It contains a series of homes on water with a cellular glass ponton.

Denmark is a country with a profound ecologic culture, where cellular glass becomes every day more and more popular.

We found a public document of Owens Corning, which contains a comparison between PUR foam and mineral wool. I guess that this kind of comparisons are not allowed in Europe and that is a pity. We can expect that the document will not be negative about mineral wool like the following citation shows:

Compare Thermafiber Mineral Wool to Spray Foam Insulation (Closed Cell). Compare the efficiency. Compare fire protection. Compare aesthetics. Compare prices, too. Whatever your criteria, the more you compare, the more benefits you’ll find with Thermafiber.

Non-combustibility, fire resistance and condensation possibilities are the main advantages of mineral wool. The same arguments could be used for cellular glass.

A lot problems with PUR, PIR, EPS, XPS, …. can be solved by combining them in a sandwich with cellular glass. GLAPOR cellular glass is working together with large manufacturers in that perspective.

Copied from the above document

The trend to passive housing forced people to build an air tight envelope. The importance of this tightness is already shown in a previous post. But the consequence of air tightness is that we have to check the quality of the indoor air (IAQ).

The first method to improve the IAQ is to avoid emitting dangerous gases and particles. No smoking inside  is trivial but also diluting by ventilation with (clean) outside air is a must. Ventilation standards in Belgium (always inspired by Europe) give as general rule 3.6m³/h  per m² floor surface or 720m³/h for a 200m² house (10×10 with 1 stock).

Heating 720m³/h air from 0°C to 20°C takes about 4800W. This is unacceptable for passive housing and for that reason heat recovery ventilation is needed with a heat exchanger, which can have an efficiency larger than 85% in case a cellular one is used.

Passive housing means a huge thermal insulation (40cm thickness and more) and an expensive (cellular) heat exchanger for the necessary ventilation with high efficiency recuperation of the heat. The large thicknesses of the thermal insulation are in conflict with elegant architecture and increase largely the building cost besides thermal insulation.

On the other hand, ventilation through permeable thermal insulation with an under pressure (generated by a fan) is a another way. The heat, leaking away in the insulation is used to heat up the ventilation air. The exhaust of the fan has an heat exchanger to recuperate the heat, reintroduced in the envelope.

In case of a windless day, an extreme small U-value can be obtained. In case of an efficient heat exchanger, the heating power will be negligible, reaching the lowest passive values possible. On the other hand, on a windy day, a large under pressure must be generated to avoid that air is going outward through he insulation.

The last system (ventilation through permeable insulation) guarantees a very high IAQ (Indoor Air Quality) and a perfect ventilation of the structure, avoiding any problems associated with humidity. In case open celled cellular glass is used, we obtain a system with an extreme long lifetime. This system can withstand extreme low temperatures with negligible power on a windless day, but is more sensitive to wind.

Swimming pool ventilated through the thermal insulation

Open cellular glass was already discussed in this blog with recipes based on CRT glass and Manganese oxide. That seems to be a good starting point. I guess that this application, dynamic thermal insulation, will induce a lot of competition with impermeable insulation if open celled cellular glass will be available soon at prices around 150€/m³.

Dynamic thermal insulation allows passive house (dynamic) U-values with moderate thickness and standard thermal conductivities below 0.050 W/mK. It is a real proof that the race to the best thermal conductivity is useless for buildings.

The principle is already used in many buildings like demonstrated in this dynamic insulation paper of the Gaia group. These architects construct buildings with clean air as priority.

In short, a small under pressure (5Pa) is created in the space surrounded by the insulation envelope and this induces a small air flow through the permeable insulation. The air, used for ventilation is heated with the heat, leaking away through the insulation. In fact, the thermal insulation acts as large heat exchanger. Passive U-values smaller than 0.1 can be obtained with only 10 cm insulation of 0.040 W/mK. The under pressure is generated by a fan and the heat of the sucked air is stripped with a another heat exchanger in the exhaust of the fan.

Passive housing with moderate thicknesses and totally no building up of moisture (which is easily achieved in passive housing, as  demonstrated at the KU Leuven, Belgium) is possible due to the perfect ventilation.

The key is to find a material, which has the right permeability and thermal conductivity. This material should by preference be fiber free and should also allow to build a mechanically stable envelope free from gaps. Indeed, all the air should flow through the thermal insulation.  On top of that, it should have a lifetime, exceeding the one of the building.

We think that open celled cellular glass would be an option. This can be done with recycled glass and some foaming agents, which induce crystallization. GLAPOR cellular glass is able to produce 3 x 1.5m boards, which should be ideal to reduce the amount of joints as much as possible. It is also important that foaming agents without any doubt about health are used due to the intimate contact of the ventilation air with the internal foamed glass surface.

Wind is known to bring a lot of clean energy like shown in this document about wind of the European Environment Agency. Indeed, we have a lot of wind energy in Europe like shown in the following map.

But there is the less popular message that wind during winter also extracts a lot of heat from the houses due to their permeability. It is seldom realized than 1m³ air has a weight of 1kg. Heating 1m³ air takes 25% of the energy to heat 1l water, although the last one is a much larger concern for most people.

As an example, I made a calculation for a fictive passive house, build with high density mineral wool in such a structure that the mineral wool is fully exposed to the wind and inner climate. This is never the case but it shows the orders of magnitude of the wind effect.

We developed a simple permeability spreadsheet (green fields can be adapted), where we used the published  Air Flow Resistance (inverse of permeability) of mineral wool. We assume a thermal conductivity = 0.040 W/mK and used a thickness of 0.4m to reach U=0.1 for passive housing. We also assumed as default 120 kg/m³ density and a temperature gradient of 20°C. Width and length of the house are 10m, height is 5m and the wind blows on two walls.

We observe that with a wind of 5m/s (15 Pa wind pressure) that we extract 2000 W while only 800W is needed when there is no wind at all. This fictive situation shows that permeability, even at high density induces a heat loss 2.5 times the declared heat loss.

But if we had used 120 kg/m³ density GLAPOR cellular glass (0.050 W/mK), we should consume only 1000 W, even with much larger wind speeds. The price of both is comparable because GLAPOR foams recycled glass at 800°C and mineral wool involves a melting step up to 1600°C.

Mineral wool, even at high density, has to be used with a severe wind protection, while this is not needed for cellular glass. This is clearly mentioned on the Paroc website.

Wind induces draft over the thermal insulation on an attic. This was already shown in a previous post. Indeed, in “Convection in horizontal loose-fill insulation below an air layer”, we can read: “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.”  This is demonstrated in the folowing figures:

The left figure shows the draft above the thermal insulation for a certain wind velocity outside. The right figure shows the extra heat transfer through the thermal insulation due to the draft, generated by the outside wind. It is clear that severe wind protection would help a lot.

Also in this case, GLAPOR cellular glass would have done the job without extra heat loss due to the wind outside.

We have already discussed the effect of humidity on the thermal conductivity of mineral wool. We have shown the following graph, where the measured thermal conductivity is given versus the absorbed humidity. Indeed, the thermal conductivity doubles for a huge humidity absorption.

But the laboratory measures according to the standards with a guarded hot plate or a heat flux meter like already discussed. And these measurements don´t include the heat pipe effect, which can be present on a flat roof with a membrane on top of the thermal insulation. The heat pipe is a very efficient heat conductor and works as follows: a liquid is evaporated at the warm side, the vapour flows to the cold side and condenses. The condens flows back to the warm side. Because water has a very large evaporation / condensation heat value ( 2257 kJ/kg), it can be used (wanted or not wanted) to transport heat in a very efficient way. The principle is shown in the following:

We made a small calculation for a flat roof with 20cm mineral wool at 60kg/m³ density and 0.040 W/mK (declared) thermal conductivity. We assume that the mineral wool contains 1% humidity and that in winter (when we heat) this water evaporates. The vapour migrates to the (cold) water proofing membrane through the permeable insulation and condenses. We assume that this is happening daily: evaporation during the day, condensation during the cold night. In that case, the heat transfer through the thermal insulation has increased with 78%. The laboratory measurement, where the heat pipe effect is not present,  shows an increase of only 5%.

It is an open question whether in a certain mineral wool roof the heat pipe effect is present or not. But it is absolutely sure that it is not present in a GLAPOR cellular glass roof, with a comparable price as the high density mineral wool. The above shows that the academic declared value is giving only little information because nobody can avoid 1% humidity in a roof.

It is a habit to use a double wall in a sea climate. The outer wall becomes wet due to rain and the inner wall remains dry. The increase of the energy price forced people to use that cavity for insulation.

Jan Lecompte from the KU Leuven building department measured the effect of eventual natural convection in such a cavity and reported this to the society. He found that gaps around and in the thermal insulation induce a large thermal loss, up to 200% more than what should be expected from the declared thermal insulation value of the used materials.

In case permeable thermal insulation is used (glass wool or stone wool), natural convection through the thermal insulation becomes important and higher densities are needed. In case of low densities, another 150% extra heat loss becomes possible.

The solution is that the path around the impermeable thermal insulation should be 100% interrupted. This is hard to fulfill with polymer foams, which are continuously deforming due temperature and humidity. On the other hand, in case of mineral wool, rather high densities have to be used to reduce the permeability.

In a previous post, we have already shown that intrinsic natural convection in low density mineral wool is possible. Now, we learn that natural convection around the thermal insulation is also possible.

It is clear that well installed GLAPOR cellular glass will do the job perfectly and at a price comparable with high density mineral wool. It is also obvious that accurate thermal conductivity measurements in laboratories are not telling everything. The race for the best thermal conductivity at the expense of a high cost is not helping our environment.