In some publications, including various articles on the internet, emphasis is placed on the influence of the thermal storage capacity of buildings, claiming that improving the thermal insulation of external walls is pointless or even damaging. Allegedly, the effects of the thermal storage capacity of a wall and the heat gains from solar radiation are insufficiently, or not at all, taken into account by scientists.

The author of this article has already dealt with this topic systematically in 1987, under the same title. In the meantime, many new findings have become available, all of which support this publication. The full (German) version of the summary given here can be ordered from the following link: [Feist 2000] Feist, Wolfgang: Ist Wärmespeichern wichtiger als Wärmedämmen? (Is thermal storage more important than thermal protection?) Passivhaus Institut, Darmstadt 2000

The latest research proves ¹⁾ beyond any scientific doubt that

• The thermal protection of the external envelope (U-value) and the air exchange are mainly responsible for the heating energy consumption of a house in Central Europe.

• Irradiation on external wall surfaces in the average heating period is usually an insignificant effect with very small energy gains, which is reduced even more by the heat radiation into the cold sky. However, the passive use of solar energy can be considerably increased through measures such as a selective coating or a transparent (translucent) insulation.

• After all, the influence of the thermal storage capability of the external walls is extremely small (less than 0.5 %).

• The heat capacity of the interior building components facing towards the interior has a perceptible influence on the temperature stability and thus on summer comfort – the interior walls and intermediate ceilings are important.
  

Evidence of these facts is provided and explained in detail in the full version. The main conclusions are asfollows:

• For external building components, it is the insulation that is effective against heat losses. Whether internal or external - insulation is always efficient. However, the prevention of constructive thermal bridges and airtightness are essential for the effective functioning of the insulation.

• The thermal storage capacity of the external building components is insignificant.

• The absorptivity of the external surface for solar energy and the emissivity of the surfaces for long-wave heat emission is important only to a small extent.
  

The thermal storage capacity or specific heat capacity (this is the term used in physics) is defined as the ability of a material to take up heat quantities in a temperature gradient. We have been using this storage effect for a long time, e.g. for hot water bottles, boilers or storage heaters. Basically, heat storage does not provide additional energy – every quantity of heat taken from storage must originally have been supplied to the storage, e.g. byheating the water for a hot water bottle.

An uninsulated hot water bottle (one which is not under a well-insulating duvet) releases its heat content within a short time and becomes a “cold water bottle”. It is actually good insulation which makes thermal storage effective – this applies to a greater extent for keeping buildings warm. The period of time spanned by the storage (more than 3 months) is much longer here than it is for a hot water bottle (8 hours).

Heat storage only works in combination with thermal insulation: A poorly insulated storage device (normal coffeepot, right) quickly loses a lot of heat which has to be constantly supplied by the hot-plate. A well-insulated storage container (left, thermos flask) keeps the contents hot for many hours. The same situation applies for buildings in winter (following picture).

This thermographic image shows an uninsulated old building on the left (behind the trees) and a building that has been retrofitted with facade insulation on the right (20 cm plastered thermal insulation): → On the left (multi-colour): the uninsulated wall conducts the heat towards the outside surface which radiates heat into the surroundings. This is reflected by high surface temperatures between 6 and 7 °C. → On the right (deep blue): the thermal insulation reduces the flow of heat from the inside to the outside quite considerably. The new plaster surface has an even low temperature of less than 4 °C - barely any different from the tempera- ture of the surrounding trees - which shows that the heat loss is extremely small. The heat loss through windows is higher. And the tilted window (above left) proves that the house is heated.

Both are described by the basic equation for heat transport. This has been known in physics since 1822, when Joseph Fourier (Wikipedia page) proposed his law of Heat Conduction (Wikipedia page). This equation describes the interaction of thermal storage and thermal conduction in fixed materials.

The heat equation in general formulation describes the time variation of a temperature field T(x,y,z) in fixed matter (e.g. in a solid body).

• Differences in the temperature (gradient grad, on the right) propel a heat flux which increases proportional to the relevant component of the thermal conductivity tensor $\Lambda$. ²⁾ ($q = -\Lambda \,grad\,T$ is the heat flux).

• The negative divergence of the heat flow is the change of the heat content in the infinitesimal volume element.

• This is the same as the temporal change in temperature $\frac{\partial T}{\partial t}$ multiplied by the heat capacity $\rho c$(left side of equation).

This equation has proved to be consistently effective in physics and technology. Such different things like heat transfer in stars, in semi-conductor devices, brake pads and many others can be calculated in good correlation with measurements. This equation also applies in building physics – and the calculations made using it correspond just as well with building physical measurements as shown in the following example.

Today it is possible to apply this differential equation with the help of mathematical software for various wall structures for example, and thereby obtain an exact representation of the temperature courses varying with time. Programmes like HEAT2 or HEAT3 can even do this for two or three dimensions. The values thus calculated correspond very well with measurements. The same is true for processes varying with time.

Also simulation programmes (e.g. “Dynbil”, “Derob”, “Transys” etc.), with which the energy flows in building components and buildings are mathematically calculated, apply the aw of heat conductance in full in each case – they take into consideration the heat storage effects as well as the heat conductance. Three important findings result with these numerical calculation methods:

• For normal building components, it turns out that to a great extent, the heat storage effect already averages out over a period of a few days (see the explanation in the next section).

• “Indirect” heat flows in the three space dimensions are even more important: These so-called thermal bridge effects can result in high additional heat losses, therefore they must be avoided meticulously if the insulation is to be effective.

• In simulations of complete buildings using Fourier's Law, the Passive House turns out to be a particularly energy-conserving solution for thermal comfort in winter as well as in summer [Feist 1993] .
  

If long periods of time are observed, the inflow and outflow of energy for the heat capacity can be averaged out from the energy balance, because the same amount of energy has to be stored up as the amount which is available at the end again, if the temperatures at the beginning and end are the same.

→ How long are “long periods of time”? This depends on the system being considered.

• For a sheet of paper, one hour is “long”,

• for a 160 mm thick concrete ceiling three days are “long”,

• however, for a several meter thick layer of earth, 6 years would be “long”.
  

Such solid buildings are unsuitable for significant storage “between the seasons”. Efforts for annual storage for solar systems show the mass required (mostly many tons of water) and the huge layers of insulation required for preventing self-discharge (500 mm and more of high quality insulation material – in this case too, storage only is only possible with insulation. A promising method would be to use the soil under the house for storage).

Stationary approximation can be used successfully for ordinary building components in building envelopes when heat losses during the heating period are being considered, because then the temperatures at the beginning and the end are about the same and the net storage balance is zero. This approximation leads to the well-known thermal transmittance coefficient or U-value (formerly k-value). Calculations using the U-value are sufficiently accurate for buildings of various types; for example, the simplified method of the Passive House Planning Package (PHPP) uses this approximation – and the results are in good agreement with measured results (see the page about energy balances with the PHPP).

How well theory and practice correlate regarding heat conductance is shown by the temperature curves of the measurements recorded during the monitoring programme for the Passive House in Kranichstein. The two graphs show the measured values (coloured symbols). The results of the calculation with the simulation model are represented by black lines. The correlation between the measurement and the theory is so good that differences can only be identified by magnification of the image (magnifying glass). Any deviations present are +/- 0.2 °C at the most.

Measured values of the temperature in the wall build-up are re- presented by coloured symbols, values calculated on the basis of the heat equation are represented by black lines (“theory”). ⇒ Theoretical and measured values conform so well with each other that deviations are not noticeable on this scale and can only be seen in the magnified image.	Enlarged diagram: The maximum deviation between the calculated values (black curves) and the measured values (coloured symbols) is 0.2 °C. This deviation is within the measuring accuracy range.3)

The wall build-up and the position of the highly accurate measuring points of the Pt100 sensors is documented in this note ⁴⁾. The insulation layer was 275 mm thick. Based on these results, many other characteristics of the insulated wall become apparent – a more detailed discussion of these can be found in [Feist 1987] , and a discussion on this page : Thermal protection works.

What is meant by the (effective) interior heat capacity? That is the total heat capacity associated with the room through the interior surfaces of all the building elements on the inside. It is inside the insulating envelope, like the fluid inside an insulated thermos flask. This heat capacity has a cushioning effect on temperature changes in the room, e.g. those due to solar radiation through the windows. In the main heating period this is not really important - but in the summer, when mainly the daily peaks in temperature have to be reduced and night-time cooling is possible, the interior heat capacity is advantageous. Good insulation is also helpful in the summer because it reduces the infiltration of heat into the rooms.

It is the insulation that matters and not the heat capacity. This is true not only for buildings, but also for many other situations in daily life:

• If we want to keep tea or coffee hot, we use a tea-cosy or thermos flask – the alternative to insulation is not storage but constant energy expenditure for heating (tea-light or hot-plate).

• In cold weather we put on insulating jumpers, stockings, hats etc.

• In cold bedrooms, we keep beds warm by using “warm” duvets. Of course, the duvet itself is not warm, it is just very insulating, so that the human body loses less heat.

• Farmers are warned regularly about the occurrence of ground frost. Frost always occurs on the ground first because of the heat emitted into the night sky (in spite of thermal storage and solar radiation). The farmer can protect his plants with hay (insulation!) or sheeting (translucent insulation).
  

The best evidence for the effectiveness of good insulation is the Passive House itself. In autumn the Passive House remains warm for a long time because it loses very little heat due to the excellent insulation and heat recovery. Even if heating is needed in winter,the heating power that is required is extremely small. Thousands of built examples show that this concept functions in accordance with the laws of building physics. Good insulation of buildings has proved to be extremely successful. Everyone can convince themselves of that, for example by taking part in excursions during the International Passive House Conference, or on the Passive House Day, on which residents of Passive Houses open their doors to the general public so that visitors may experience for themselves what it is like to live in a “Passive House”.

The scientific context can be checked by anyone – no authority by any Guru is required for this. Incidentally, that is the most important demand that can be placed on serious scientific work: it must be verifiable. The test must also be verifiable, boundary conditions must be documented, measurements must be carried out with due accuracy (with ordinary room thermometers it is only possible to measure the temperatures to the nearest 1 or 2 degrees). You don't have to believe in physical interrelationships – you can check them yourself.

The topic discussed here is very suitable for school projects. In secondary education and sixth form classes a fundamental understanding of physics and the differences between extensive properties (like enthalpy/internal energy) and intensive properties (temperature) as well as the First Law of Thermodynamics can be worked out. Small models (e.g. boxes made of insulation material as a container with hot water) can be easily made to allow students to independently verify the interrelationships – this relates closely to daily experience. As Albert Einstein said regarding education and school: ”Characters are not shaped by what they hear and say but through their work and actions”.

Don’t save on the insulation

Thermal protection works

[Feist 1987] Ist Wärmespeichern wichtiger als Wärmedämmen? 1. Auflage, IWU 1987; 2. Auflage, Passivhaus Institut, 2000 Link to PHI Publication
(“Is thermal storage more important than thermal insulation?”, 1st edition, IWU 1987; 2nd edition, Passive House Institute, 2000)

[Feist 1993] Passivhäuser in Mitteleuropa; Dissertation, Universität Kassel, 1993
(“Passive houses in Central Europe”; Dissertation, University of Kassel, 1993)

[AkkP 5] Energiebilanz und Temperaturverhalten; Protokollband Nr. 5 des Arbeitskreises kostengünstige Passivhäuser, 1. Auflage, Passivhaus Institut, Darmstadt 1997 Link to PHI Publication
(“Energy balance and temperature behaviour”; Protocol Volume No. 5 of the Research Group for cost-efficient Passive Houses, 1st edition, Passive House Institute, Darmstadt 1997)