G. Schulz, Sanders Bros Inc. & Co., Bramsche
S. Ripperger, Technical University Kaiserslautern, Kaiserslautern

About a quarter of all Germans doesn’t sleep well. This leads to a lack of efficiency, frequent illness, exhaustion and listlessness. The bed furnishings, which include the mattress, duvet and pillows, are a crucial factor that can contribute to good sleep. The duvet plays an important role, since it influences moisture and heat regulation and determines the microclimate in the bed space. With the „ClimaBalance“ duvet, a duvet has successfully been combined with a hitherto unknown functionality for the first time. In this article the innovative „ClimaBalance“ climate duvet is introduced and described on the basis of computational results for heat and moisture dissipation and the benefits involved with this.

This climate duvet consists of stuffed heat isolating zones with intermittent so called climate zones. Heat transfer is not significantly changed through this construction, while moisture dissipation is increased. Moisture is wicked away as water vapour through diffusion and through the convection of the air in the climate zones. The microclimate in the bed space is thereby noticeably improved. In this way it possible, for the first time, to wick away from the bed space all the water given off by the body during the night.

Principles

A bed provides comfort during sleep and thereby supports its restorative effect. It is essential for comfort that the temperature and air humidity do not fall below or exceed certain values. In the process it must be considered that the human body also generates heat and moisture due to metabolism during sleep. Empirical established figures indicate that heat generation amounts to approximately one W/kg of body mass and that 200 to 500 g of condensation is discharged from the body over a sleeping period of eight hours. During a sleeping period of eight hours, that corresponds to a temporal average of 25 to 62.5 g/h. The bed environment, particularly the mattress and duvet, should support the body in keeping its temperature constant. The stabilisation of body temperature is only possible when heat generation is in balance with heat dissipation. For the dissipation of generated heat under variable climate conditions, the body uses the mechanisms of

• thermal conduction and convection,
• thermal radiation and
• heat dissipation through water evaporation.

Parallel to these mechanisms, the body’s heat emission to the environment is influenced by a series of environmental factors, whose entirety is summarised by the term climate. In bed, the microclimate which appears there is crucial. Certain parameters for this climate are air temperature, air humidity, movement of the air and the surface temperature of objects surrounding the body. Because human skin is never dry, a portion of the heat is emitted through the evaporation of water (evaporating heat dissipation). Perspiration sets in at an ambient temperature of 30 ˚C, whereby this amount is substantially increased.

In the range of 28 to30 ˚C (relative air humidity 50 %, windless), the body’s heat generation is minimal for an unclothed, resting adult. Below 28 ˚C, heat generation steadily increases with decreasing ambient temperature. This is necessary to offset the increased heat generation that is essential to the stabilisation of the core temperature. Above 30 ˚C, heat generation also increases as a result of the activation of perspiration (picture 1).

At an ambient temperature of 36 ˚C, this heat is almost completely dissipated to the immediate environment through evaporating heat dissipation. Connected to this is the dissipation of an amount of condensation of ca. 115 g/h. At a temperature of 30 ˚C, the evaporating amount at rest is reduced to ca. 27 % of the total generated heat. At a temperature of 20 ˚C, water evaporation (without perspiration) is reduced to a value of 22 g/h. These values illustrate how the body can adjust to various conditions. The cover surrounding the body should exhibit a similar degree of flexibility to achieve optimum temperature regulation and the related dissipation of moisture.

The achievement of a comfortable microclimate in bed is essential to restorative sleep. For this purpose, the temperature should remain in the range of 32 to36 ˚C. The microclimate in the bed is influenced by the heat and moisture dissipation of the human body on the one hand and the wicking away of heat and moisture by the duvet, mattress and open channels to the surroundings as well as the indoor climate of these surroundings on the other. The interaction of these processes determines the temperature and the air humidity in the bed and the body’s heat dissipation in turn as a result. One can assume a body heat generation of ca. 80 W during the sleep of an average grown person.

The problem with previous duvets lie therein that the wicking away of water vapour is severely hampered by heat insulation. This results in condensation not being wicked away to an adequate extent at an increase in temperature, whereby the balance between the body’s heat generation and heat dissipation cannot be struck. In special cases, e.g. with invalids who are limited in their movement and with small children, even dangerous hypothermia can occur. For healthy people, the feeling of comfort is thereby compromised and restless sleep is caused. The associated movements cause a pump action in the bed, whereby the dissipation of moist air is promoted. Generally, however, a large part of the condensation given off accumulates in the mattress, bed linen and in the duvet in a conventional bed setup. This is also indicated by the results of trials that were conducted with an artificial perspiring body in simulation of a body [3]. In a conventional bed setup, great importance is therefore assigned to the ventilation of a bed after sleeping.

The previously described circumstances apply to average, healthy adult persons. Extreme conditions occur with people who tend to sweat excessively and with invalids. The bed plays a major role as sickbed. In this case, the desirable or undesirable deviations from the normal climate related to the respective illness must be considered. A fever is thus an accompanying symptom of many infectious diseases. Thus, the desired value of body temperature sets itself at a higher value. During a fever, the normal body temperature value of 37 ˚C is already experienced as hypothermia. At the remission of a fever and a conversion of the target body temperature, the body is too warm, whereby it reacts with bouts of perspiration to adjust the normal temperature again. In a case like this, the comfort in a bed with normal duvets cannot be guaranteed.

Climate duvet

The Dr. Schulz climate duvet combines the advantages of a down duvet with those of a light duvet. Through the combination of the down-filled areas and ventilation zones (climate zones), the joint heat and moisture dissipation is optimised in such a way that the moisture dissipation increases stronger with increasing temperature than is the case with conventional bedcovers. The ventilation zones are multiple rectangular or streaky openings in the bedcover that are covered with an exposed fabric (picture 2).

The flow resistance of this fabric should be adapted to the respective construction of the duvet. Depending on the thickness of the duvet and the resistance of the fabric, a convective airflow occurs because of the temperature difference between the bed space and the environment (picture 3). This mechanism of airflow corresponds to the chimney effect that is used in many areas of technology. Because of this airflow, a state of comfort in bed can be achieved despite a rise in temperature and the resulting increase in perspiration.

Heat and moisture dissipation in duvets with and without climate zones

- Purpose of the modelling
A calculation model and corresponding computing programme was developed for the construction and optimisation of the climate duvet. With these, the differences between a conventional duvet and the climate duvet in terms of heat and moisture dissipation could be calculated on the basis of experiences and measurement results to date, and the specific properties and advantages of the climate duvet could be investigated.

- Principles of the modelling
The physical laws of heat and matter transfer and fluid mechanics form the principles of the modelling. The heat dissipation from the bed space with a duvet with climate zones consists of the following components:

• heat conduction through the climate zones and the filled areas of the duvet,
• heat dissipation because of the diffusive wicking of water vapour through the climate zones and the filled areas of the bedcover,
• heat dissipation because of the convective wicking away of air and water vapour through the climate zones.

Fixed conditions are assumed in the calculation. The convective ratio behaves according to the density difference between the warm air on the inside of the bed and that of the ambient conditions. It is essentially determined through the resistance of the covering fabric in the climate zone sections. The surface ratio occupied by the climate zones is also decisive for the individual ratio. In a conventional bedcover, the ratio of the climate zones and the convective rate are cancelled. Moisture dissipation is linked to diffusive and convective heat dissipation.

- Entry parameters and assumptions
In the calculations, it was taken into account that, for example, in a cover with outline dimensions of 2 x 1.35 m, only an area of 1.8 x 1 m is in contact with the inner climate in the bed and therefore relevant to the heat and matter transfer and has to be considered. This area is described as the active cover surface area. The climate zones are arranged in this area. The thickness of the climate zones and the thickness of the areas that are filled with down can be varied (picture 3). In water vapour diffusion, the surface porosity of the fabric is taken into account. In the calculation, values for describing the climate inside and outside of the bed (temperature, relative humidity) are specified. Furthermore, the necessary material values (heat conductivity of the air, diffusion coefficient of water vapour in the air, heat of vapourisation of water, etc.) were taken into account. Because the temperature range of interest which lies between 15 and 35 ˚C is relatively small, many of the material values could be assumed as constant and included with an average value. The dependence of the vapour pressure of the water on the temperature is nevertheless taken into account.

The heat transfer resistance of the down-filled areas was determined by measured data [3]. For the diffusion of water vapour through this area, a porosity of 50 % and a diffusion distance amplified by a factor of 1, 5 was assumed as opposed to the actual thickness. As will be shown subsequently, these values of the experimental results obtained in [3] could be recalculated very well.

The considered flow resistance of the covering fabric for the segments is based on measurement results. The exact information of this value is important, because it significantly influences the airflow through the climate zones. The additional influence of a cover was not considered in the calculations.

Results

- Recalculation of trial results
Measurement results for the heat and moisture transfer of conventional bedcovers that were obtained with an artificial perspiring torso were available [3]. The climate inside and outside of the bed was recorded. Thereby values were available by means of which could be monitored if the calculation estimates and the assumptions that were made mirror reality. Table 1 shows the comparison of the measured and computational results. The values in column 2 were determined according to the measured course of temperature and moisture progress and were based on the evaluation of a stationary condition. The measured water vapour transfer emerged as an average value from the evaporated condensation over the trial period of eight hours. A duvet thickness of 25 mm formed the basis for the calculation.

The comparison between the measured and computational values showed that the measured values were calculated with the programme with good accuracy. Furthermore, the calculation showed that vapour condensation during transfer to the outside cannot occur under „winter climate“ conditions. This was confirmed by the experimentally determined very low degree of water absorption in the duvet during the trial. In the chosen conditions of a „summer climate“, the measured and computational values showed that condensation of the vapour occurs during the transfer to the outside. Considerable water absorption by the cover occurred here.

- Computational values for an optimised climate duvet
The computed climate duvets present a certain optimum under the prevalent climatic circumstances in bedrooms. In the down areas they are 60 mm thick and contain 16 climate zones with an edge length of 150 mm in the area of the „active cover surface area“. The climate zones are covered with an open mesh fabric whose flow resistance is optimal for this duvet construction. The climate zone section boasts a thickness of 25 mm. Initially, heat and moisture transfer is calculated for the following climate conditions:
Outside the bed (winter climate):
Temperature 15 ˚C, rel. air humidity 50 %
Inside the bed: Case A
Temperature 34 ˚C, rel. air humidity 50 %
Picture 4 illustrates the computational results for water vapour transfer in case A for a duvet without and a duvet with climate zones. One can see that the moisture given off by the body is by far not transferred by a duvet without climate zones. With climate zones, the moisture transfer is increased by a factor of 3.5.

In duvets without climate zones, water vapour is only transferred by diffusion, with climate zones by diffusion and convection. In case of diffusion, a distinction must be made between sections with down and the climate zone sections. The relatively open diffusion sections of the climate zones and their substantially smaller thickness promote diffusion so that the climate zones also promote diffusive water vapour transfer. With airflow it must be considered that air escapes through the climate zones because of the resulting pressure difference, with a moisture load as it is prevalent on the inside of the bed. It is assumed that an equal amount of air streams into the inside of the bed through openings in the covers. For moisture transfer out of the bed, the difference of the water vapour content of the air between the inside and outside is decisive.

Picture 5 illustrates the heat transfer in the described case. This is increased by the climate zones. It amounts to 71 W in a duvet with climate zones and thereby falls a bit under the 80 W of heat generated by the body. One can assume that the heat transfer via the duvet dominates, whereas the mattress presents considerably better isolation. Thus a balance is achieved faster between the emitted heat and transferred heat in a bed with climate zones. If the transferred heat is considerably less than the emitted heat, the temperature in bed rises above the assumed value of 34 ˚C. This is certainly the case with conventional duvets, according to the computational values. A heat build-up occurs which causes the temperature in the bed to continue to rise over a long period of time, until a state of equilibrium is reached. Experimental results [4] also show this correlation. In the case of such a heat build-up, the temperature in bed can rise to such a degree that the body attempts to counteract the temperature rise through perspiration. Another possible reaction is „restless sleep“, during which movement and thereby caused pump action increase the air exchange in the bed and boost moisture and heat transfer. These reactions are not required to transfer the heat in case of the values of the duvet with climate zones.

If one considers the ratios of heat dissipation in picture 5, one recognises that the heat generated by the body is transferred through heat conduction and through diffusive and convective water vapour transfer (evaporating ratios). Because the diffusive and evaporating ratio is increased by the climate zones, the heat dissipation through the duvet better adapts to the body’s heat dissipation mechanisms at a rise in temperature. As shown in picture 1, for the temperature range in bed, the body’s heat dissipation at a rise in temperature is almost exclusively raised by an increase in the evaporating ratio. It is therefore beneficial if the system of bed and duvet reacts in the same way, that is: heat is dissipated by an enhanced moisture transfer in case of rising temperature.

Next, case B is considered, where the outside climate matches that in case A, though the inside climate falls below the comfort threshold. The construction of duvets is similar to the ones in case A.
Outside the bed (winter climate):
Temperature 15 ˚C, rel. air humidity 50 %
Inside the bed: Case B
Temperature 36 ˚C, rel. air humidity 60 %
Because of the microclimate in bed, the body will attempt to change the temperature proportions through a high degree of perspiration. When looking at the moisture transfer for this case (picture 6), one can see that the perspiration given off by the body is not dissipated much by a conventional duvet. It is mainly accumulated in the bed, so that areas become damp and the microclimate in the bed deteriorates further. On the other hand, the majority of the perspiration is dissipated by the climate duvet.

When looking at the heat dissipation (picture 7), one can distinguish that cooling is possible with the climate duvet, because the total dissipated heat flow is a little greater at 94 Watt than the heat generated by the body. Perspiration should abate at the cooling of the microclimate in the bed. Furthermore, the heat dissipation out of the bed decreases again (case B then changes into case A), so that an equilibrium in a comfortable microclimate is once again reached.

According to the computational data, the temperature will continue to rise with a conventional duvet in spite of a high temperature on the inside of the bed. It can be reckoned that only 46 Watt is dissipated with a conventional duvet, even at an inside temperature of 37 ˚C and a relative humidity of 90 %. Thus no equilibrium can be reached at rest between the heat generation of the body and the heat dissipation from the bed with a conventional duvet.

With a climate duvet, on the other hand, a heat flow of 80 Watt is already dissipated at a temperature of 35.5 ˚C and a relative humidity of 50 %. The moisture dissipation amounts to 49 g/h in the process. It is to be expected that the body also generates considerably less perspiration in this climate. Furthermore, with a climate duvet it is insured that the heat and moisture dissipation increase sharply at a rise in temperature and an increase in the relative humidity in the bed. The mechanisms of moisture and heat dissipation in a climate duvet therefore help to create a comfortable microclimate in the bed once again. Through a skilful arrangement of the climate zones in the duvet, it is also possible for the heat dissipation to adjust to the body’s heat generation areas. Chilly feet in particular can be avoided as a result.

- Extreme performance of the climate duvet

The facts about the following duvet correspond, when no differences are otherwise indicated, to those in the previous section. Values for the climate conditions of case A are calculated. Then the heat transfer through a climate duvet with a duvet and segment thickness of 70 mm is calculated (climate duvet A). The results are illustrated in picture 8. The high ratio of convective heat dissipation is revealed. This increases with the thickness of the segments. Thereby a high degree of heat dissipation can also be achieved through thick covers. The discharged heat flow of a conventional duvet with the same thickness under these conditions amounts to only 24 Watt.

The heat flow of a climate duvet can be influenced by the flow resistance of the segment covering. Picture 8 shows that the flow resistance of the segment cover in climate duvet B is twice as high as in climate duvet A. It can be seen that the convective heat dissipation is thereby considerably reduced. The computational values illustrate the influence of the cover thickness and the resistance of the segment covering. Theoretically, it is possible, with a subtle choice of these sizes, to also develop a thick down cover for a summer climate with climate zones that will do justice to the claims regarding moisture and heat dissipation.

Summary

The advantages of climate zones in duvets were illustrated according to computational values. Through these zones, moisture and heat transfer is brought about through convective airflow in addition to the mechanisms of diffusion and heat conduction. Through an appropriate choice of covering fabric in the climate zone sections, a duvet was developed that exhibits improved wicking away of moisture generated by the human body from the bed. Less moisture is thereby accumulated in the bed and a dry sleeping climate in the bed space is guaranteed. The clumping of down is thereby also prevented. Climate zones enable an improved assimilation of moisture and heat dissipation to the conditions of the human body and open the possibilities to adapt the features of duvets to the requirements at hand in a given situation and certain climate conditions better than ever before.

References
[1] G. Thews, E. Mutschler, P. Vaupel: Anatomie, Physiologie, Pathophysiologie des Menschen; edition 5, Wissenschaftliche Verlagsgesellschaft (1999)

[2] S. Silbemagel, A. Despopoulos: dtv-Atlas der Physiologie (1979), p. 179

[3] Eidgenössiche Materialprüfungs- und Forschungsanstalt (EMPA), St. Gallen, test report nr. 417698 of 2 May 2001, test leader: W. Weder

[4] Dalien Cable; Memo TNO-TM 2001-MOXX of 11 November 2002; The analysis of the thermal characteristics of 3 different bed duvets with and without their covers.

Picture 1: Heat generation and emission of an unclothed person at rest at various temperatures (according to [2])

 Total heat emission in Watt

Through evaporation
Through convection

Through radiation
Room temperature

Picture 2: Climate duvet with 16 square climate zones

Picture 3: Depiction of the climate zones and their function regarding moisture dissipation

Picture 4: Moisture dissipation for case A

Picture 5: Heat dissipation for case A

Picture 6: Moisture dissipation for case B

Picture 7: Heat dissipation for case B

Picture 8: Heat transfer through a climate duvet with a thickness of 7 cm

• Climate duvet A
• Climate duvet B

Heat dissipation through convection
Heat dissipation through diffusion
Heat conduction

Table 1: Comparison of computational and measured results