Quantifying Thermal Performance in Bedding: How Effusivity Improves Your Sleep

Written by Arya Hakimian, Laboratory Scientist (BSc, MSc)

Effusivity and Sleep

There are multiple aspects related to sleep quality, and the thermal performance of bedding is one of them. This topic has been the subject of studies for decades, using different approaches. A study from the 1980s, for example, revealed that a blanket made from wool confers a higher degree of thermal insulation and possesses better moisture-transport properties under stationary conditions, combined with better moisture absorbance and buffering capacity under transient conditions, in comparison to a blanket made from an acrylic-fiber–cotton blend. In this particular study, participants subjectively preferred the woolen blanket for its thermophysiological comfort [1]. Just like the products, the technologies related to testing these products have also evolved, allowing for a much better scientific perspective, especially for Research & Development.

While we rest, our bodies release thermal energy that is absorbed by our surroundings, which can help maintain a comfortable temperature – not too hot nor too cold. If we get too hot, this can have negative side effects on sleep quality. According to [2], in situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Several bedding brands explore concepts such as “cool-to-the-touch” feeling, as well as other “cooling” features, to promote their products within this context. This allows for balancing people’s sensations and the bedding thermal insulation capabilities, which is also a variable to solve Fanger’s Comfort Equation, the combined quantitative combination of the environmental and individual variables to determine an individual’s thermal comfort [3, 4].

An analysis published in 2007 explains that it is possible to determine a thermoneutral zone, which is defined as the range of optimum temperatures in which the human body feels thermally comfortable [5]. This thermal comfort zone is directly influenced by clothing and bedding, and the way these elements interface with the environment. When designing mattresses, bed coverings, and blankets, the selected materials’ ability to transfer and exchange that thermal input is crucial to the product’s overall performance. This key attribute is directly correlated to the material’s thermal effusivity.

Thermal effusivity is an intrinsic material property that describes the ability of two materials in contact to exchange thermal energy, it is the rate at which a material can absorb heat. It is what many industries use to describe a material’s warm/cool feel, such as the cool-to-the-touch feature presented in many bedding-related products. You can learn more about thermal effusivity here.

Measuring thermal effusivity

Quantifying the thermal effusivity of bedding materials can represent a competitive advantage considering how materials are selected depending on the expected results and sensations expected for a particular product.

Figure 1. C-Therm’s Modified Transient Plane Source (MTPS) with Guard-Ring™ technology

The thermal effusivity measurement process in this context is easily accomplished using C-Therm’s Modified Transient Plane Source (MTPS) with Guard-Ring™ technology. Not only is this the sole method that conforms to ASTM D7984 but being a single-sided measurement device, it easily allows for representative testing conditions under varying force loads, temperature, and humidity conditions. This supports the development of bedding components that are supposed to have cooling/warming properties that might be better suited for different seasons/climates/geographies (think flannel sheets for winter and cool summer sheets). Let’s take a look at some examples.

Weighted blankets were originally used as a therapeutic tool for individuals in the autism spectrum and/or people who suffer from different mental health conditions. These products have been popularized in recent years as a sleep aid and to help reduce anxiety. It is important to mention that, to date, scientific data on efficacy is inconclusive. Many companies offer add-ons to these blankets, such as “cool feel” covers. We decided to test the thermal effusivity of both the cover and the weighted blanket, which is composed of different layers.

Figure 2. Weighted blanket (a) and “cool feel” cover (b)

The “cool feel” advertised by the cover manufacturer could be seen in the data gathered during testing: the cover presented approximately 56% higher thermal effusivity in comparison to the weighted blanket (Figure 3), which can be essentially translated as a much “cooler” initial feel upon contact. The same idea could be applied to sheets and pillow covers, for example.

Figure 3. Thermal Effusivity comparison (Cover vs Blanket)

Another interesting application is sleepwear. People might prefer a warm feel for Winter sleepwear and a cool-to-the-touch sensation for Summer sleepwear, for instance. The materials chosen during the development of this type of product have a significant impact on this characteristic. Recent data collected for a perception study underway at the University of Oregon (Figure 4) showed that Thermal Fleece has a much lower thermal effusivity in comparison to 100% cotton fabric. Interestingly, the fleece/cotton blend exhibited a thermal effusivity closer to the pure fleece than the 100% cotton. Additionally, higher levels of thermal effusivity could be achieved by combining, for example, Spandex and Cotton, increasing the cool-to-the-touch feel and providing additional mechanical benefits to the fabric structure.

Figure 4. Thermal Effusivity comparison for different sleepwear fabrics [Oregon University]

In all these cases, the effusivity can be leveraged, for example, to promote products in a particular context, such as seasonal approaches like Summer versus Winter products/features. Besides that, different materials can be tested for thermal effusivity during R&D processes in order to determine the best options to move forward with a product’s design and development depending on the desired “touch feel”.

Using C-Therm’s MTPS technology it is also possible to test multilayered systems and different compression levels, simulating different situations and contact duration, for example. While operating outside the standard ASTM D7984 requirements, this type of study can help to understand the relationship between multiple materials and the thermal effusivity of the system over time, simulating a “prolonged touch time”. For this test, we analyzed a fabric and a foam thermal effusivity when isolated and then combined in a multilayered structure (Figure 5).

Figure 5. Fabric and foam multilayered structure

As shown in Figure 6 (a), the fabric had a much higher thermal effusivity, giving it a more “cool-to-the-touch” characteristic in comparison to the foam. However, as presented in the Figure 6 (b), the multilayered structure has an initial thermal effusivity below that of the pure fabric due to the effective probing depth of the measurement. Over time, with increasing probing depth the influence of the foam begins to dominate the measurement, indicative of the warmer feeling material underneath.

Figure 6. Fabric and foam: thermal effusivity comparison – isolated (a); multilayered(b)

Whether developing the next generation of mattress technology or simply looking to better quantify the performance of materials in general, the MTPS technology is a great fit for the characterization of a range of common bedding materials. Thermal conductivity testing using C-Therm’s MTPS technology is a way to improve and leverage a product’s thermal features with scientific and accurate data. This topic was further explored in one of our webinars, which can be accessed here.

Works Referenced