Quantifying Comfort with Qmax and Clo

18 November 2021

Manasseh Franklin

Comfort is subjective. We might feel like an insulated down jacket is warmer than, say, a fleece, or that a synthetic t-shirt is cooler than a wool one. But these observations are opinions, not actual measurements, influenced by the conditions—temperatures, wind, humidity, etc.—around us.

To accurately determine how warming or cooling fabrics really are, and how they compare to other fabrics, you need something more objective. You need scientific measurements.

Cindy Lau is a scientist with a PhD in Chemical Engineering focusing on Materials Science. Part of her job as a scientist at LifeLabs Design is to quantify comfort by objectively measuring the cooling and warming ratings of fabrics. She does this with two specific ratings systems: Qmax and Clo. Here’s an explanation of both and why they are important in the clothes you wear.

Qmax––the Instant Cool Touch

Think about the last time to you touched a piece of metal. Chances are, unless the metal was in direct sunlight, it felt cool to the touch. Now consider a piece of wood. Even without direct sun, the wood likely would feel room temperature. In either instance, you probably didn’t realize that you were experiencing Qmax in action.

“Qmax measures how quickly the heat flows through a material upon contact,” Cindy explains. This is also known as instant cool touch.

A material’s Qmax measurement is heavily influenced by the nature of it, that is the form it’s in and the molecular arrangement within the polymer. For instance, LifeLab’s CoolLife clothing line relies on Polyethylene, a highly infrared-transparent polymer with an inherently high Qmax.

Plastic grocery bags also contain PE, but the Qmax of CoolLife is much higher because of the contact area created when the PE yarn is made into a fabric, as well as the molecular structure. In the grocery bag, the molecules are not as well aligned as in the PE fabric, so there is less connectivity.

PE’s Qmax quality is inherent says Cindy, so it’s different than a finishing that you put on fabric. This makes the QMax of PE especially durable.

To measure Qmax, scientists use a special machine called the Thermo Labo KES-F7. The machine has two parts: a heated flat surface and a cooler block that gets placed on top. To measure a fabric’s Qmax, you put the fabric on the flat surface, cool to 25 ° C and then place the block, which is heated to 35 ° C (approximate skin temperature) on top. The machine will measure how quickly heat moves through the fabric to the 25 ° block.

When it comes to clothing, Qmax gives a clear measurement for the rate at which a fabric can pull your infrared body heat away from the skin, which ultimately helps you stay cooler.

Silk is one of the materials that also claims to have a good Qmax, Cindy says, but the CoolLife fabrics she and the other scientists at LifeLabs are developing rate higher, .247 for CoolLife versus .194 for silk. “So we’re proud of that.”

Clo––Keeping the heat in

Clo on the other hand measures continuous warming or cooling. We can also think of it as how much heat is trapped by fabric. For instance, thin fabrics like mesh have very low Clo ratings, whereas a bulky down jacket will have a very high rating. The two release vastly different amounts of heat.

“In LifeLabs fabrics, Clo is used to measure both WarmLife —where we want to retain heat—and CoolLife—where we want to retain less heat,” says Cindy. It can be measured at the fabric level, before it’s made into anything, and once it’s been sewed into a garment, where the design and the fit play into the Clo rating.

Scientists have two ways to measure Clo: the Sweating Guarded Hot Plate (SGHP) and the garment manikin. Both work essentially the same way with the SGHP being used primarily to test in the fabric stage and the manikin for complete garments.

Both offer measurements of continuous cooling by tracking how much power it takes to maintain a temperature of 35 ° C.

The SGHP features a chamber kept steadily at 20 ° C with a light wind that mimics the airflow you would produce while walking. In the chamber is a hotplate at 35 ° C, to represent skin temperature. The fabric you’re testing goes on top of the hotplate, and if it retains more heat, the machine puts out less power to maintain that 35 ° . This allows you to calculate the thermal resistance of the fabric, a.k.a. Clo.

The manikin is also kept at 35 ° but has 20 different zones that keep track of how much power the machine produces to maintain that 35 ° body temperature. “The different zones separately monitor power output on the arm, lower arm, hands, chest, so we have a very accurate reading of what the actual number and how is it measured,” Cindy explained. The different zones also allow designers to tweak the fit, especially if they notice particular cold spots on the manikin.

When using traditional materials, Clo ratings are typically increased by adding more bulk. But because WarmLife incorporates infrared technology that actually reflects body heat back to the skin, you need less of the bulky insulation commonly used in coats and jackets.

The Clo rating is also useful for quantifying the cooling effect of CoolLife fabric. “One thing that’s very special about PE,” Cindy says, “it is only made of carbon and hydrogen. What that means is when radiation or body heat gets to it, a lot of it passes through. We are the only brand able to offer an infrared transparent fabric.”

And when it comes to both WarmLife and CoolLife, the Qmax and Clo measurements give us the ability to see how LifeLabs garments—and claims about their performance—stack up against competitors. By design, these measurements take the ambiguity out of warming and cooling. Rather than relying on feelings or subjective comparisons, we can look to verified data that creates a new frame of reference for the accuracy and efficacy of fabric ratings.