When we think about heating our homes, we instinctively turn to radiators, underfloor heating or heat pumps. But rarely do we consider one of the most efficient and overlooked heating systems available to us: our own bodies.
The human body is a highly sophisticated thermal appliance, constantly radiating warmth into its surroundings. With the increasing shift towards low temperature heating systems (such as heat pumps) it’s high time we reframe our understanding of thermal comfort. It’s not just about heating air; it’s about keeping people warm. And that opens the door to smarter, more sustainable approaches.
This may not be an approach for everyone, but with energy prices rising and heat pump efficiency closely tied to lower flow temperatures, learning how to retain our own body heat could be a game-changer for some households. For those struggling with heating costs or trying to get the most from their renewable system, personal insulation (through smart clothing choices) can make a real, measurable difference in comfort, energy use and bills.
Our Built-in Heat Source
Our core body temperature hovers around 37C, with the surface of the skin typically around 33-34C. When the surrounding air is cooler than this, heat naturally flows from us to the environment. This process is continuous… our bodies lose heat through the skin and via breath.
Unlike other mammals covered in thick fur, humans rely on clothing to trap a layer of warm air close to the skin. This thin buffer zone acts as insulation, much like the cavity wall insulation in your home. The principle is exactly the same: retain warmth by slowing down heat transfer. But while buildings require extensive materials and construction to achieve high levels of insulation, keeping a single human warm requires just a few millimetres of thoughtfully chosen clothing.
And here lies the opportunity: by insulating people rather than spaces, we can radically reduce the energy needed to maintain comfort indoors.
Understanding Clothing Insulation: What is a ‘Clo’?
To quantify how well clothing insulates, scientists use a unit called the clo, short for “clothes.” One clo is the amount of insulation needed to keep a resting person comfortable at an ambient air temperature of 21C. It roughly corresponds to a typical outfit of trousers, a shirt and a light jacket.
The clo isn’t an international SI unit, but it is widely used in thermal comfort modelling. For those more familiar with building insulation, one clo equals approximately 0.155 m²·K/W, or 0.88 R-value.
Here are a few typical clo-values for individual garments:
- T-shirt: 0.10 clo
- Long-sleeved shirt: 0.25 clo
- Sweater: 0.30 clo
- Trousers: 0.30 clo
- Full business suit: 1.0 clo
- Thermal underwear (top and bottom): 0.60 clo
- Heavy winter ensemble: 3.0-4.0 clo
These values can be added together for total clothing insulation. Want to stay comfortable at 17C instead of 21C? You’ll need about 1.4 clo rather than just 1 clo, meaning you might just need to pull on a sweater and some thermal leggings.
Layering Smarts: Less Heating, More Comfort
We know that heat pumps perform most efficiently when operated at lower temperatures, ideally around 35-45C flow temperatures. This is perfectly adequate for well-insulated homes, but what about older properties or underperforming radiators?
Instead of overcompensating by increasing flow temperatures (and reducing COP), we can reduce the thermal demand by managing comfort at the individual level.
Layering clothing intelligently is the easiest and cheapest way to maintain warmth without cranking up the thermostat. The trick lies in trapping warm air close to the body while minimising the movement of air within and between clothing layers, a concept known as the pumping coefficient.
Tight-fitting base layers, especially thermal underwear, are extremely effective at preventing warm air from escaping. In fact, long underwear has better insulation performance than many loose, bulky items, even at the same clo-value. This is because movement disturbs air layers in loose garments, reducing their effectiveness. So, a thin, snug base layer can outperform a big jumper in many situations.
Why This Matters for Low Temperature Heating
If you’re running a heat pump, the cost and efficiency of your system are directly tied to how warm you’re trying to keep your home. Every degree you lower your thermostat translates to around 6-10% energy savings, and possibly more if your unit is struggling in colder weather.
Let’s look at a quick example:
- Wearing a light t-shirt and jeans (about 0.6 clo), you might need 22C indoors to feel comfortable.
- Add a sweater and thermal base layer (taking you to 1.6–1.8 clo), and you can remain perfectly comfortable at 17C.
That 5C reduction could cut your heating bills by 30-40%, and in off-grid or high-cost electricity households, that can be hundreds of pounds per year in savings.
Heat Pumps + Human Heat = The Efficient Partnership
Modern heating philosophies, particularly those aligned with Passive House or low-energy retrofit principles, tend to focus on lowering background heat demand, improving air tightness and optimising renewable systems like heat pumps.
But here’s the thing: your body naturally produces around 100 watts of heat at rest. That increases significantly with even light activity. So the more you’re moving about (cooking, cleaning, walking) the more your body contributes to heating your space.
At rest (watching TV or working at a desk), your body still emits enough warmth that, when paired with effective clothing, you could remain thermally comfortable in a room at 16-18C. For homes running heat pumps, especially during shoulder seasons (spring/autumn), this can be the difference between switching the heating on or not.
Even better, if your family adapts to a slightly cooler indoor climate, your entire system can operate more efficiently year-round.
Modern Thermal Underwear: Not Just for Hikers
Thanks to decades of research by outdoor clothing brands, space agencies and military institutions, thermal wear is no longer bulky or unattractive. Today’s thermal base layers, especially those made from merino wool or synthetic blends offer high insulation at low weight, and many are stylish enough to wear casually.
A single top-and-bottom merino layer can add 0.6 to 1.0 clo, enough to reduce the need for central heating by several degrees. Stack a second layer or add a fleece, and you’re good for 15C and below without sacrificing comfort.
Cultural Comfort Zones and ‘Thermal Adaptation’
One reason indoor temperatures have crept up over the decades is cultural expectations. In the 1950s, the average UK living room temperature in winter was around 15-16C. Today, it’s closer to 21-22C.
But our bodies are capable of adapting to lower temperatures, provided the drop is gradual. Studies show that people who consistently live in cooler homes become acclimatised, feeling just as comfortable as those in warmer homes. This thermal adaptation can take just a few weeks.
In fact, many European and Asian countries already accept cooler indoor conditions as normal, particularly where energy prices are high or where passive home design is more common.
Extremeties
If you’ve ever sat in a cool room and felt fine (except for freezing feet) you’re not alone. Hands, feet and head lose heat faster due to their surface area and vascular structure.
However, this doesn’t mean you need a hat and gloves indoors. Often, if your core body is properly insulated, blood circulation to extremities is maintained, and fingers and toes remain warm.
But for those in particularly draughty or under-insulated properties, warm slippers and a thick pair of socks can make a world of difference, often more than turning up the thermostat.
Your Clothing is Part of Your Heating Strategy
As we embrace low carbon heating technologies like heat pumps, we must also rethink how we define comfort. Thermal comfort isn’t just about air temperature… it’s about the balance between heat production (metabolism), insulation (clothing) and air temperature.
Lowering your thermostat doesn’t have to mean sacrificing comfort. By dressing appropriately, using modern thermal wear and embracing the idea of personal insulation, you can maintain comfort, reduce costs and help your heat pump system run more efficiently.
In fact, incorporating clothing strategy into your home’s heating plan is arguably one of the most cost-effective, low-tech energy efficiency upgrades available. And it’s one that every household, regardless of age or income, can act on immediately.
Heat the person, not the space. With just a few mindful changes, your body can become the most efficient heating system in your home.