The Search for Thermal Comfort

June 09, 2015
July/August 2015
This online-only article is a supplement to the July/August 2015 print edition of Home Energy Magazine.
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In the beginning there was woman and man, naked; I assume.

The body’s core temperature is stable at about 98 °F. In an outdoor environment with a temperature between 65 to 102 °F, a naked body, depending on activity, can maintain a comfortable level of thermal stability. If body temperature goes much above or below its core temp for very long, the body will descend into thermal shock and eventually death. Add air movement (wind) and or wet, and thermal shock will happen faster.

The anatomy of the human body is much like the anatomy of a building. It starts with our outer continuous layer of air tight, waterproof, and a vapor smart, skin (plywood and Tyvec). A layer of fat (insulation) follows the entire cover of skin (the perfect wall). Under the layer of air seal and insulation, the body’s furnace (metabolism), fueled by food, is running 24/7 to keep the inner core temp of the body at about 98 degrees. When the environmental temperatures get too hot, the body even has an evaporative cooling system know as sweat. You might say our breathing system is like an HRV: fresh air in, stale air out and a heat exchanger (respiratory system).

Terry Nordbye
is the owner of The Practical House and has been a general building contractor in Northern California for 35 years.

The advent of clothing, skins and blankets allowed man’s ability to stay thermally comfortable in more extreme conditions (Delta T). When fire was discovered over a million years ago it allowed even greater thermal control and comfort of the body: but only while the fire was burning.

A paradigm shift occurred in human evolution as humans learned how to replicate a thermal shell outside their own (naked) shell by developing clothing and shelter. The primary benefit of an external shelter was not only to contain the heat (fire) longer, but to keep the wet out and equally so, reduce the thermal intensity of heat exchange from movement of air (wind).

Shelters According to the Environment

In the beginning, manufactured building parts did not exist; shelters were built with materials nearby on the ground.

In Cappadocia, a limestone and woodless region of Turkey where I traveled recently, large 2,000-year-old cities were carved into hillside rock as caves and underground cities. North American tribes build out of ice and logs and soil and in Southwestern US the adobe soil was well suited for dirt houses. In early America, settlers built with nearby logs and timbers and stone.

These dwellings combined with fire would allow the inhabitants to control their thermal comfort against the outdoor elements. The ability to control indoor thermal comfort increased humans’ ability to flourish and expand into new hunting and/or resource areas with temperatures above or below their bodies’ ability to cope.

Outdoors, a naked body can maintain thermal comfort at temperatures of 60–105 ºF. Outside that range, survival becomes difficult.

The human body can be insulated with layers of clothing.

cl Nordbye-sleeping on fire escape
Two kinds of comfort. Skin provides an air seal; fat provides insulation; metabolism provides heat. Combustion outside the body can add heat (the baby), while sleeping outside can provide cooling—if there is a breeze.

Some Native American homes were built with unfinished wood framing and animal skins.

The earliest human shelters were built with materials found nearby—in this case, ice.

Building materials evolved over centuries however the thermal comfort of dwellings did not significantly change for thousands of years: when the fire went out it got cold. Buildings with greater thermal mass (stone and adobe) would fair better in heating and cooling: once heated, they would continue to exude heat when the fire went out, and unheated mud or stone would hold the night cool in hotter weather (thermal lag). And of course, buildings which passively used the sun for heat always faired better.

Thermal Comfort and the Industrial Revolution

The first circular saw blade was invented in 1831. Before that, lumber dimensions were somewhat random and continued to be random until 1924 when lumber was standardized. This led way to standardized framing of stick houses. Thirty-two and twenty four inch framing centers gave way to sixteen-inch centers. Balloon framing gave way to platform framing. And in 1956 the American Lumber Standards ruled that a milled one inch by four inch piece of wood be a consisitant three quarters by three and half inches.

Coal fired central heating systems delivering steam heat came into being in the end of the 19th century In the late 1800s coal began to replace wood for combustion heating systems in rural areas and coal fired the first forced air furnace in 1924. Fuels such as heating oil (kerosene) gave way to natural gas and light propane gas (LPG). Electric resistance heating became popular after 1930s.

Early plywood was used mostly for furniture making, but in WWII it proved to be valuable in large sheet form, for boat making. The sheets showed up in buildings in the fifties and became mainstream by the '60s.

Plywood was a great leap forward in air sealing the exterior shell. Tar paper introduced in 1803 began to control moisture and vapor drive, but did not become more widely used in shell assembly for another hundred years.

Fiberglass insulation was invented by Dow in 1939 and became widespread in the '50s. The combination of sleeker central heating systems, plywood wrapped buildings and cavity insulation turned buildings on their heels in terms of thermal control and comfort. And all that came together only 60 years ago!

For some mysterious (dumb) reason two by fours dominated the stick-frame game. A house in Iowa or Arizona or Oregon was framed with two by fours. This of course put great limitations on the overall thermal values of the house. Air sealing was almost entirely ignored in most buildings and large holes in the assembly remained unabated. But then again, with cheap heating fuels to achieve indoor thermal comfort, all you had to do is crank up the AC or the furnace.

Dual glazing caught on in the 70s, but large air conditioners and furnaces run on cheap fuel could keep up with any environmental conditions the seasons could throw at a leaky low R/U value window.

That is pretty much where building stalled out in terms of its ability to stabilize thermal control. Title 24 increased insulation values in California, which bumped up the thermal value of all new buildings, but we were, and still are, building gas-guzzlers.

Dwelling technology has seen a lot of changes since naked man, but what has never changed in the evolution of shelters is the need for humans to seek thermal equilibrium inside them.

Thermal Complacency

Thermal comfort is guided by two things- feel and physics. Feel is what tells the body if the physics are working or not.

Thermal fluctuations in living spaces are caused by drafts, leaks, pressure differences, cold walls, hot ceilings, cold windows and floors, cold or hot spots from thermal bridges, stack effect, high velocity fans blowing hot or cold air into parts of a room, overheating from too much glass in the wrong place at the wrong time, etc.. Every time our body gets near one of these thermal adventures, it reacts, in small ways we don’t even notice: because that is what we are used to. It is those very fluctuations that interrupt thermal body comfort.

Because most of us were born in and inhabit houses that are old school or at best, a ten-year-old state of the art building, we have never experienced the level of thermal comfort that a High Performance or Passive House house delivers.

Our bodies are used to extreme thermal shifts. We don’t notice a cold leaky window sucking the heat out of our backs or an air conditioner blowing cold air on our face while the sun is heating our neck. It might be normal to wear a sweater in the morning and turn on the AC at two PM when its 82 degrees outside. If you went around a typical house in almost any season and checked the surface temperatures of objects, you are likely to find up to a ten-degree difference between surfaces.


I spent a total of four nights in a Passive House. I monitored surface temperatures of different parts of the house- walls, floors, furniture, ceiling, fixtures, etc. A few surfaces were more than four degrees different; most variations were around two degrees. In the morning it was 36 degrees outside and inside the house it was hovering at 64 (with no heater on since the night before when I ran a 1,200 watt heater for two hours). I roamed around, comfortable, in my pajamas, with no heater on. I was confused as to what I was “feeling” being in the house. I described it to a friend as “kind of eerie” but later came to recognize it as extreme thermal comfort.

A common worry or complaint about extreme thermal and air control is; it will be a prison we cannot escape from. To those who have such fears, let it be known, you can open doors and or windows any time of day or night and let in the wild.

We now have all the knowledge and materials to build interiors that do not challenge our internal thermal regulation. To do so, requires only a baby step out of where we are now. Change often seems like a giant step when it is in front of time, but we are currently in the middle of the change so it’s really not that big a leap.

NOTE: Passive Solar houses brought in a refreshing and promising light to reducing heating loads but they have a tendency to over heat certain times of the seasons and days and loose a lot of heat at night or on cloudy days. The thermal comfort levels are often too random and difficult to control.
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