Gettin’ Cozy With Thermal Mass


A fig tree thrives in a greenhouse at Millsap Farms in Missouri, where winter temps can dip into the 20’s.

Greenhouses help to extend the growing season on farms across the United States and around the world. However, moderating the interior climate is key as plants have a hard time withstanding extreme temperature shifts. This means many farmers spend significant amounts of resources on energy to heat and cool.

At Millsap Farms, greenhouses are critical and Curtis and Sarah Millsap employ a variety of different shapes and sizes. In 2012, they built a “Chinese high tunnel” and even though it took extra money, time and machinery, they are ever so glad that they did. (Read more about Millsap Farms here.)


Curtis Millsap in his “Chinese high tunnel”. Massive concrete bricks buried beneath a berm on three sides helps keep temperatures moderate. Photo credit:

Much like an Earthship, a “Chinese high tunnel” utilizes passive solar energy and thermal mass to maintain a temperate climate for the plants growing inside. The south wall and ceiling are transparent – made of plastic sheeting that can be raised when it gets too hot inside – and in the Northern hemisphere, south facing to capture the maximum amount of sun.

The remaining three walls are made from giant concrete blocks and are buried beneath a hill of soil. The mass of the concrete blocks and the berm help regulate the temperature inside the greenhouse – absorbing warmth during the day and radiating it back into the greenhouse at night. Also, when temps outside dip down into the 20s and 30’s, the lion’s share of the berm stays almost toasty at 55 degrees, sharing its relative warmth with the air in the greenhouse. Thus it is a greenhouse that stays warm in the winter using only passive solar energy.  In the summer, the berm helps keep the heat manageable.


Thanks to an earthship-style greenhouse atrium, bouganvilla blossom in at a chilly elevation in Maxwell, NM.

Temperature extremes plague farmers’ greenhouses. From a high of 120 in the height of a fall day, temps can drop into the 30’s at night. Most plants are not hardy enough to take these extreme temperature shifts. Thermal mass helps moderate the swing by forcing the changes to move more slowly and thereby softening the extreme ends of the spectrum. Curtis compared temperature fluctuations in the Chinese greenhouse with his other greenhouses and found that its temperature swing range was smaller by more than 20 degrees.

In fact, the Millsaps have found that without any supplemental heat, their Chinese greenhouse stayed in the mid twenties right through a negative 10 degree cold snap this past winter.  This is plenty warm to keep their lettuce, carrots, chard, kale, arugula, and celery safe and healthy through the winter.  Meanwhile, their 6,000 s.f. conventional gutter-joined greenhouse, needed two wood furnaces burning full tilt to maintain the same night-time temperatures.


The Millsaps conventional greenhouse requires significant additional heat in the winter to maintain plant-healthy temps.

Water is also a popular thermal mass medium. At Happy Hollow farm in Jamestown Missouri, Liz Graznak simply uses a dozen 50 gallon drums filled with water on the north side of her tiny greenhouse.


One of the most adorable greenhouses at Happy Hollow farm.


Drums filled with water line the north end of the greenhouse, above the shelving…


…and below.

On the other end of the spectrum, at Perfect Circle Farm in Vermont, Buzz Ferver puts water to dual use in his greenhouse which is attached the south side of his home. Sub-divided 1200 gallon tanks are home to a hundred tilapia. Poop from the tilapia fertilizes the filter bed where plants flourish. The gravel filter bed and plants “clean” the water before its pumped back into the tilapia tank. (More on “living machines” like Buzz’s aquaponic system in a later post.)


The tilapia live in the grey tank in the back which feeds nutrient rich waters to the wood-paneled tank in the foreground where a variety of plants enjoy their living machine. Photo credit: Buzz Ferver, A Perfect Circle Farm


Greenhouse going bananas. Photo credit: Buzz Ferver, A Perfect Circle Farm

It’s not a foolproof system to be sure. In the northern wilds of Vermont, Buzz still needs to pump some heat into his greenhouse in the winter which comes from a wood boiler in another building. (In fact, he uses more heat to maintain temps in the greenhouse than he does to heat his house.) But he’s moving in the right direction. (And who knows, with climate change, he may not need the boiler in a decade or two.)


Earlier this month, Buzz harvested dozens of 3.5 lb  tilapia from the tank. At that time, about 100 little babies showed up. A system that helps sustain itself. Priceless!! Photo credit: Buzz Ferver, A Perfect Circle Farm

The point is, there exist a range of solutions on the path towards energy independence. Many of them don’t cost more in terms of money, time or labor. In fact, many permaculture solutions save on all three.

We simply need the clarity to recognize the need to reduce our carbon footprint, the desire to seek out solutions and the will to implement them.


Architects, engineers and designers are creating new and beautiful ways to incorporate thermal mass in both residential and commercial buildings. Photo credit:

About Anastasia King Jaress

Anastasia is a former media producer who hit the road to sustainability in April 2013 with her husband JC and Mattie, the dog. She writes about food, community, sustainability, travel, family and the myriad questions that boggle them.

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5 Responses to Gettin’ Cozy With Thermal Mass

  1. Curtis August 1, 2014 at 7:39 PM #

    Great article. Thanks for taking the time to write about your adventures WWOOFing, I’ve really enjoyed all the posts. It’s a bit like getting to visit other farms, just by reading a blog post.

  2. Lloyd Vivola August 6, 2014 at 6:04 PM #

    Evocative article, not least of all for how it reminds us that there are many different solutions for achieving viable greenhouse conditions – or for that matter, ecologically sound irrigation systems or composting regimens – and that good solutions are not seldom related to particulars of place as well as a grower’s own ingenuity and personal style of attentiveness..All of which, in my opinion, lends much to the spirit and application of sustainable farming and its impact on local markets and community. And yes, I remember those Lady Liberty champagne currants well.

    So thanks again for the journey. And the blog which should endure as an informative, entertaining reference..


  1. Getting Cozy With Thermal Mass | Millsap Farms CSA Blog - August 3, 2014

    […] Written by Anastasia King Jaress Full article with photos is at […]

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    […] More on Millsap Farm. […]

  3. Earthship Amazing - August 8, 2015

    […] The face of an Earthship is basically a windowed corridor, much like a greenhouse. (In our hemisphere, this is south-facing.) During the colder months, when the sun is low in the sky, the sunlight enters the windows, heating up the greenhouse. In this space, you can grow all manner of fruits and vegetables using recycled greywater. On the inside of the greenhouse corridor are the living spaces – spacious, stylish and temperate. Many of the interior walls are built using empty cans and bottles. Sometimes these are arranged in a pattern and allowed to show through, creating a whimsical effect. The roof hosts a rainwater catchment system, multiple solar panels and several vents and skylights that help with cooling and lighting. The back wall of the house is constructed of recycled automobile tires filled with rammed earth. On the inside, all of the walls are finished in smooth, inexpensive adobe. On the outside, the back wall of the house is covered in sloped earth, otherwise known as a berm, providing additional thermal mass. […]

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