Thermal Mass vs Insulation: Materials Choice

While many in the Permaculture and Natural Building movements gravitate towards a particular building material selection based on certain priorities, in this article I will present two vital factors to consider.  Along with the access to local materials, the properties of thermal mass and insulation should be weighed paramountly, while also meeting your social and environmental context.  For example, while cob is a fantastically moldable choice for funky and functional buildings, they are an example of thermal mass.  Does this wall type fit into your climate and your particular property and the building site itself?  A

Taipa wall, A thermal mass form of rammed earth in Portugal.

Taipa wall, A thermal mass form of rammed earth in Portugal.

look into microclimates is an important choice when you get down to the detail and knowing well your overall climate is crucial.  Remember Permaculture is not rubber stamp development so cob in one locale might work but in another it will be energy inefficient.  Thus this is what ecological design hinges upon, the conservation and generation of energy through a holistic framework.

Henceforth, thermal mass and insulation can be looked at in the context of energy exchange and interaction.  In short the difference is the following with further explanation below that is displayed through pictures with many examples including a cob oven project I did on my families land in 2012:

Thermal Mass: heavy materials that absorb temperatures slowly and act as a battery of temperature and then release them again

Insulation: airy materials that slow the exchange of temperature between the inside and the out

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Thermal Mass

As seen in the slide above, thermal mass materials are made of heavy materials such as rocks, cob, rammed earth, or adobe.  The greatest thermal mass on earth is water.  It’s extremely heavy and very slowly absorbs temperatures and rereleases them.  It’s an amazing substance and this is just one of many of its outstanding properties.  This means for water bodies that it will over summer continue to absorb warm temperatures and release them into the fall and even keep frost away from a small microclimate.  It also does this on a daily basis where the sun really charges the water with heat, slowly increasing its ambient temperature. This also creates a more frost free environment close to the water body as temperatures approach freezing because heat is released at night.  Rocks do this as well ,which is also used to make

rocks coming out of pond, both thermal masses.

rocks coming out of pond, both thermal masses.

microclimates with for say trying to grow something cold sensitive which needs some extra heat (i.e. Carob in the central and northern parts of Portugal).  So essentially a house that is well designed with proper materials (including thermal mass vs insulation balance), dimensions, glazing placement, roof overhangs, and orientation will provide you with the exact microclimate you are looking for throughout the entire year with only minimal input of energy to maintain.  I put this in bold because some houses will preform great in the summer in terms of keeping you cool, but in the winter they also keep you cool unfortunately (i.e. the conventional concrete house of Portugal).

So what a thermal mass wall of a house, a floor, or even a wall of water theoretically does is absorbs sunlight, fire heat, or ambient heat or cold energy and releases this later.  In the context of the pictures you see, a cob oven being constructed, you can see the dense cob mix going over the sand dome covered in newspaper. The sand is removed once the thermal mass laden cob dome is constructed which leaves behind the hollow for the burn chamber.  Thus the intense heat of the flames are slowly absorbed into the walls and will start to release that heat over an extended period of time.  An average sized oven of this sort takes about 1.5-2 hours of wood burning to be able to cook some pizzas and bread.  In Costa Rica we would leave the fire burning for three hours and cook for hours and dehydrate a cake like breakfast bread for the morning with fruits from the farm inside. After about four hours of cooking other things, we would place this in last and leave the dish there overnight cooking and it would still be warm 10 hours later.

Pizza cooking the cob oven at the lake, authors family land in Kentucky, USA

Pizza cooking the cob oven at the lake, authors family land in Kentucky, USA

Furthermore, in the context of the house you see in the slide above picture below right, you can see a traditional rammed earth building style from Portugal known as Taipa.  It’s great for absorbing the sun in the winter and releasing this storage of energy in its battery at night. Taipa is a clay and small stone/ earthen mix rammed in linear forms giving it a high thermal mass property.  For the dry and hot summers of Iberia this is perfect as the temperature swings of each day in the summer call for this technique.  The nights get quite chilly as drylands areas have high daily temperature swings so the space actually needs a bit of heating to be comfortable sleeping weather.  This is facilitated by the dense earthen materials ability to absorb day time heat and some direct input from the sun.  If this was a warm evening climate you would not want such materials.  For example where I grew up in the humid tropic like conditions of the temperate summers in the Ohio River Valley, USA, our brick house would rerelease the heat at night making it miserable to try to sleep since the air only cools from 30 + C (90 F) to only 23 C (75 F). It was a poorly designed fifties house that was very energy inefficient as the thermal mass brick walls absorbed tons of input from not properly design roof overhangs.  Also if a Taipa house is not properly designed for winter, it will be cold and damp since the regions of Portugal where they are common regularly have night natural building 1.030temperatures around zero degrees and a rainy and humid winter.  Consequently one needs to site the house and design it so that the sun can charge the walls and also parts of the floor or interior walls through glazing at the right spots on the southern part of the building.  Because temperate areas have such differences in sun angle this is an extremely important facet of ecological building design.  One must really get to know their winter and summer sun sectors to fully utilize this thermal mass battery.  Otherwise the heavy materials only absorb cold in the winter and the ability to warm and dry out the space requires huge amounts of firewood all the while there is always an air of uncomfortability.  If winters are cold, damp, and grey like most west coast climates, then the winter recharge is often sparse or simply not possible.  Also in mountain landscapes or on a northern facing slope in the northern hemisphere this may not be possible due to the low winter sun angle.  This, and other designs, also need appropriate levels of insulation, especially in the roof and floor, because this is one of the places where temperature exchange occurs most commonly. This is also where the rocket mass heater comes in for space heating as to slowly release heat throughout the night since it relies on the thermal mass property as well.  If there is no insulation in the house, however, it also can only do so much as the heat will continue to leak.  Few use this rocket stove energy technology so the common house stove simply is not able to space heat efficiently putting further pressure on forest reserves of stored sunlight, firewood.

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Insulation

Insulation, again, or as they say in Europe isolation, slows the exchange of temperature between two temperature regimes.  In the slide below on the top right you see the same cob oven project.  You can see a light clay straw layer is being applied around the dense thermal mass where the flames interact.  Without this insulation layer, the heat would be rereleased both inside the baking chamber but also out into the environment where you are cooking.  Through adding the insulation layer, your use of firewood has a greater potential for cooking since it is not being wasted into the air and the heat is retained for a greater time. Light clay straw is a straw bale broken apart then tossed in a clay slip to preserve and bind the straw.  You normally apply it lightly in forms, as the

light clay straw

light clay straw

name suggest, which leaves plenty of small air pockets.  This prevents the temperature to diffuse outwardly as the light clay straw effectively insulates. Also in the slide below you see the interesting factor of roof material selection and its influence on the house.  Roofs take in a lot of sunlight energy, especially in the summer in temperate regions or in the tropics where the sun is always high in the sky.  The metal roof commonly used in the tropics is a conductor of extreme heat making many spaces quite uncomfortable during the day.  The traditional material would have been thatch, often a mix of palm leaves or rushes, which are insulators not conductors of heat.  Having made this switch of materials all the while destroying our thatch producing wetlands, we have caused an increase in energy consumption to maintain spaces with air conditioning and fans. This in turn further destabilizes climates through energy production outputs like carbon dioxide, and inputs derived from mining.  Moreover, in the Slovakian mountains, where I taught a PDC in 2015, the main building was built with thick rock walls, an extreme thermal mass.  The building was nice and comfortable in general in the summer but I heard stories of persistent cold in the winter.  This is in part because the original roof choice was thatch, an insulator, but had been switched to industrial tiles, a thermal mass.  So the insulation factor was lost so when heating took place as it leaked out and kept the space cold all winter.

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In cold climates, where temperatures go near or well beyond 0 C (32 F), insulation is quite crucial in the walls, ceilings, floors, and windows.  With windows there is a general trend in the states to use double glazing as a mandatory step towards energy efficiency.  Between the two panes of glass air is trapped, which helps to resist temperature swings, both hot and cold. This helps to ensure the space is kept at a more comfortable temperature as pesky drafts are reduced dramatically.  The glass still lets the sun enter, which is important in the winter but with sound design it should be kept out in the summer.  Thus roof eaves are calcualted appropriately for the building and your precise latitude to prevent summertime buildup of heat by shading windows on the south (North in Southern Hemisphere) with the high sun angle.  Insulation in these cold climates can be a better choice than thermal mass like a light clay straw house vs. a cob house.  This is especially true in building locations where ample sun can not charge the thermal mass battery of the building.  Strawbale is another insulator wall choice and is great for those more extreme winter climates like continental temperate locations.  For the more Mediterranean ones light clay straw is a sufficient insulator.

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At the end of the day, all natural buildings should be a balance of the two and most commonly are a hybrid between the two.  Again go back to your climate and context and

cork insulation panels in roof schematic

cork insulation panels in roof schematic

find what is the most appropriate choice by also thinking local.  In Portugal, the common taipa house is now being augmented by the cork industries use of non cork stopper quality cork to make insulation panels. These are fitted into areas below the roof tiles and sometimes tongue and groove wood to hide them and further insulate. In a local convent near where I live currently in Sintra, Portugal (2016), the monks at one time had coated the stone walls with pieces of cork to act as an insulation layer.  Same local materials, cork, two different approaches but both solutions to insulation.

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When you combine the two and add some art into the building landscape, then you, in my opinion, have a general pattern of natural building.  Add the good hat and good boots part (well placed and appropriate roof overhangs and strong foundation) and you are able to further the design process.  Even though the adage of water, access, structures is thought more of in the landscape, house design also applies once you are doing more of the nitty gritty interior design.

As a review of what materials fit into what category, I leave you with the following list:

Thermal Mass

  • Rammed Earth (Walls and Floor)
  • Adobe (Walls and Floor)
  • Cob (Walls and Floor)
  • Stones (Walls and Floor)
  • Wattle and Daub (Walls including interior)
  • Stabilized earth bricks (Walls and floor)
  • Concrete (Walls and Floor)
  • Brick (Walls)
  • Tiles (roof and Floors, sometimes walls interior and exterior)

Insulation

  • Strawbale (Walls)
  • Light clay straw or light earth (Walls)
  • Thatch (Roofs)
  • Straw (Roofs, on top of liner for green roof)
  • Cork (Addition to walls or ceiling or floor)
  • Wool (Addition to walls or ceiling or floor)
  • Carpets, rugs tapestries from cotton or wool (Addition to walls or ceiling or floor)
  • Double glazed windows

Enjoy the design process and find the right blend of materials and an ecological design will come to fruition.

Written by Doug Crouch

Header Art Rivca Doukarsky

Natural building