Tag Archives: Thermal Inertia


Posted on September 7, 2012 by

Understanding the Earth Beneath Us

Understanding the earth and how it behaves, particularly thermally, is key to Earth Sheltered home design.

The earth is not some magical heat sink or constant temperature material that so many people believe it to be.  It can conduct and store heat in a way that is very similar to other materials.  It has a specific heat capacity less than 1/4th the specific heat capacity water, and its R value is about 1/12th of rigid insulation.   So, pound for pound, it is not the best heat storage medium and is a relatively poor insulator.   However, we have a lot of it and it is, as everyone knows, dirt cheap (or even free).

Soil temps moderate and lag behind air temps

It is well know that the temperature of the earth is relatively constant.  In south eastern Michigan, it is about 54 degrees Fahrenheit 20ft below the surface.   Further south, it can be even higher.   This constant temperature is roughly the average of the air temperature over a period of time.   The air exchanges heat with the earth, which stores it and slowly conducts it downward.  In the winter, this reverses and the earth slowly gives up its heat to the cooler air and the earth cools as its energy is conducted toward the surface.  In both directions, the earth has a somewhat moderating effect on the climate, which (in addition to the larger effect of water doing the same thing) is why the air temperatures lag behind the seasons.   Some refer to this heat stored in the ground as “geothermal”, but really is just a form of stored solar energy.  True geothermal energy is found near tectonic fault lines (due to friction between the plates of the earths crust) or deep (miles) within the earth, emanating from the earths core ( actually due radioactive decay and friction caused by gravitational forces as the massive core of the earth interacts with the moon )… but I digress…
This thermal exchange with the mass of the earth is slow enough that, increasing with depth, daily changes and even seasonal changes in the air are moderated (averaged out).  At the 20ft depth, the soil temperature is stable, roughly averaging the whole year.  Actually, there are a variety of other smaller influences including reflectivity and permeability of the ground cover, transpiration of plants, the water table, etc., The net effect of these influences tends to keep the temperature a little below the average air temperature.


At about 10ft below the surface, the temperature is said to be roughly the average of the past 6 months of air temperature ( not quite, see the results of my experiment).  This convenient “thermal capacitance” means the earth temperature at this depth is at its warmest when the air temperature is the coldest.  Over the next 6 months, it cools until it is at its coolest just before the air temperature is the hottest.

When we place a home in the ground and start to control the temperature in the home, the soil temperature profile starts to change.  The soil temperatures near our home adjust to an average of the air temperature we prefer in the home (rather than averaging the extremes outdoors).    If our means of heating or cooling the home is interrupted, the thermal inertia in the walls and earth can continue to moderate the environment within our homes.


Umbrella Basics

Posted on June 28, 2012 by

Earth Sheltered Umbrella Basics

Umbrella Origins

The idea of extending the insulation out horizontally from the roof line of an earth sheltered home  (an earth sheltered umbrella) was first presented to me by Earth Sheltered Housing Design, prepared by the University of Minnesota.  (In Minnesota, with its very cold, but very sunny, winters, passive solar earth sheltering makes a lot of sense.  This lead to government incentives and funded research which further increased its popularity.) The idea is very similar to the “Frost Protected Shallow Foundations” idea that has been widely used in Scandinavian countries since the 1940’s.    John Hait  is the most famous for developing and popularizing this idea, I think he also coined the “Umbrella” name.  In his book, he talks about originally coming upon the idea as a way to simplify the application of insulation to his geodesic dome home.  I hope to further improved on the concept with my “by-passive” heating idea that avoids overheating the home.

Umbrella Theory

The basic idea is to use a few inches of rigid insulation to form an underground thermal umbrella that further separates the home and the soil around it from the above ground air temperatures.   It is also important that the mass of earth be thermally connected to the home its self.   When summing up static R-values, the order of the layers is not very important, but by placing a thick layer of earth between the home and the insulation, the thermal inertia and dynamic R value increases dramatically.  This is simply taking the basic concepts and components of an earth sheltered home and rearranging them for even better performance.

Soil is a poor insulator compared to modern expanded or extruded polystyrene (EPS or XPS), but it does have some R value (which is why it was used historically and by animals).  The R value of soil is proportional to many things, including the distance the energy must travel thru the soil, the moister content, density, soil type, etc.   Above the soil, vegetation, and even snow, can add additional insulation.  In terms of “dynamic” R value, soil brings low cost thermal inertia to the design.

Soil can also absorb and retain warmth like a capacitor.   You know that heat flux thru a medium is inversely proportional to its R value… but Fourier’s law also states that it is proportional to the temperature difference across the medium.   Just as pressure pushes flow or voltage pushes current, it is the temperature difference that pushes the heat thru a conductive medium.  However, in a material with high thermal capacity, such as soil, some of the heat is absorbed instead of transmitted.  As the material heats up locally, there is less and less local temperature difference remaining to drive the heat transfer.  In above ground cement walls, this is known as “dynamic R value” and can dramatically improve the thermal comfort of a building in certain situations.  Below ground, the much greater mass leads to a much longer cycle and an almost steady state R value improvement.  This phenomenon is the same one that leads to the soil temperature profiles we discussed in the Earth Sheltering Basics.

Above Ground Note: Dynamic R value is often “sold” by ICF dealers and proponents.  It basically describes the additional thermal inertia (resistance to change) in the wall during temperature swings from one side of room temperature to another.  For instance, in the southwest, you may have a very cold night followed by a very hot day.  High mass walls cool down during the night and provide dynamic insulation into the hot day as they warm up again.  If the following night is cool, the warmed up walls continue to stabilize the home temperature as they cool down.  However, this benefit is dramatically reduced in areas that are colder (or warmer) than room temperature both day and night.   In temperate areas like Michigan, air temperatures do fluctuate over any 24 hour period, and dynamic R value can bring some moderation, however, both daily extremes are typically on one side of the desired internal room temperature, so the energy transfer is one way and “dynamic” effect doesn’t really happen.   In temperate regions, wide temperature changes are seasonal and the Static R-value and the temperature outside the walls is much more more important for calculating heat loss.  On the other hand, an earth sheltered home provides a lot more mass than an ICF wall, enough to moderate seasonal changes.  Also, 6 inches of insulation with static R-values of 60 to 100 are typical for an earth sheltered umbrella and the dynamic-R value of the soil on both sides is a bonus.


When you bury a wall, the deeper portions are further from the surface.  The heat must travel further to reach the surface.  Distance to the surface increases the resistance to conductive heat flow presented by the soil (aka R-value)…

Typical earth sheltered insulation

Uniformly applied insulation is a throwback to above ground construction and is less efficient for an earth sheltered home

Due to the longer path to the surface, the total resistance (R-value) is greater near the bottom of a fully insulated wall.  Taking the path of least resistance, heat flux is therefore greater at at the top of the wall.  We could reduce the heat flux at the top by adding insulation there, or we could make better use of the same amount of insulation by taking some from the bottom and using it higher up the wall where the heat flux is greater.   (Again this is similar to what many smart above ground builders have done for years in terms of concentrating insulation under the perimeter of the slab.)


Re-position the insulation for improved efficiency

Re-positioning the insulation further up the wall can balance, and overall reduce, the heat loss without increasing the quantity or cost of materials.   It is important to note that, for conduction, The R values are simply added up (commutative and associative properties of addition) so it doesn’t matter what order the R value components come in, the total R value is the same regardless of the order of the layers. These two concepts open a world of possibilities for optimizing both the insulation and the thermal mass.

Increased thermal mass increases thermal stability and capacity for passive solar homes


We can increase our thermal mass simply by including some soil under the insulation.   Heat energy follows the path of least resistance and will either pass thru the the rigid insulation or take the longer path down and around the insulation to the surface.   The parameters of the design (thickness and extent of insulation) can be adjusted to minimize the heat loss for a given quantity (cost) of insulation.   One major flaw with this idea is that it could be difficult to install.


Extending the insulation horizontally past the edge optimizes the amount of thermal mass between the interior and the insulation

Another idea is to take the same quantity of insulation and extended it out horizontally from the edge of the buried roof.  The R value of the earth plus insulation can be the same or greater, but the thermal mass is definitely maximized.  This can be done after the walls are backfilled so that construction is as straight forward as laying insulation on the ground.  Tilting the edges down a little at the perimeter would help with diverting any sub surface water away from the home.


The University of Minnesota wanted to use about the same amount of insulation as a regular home, so they tested this with a 5.6ft extension and showed overall thermal performance, particularly for the summer time.   John Hait was trying to insulate a geodesic dome, so he included some soil over the dome to simplify the shape and then extended the “umbrella” 10 feet past the sides.  He also saw very good results, but wished that he had extended 20 ft.

John Hait’s schematic to illustrate the layering of his umbrella (on the left)


Rain Umbrella

One topic often discussed with these earth umbrellas is water penetration.  The concern is that cold water (from cold rain or snow melt), with its greater thermal capacitance, would percolate thru the soil, steal much of the absorbed heat from the earth, and dump it into the water table.  This is why John Hait used his three layers of plastic sheeting to waterproof his umbrella.   I also plan to layer my insulation with waterproofing, but I am less concerned about this theory since conducting a full scale experiment.   In my experiment, I buried temperature and moisture sensors at various depths below ground.  I had two sites.  One was the control site and the other was covered in a few hundred feet of plastic water proofing buried 1 ft below the ground.   I expected to show how much heat was lost from the earth due to this percolation.   However, watching the data come in over the past year, it is clear that the effect of the rain on the soil temp is only in the first few ft near the surface.

The waterproof umbrella does have several other benefits.  Primarily, it helps to reduce the amount of water in the soil around the structure.   Any time you build below the earth, you need to be very concerned about water in the soil.   Reducing the water content of this soil reduces the lateral pressure on the structure, it reduces the chances of water penetrating the cement and causing other damage, etc.  Traditionally, earth sheltered home builders dealt with this water by severely over draining the soil around and above the earth shelter.  In many cases, this actually lead to surface conditions that were too dry to support plant life.   In contrast, the umbrella serves as a divide between the moist plant bearing soil above and the well drained earth shelter friendly soil below.  As a bonus, the dry and almost airtight soil under the umbrella is also unattractive for most digging pests, tree roots, etc.

It may be a good idea to provide a vent in the top of the umbrella to let radon (if any) escape…


Umbrella Construction

The robust umbrella construction recommended by John Hait includes 2 layers of  XPS rigid foam insulation (thickness tapers off toward the edges) between 3 layers of 6 mil plastic sheeting (or pond liner in some cases).   The XPS foam is recommended because water absorption tests show that it will absorb no more than 0.3 percent (by volume), this is 1/1oth of what the EPS foam will absorb.  Water absorption reduces R value.

Note: I have since discovered that EPS may actually be a better plan…  For more info, see my page on insulation.  A switch to EPS cuts the cost of the umbrella in half, so it is well worth considering.  But when I started my own construction, I bought 2″ XPS ;^)


I suspect that thinner plastic (4 mil or 2 mil) could be used on the lower layers.   The layered construction is intended to make it difficult to puncture the umbrella.   If one or more layers are punctured, any drips that pass thru tend to run along the underside of the plastic which is sloped towards the perimeter.   The primary downside to this plan is that  if moisture does get thru, it becomes almost impossible to determine where a leak has happened because it may run along the plastic and enter the home some distance away.  

I discovered that there are many places selling used billboard vinyls.  These are 15 to 17 mil thick, waterproof and tough.  They are way over designed for their short life on the side of the highway and still have a lot of life in them when they are taken down.  Some companies will even glue them together into large 200’x200′ pieces…  The prices are great, but shop around a little.  I found that the store I linked to in this section had prices about half of what other more generalized building material re-sellers were charging.


The layer of soil below the umbrella is there to facilitate installation (easier fit was one of the reasons the idea came to John Hait in the first place) and to keep the umbrella from rubbing against the cement.  It will be important to keep sharp rocks and other “dangers” out of these early layers.

Anatomy of our planned Umbrella (plans have changed a little now.  I will redo this pic when I get a chance ;^)


Our early plan was to waterproof with Zypex (or equivalent) and perhaps even a layer of bentonite clay in some areas.   However, I am now more likely to go with a painted on rubberized waterproofing…  Additionally, many earth sheltered home builders also add a layer of used carpet over the top of the umbrella for further protection.  I have not tried it myself, but I imagine that it is actually quite difficult to accidentally put your shovel thru polyester carpet.  And the synthetic nature of most carpets ensures that the protective barrier will have a very long life.

Initially, my design also included some cellular concrete (aircrete) on the roof between the vaults to fill in the gaps without adding too much weight.  This is not shown in the above image, but would be found below the top layer of cement.  Cellular concrete has a pretty decent R-value and can also be formulated in a way that makes it naturally moisture resistant.  However, due to the lack of availability of reasonably priced cellular concrete in my area, I briefly considered starting my own company and then decided I had enough on my plate and would just fill the gaps with EPS rigid insulation.  Either method could be covered with a thin (2″) topper layer of waterproof cement.

So, in summary, the umbrella is an alternative placement of insulation that allows the many tons of earth around the home to be incorporated as part of the thermal mass of the home.  An umbrella extending 20 ft around the home would be similar to an additional R100 (20 ft x R5/ft) against a normal wall.  The high thermal capacity of the soil also stores heat and increases resistance to heat flux.  While slightly reducing the volume of thermal storage under the umbrella, the edges are sloped down to enhance runoff (water management).

Simplistic comparison between direct insulation and umbrella insulation




Earth Sheltered Basics

Posted on June 23, 2012 by

Earth Sheltered Basics


“No house should ever be on a hill or on anything. It should be of the hill. Belonging to it. Hill and house should live together each the happier for the other.” ~ Frank Llyod Wright

I am sure Frank wasn’t actually talking about earth sheltering the house, but I think he would have liked the idea.  During Frank’s time, the technology wasn’t right yet.  Even during the 1970’s, when it really took off, many people made lots of basic mistakes.

This website will try to cover the basics of earth sheltered home design and construction.  In this section on Earth Sheltered basics, we will look at the properties of the earth, passive solar, insulation, earth tubes, etc…


Earth Sheltered Basics

Building on the surface results in high heat flux due to the large temperature gradients

When you build a home on the surface, you set the temperature on the inside to around about 70 degrees Fahrenheit that humans find comfortable (note, you can find whole books on the complex relationship between temperature, humidity, emissivity, etc. and human comfort, so this is simplified).   Meanwhile, the temperature outside our homes varies considerably, even within the period of a day or an hour.   The rate of heat transferred thru the envelope of your house  (heat flux) is proportional to ΔT/R, where ΔT is the difference in temperature between the inside and outside of your home.  This is known as Fourier’s Law of heat transfer.  If we assume that two homes are are constructed the same way (the same wall cross section with the same R value), then you are left with ΔT as the primary driver of heat loss or gain.  At this point, we are not yet discussing passive solar or earth tube cooling, so lets assume that you paid for that heat or cool.  We are also saving “infiltration”, the main cause of heat loss or gain for another section.

Summer heat is transferred into the home, and winter cold draws heat out of the home.   Assuming a well sealed building envelope, the majority of the winter heat is lost thru the conventional roof.   Walls and windows also lose a lot of heat, proportional to their area and inversely proportional to their R values.  This heat exits the home and is lost forever; some thermal energy is carried away by the wind, some is radiated out into space or contributes to the green house affect.  However, under the floor, something different (and not generally well understood) is happening…  Heat is leaking out and saturating the earth under the middle of the floor.  As the ground absorbs the heat and warms up, the temperature gradient levels off and the heat flux slows.    Near the edge of the floor, heat is still conducted to the outside, so the temperature is closer to the outside temperature and more heat is lost.  This is why many builders, even in cold places like Sweden, only insulate the perimeter of the floors.  Insulating the outside of the wall or even using “Frost Protected Shallow Foundations” can help retain some of the perimeter heat that would have been lost from the outside of the foundation.  But there is much less need to insulate the center of the floors.

This is why many Home Heat Loss Calculators (such as this excellent one), used for calculating the heating requirements of above ground homes, ask for area and R value for the roof, walls, windows and floors, but only want the perimeter for slab on grade floors…  “Heat loss from slab on grade floors is primarily dependent on the length of the perimeter and not the area of the floor.” ~ Gary Reysa.     Imagine what happens when you bury the walls and roof?

Earth Sheltered heat transfer with the underground environment

Sheltering the building with earth reduces the temperature gradients and the heat flux


Home in the Earth

If we take that same building, with the same R values, and move it under ground (an earth sheltered home), it is like moving it to a more moderate and stable environment.    This means a significantly reduced ΔT between the inside and outside of the building envelope, and therefore a significantly reduced heat flux for a given R value and area.  When the air outside is freezing, the soil on the other side of the wall is well above freezing, and when the air outside is sweltering hot, the soil temperature is still mild.    Above the roof, where the soil depth is less for practical structural reasons, transpiration from plants helps provide cooler temperatures.   Along the walls, the temperature becomes more stable the further down you go (research suggests you need much less insulation near the bottom of the wall).   The time lag effect of the soil is another benefit.  As we mentioned earlier, the soil outside the walls lags behind the air temperature by 3 to 6 months providing the coolest earth in the middle of summer and the warmest earth in the dead of winter.

Some refer to the earth as a “heat sink”, which is true to some extent, but at least it conducts the heat away slowly.   The air around an above ground home can actually transport the heat away much more quickly, without any potential to store or return it.

Of course, the performance of the system is highly dependent on a number of factors including the rate of heat transfer thru the walls and windows and the rate of heat conduction away from the earth sheltered home, deeper into the earth, as well as more dynamic factors like solar heat gain, external temperatures, etc.  Achieving an ideal balance is the challenge for an earth sheltered home designer, and it likely requires some controls that can be tweaked over time…


Keys to Earth Sheltered Success

Ancient cave paintings show that humans spent quite a bit of time (historically) in caves, most likely for protection from the elements.   Those caves were likely too cold or damp to be comfortable at the start (but still better than the freezing cold or sweltering heat outside), but over time, as the inhabitants lit fires for heat or cooking, some of that heat would have been stored in the rock and the caves would have warmed up and become more comfortable.  I am sure south facing caves with solar gain were the most in demand… location location location.   With few exceptions, those caves were part of massive thermally-conductive rock formations and the heat probably leaked away from the relatively small fires rather quickly.  But we have some technology they didn’t have;

       A way to heat the home efficiently with passive solar gain.

       A way to isolate our portion of the earth, our micro climate, away from the rest of the environment (heat sink and infiltration) with modern construction and insulating materials.

The hardest part about designing an ideal passive solar home is to balance the amount of free solar energy that you allow into the home with the amount of thermal energy the home can store.   Too little mass and the home will overheat, too much mass and its temperature will respond too slowly.   Above ground passive solar designs work to ensure they have enough mass, usually in the form of cement floors and maybe some thick masonry walls or even trombe walls, to store some low-angle winter solar energy for about a winters day.  In Michigan, in January, only about 1 day in 5 has sunshine, so needing a daily recharge won’t work for us.    These same homes must be careful to keep out the sunshine in summer.  They take advantage of the earths tilt and design awnings to keep out the hotter sun which would certainly cause overheating.

Earth sheltered homes are surrounded by a lot more thermal mass.  And unlike the expensive thermal mass often used by above ground solar homes, dirt is, well, cheap.  Unfortunately, most earth sheltered homes are designed with the insulation attached to the outside of the cement walls. I won’t even discuss those who put the insulation on the inside of the walls!  This insulation (aka out-sulation) does its job and isolates the home from the thermal mass of the surrounding earth.  The passive solar storage is limited to the cement or other thermal mass the home owner built into the design.   Since these homes are carrying an earth load, they are typically more substantial than the above ground passive solar home and can store more energy, but they are still missing out on a lot of “free” thermal mass.

John Hait came up with his idea for Passive Annual Heat Storage.   He found that positioning the insulation further from the home incorporated many tons of earth into his thermal mass and allowed him to store thermal energy for much longer periods of time (theoretically for the whole year).  He had large un-shaded windows that let passive solar energy into his solarium during the summer.   His design was reasonably successful, except that the heat transfer rates from the living space into the earth were not high enough and his living area tended to be a bit too warm in summer (energy couldn’t get into the earth fast enough).  His book suggests improvements to his design including extending the umbrella out to 20ft around the home (his only extended ~10ft.)

I plan to take the next theoretical step.  My home will be massive (hundreds of thousands of pounds of cement, which, surprisingly, doesn’t actually even cost that much at about $100/3240lbs + instalation) and I will use an insulation umbrella to increase my thermal mass with many times more earth.   But instead of letting the sunlight into the home directly where it can make living conditions uncomfortably warm, I plan to “by-pass” the home  and collect heat all summer thru solar collectors which should heat up the earth under my home…   The idea is to heat up my earth tubes with very hot air (well over 120°F) and let the energy slowly conduct thru the earth, timed to reach my home just about when I need it, 6 months later.   I will still augment this with conventional passive solar in winter.

I plan to provide many of the details for how to setup this passive system over the next few months.