Tag Archives: Thermal Bridging

The biggest difference between a cold damp cave and a warm modern earth shelter is insulation. No, the earth does not provide good insulation for your earth sheltered home.  While some estimate that a foot of moderately dry earth has an R value of ~5, it is not structurally practical to use earth for that purpose (above ground).  Instead, we see the earth as a large thermal capacitor and cover it with an umbrella of insulation.   Without the insulation, your home would be moderated to something a little less than the annual-average outside air temperature.  With the insulation umbrella, the walls and earth temperature within the envelope will eventually stabilize near your indoor air temperature, which is a lot more comfortable.

There are a number of types of insulation, as well as a number of different locations where it needs to be used.

Inside/outside

For passive solar design, we want thermal stability, which means we want as much mass inside the insulation envelope as possible.  Earth sheltered homes, particularly concrete ones, have a lot of thermal mass.  Insulating the inside and leaving most of the mass outside the thermal envelope would result in greater temperature swings and basically miss the point of earth sheltering.   PAHS or other earth sheltered “umbrella” concepts (such as my “by-passive annual heat storage”) relies on taking that concept further and including many tons of earth within the envelope.

If we assume that we are going to insulate outside of the home instead of inside, then we must rule out all the light and fluffy insulation options such as spun fiberglass (batts or blown in), wool, hemp, wood chips, etc.   Those types of air based insulation have no real structure of their own.  They can only be used in a framed wall situation where they can be kept dry and uncompressed.   Since the layer outside of the walls is often damp and very compressed, we are left with with rigid types of insulation as our only real option.

Most earth sheltered homes do have some conventional walls that are not earth sheltered.   The fluffy insulation may be applied in those locations, but spray foam insulation would do a much better job of keeping out infiltration.

Rigid Insulation

This brings us to the types of rigid insulation.  In traditional construction, this is used under the slab or around the basement walls.  A slightly less conventional application may be to cover shallow footings for frost protection.  Those environments are all somewhat similar to the earth sheltered umbrella concept (but the geometry is not).

There are several main types of rigid insulation.

Note that you can also purchase used rigid insulation for a fraction of the price of new…  It is worth looking for suppliers in your area.

Polyisocyanurate

Polyiso is considered to be the “best stuff” for above ground insulation.  It is the yellowish stuff with the metal foil backing.  It has the highest R value per inch (5.6 to 6.8 per inch, although many claim R7/inch), which means you can fit more R value into a given wall thickness.  It also has “green” attributes such as being mostly made of recycled materials and not including any “global warming” ingredients.

However, it is also the most expensive stuff.  For an earth sheltered application, we don’t care too much about thickness so it doesn’t make sense to pay for thinner insulation.  But more importantly, Polyiso absorbs water like a sponge and is not considered suitable for below grade use.  Just don’t even consider it.

XPS (Extruded Polystyrene)

Extruded Polystyrene is the pink stuff or blue stuff or green stuff.  This is the one you usually see on building sites, particularly for below ground applications.

To make XPS, crystals of solid polysyrene are combined with additives and a patened blowing agent.  The combination is melted under high pressure into a viscous (thick) liquid plastic state.  The goop is extruded through a die (a slot the exact thickness of the panels ) into air at standard pressure.  The blowing agent expands the foam immediately, and is trapped inside (increasing its Rvalue) as the panel is shaped by the die.  The extruder makes a continuous panel that comes out of the machine onto rollers where it cools and the sides are trimmed (sometimes cut with tongue and grove) and eventually, the boards are sliced to length.   Here is a video

The manufacturing process is what gives XPS its desirable properties; its precise size, its hogeneous structure, its compressive strength and tough skin.  The trapped blowing agent gives it a superior R value and premium price.

XPS is recommended by John Hait in his book about how to create a PAHS umbrella.   Proponents are quick to mention that it has a higher compressive strength and lower water permeability (compared to EPS).  It also has a decent R value of about ~5 per inch.  You can also buy even higher density XPS that has 25 PSI rating where you need it.

But the more I look into it, the more I suspect some of the difference is marketing to help sell products that are still patent protected.  Big companies like DOW (blue stuff) and Owens Corning (pink stuff) need to promote their products to compete with the much more widely available and lower cost EPS (Expanded Polystyrene).

Here is an article questioning the supremacy of XPS, and here is a website Owens Corning put together to protect their brand…  Personally, the Owens Corning arguments didn’t sway me.  Yes, they show water passing thru EPS, but it passes right out again (that could be a good feature).  The water injected into the XPS stays inside and permanently degrades it (by forcing out the blowing agent), just as the EPS people say it will.

Traits like dimensional stability, R-value per inch, and even the higher compressive strength and water resistance are not as critical as you may thing when constructing an insulating umbrella.  I explain why in the EPS section below.

If you actually expect that your rigid insulation will be submerged in water much of the time, we recommend you reconsider building an earth shelter on that site.

XPS was originally invented by DOW in 1941 (from Polystyrene which was invented over 100 years before that).  Its first application was to help float life rafts for the US Navy.  They have been promoting and defending their products aggressively ever since.  Polystyrene is less aggressively defended because it is not owned by any one company.  Competition between polystyrene producers reduces costs but leaves less money for marketing and little time for cooperation.

EPS (Expanded Polystyrene)

This is the stuff white coffee cups or packing peanuts are made of.  It can be made (steam compressed) into any shape, which is why it is also the stuff that ICF (Insulated Concrete Forms) companies use.  Its manufacture is not so limited to a handful of competitors, so the prices are much more competitive.  It is often sold without a brand name, or you can find branded versions that add features like a polylaminate waterproof facing.  It does have a lower R-Value per inch, but a higher R-Value per dollar.

To make EPS, the factory starts with tiny 1 mm polystyrene pellets.  These are expaned with steam  to 40 times their original size (pre-expanded as the blowing agent, pentene gas, escapes and is replaced by air).  The now larger pellets are cooled, dried and stored until needed.  To make anything from the pellets, they pack them into the right shaped mold.  In this video, they use a block mold because they are just making rigid panels.  The amount they pack into the mold affects the density, compressive strength, impermeability and the price.  The pellets are steam fused together.  In the case of rigid foam boards, the large block is sliced precisely with hot wires strung across the assembly line.  This second video shows how cups are made in much smaller molds.

Waterlogged?

The primary criticism against using EPS below grade is that it absorbs water more easily than XPS.  However, studies show that while it does absorb water more easily, it also dries out more easily.  Studies done on samples that were buried for over a decade show that the EPS actually retains its R-value much better than XPS.

Manified cross section of EPS foam showing the microscopic gaps between the beads where water can pass through

EPS is basically bubbles of air surrounded by rigid polystyrene.  If the air is displaced out by water, other air can replace it when the water leaves.  On the other hand, The R-value for XPS depends on the special blowing agent gases trapped in its cells.  When that gas leaks out over time, the R-Value is permanently reduced.  In an effort to keep the gas from leaking out in the first place, XPS has more polystyrene around its bubbles.  This gives it extra density and compressive strength, but also means that once water gets in, it is very difficult to dry the XPS out again.

But the ASTM C272 test shows that EPS absorbs water easily…?   In that test, a 3 inch square of a half inch thick sample is completely submerged for 24 hours and then immediately weighed without giving the water any time to run out.  This is hopefully not what will happen to your panels.  EPS industry excavations of real EPS and XPS boards that were buried for 15 years tell a different story. www.epsindustry.org, Or here, or the technical bulletin here…

Crushed? Lets do some math.

Some builders are concerned that EPS has a lower compressive strength than XPS.  This is true, but the real question is how much compressive strength do you need?  Lets do some math.

What compressive strength do you need to support 6 inches of concrete (assume 150lbs/cuft)?  Lets take that cubic ft and cut it in half to get down to a 6 inch thickness weighing 75 lbs/sqft.  Then lets divide by 144 sqin/sqft to get 0.52 psi…  A lot less than the 10 to 15 psi offered by low end EPS.

Lets park a 10,000 lb forklift on the concrete, that weight gets spread wide by the concrete and rebar.  Even if we assume that the weight is all applied in a small 6ft x 6ft area (it would really spread much further), the pressure on the underlying insulation never exceeds 2.5 psi.

Now lets imagine we are talking about an earth sheltered umbrella without a concrete layer to spread the load out.  Lets imagine that we have 3 ft of earth over the umbrella (assume 120lbs/cuft).  That is 360 lbs/sqft divided by 144 sqin/sqft equals  2.5 PSI.

What if we drove a 35,000 lb bulldozer over it?  If we assume that the bulldozers treads spread its weight across a 10 ft x 10 ft section of the surface and assume that the load comes down at about 45 degrees (angle of repose), then we would be spreading that load over 256 sqft of EPS.  That comes to less than 1 psi of additional pressure.  Interestingly, the pressure would be higher if the earth layer were thinner.

Falling apart?

A third concern is that the EPS beads can crumble apart.  This is less of a concern with the higher density EPS, but you could certainly have thicker boards crack or break apart when trying to build your earth sheltered umbrella.  This problem only gets worse with thicker boards (they are less flexible).  I don’t have a good solution for the cracking…  But perhaps an umbrella design, featuring sheets of waterproof vinyl or plastic between the layers of rigid EPS insulation, will mitigate the problem or at least trap the cracks.

My Rigid Recommendation:

I would recommend not buying in to the marketing about only using XPS for underground applications.  Instead, look at the research that shows that buried EPS outperforms XPS over time, and for a lower price.  Shop around and go for the best r-value price.

I ended up with a truck load of new XPS foamular 250 sheets that I got a great discount on. However, I also plan to buy a load of used EPS insulation from another source.

Since my umbrella design is layered, I may try a hybrid design with XPS on the top layer to help distribute point loads out a little further and reduce compression of the less rigid EPS…

Sourcing?

One of the most pleasant surprises that I found was that I could get good recycled EPS for a fraction of the price of new.  One company offered me a truckload of more than 20,000 sqft of type 1 EPS for under $1000 (this deal evaporated later when I tried to order).    Another company offered me a 7x higher price on a truckload of EPS that was 2nd hand, but never actually used (and therefore in better shape). Google “reclaimed EPS rigid insulation” to find companies near you.

XPS is also available, but tends to fly out of the yard as quickly as it comes in.

Reading various discussion boards on the subject, such as this one, you should be careful to make sure you know what condition to expect.  The reused EPS may have some small holes or other minor imperfections, but that doesn’t matter much when you are laying it under your slab or making an earth sheltered umbrella.  From time to time, you may also be able to get “new” EPS from the recycling place.

While you are looking for recycled EPS, you could also find used billboard vinyls to build your umbrella with.

For building a long lasting, relatively low cost, high thermal capacity and long lasting earth sheltered home, concrete is my preferred material of choice.  I have seen many concrete designs from small earth sheltered homes made of 12 ft diameter precast concrete drain pipe designed by Michael Janzen to the very sculptural earth sheltered homes designed by Peter Vetsch.  These designs look great, but probably wouldn’t perform very well in a norther climate where their solid concrete walls would make it very difficult to maintain a thermal differential between inside and outside.   In other homes where the designers have striven to separate the inside from the outside thermally, there may still be some direct connection points that allow heat to conduct outward.  This path for heat loss is known as a thermal bridge and it is a serious problem in concrete construction…

I like how Joseph Lstiburek puts it at the start of his building science.com article on the subject;

If an alien from another planet looked at our construction practices he would conclude that we have too much heat in buildings and we want to reject that heat to the outside. We expose our concrete slab edges and our concrete frames. We build our structures like heat exchangers with protruding fins that transfer every last available BTU across them—like huge concrete “Harleys” with air-cooled structural frames ~ Joseph Lstiburek

In this somewhat humorous article, Joseph is talking about high rise buildings where the slab for each floor also forms the balconies that cantilever from the building.  This is a bit of an extreme case.  However, I have seen cases where earth sheltered or earth bermed homes are constructed with some exposed structural concrete or perhaps with a steel stud front wall leaking heat, which I discussed in an earlier post.

He ends with this;

Who says we have to live with those thermal bridges?  You want to get serious about energy efficiency?  Get serious about thermal bridges.  That means exterior insulation on steel studs and structural frames, off-set relieving angles for brick veneers and some serious structural-thermal thinking for balconies and projecting structural members. We mechanical engineers are going to have to get to know those structural engineers better. And then we both have to have a chat with the architect. Some interesting times are coming . . .   ~ Joseph Lstiburek

The point is that, whatever you build, you need to be very careful about thermal bridging.  This is even more true if you are building in the earth with the purpose of energy conservation.  It would be a shame to get it 99% right, and then lose much of your heat thru that one thermal bridge you didn’t worry about…

I saw pictures of a concrete dome home in Colorado (not earth sheltered, but could have been).  The owners had constructed the dome out of cement, then added several inches of insulation over “almost” all of it, and then covered that with a second layer of steel reinforcement and concrete.  There was one spot at the peak of the dome where a steel connection plate connected the steel reinforcing between the inner and outer concrete shells.   This one square ft of steel prevented any insulation from being placed between the inner and outer domes at that location.  The insulation covered 99.9% of the dome, but that one spot was a steel-reinforced concrete thermal bridge.  Even worse, because it was the structural connection to the reinforcement on both sides, it was essentially the highly conductive hub of a heat collection and distribution array.  It allowed heat to efficiently travel around the carefully placed insulation and radiated enough heat to keep a 5ft radius area on the roof snow free all winter.

Earth sheltered homes have some advantage, at least for the earth sheltered parts.  Earth sheltered builders can use a continuous umbrella of insulation to cover the majority of the home without being too concerned with structural attachments  etc.  The weight of the earth (gravity) will keep the insulation in place (assuming you have been careful to design for stable soil, including looking after erosion,  angle of repose, etc.).  Gaps in the insulation can fill with dirt which is considerably more conductive, but not nearly as bad as a steel/concrete thermal bridge.  This gap problem can be prevented by offsetting the layers of insulation and using sheets of plastic between the layers.

However, most earth sheltered homes do still have some “above ground” portions.  Many don’t seem deliberate enough in their efforts to prevent thermal bridging.

If you need to connect concrete on either side of the insulation your engineer may specify rebar.  It is not the end of the world if a little energy is leaked thru the rebar thermal bridge, but it would be even better if you could reduce or avoid this.   In my case, I had several horizontal concrete structures that would need to be outside of the insulation and would carry spill over earth.  I managed these in two different ways.

For the eyebrow sunshades which needed to cantilever out away from my building, I extended the structure backward to more than counterweight the cantilever.  The eyebrow concrete is completely isolated from the structural concrete of the home.  The majority of the eyebrow structure will be buried under the earth the on the roof.   The weight and shape of the eyebrow should be more than enough to keep it in place.  If I need to add any rebar, one option would be to use fiberglass rebar which would be much better and preventing heat loss.

Instead of a parapet (which can be a huge thermal bridge if not designed carefully), I have chosen to go with a horizontal sunshade and slope the earth down on to it.  This will make the green roof more obvious from the front, instead of hiding it behind a high parapet.  But I needed a way to support that sunshade against the front of the house.  Instead of cantilevering it out and worrying about such heavy loads hanging from the front wall of the house, I designed separate foundations and columns that carry all the load.  The overhang appears against the house, and the columns appear as pilasters of the main home, but thermally and structurally speaking, it is a totally separate structure.

(work in progress, images to come, etc)