Now that I have sorted the toilets out I have gone back to thinking about the pros and cons of laying insulation under the floor of the building.
The Fundamental Conflict
I have been struggling with this issue since starting my house design. In a standard house floor insulation would be a no-brainer. Insulation is placed under the floor to prevent the warmth built up in the interior of the house from being leached away into the somewhat colder surrounding earth during the winter months.
Earthship purists argue that floor insulation cuts your building off from the massive thermal battery also known as the Earth. Consequently, although your bermed building has substantial thermal advantages over a typical stick framed house, it is not taking advantage of the earth’s constant temperature and is consequently missing the boat (or ship so to speak).
My Take
To me this insulation issue has two practical realities; physical comfort, and the energy consumption required to maintain the house’s environment at a livable level.
Typical house construction achieves physical comfort by using energy (fossil fuel, electric, wood) to maintain the inside of the building at a comfortable temperature. Take away the energy and the building quickly matches the surrounding temperature (frozen in the winter, and really hot in the middle of summer).
Our current house (two 1970’s era 10’x30′ office trailers placed side by side) adequately demonstrates using lots of energy to maintain a comfortable inside temperature in the winter. The walls and roof are very poorly insulated. Last year when it started warming up in the spring our ceiling developed ‘leaks’, despite having been re-roofed recently. In spots the roof insulation and vapour barrier is so thin or non-existent that the warm moist air rising through the ceiling condenses and forms ice between the ceiling and the roof. Presumably when the ice gets thick enough the formation of ice drops off because it is now insulating the ceiling! In the spring this ice melts and leaks back into our living space. We heat with electricity and due to our poor insulation we spend a lot to maintain a comfortable temperature in our 600 sqft.
A typical house built today has better insulation but the problem is the same. If the energy is shut off the house rapidly becomes a really well insulated (by current standards) ice box. So, we achieve a comfortable living space at a high energy cost.
The approach taken in earthship design is to passively heat the house with solar power stored in thermal mass. The idea is to achieve a comfortable living space at no or minimal cost. The issue becomes one of defining comfort. By today’s standards comfort means a constant inside temperature in the neighborhood of 20 degrees celcius.
I think comfort may need to be redefined when high energy cost heating is limited. The reality is that the inside temperature will fluctuate somewhat (say in a range from 15 degrees celsius to 22 degrees celsius). This implies a willingness to use sweaters as required or supplementing the passive heat actively when the solar gain is not enough due to cloudy periods or cold snaps.
In many parts of the world a home that guaranteed a livable space with minimal energy might be considered a slice of heaven on earth. As little as 60 to 70 years ago many people in North America would have whole heartidly agreed. We have been spoiled by cheap energy to the point that acceptable comfort is quite rigidly defined!
This comfort versus energy usage trade off is highlighted in cold climates by the issue of floor insulation. Without floor insulation the building taps directly into the constant heat sink provided by the Earth, thus making the building easy to heat. In fact, you could probably lock the door on your building for the entire winter without taking any precautions with regard to frozen pipes or frost/water damage. The constant earth temperature and passive solar gain will guarantee that the living space maintains an above freezing temperature. The trade off for this low maintainance living is an inside space with fluctuating temperatures and a cold floor.
What Other Thoughts Exist on this ?
Still not feeling comfortable with this insulation issue, I read fairly extensively over the last couple of weeks to get a better handle on it.
Turns out a bermed earth house builder named Rob Roy has built earth bermed housing both with and without floor insulation and lived in both buildings. His buildings are both close to the Canadian border in New England so the winter temperatures he is dealing with are similar to ours. He documented his experiences in two of his books that I listed in the references. His first building had no floor insulation and his second one did.
His take on insulating a bermed building is very pragmatic and dependent on your climate (he differentiates between the northern and southern states). In a warm southern climate your primary goal is to maintain a cool inside climate during hot weather. Using no insulation under the floor of the building makes sense as the earth’s temperature acts as a passive air conditioner.
In northern climates your goal is geared more towards maintaining a warm inside climate during cold weather. Quoting from Earth Sheltered Houses, “Without insulation … the fabric of the building becomes one and the same with the earth’s mass … In order to control the mass fabric of the home itself, we must place the insulation between the home’s mass and the earth.” He is re-stating basic thermodynamics and his argument makes a lot of sense.
I suspect the earthship design has been successful without floor insulation because most earthships are being built in New Mexico and are more concerned with summer cooling than winter heating.
My Conclusions
We will provide alternate heating in our home with the two wood stoves shown in our plans. These stoves are more aesthetic for us than practical. We have had wood heat in the past, enjoyed it, and miss it now that we rely on electric baseboard haters. We hope to construct these stoves ourselves as detailed in the Earthship Volumes as opposed to purchasing.
We are also debating putting radiant heat in the floors to provide an active means of heating the thermal mass of our home. We have some concerns about our solar gain in December and January. We live in a fairly narrow river valley and get minimal sunlight due to clouds and fog during these months. We have access to significant quantities of waste wood due to our location so a radiant heat system fueled with wood seems practical to us. The decision to install radiant heat will be governed ultimately by cost.
In a typically Canadian manner I have chosen to straddle the fence on the issue of floor insulation. My intention is to insulate under the footings with 1″ of rigid EPS (R5). I also plan to insulate under the floor spaces (again R5). I will NOT insulate under the planters or cistern. I also am thinking of leaving a percentage of the central floor space uninsulated.This practice of leaving some of the floor uninsulated is used sometimes in conjunction with radiant heat floors to guarantee that if the building is left unheated for periods of time over the winter the facilities sensitive to freezing will not be damaged because the building is coupled to the Earth’s constant temperature. I think this decision will have minimal impact on my ability to control the temperature of the mass fabric of the home, and keep it hooked up to the Earth’s thermal battery.
Hi Chris,
I’m interested in how your earthship came out, with the floor insulation choices that you made. No judgement, just wondering.
Let me first say that I am neither a builder, nor a thermal physicist. But I have been paying attention to renewable energy and energy efficiency for a long time…and this is my understanding (it did take my brain a while to wrap around):
Heat only moves in “one” direction–from the hotter body to the colder body. You will never “gain” heat from the thermal mass of the ground unless (like you mentioned, freeze protection on vacation) the interior space drops below the ambient temperature of the earth (55°F or so). But this 55° is far below a preferred living space temperature. If you like your living space 65 or 70 degrees, that is hotter than the ambient earth temp, and so heat will “always” be moving from your home to the earth.
The benefit of earth-bermed house then is twofold: in the summer, you can take advantage of that movement of heat to cool your house by dumping heat to the cooler earth. And in the winter you can be happy that the heat leaving your house (even though you’d like to keep it) is leaving slower than it would in a non-earth-bermed (and non-insulated house). This is because the “rate” of heat loss is based on the difference in the temperatures. Your 70° home will loose heat slower to the 55° ground than it would to the 30° winter air. But it’s still loosing heat, never gaining it.
The thing about insulation is that it slows that heat transfer, slows that loss. Just as insulation will help prevent you from running your furnace to heat up the frigid outside air in the winter, it will also help prevent you from using your furnace to heat up the chilly earth all winter too. Walking into a 55° cave on a cold winter’s day may feel warm at first, but eventually it’s still a chilly 55° cave. And heating that cave would take a lot of energy because that thermal mass of the earth can suck up a lot of heat and still stay 55°. Insulating that cave would be the best of both worlds.
I’d say, use insulation, even on an earth-bermed home.
Then what about the advantages of thermal mass? you ask? They’re really a different concept than the reduction of the delta-T (temperature difference) mentioned above. Yes thermal mass can warm up all day from solar gain, and dissipate that heat back into the room at night (if it’s hotter than the air). And in the summer (in a cooling climate) it can help prevent over heating (solar or otherwise) by sucking up a lot unwanted heat during the day. The deal trick is designing the right amount of thermal mass for your heating and/or cooling cycles, and getting them into your living space, but on the “inside” of your insulation. A primarily cooling climate will need one kind of mass ratio, a heating environment another, and a mixed environment another, all to help balance out the diurnal (daily) temperature swings from daily heat gain and loss.
Yes, there are some people experimenting with “annual” heat storage…trying to warm up the earth under their house all summer so they can “use” that heat in the winter. But I believe that they still have insulation around the living space, and instead are “actively” moving that heat (pumps or fans) to their storage mass under the house. Then remember, unless they are actually heating up that mass to hotter than 70° they will never “gain” that heat back…but rather will just use it to slow their losses in the winter.
I hope some of this makes sense. I wish someone in the industry would really do a good job of describing the mass/insulation balance, and how it pertains to different climates. Too many people think that the two are interchangeable, as in “Cob walls don’t have great insulation, but they make up for it in mass”. Well concrete block has good mass too, but I don’t want to live in my garage.
Peace,
Ben
You have very nicely summarized this issue … very well written.
We did insulate under the floor (a fairly common decision in colder climates, and a building code requirement in our region) and we also have insulation in our berm (this has become pretty much standard in earthship construction and referred to as thermal wrap). The goal was to create a large thermal mass isolated from ground temperature to be able to better control the inside climate. In other words we are attempting to increase the ambient ground temperature in a pocket around our building.
As you have described the trick is to balance the thermal mass and insulation to get the desired result for a given size of home in a specific climate … there is very little information on this kind of design that I was ever able to track down so it mostly seems to be trial and error.
We are now into our second winter in our home and are quite happy with the results. I have no data on ground temperature inside our berm so I cannot give you any answers based on hard data. I can say that the house seems to retain heat better this second winter than it did during our first … I am assuming this is partially due to the earth mass that is captured inside the thermal wrap being at a higher temperature than ambient earth temperature. I should also add that we have a masonry stove (kind-of … it is really a rocket mass heater) built into the wall and floor of our living room that I suspect adds a fair boost to the heat storage in our thermal mass.
I did not know very much about ‘annual’ heat storage when designing and building our home but I am more convinced now that this would be a good active heating addition to an earth sheltered home.
I hope my response made as much sense as your comment!
P.S. – I agree … I wish there was more design/building knowledge on this topic.
Actually there is a fair amount of information about insulation in and around your home.. if you looked a passive annual heat storage PASH you would have discovered.. that you put the insulation in the wrong place.. not under the house but around the perimeter to allow the ground to act as a true heat sink. 20 feet around allows you to keep the ground dry and put a lot of heat into the ground that can be re absorbed into your earthship as needed..
6 months supply of stored heat is way better than three weeks worth
During the design of our home I did some investigation of passive annual heat storage … and learned more about it when we were well into construction. I believe there is some amazing potential with seasonal heat storage but did not go beyond the implementation of a thermal wrap around the perimeter of the earthship as is typical with any recent earthship construction.
I am more than satisfied with our decision to insulate under the travelled portions of our floor space … it allowed us to build to code and if we ever finish our radiant floor we will have warm feet as desired!