2 Comments to “Floor Insulation … what’s the decision?”

  1. Ben Root

    Jan 22nd, 2014

    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

  2. Chris

    Jan 22nd, 2014

    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.


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