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R Value Of Soils-Underground House Roof

bnjmn Member Posts: 53
Have a job coming up just starting to work on preliminary planning now.  Underground house, 10,000 square feet built into the side of mountain.  House is all 12" ICF with 16' interior walls.  Entire house will be below grade except for east facing exposure, all timber frame and plate glass.  Roof system will consist of ICF panels with an r value of 10, roof concrete will be 10" thick with a tapered layer of urethane foam sprayed directly to concrete.  Foam will be around 6" thick at center of building and taper to exterior down to 1" thick.  Foam will be providing some insulation and slope for roof drainage.  Roof will have membrane over foam then 4 feet of soil then grass.

My real question I guess is the R value of soils.  My searching has only come up with about R .25 per inch of soil.  I know there are all kinds of arguments whether to insulate the roof or not as well.  This place is not going to be passive or net zero, we just want to have accurate numbers in determining what it will take to heat and cool.

Any Thoughts or Idea's


  • icesailor
    icesailor Member Posts: 7,265
    Underground Insulation:

    Someone may say that I am FOS, but a buried foundation, has very little heat loss. Because it never sees the OAT at the moment of the exposed parts or walls. That's why basement/cellar calculations are taken at two points. Above grade/exposed and below grade. Even if you get 4' of frost and it is zero outside, it isn't zero where the frost stops. So, that below grade number is taken as an average. The factor is very low. What they are proposing on the roof is not something that you can easily calculate. You'd be better off taking some adequate number and designing to that number. You can pick lower numbers but you just increase the cost of installation with a cost that can never be recoverd,

    I hope that they aren't going to try to put any sort of foam insulation on the outside surfaces where there will be earth in contact. Insects like termites and ants will just love it. Take a infra-red thermometer gun and shoot a cellar wall (that is un-insulated) when it is freezing outside. You will be surprised at the results.
  • bnjmn
    bnjmn Member Posts: 53
    Foam Insulation

    Very little heat loss or gain in this place for sure.  My biggest loads are on the front exposed portion of the house and the roof.  That's why I'm looking for some soil info.  There will not be any exposed foam insulation, entire roof and walls down to the footings will be covered with waterproof membrane.

    Cant disclose location but outdoor design temp is -20 winter 100 summer.
  • hot_rod
    hot_rod Member Posts: 19,860
    plenty of info

    about soils at the various GEO sites.

    A neighbor in Utah had an underground home, 20 plus years ago. He struggled with water infiltration from day one, maybe still does :) The membrane installation and drainage is a critical detail.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • icesailor
    icesailor Member Posts: 7,265
    Soil resistance:

    Anyone offering soil heat resistance is making guestimates. What is the type of soil? Will it be soaked and frozen ( no resistance) or have a lot of organic matter and offer some resistance. How much resistance for the type of soil?

    I've always planned for the worst and been grateful for something better.

    I always wonder about jobs like this about how they plan for the dew point/condensation on walls in the summer. Do they just pretend it isn't happening when it is?
  • Jamie Hall
    Jamie Hall Member Posts: 21,524
    It's a lot more complicated

    than it looks!  The problem is that the temperature which you have on the "other" side of the wall and insulation is not the design temperature, but rather a mix of the design temperature and the structure temperature -- and conventional ways of predicting the heat loss simply don't work, as the soil itself has a tremendous capacity to absorb or release heat, depending of course on whether the interior is warmer or colder (almost always warmer, actually).

    Soil itself, particularly if it is wet (which it usually is) has a rather poor insulating capability -- not all that different from water.  But, since it can't convect or radiate, all the heat transfer is by conduction -- unlike a conventional wall, where the heat transfer on the outside side (inside, too, for that matter) is mostly convection.

    Anything much over a few feet from either the surface or the building  the soil temperature will be very close to the annual mean temperature, year 'round.

    Have fun!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    edited May 2014
    Underground house heating system

    You may have to size the total btu on the DHW load, so this may be a case where SWEI's asynchronistic dual boiler sizing could be used: a small one for the heating, and a another one for the additional needed, when heating the water. If you use gas boilers, how will they be vented? You don't want too many penetrations through the soil!--NBC
  • bnjmn
    bnjmn Member Posts: 53

    HVAC equipment consists of water to water heat pump, propane condensing boiler for dhw and auxiliary heating. High velocity air handlers with chilled water coils for distribution. HRV's used for bath ventilation and house ventilation.

    Wall condensation should not be a problem being the walls are all insulated concrete forms. There is a 5000 ft shelter on this same property that we did a few years back. Built the same way except for no eastern plate glass exposure. It has no central heating or cooling only a fresh air ventilation system and it maintains 52-55 degrees all year round with no people living in it.
  • Jamie Hall
    Jamie Hall Member Posts: 21,524
    Almost exactly

    what I would expect it to maintain.  That being very close to your annual mean temperature.

    The actual heat loss of the new structure won't be that much, really, since that mean temperature is basically what you're low side temperature is (somewhat analogous to the "design temperature" for a conventional structure).  That soil temperature will be reached somewhere around 4 to 5 feet out from the buried walls of the structure, and you can figure your wall losses based on that much saturated soil.  The roof should have a lower "design" temperature -- in fact, unless it is much over 3 feet of soil, I'd use the actual design temperature of the site.  I'd use that, too, for the exposed window wall.


    What you will also find is that unless you have a large amount of extra capacity in there, like two or three times what the heat loss says you need, it will take days to weeks to change the interior temperature in such a structure.  Literally.  This is something you might want to consider.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gordy
    Gordy Member Posts: 9,537
    Agree with Jamie

    That's definitely a set it and forget it envelope. Ramp up on seasonal startup.