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home page » passive solar design chapter » earth tube detail page ... you are here
Earthtubing utilizes conventional, thin wall, plastic pipe to passive solar pre-heat your home's air intake.
Fresh air enters a system of these pipes which are laid around the exterior and interior of your home's foundation.
You can let the air draft naturally through your earthtubes for a truly sustainble, non-electric, passive geothermal system or add fans and filters to supplement the home's back-up heating and cooling system.
Be careful with Earthtube details, though ... while architects are warming-up to the idea, they are making very elementary mistakes, like using large concrete (cement) pipe that is impossible to clean.
The Adolo Leopold Legacy Center earthtubing details are a classic example of very poor design that relies entirely upon mechanical means to attempt to clean the air coming out of the tempering tubes.
Earth tubing (fresh air return tempering) is not suggested for low/zero energy, sustainable applications in humid climates where the designer is concerned about air quality.
There have been numerous designs over the years that attempt to deal with the issues, but they invariably are forced to develop high-energy, mechanical systems to deal with lowering humidity and sterilize air prior to passage through the tempering earth tubes.
Every low-tech passive energy system we have ever encountered fails to properly address the air quality issue.
Hybrids tend to rely upon UV treatment of the air, which is sketchy, at best.
Conventional homes are typically High R value and "airtight", so all of the heating and cooling effect is stored within the air temperature (convection).
If you let the air escape, you lose all your comfort.
High thermal mass building materials allow the heating and cooling effect to be stored within the home's walls and floor (radiant).
This allows you to vent the inside air of an HTM without "losing" all your heating or air conditioning comfort.
Constant venting in the winter is common to HTMs, with fresh air being pre-heated or pre-cooled before it enters the home.
Please note that Earthtubing is not meant as a summer cooling system in hot, humid climates where moisture would reach dewpoint and collect in the tubes.
Weep holes are drilled in the bottom of corner fittings, but these drains can be quickly overwhelmed in a very humid climate.
If you must alter your indoor climate in the summer for personal comfort level, you will need mechanical means.
There really is no truly healthy passive method to de-humidify.
The drawing above illustrates a set of four earthtube ventialtion pipes as they're installed with a monolithic slab. Please note the use of 45 degree fittings which help accentuate airflow and make alignment and spacing easier. Greater spacing between pipes (see below) allows for more surface contact with thermal mass and thus better heating and cooling qualities. Monolithic slabs are confined in terms of space available to separate pipes, but their being imbedded in solid concrete helps make up for the design loss. Please note that this drawing shows insulation under the interior floor slab. With sloped glass, this layer of insulation is typically not installed (see wing insulation detail at end of page). The typical number of earthtubes is four, which is enough to supply average household air volume. One pipe is generally considered to carry enough air for one room. In theory, you could have as many earthtubes as you wish, but the slower the draw-time, the warmer the air moving through it (or cooler in the summer).
Cleaning earth tubes is an easy matter - simply lay nylon cord in the pipes while constructing and use it to pull a bleach or disinfecting solution soaked towel through the tubes. Tie a second cord to the towel so that you leave a cord in the pipes after the towel has been pulled through. We suggest installing a standard furnace filter at the earthtubing inlet grate to prevent dust and insects from entering the pipes. Keep in mind that mold only grows on surfaces with fuel to support it - clean pipes will not support mold growths. Many people worry about keeping the earth tubes clean, but you rarely see folks cleaning their home heating and cooling ducts. Earth tubes are nothing more that air intake pipes - and round pipes are much, much easier to clean than rectangular duct work. Earthtubing is a very simple and cost-effective means to temper air intake for any home. They are especially important for high thermal mass homes, which can be very aggressively vented, ensuring quality indoor air (versus air-tight conventional construction).
The photo above shows a set of four passive geothermal earthtube pipes being installed around the HTM's foundation. Shown below, are these four earthtubes going around an inside corner of the home's frostwall. Later, these pipes will be backfilled with fill dirt and covered with a concrete floor slab. Please note that earthtubing technology is not geothermal heat pumping. Earthtubes are sustainable, non-electric, passive heating and cooling systems. Unlike geothermal heat pumps, earthtubes do not involve any exotic machinery or special type of pipe that we are trying to sell you.
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The waterproofing treatment shown above is a simple mop-on coating.
If you have the money in your budget, we highly recommend using vertically applied, roll-on roofing underlayment, such as Grace® Ice and Water Shield™, for the exterior of your foundation walls.
Don't forget to use their primer first, though!
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The most important factor to successful earthtubing is DRY EARTH. As the sketch below outlines, we stress the need for waterproofing and insulating up to a 20 foot perimeter around the home. Wet earth acts as a constant heat sink pulling energy away from your foundation walls. Dry earth under and around an HTM stores an amazing amount of energy. Earthtubes utilize this energy by allowing fresh, incoming ventilation air to passively gain or lose heat energy before it enters your home. This sustainable ventilation system exchanges indoor air more often, keeping your home's environment fresh without the drawback of "losing all that energy".
With any earth tube system, it is key to note:
It is common to bring all earthtubes into the central utility room. Terminate the pipes in a small junction box, if you wish to filter the ventilation air and/or add a fan to power draft and/or use as fresh air intake for HVAC system. The junction box can be a simple wooden cabinet sized for the filter being used. We have seen "D" Boxes (like those used for septic systems) used to create a very effective interior junctio/filter box. It is also possible to simply terminate the pipes in a wooden framed "wet" wall and install standard galvanized grate/vent duct box (cold air return) for each individual room.
The outlet ends of your earth tube vent pipes can be spread out around the home or clustered in a single room. Using tempered air tubes as the "cold air return" for boiler or furnace is common commercial construction practice. A common upgrade to any earth tube system is powered venting to assist passive draft. Your home is constantly venting out through open windows, bathroom vents, furnace, water heater, dryer and range hood This air is coming in from somewhere - the earth tube pipes (cold air return system). Earth tubes can end in a central terminus box ("plenum") and be fan powered (air handler) or you can individually fan assist each earth tube pipe (shown below). We commonly suggest installing a 4 inch axial fan (12 VDC computer cooling fan) in the end of each earth tube pipe and covering with a protective vent grate. Common computer fans fit nicely into a four inch pipe and can be run on a small solar panel, car battery, or just use a transformer (standard 110 volt AC). A nice touch is pulling cheesecloth across pipe end for filtration and aromatherapy (drip oils onto cheesecloth).
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