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Community and Q&A

Can I use my basement as an earthtube?

Sal_123 | Posted in Energy Efficiency and Durability on

I am building a house in Zone 5, stick framing, 1.5″ exterior outsulation, 2×6 walls with a full basement. As it starts warming up (finally!) I can already sense the second floor is notably warmer than the first floor. Two basement stair openings are wide open, no doors, literally exuding a cool air that lingers in the first floor, almost making it feel like the AC is on. Entering the first floor from outside there is significant temp drop. I am attributing it to the well insulated home (caulked framing, caulked and taped XPS panel seams, caulked plywood seams) that retains the coolness of the basement. It’s still all open still, no sheetrock, no doors, and am thinking of “pumping” that cool free air from the basement into a duct system that brings it to the second floor. Kind of like an earthtube concept. I am in the process of designing my own ERV system, and not sure if I can (or should) marry the two ideas. Perhaps a system that draws air from the basement, rather than outside (tempered air, warmer in the winter, cooler in the summer). I have a small room off the main basement that can be sealed and isolated as a plenum of sorts. So outside air enters freely into the “plenum room” by a damper, where it is naturally pre-treated, again – warmer in the winter saving the ERV from getting hit with 5 degree air and cooler in summer where it may be 68 degrees rather than 80 – it is picked up by a 10″ duct and fed to the ERV in the attic which then distributes the air to the second floor. Will make sure that basement room has low radon levels (for what it is worth).
What do you think?

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  1. GBA Editor
    Martin Holladay | | #1

    The main reason that your idea won't work is that the volume of cool air in your basement is very small compared to the volume of cool air you would need to cool the upper floors of your house in hot weather.

    If you pull cool air from your basement, you'll have to replace the air you've removed with a return-air duct. It won't take long for the air in your house to be thorough mixed, and then you won't be getting any cooling effect.

    During the first few weeks of summer weather, you can always sit in your basement to enjoy the cool temperatures. That's the easiest way to take advantage of the pleasant temperatures down there.

  2. Sal_123 | | #2

    Martin thanks for the prompt reply. I mentioned in my question that the "plenum room" would have a damper system to draw outside replacement air freely. I would set up a filtration system, possibly one of these in-line fans to draw in the air and run through a Merv filter, then dumping it onto the plenum space. So the "plenum room" either has an open damper. it will draw outside freely as needed, (or keep it positive pressure) while the ERV will dump the "used" stale air outside, so the air does not re-circulate. New air is constantly entering the system, however, rather than pass via an earthtube, it enters the basement. In winter months, when the air is 5 degrees outside, the basement room designated as what I am calling a "plenum room" is isolated from the rest of the basement for obvious reasons.

  3. GBA Editor
    Martin Holladay | | #3

    My answer is unchanged. Replacement air has to come from somewhere.

    If you don't believe me, go ahead and build it. Then report back with your results.

    The potential energy savings are so low that the savings won't justify the cost of the ducts and fans.

  4. Jon_R | | #4

    In most climates, you will have humidity problems (in addition to the lack of sensible btu).

  5. Sal_123 | | #5

    I must not be explaining myself well. Replacement air enters the designated "plenum room", Ie the small room of the basement, it can be set up with a damper to allow air to enter as the ERV draws it in(or it can be positive pressure via an in-line fan with a filter system, <$200 for matls). So there is replacement air, there has to be or we'd be rebreathing our own stale air and the ERV is of no use.
    The cost is not in materials, that's negligible. The true cost is inconvenience and labor. If it does not work, you need to re-run the ventilation tubing.
    Thanks for considering my question nonetheless.

  6. Sal_123 | | #6

    Jon R, yes, the humidity in the basement air is much higher. I agree this is an issue, unless it is dehumidified, either in the basement, or on the second floor.

  7. GBA Editor
    Martin Holladay | | #7

    Let's say that your plenum room is 8'x8'x8'. It's filled with cool air -- maybe the air is at 50°F.

    The fan on an air handler (the type of fan used for cooling) might move 600 cfm. You have 512 cubic feet of 50°F air in your plenum room. You turn on the fan. All of the air is evacuated out of the plenum room in less than a minute. Now your cool air is gone. What happens next?

  8. Sal_123 | | #8

    Their is an opening on the external wall of the plenum room with a damper, new outside air replenishes the air in the plenum room.

  9. GBA Editor
    Martin Holladay | | #9

    If the fan is pulling 600 cfm, the outdoor air will enter the ductwork without spending enough time in the plenum room to cool off.

    Even if the plenum room has walls that start out at 50°F, there's not enough (a) time, or (b) surface area, or (c) delta-T, to lower the temperature of the air rushing from outdoors into the plenum room, and then into the cooling ducts.

    It's probable, however, that the walls of the plenum room will soon be damp with condensation.

  10. Sal_123 | | #10

    And there is my answer. The small capacitance of the room and brief time spent in the plenum room will not allow the new incoming air to be tempered to a useful degree and the moisture in the air will end up likely condensing on the walls.
    Possible fixes: Use the entire basement as a plenum (increase capacitance and thus time spent to alter Delta-T) with constant dehumidification of the air.
    Overall, seems like not a great idea, thanks.

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