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Community and Q&A

What is the R-Value of water?

Kevin_Wyckoff | Posted in Mechanicals on

I am exploring the utility of adding a low profile rainwater catchment system to the outside of a building. I would like to quantify the insulative value of adding a 9.5 inch water tank to the outside of a building.

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  1. GBA Editor
    Martin Holladay | | #1

    For all intents and purposes, the R-value of this water tank is 0.

    More accurately, the water in the tank will follow the rules of convection. If one side (the side nearest your house) is warm during the winter, and the side of the tank farthest from your house is cold during the winter, a convective loop will be established. This convective loop will hasten rather than retard heat flow from your siding to the great outdoors.

  2. Expert Member
    Dana Dorsett | | #2

    The thermal mass of a 9.5" of water-wall is pretty high, and will give you some thermal benefit, but that doesn't translate into an R-value. There will be convection losses, evaporative cooling losses etc.-there is no simple model to work with that would be up to the task of accurately estimating the benefit.

  3. Stu Turner | | #3

    The U value of liquid water is somewhere in the neighborhood of 250 (BTU/hr-ft2-F), so it does have some R Value in an academic sense, approx R 0.004, So Martin is correct and it rounds to zero. Water can be useful as a thermal mass in a passive solar design, it has a very high heat capacity, about 5x that of brick or concrete - but it needs to be in conditioned space or have some insulation in front of it. (For ideas, Google "Harold Hay Skytherm house", or "Steve Baer water walls"). So short answer is no, the idea won't work unless it is part of a fairly elaborate passive solar design.

  4. GBA Editor
    Martin Holladay | | #4

    I agree with Stu. If you were hoping to benefit from the thermal mass of the water tank (rather than the water's R-value, which is negligible), the thermal mass would have to be inside the home's thermal envelope.

    For more information on this topic, see All About Thermal Mass.

  5. wjrobinson | | #5

    Whatever happened to the water windows idea for solar collection?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Whatever happened to the water windows idea for solar collection?"

    A. It moved to the vegetable garden. This application is called the "Wall-o-Water."


  7. Expert Member
    Dana Dorsett | | #7

    Thermal mass on the exterior reduces peak & average cooling loads, doesn't do much for heating loads (except when outdoor temps area crossing through the 0C freezing point, when the thermal mass is water).

    To be sure the benefit of mass is much smaller-spuds when it's outside of the insulation, but still non-zero.

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