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Wall design hot humid, passive cooling

williamfinch | Posted in Green Building Techniques on

Most wall installation guidelines seem to assume buildings will be conditioned continuously. But suppose in zones 2A, 3A or even 4A you had home operators contrary enough to avoid use of AC for as much of the year as possible — but who  may rely on AC for a few particularly hot and humid weeks of the year. I hear such people still exist.

State of the art wall design for hot humid seems to desire the vapor retarder or barrier external to the insulation and the framing, so that the wall can dry to the interior, aided by the dehumidification potential of the … AC. But what if the conditioning unit is not being operated for much of the 7 months of the year? And consider that some of the best temperatures for passive cooling during the long shoulder seasons may be accompanied by consistently high humidity. Perhaps under those conditions it’s a moot point, since humidity would be high both sides (though sometimes higher inside or out depending on time of day), and drying might not occur regardless of where the vapor barrier is. But is there any reason to believe an external vapor barrier would actually increase wall moisture issues in (mostly) non-conditioned buildings?

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  1. Jon_R | | #2

    If you have a bulk water wetting event, drying to both sides will outperform a single side. General design principle - don't make walls/ceilings any more vapor closed than necessary.

    Regarding mold prevention, consider low moisture retentive designs that efficiently allow sporadic dehumidification.

  2. JC72 | | #1

    There's a big difference between the relative humidity in zone 2A vs 3A/4A. 3A/4A are considered mixed-humid. Passive cooling is used in late fall and early spring.

    As for your question, it's not a problem for the wall. The problem is for the rest of the house because mold is always a concern in 2A (Carpeting, drywall, furniture fabrics, HVAC system).

    1. williamfinch | | #3

      As we plan a number of projects for our area, we're trying to rethink the architectural details to minimize humidity issues before resorting to air conditioning to solve all problems. There are many reasons practical and philosophical for doing that. It's a thought process that I think is gaining some ---ugh-- steam in many warm/hot humid areas of the world. Some of that process is (and has long been) considering what type of materials are used in houses.

      Having lived and worked in the most humid conditions in all three, I'd note that humidity issues differs greatly even within each of those zones, depending on where you live, even within a few miles in some cases. But even though night temperatures differ in each of those zones, the resulting humidity issues are similar (and perhaps effectively worse in areas where temperatures don't demand conditioning -- for various reasons, we've found humidity issues in some of our 3A abodes to be as serious as they were in coastal 2A).

      I recognize that I'm ignoring the "rule" that passive cooling should be restricted to late fall or early spring, but much of the rest of the warm humid world happily ignores that rule as well.

      1. JC72 | | #5

        Don't assume people who live in humid climates prefer to live with only passive cooling. Those that do typically don't have the choice.

        There's nothing philosophical about the need for air conditioning. Air conditioning/de-humidification improve occupant comfort and by extension well being. As a result it has allowed certain areas of the world to experience a vast improvement in standard of living. Hell, it happened in the US with the economic expansion in the SE US and Florida.

  3. williamfinch | | #4

    Jon R -- yes, that thought keeps banging around in my head as I think about, for example, effective vapor barriers like multiple layers of taped polyiso in roof, wall and floor assemblies-- even if external. My guess is the additional problems in non-conditioned spaces are more theoretical than realistic -- inevitable roof heat gain may limit problems there, but wall and pier-elevated floor may be a different animal -- wonder if anyone has tried to test it under those conditions?

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