Hi, All,

My normal territory is in Northern California, where it's hot and dry.  Recently, however, I helped with an evaluation in the tropics, where the temperature was regularly in the mid-80s, as was the relative humidity.  It was their winter.  One of the chief goals is to reduce electrical usage, which in turn means reducing the use of air conditioning.  Much of the load on the AC is to remove moisture, so it strikes me that one strategy is to limit moisture intrusion. 

But how do you determine how the moisture is entering?  Let's assume:
     1. We've performed blower door tests and know how much air leakage there is.
     2. There is no bulk water to deal with.
     3. Wherever possible, capillary breaks have been added.

One of the buildings I looked at is two story, slab-on-grade (with no moisture barrier and a high water table); walls are styrofoam blocks with concrete cores (essentially like our ICFs) and stucco cladding (no rain screen).  How do we determine what percentage of the moisture is due to infiltration; due to diffusion through the walls; diffusion through the slab?

And a follow-up question: if we were to treat the slab to block diffusion through the floor, would that increase the rate of diffusion in inaccessible areas, i.e. beneath the sole plates of the walls (several of the other buildings are slab-on-grade with wood frame construction)?

Thanks.  I look forward to hearing how you would go about analyzing this.


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Someone smarter than me could calculate the rate of vapor diffusion through the various materials you mention (such as the slab floor), given a few facts. I do think it would be worth exploring how much the slab might be contributing to interior moisture load, but I wonder if the primary issue is going to be air infiltration. What sort of blower door numbers were you getting? Unless these are well air-sealed, and the windows and doors are always closed, I would think that it's infiltration that you're fighting. 

Hi Bruce ad David,

The outside air, temp and humidity, sets a baseline for how low you can go without some form of powered mechanicals to remove moisture and or cool the space.  Even a tight house will need to exchange a lot of air to reduce further contribution from the house and occupants.

I'm sure some of the lab folks can tell us how to measure or calculate the moisture contribution from each source, but what comes to my mind is an extension of the old plastic square on the floor.  If you created a sealed insulated box, open on the bottom and taped it to the floor with a temp and humidity gauge inside (a logger would be better) and recorded the rise in humidity over a period of time, that curve would be interesting. 

As for reducing the moisture, most other contributors will have been well documented along with suggested solutions.  What are they currently doing that we may not be aware of?


Hi, David and Bud,

Thanks for your replies.  Full disclosure: I never performed a blower door test.  The island is in Fiji, and remote even by Fijian standards.  Also, the current is 220V.  So I wasn't able to arrange the transport of the right equipment.  I think you're right that air leakage is the major source of moisture intrusion, and the residents of the island have air sealed as best they can without the benefit of test equipment to guide the process.  I'm now working to get them a blower door and IR camera [know of any potential donors to a U.S. recognized non-profit?].

I've been told that diffusion is probably an insignificant source of moisture intrusion.  But there's this side to me that would like to be able to either verify or disprove this.  To gain a deeper understanding of moisture transport.  It may be, Bud, that your suggestion of using a sealed box is the most practical approach.  Curious to see what other responses we get.


I have an approach for estimating air leakage using just a manometer.  Only slightly better than a wild guess, but so much of what we do is based upon guessing, a little more can only help. 

An option for a blower door might be Australia, they are well into energy efficiency and perhaps a rental or loaner.  A little closer.  Or, you could use what they have available over there and build your own, plywood, attic fan, and air flow meter.  I've always been a scrounge.


I think you could do a passable job with a box fan, an anemometer, and a manometer.

From his one email notice I just have spent the last 6 hrs. reading and downloading and printing out all that I can about moister removal. I live in central Virginia and the last 2 two weeks seen more condensation on exposed fiberglass bats than I care to talk about.  http://us-mg5.mail.yahoo.com/neo/launch?.rand=fhv8picl0ch0k


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