Post Requesting Simulations

 

In “Extreme Weather Hits Home, I indicated I would be posting ideal wall simulations. Rather than limiting myself to the ideal – here’s some simulations that show acceptable and problem situations.   – John Banta

1. Looking for latest test results of the “Ideal Wall”and is any of it applicable to commercial buildings?Comment by Jack Lofstrom — October 29, 2007 @ 2:10 pm | Edit This
2. Hi Jack,

The wall assembly types I have been using are modeled for residential structures, but should apply to commercial buildings as well – providing their indoor moisture levels are at reasonable levels. So office commercial should be fine. Manufacturing processes that raise indoor humidity levels (bakery, cleaners, indoor swimming pool …) have not been explored, but logically would tend to get into trouble quicker.

 I have simulated a wall assembly for Montreal Canada, and Miami, FL. The wall works well in Montreal and fills with water in Miami. Removing the 10 mil poly in Miami reduces the problem. I have posted the simulation run for this particular example below.

1. John Banta

Typical Brick wall assembly in Montreal

This is a typical construction style for brick wall building in Montreal, Canada. The wall layers are from outside to inside:

4″ Brick

1″ Airspace

60 lb felt paper

3.5″ fiberglass insulation

Gypsum Board

Latex Paint

The simulation was run using WUFI for the projected period October 1, 2007 to October 1, 2009 assuming these are normal years (not significantly different from influences by climate change).

The blue represents moisture. The wall assembly doesn’t get into trouble because the water vapor doesn’t condense in the wall cavity. Note there is condensation in the air space, but this is okay because it can drain down the 60 lb building paper and drain from the space behind the bricks. 

Typical Brick wall assembly in Montreal Moved to Miami

Here is the same wall assembly if it were moved to Miami, Florida.

 As climate changes and higher levels of humidity occur it is like moving a building northward. Notice how the fiberglass insulation layer has reached the condensation point. That water will tend to be trapped in the wall and would be expected to cause deterioration.

Typical Brick wall assembly in Miami (no interior vapor barrier)

The only difference between this wall and the last is the interior vapor barrier was removed.

These three models when compared demonstrate the significant difference in a wall assembly where a vapor barrier is installed inappropriately.

In my book I predict that climate change resulting in higher levels of humidity will at some point reach a tipping point where many northern buildings will begin to deteriorate inside the wall cavities. Because the developing problem is hidden from view and may not be recognized quickly – severe deterioration may result.

Study ties Increasing Humidity to Global Warming

 The October 11, 2007 edition of the Journal Nature reported humidity levels are increasing and that this is consistent with predictions for global warming. http://news.bbc.co.uk/1/hi/sci/tech/7038278.stm

This further supports my contention that many buildings in the northern part of the U.S. and Canada are going to be more likely to trap moisture in the walls and other building cavities. As buildings in the north experience higher levels of humidity, they will experience conditions more typical of the southern United States. The presence of a vapor barrier on the interior of a building is more likely to allow condensation moisture to form in wall cavities when outdoor conditions are hotter and more humid. I provide a much more detailed explanation of moisture flow in buildings in my book. I also discuss how to predict when conditions will allow moisture to become trapped and how to prevent the problem. One of the tools I suggest is the WUFI model for moisture flow in buildings. This can be downloaded at http://www.ornl.gov/sci/btc/apps/moisture/index.html