This Old CFD House: Part IV

Welcome to Part IV of a multi-part series, where in this installment Travis continues the “virtual” look at his crawlspace.tm_bio

In the previous installment of “this old CFD house”, we started to peel back the veil on my crawlspace and its moisture issue.  I analyzed the problem as it is, and we made some enlightening findings.  First, really the only way air is getting into the crawlspace is through vents at the front of the house.  If the predominant wind direction isn’t towards the front of my house, my crawlspace will not get the fresh air it needs to get rid of any moisture.  We also saw that the air is stratified in the crawlspace, due to the cool damp nature of this cave-like environment, making it hard for the fresh air to pick up moisture as it travels through the crawlspace.  Lastly, we found the humidity (aka percent saturation) values calculated closely matched those from the crawlspace inspector.


Humidity results with standing crawlspace water

Now that we have an understanding on the problem, we need to start looking into how to fix it.  But not only how to fix it, how to fix it as cheaply as possible, as the bay area real-estate prices are crazy high and the down payment has drastically dwindled my bank account.  While being cheap may get me some ribbing at the office, it is a typical business goal, and FloVENT is an excellent tool to help achieve cost savings for many different types of problems.
After seeing that it takes over 29 hours for a change of air in my crawlspace for a conservative wind speed, I knew getting rid of the surface water had to be the first priority.  If there are puddles of water in the crawlspace a month after the last rain, the ventilation system isn’t working fast enough to remove this water, and disease causing mold can grow in that length of time.  We could have put in a French drain system, but those are costly and would involve destroying our patio, and yet it really doesn’t address the lake we already have in the crawlspace, it just helps prevent new water from getting in (and there are a series of online discussions on their effectiveness and they can be prone to clogging).
In the bay area, there are 3 companies that specialize in crawlspace issues.  2 are actually franchises of the same company, so their prices are the same.  The 3rd is an independent contractor who quoted well below the other company.  Instead of about $14,000 for the 3 phase solution, he quoted 8k.  Still too high for me!  I was interested in getting a sump pump, phase 1 of their 3 phase solution, to get rid of the standing water.   His price was $1000 for this compared to 8k the first company priced this service at.  $1000 is still a good chunk of change, so I wanted to see if that would help fix my problem before committing to it.

A sump pump gets rid of the surface moisture, but it can’t dry out the soil.  So I was still going to have an issue with water evaporating from the ground and needing to get rid of that moisture.  But I hoped that this evaporating moisture would be a lot less then the moisture coming off of standing water.  So I did a little research to determine what a reasonable value was for soil evaporation was.
Well, after reading a lot of agriculture papers on soil evaporation, the best I could find was they quantify this with the term “evapotranspiration” which combines the water loss from evaporation with the water loss from the plants transpiring.  I don’t have any corn fields in my crawlspace, so these rates were likely high.  Also, I don’t have any sunlight down there, which would also drive the evaporation portion of this agricultural moisture loss.  But this was all I could find, so I used the lowest evaporation rate, which was that the top 5-10 mm of the soil will dry up in a day.  Awesome, so with a little more research I found some numbers on the amount of water per cubic meter for different soil types, and I know the surface area of the crawlspace so I could now model this evaporation.
Making the changes in my FloVENT model, I made a fantastic discovery.  By eliminating my surface moisture, the humidity in my crawlspace dropped substantially.  A $1000 sump pump would solve my problem, and I wouldn’t need an expensive plastic liner, or worry about expensive mold problems.   You can clearly see in the following FloVENT results at different heights, that by removing the surface water, the humidity values are reduced.  I was told a safe zone is 50-60% humidity, but even up to 70% I heard is ok.

Results near crawlspace ceiling (house floor)

Results near crawlspace ceiling (house floor)

Results at crawlspace vents

Results at crawlspace vents

You can see there is still a zone of higher humidity, by the garage and behind the porch, due to the poor airflow in this area.  Still this is a whole lot better than before when the entire crawl space had moisture issues.  Also, this is the worst case scenario, with my entire crawlspace soil being saturated with moisture.  The inspectors had only seen standing water in the rear half of the crawlspace, as my backyard slopes toward our house.  As this soil is the furthest away from its moisture source, I have to think it would have reduced moisture content then the 100% I used for this conservative simulation.  This would reduce the humidity in this area.
This knowledge from FloVENT helped me know exactly what was needed to solve my problem, and saved me a lot of money by avoiding the costly repairs that I was being told I needed.  Only by understanding your problem, whether it’s a lake under your house or any number of design issues engineers face every day, can you really begin to work on a solution.   FloVENT simulations can help not only with the understanding of the problem, but also by investigating different possible solutions, and in do so help you find the most cost effective solution in the most cost effective manner,  by utilizing your desktop computer instead of costly physical tests and prototypes.

Join me next time when I investigate another house hold problem with one of Mentor Graphic’s CFD products.

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Posted May 26th, 2011, by

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About John R Wilson's Blog

Insights into the practical application of CFD to the thermal and airflow design process. John R Wilson's Blog


One comment on this post | ↓ Add Your Own

Commented on May 28, 2011 at 11:22 am
By MastercraftHCP

I just found your blog and I love the graphics! Excellent stuff. These FLOVENT simulations are pretty impressive.

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