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This Old CFD House: Part III

John Wilson

John Wilson

Posted May 19, 2011

Welcome to Part III of a multi-part series, where in this installment Travis takes a “virtual” look at his crawlspace.

For this next installment of “this old CFD house”, and really the entire reason I started this blog series, I’d like to take a look in my crawlspace. Now, from whatm_biot I understand, Feng Shui says running water is good for wealth. Well the opposite is true when you have a lake beneath your floor. In California, houses are built on posts and are a certain distance above the ground, and the space in-between is the crawlspace. Our crawlspace (it’s officially ours now), has a bit of a moisture problem, with standing water and high humidity. It’s this high humidity that can cause a world of problems, from mold to rotting your floors.

In fact, during the inspection process we had a crawl space specialist look at the problem. After his inspection, he gave his assessment on the issue and what it would cost to remedy. He gave 2 options, or really, a phase 1 and phase 2 (a “how much do you love your family type test). I like to relate expensive costs to Ford Mustangs, my favorite car, as it’s a good way for me to evaluate what I would rather spend that amount of money on (it’s usually a Mustang). So, to get a sump pump installed with some trenching, and a plastic barrier to eliminate soil evaporation, we were looking at a low end Mustang, V6, cloth interior or maybe an older model. I was pretty sure I could dig a hole, install a sump pump and put some hefty trash bags on the ground for a whole lot less than that. But of course, I was worried about being penny smart and pound foolish, so I knew I needed to analyze this with FloVENT to ensure whatever course of action I took it would fix our prospective mushroom garden under our house.

The main issue was that our backyard was sloped towards our house, and re-grading was an even more expensive solution. Another possibility was running a French drain around our house, and destroying a lot of concrete patio along the way. The problem with that is it doesn’t help in get rid of the current moisture in the crawlspace. This brought me to my first FloVENT analysis.

We frequently heard that our house had a greater than average number of vents into the crawlspace, more than double the current code, which I believe, is one vent for every 150 sqft. This should provide more than adequate ventilation to remove any humidity, but yet we have 88% humidity (well above the safe 50-60/70 zone) and standing water a month after the last drop of rain. Seems like all this supposedly good airflow under our house is doing is making the floors super cold and driving up my heating bill. So, like you, I wanted to see how effective this “venting” strategy is at getting rid of moisture. I’m sure in the 1960 or 70’s or whenever venting became the standard way of building a crawlspace, no experiments or CFD was used to see if this design accomplished it’s design goal of a dry crawlspace, so I needed to take the initiative and analyze mine to get a handle on this problem.

I created the FloVENT model geometry based on the blueprint layout of my house. I eyeballed my vent positions and looking at the vent style (shown below), I estimated it was about mesh80% open to airflow for my air resistance. Initially, I didn’t model any obstructing geometry in the underfloor, as I didn’t know the location of ductwork. After running the analysis though, I did start to wonder how much obstructions would affect the performance of my crawlspace system, particularly floor joists which I thought would “trap” any natural convecting air up at my floor. I didn’t go into the crawlspace (…spiders…) but I know the size of my ducts and where they start/end, and I know the size of my joists and their spacing. Looking at the results, it only changed my air change rate by less than an hour, so not a huge effect, but I’m getting ahead of myself.

aspect of this problem is the boundary conditions. In my case, my house is basically as wide as my lot, with fences on the sides. So, there isn’t going to be any air getting into my crawlspace from the sides with any type of wind momentum to penetrate any significant distance into the crawlspace. Similarly in the back yard, in California land is at a premium, so back yards are small. Any wind blowing from that direction will have to go over my neighbor’s house and our fence and in my opinion unlikely to dive down and into my crawlspace. Right there, I was starting to see the issue; only the front of my house really had the ability to have any airflow get into my crawlspace, simple as that. Unlike a roof, where you can harness natural convection to drive air out the top of the roof, a crawlspace has no height change between vents and is basically a dark cave with no heat source to create that convection current (which would get caught up in the floor joists anyway). No, it needs a wind, and if your house is like mine, a good half of the front is taken up with a garage. That garage, takes up ideal ventilation space. That means that the air entering in the my only front 2-3 vents now has to drive the air in the entire crawlspace, and make a pretty good 90 degree turn to clear out the dead air space behind the garage/front porch. Oh, and this is assuming that the wind is coming towards the front of my house for a substantial amount of time (which I had to assume for the analysis).

houseNow that wind is a pretty important component of this equation, not just from an airspeed point of view, but also from a temperature and humidity aspect as well. Yes, we need to try and drive out humid air with air that also has some humidity to it as well, a less than ideal situation. Luckily, the weather underground can provide such stats, and it seems like living next to the San Francisco bay, the air is usually around the 50% humidity range. For wind speed, unfortunately there isn’t a weather vane in my front yard, and as I’ve discussed direction is just as important as speed, so I went with a conservative 1 m/s which is the bare minimum air speed a person can feel on their skin. While you may notice the breeze and gusts more, I’m hoping and praying that there is at least a slight, unnoticed, constant wind blowing across my street and into my crawlspace.

Lastly, we need to model that evil humidity. I’ve modeled pools and things before, but that math assumes things like a temperature difference between the air and pool/hot tub water driving some evaporation, and a correction for kids splashing, none of which I could use for my sub-residential ocean. Believing that this water would behave like a humidity “no slip” condition, I modeled it as a 100% humidity source at the surface of the standing water. From 3rd party accounts (again, spiders and deadly mold down there), I’ve heard the water was over about half my crawlspace, but that was one moment in time. Maybe in the worst case, it could have filled my crawlspace, so I modeled it as such.

Below are some of the results. First up are 2 plots of the humidity at different elevation above the ground. Comparing the 2, you can see the substantial difference in humidity in just 1 foot change in height, indicating pretty stratified air. You can see that for the most part, the lower crawlspace area is a pretty constant humidity of 80% and above, which confirms what the 85% humidity meter reading from the inspector (which I didn’t fully trust as he has a financial interest in showing I have a problem). Still, it’s pretty depressing to see all that humidity. The dark blue areas in the images are the garage, the concrete front porch, the chimney, and the heating ducts. You can also see the plumes of humid air coming out of the vents around the house.


image_02Looking at the speed contours, you can see that really only one of the vents is providing a nice jet of air, but for the most part the airflow in the crawlspace is non-existent (for comparison, a natural convection current is ~0.2 m/s). Oh ya, and did I say those vents are right behind a empty, but soon to be filled, flower bed and some future hedges (see previous image)? It couldn’t be in a worse location…


Sometimes it’s easier to understand the airflow by looking at streamlines, so I tracked the airflow from all the inlet vents to see where the air is going in my crawlspace. It looks like I have a nasty looking vortex in my underfloor, a big issue with raised floor data centers, but for a crawlspace more an indication of poor airflow.


Clearly, the right side of my house gets the best airflow, and behind the porch and garage the airflow is a lot worse. Also, you can see from the humidity (percent saturation in this chart) that the moving air doesn’t pick up much humidity before leaving the system. When looking at the results in 3D you can see the airflow hugs my floor (it’s ceiling), likely due to the fact the air in the crawlspace is cool and denser then the fresh wind air, resulting in a stratified air in this space. Comparing with my original FloVENT analysis without the HVAC vents, I found that the HVAC vents do help create some mixing effects and reduce the average humidity ratio in the crawlspace, an interesting finding.


For me, to really understand the magnitude of the problem, I needed to look at some numbers. I figured the real design goal of these vents is to achieve a certain amount of air turnover in the crawlspace, so I wanted to calculate this air change rate to see how long it took to completely replace the air in the crawlspace. The calculation was easy. FloVENT tells me the volume air flow rate in all my vents, and I knew the volume of air in the crawlspace (the square footage of my house times the height of the crawlspace), so with some simple math I calculated a air exchange rate of about 29.5 hours. Now I could see why standing water could exist for so long under my house. I’m not sure if this is out of the norm for crawlspaces, but clearly, air can only hold a certain amount of water, and if it takes more than a day for the air to leave the house, I could easily see it taking many weeks to dry out the crawlspace after a good hard rain, especially if the air is stratified and the new air really isn’t interacting with the moist air. Of course, you can look on the bright side and say that the denser, moister air is down near the floor of the crawlspace, separated from the floor by the incoming air. Therefore, it’s kept from rotting the wood of the floor or growing mold there. But, I don’t want mold growing anywhere, so the moisture issue has to be solved.

Now that we understand the problem, we can start to investigate my solution, and whether it will accomplish what we want, a clean, dry, mold free crawlspace. So join me next week for the conclusion of this blog installment.

Computational Fluid Dynamics, crawlspace, CFD, HVAC, FloVENT, humidity

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About John Wilson

John WilsonJohn Wilson joined Mentor Graphics Corporation, Mechanical Analysis Division (formerly Flomerics Ltd) in 1999. John has worked on or managed more than 100 thermal and airflow design projects. His modeling and design knowledge range from Electronics Cooling IC packaging level to Data Centers and Clean Rooms. He has extensive experience in IC package level test and analysis correlation through his work at Mentor Graphics' San Jose based Thermal Test Facility. He is currently the Consulting Engineering Manager, WRO in the Mechanical Analysis Division. Visit John R Wilson's Blog

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