The Crawl Space Concept

Written by Matt Leech –

President/CEO of CrawlSpace Concepts LLC

Wixom, Michigan

March 18, 2009

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Why Now?

I get phone calls from homeowners that live all over the U.S. and Canada asking me about the closed or conditioned crawl space. No one seems to wonder if it will work but rather how it works. If we look back to the 30’s and 40’s there was no such thing as a conditioned crawl space. Yet today, as some crawl space companies put it, you can die if you don’t have it done their way and right away. So how did we live without it back then? Well, the answer is much more complicated than the question. Looking back to the early part of the last century there were many things that we had never heard of like plasma televisions or a laptop computer. But that did not stop the technology and research from making it possible. The conditioned crawl space concept is no different. In the 30’s and 40’s we had a different lifestyle, the windows were open in the summer and there was no air conditioning. The homes were built with different technology and guidelines so they were not as air tight as they are today. In fact many of the homes that were built in this era have added air conditioning and have been improved and updated to today’s standards for energy conservation. This is a good thing but it also means that these homes get less fresh air than they use to. By tightening up these homes they have confined the occupants to the environment created inside the homes envelope. As environmentalist study the causes of pollution in the air outside and make headway to improve and control the emissions given off by factories and vehicles, there is very little push to improve the air in a families home. Even the indoor air in public buildings is a strong focus today with banning cigarette smoking in cities and states across the country. Yet there are no guidelines for healthy air inside a private home. It is up to the families to identify and correct this problem. It is known, however, that the air inside an average home is seven times more polluted than the air outside and up to twenty times more polluted during the winter months. How can this be? It simply comes down to the lifestyle of the occupants and the environment in which the home was constructed. In this article I will be more focused on the latter of the two but more specifically homes that are built on a crawl space.

The Issues

The crawl space environment

The crawl space environment is no different than that of a cave in the side of a mountain. It’s dark, dirty, and smelly and has small animals and bugs living in it. When the home was first built the crawl space was not considered part of the house. Excess building materials and waste was thrown down there because it was easier than picking it up and discarding it in the dumpster. I have personally taken out old refrigerator doors, tires, old pipes, a toilet with the seat and drop ceiling panels among other things. It is also not uncommon to remove the carcass of a dead animal or two. This is the condition before anyone tries to improve it. Back in the day it was required to have vents in the foundation to promote cross ventilation in order to help control the moisture that would escape from the ground. This caused a few other problems, cold air getting under the home and making the floors cold, allowing even more moisture to get in by way of the humid summer air and a constant supply of mold, mildew and fungus spores. The cold floors were addressed by installing fiberglass insulation in the floor joist cavity and the other two problems were thought to be unavoidable. Now we have a dark, dirty, smelly place that has small animals and bugs (powder post beetles, termites, creepy looking bugs about 1 ½ inches with long furry legs that run faster than my eleven year old son) living in it, we add more moisture via the vents and plenty of mildew, mold and fungus seeds. The next thing we do is add a fiberglass nesting material for the animals to bed in and then through the open vents in the foundation we make it cold enough in the winter to raise the heat bill and chase these small animals and bugs into the home in search of food and a warmer place to live.  By spring, a few mouse traps and a can of ant spray the home is ready to start this cycle over again. This is a reality for most crawl spaces so it is no wonder nobody wants to go down there.

Did I mention that crawl spaces are prone to flooding? 

We all know that water runs down hill, with that being said crawl spaces are generally lower than the ground around the home because we backfill the foundation so that water shed will run away from the home. Over the years the backfill settles to a negative grade and allows water to rest against the foundation of the home. So when it rains hard in the spring and fall the ground becomes saturated. Once the ground will not take on anymore water it begins to run to the lower lying areas like ditches, storm drains and crawl spaces. Once standing water is introduced into the crawl space the homeowner runs the risk of major

foundation failure and joist and beam rot. The major reason the standing water can be so devastating is because it goes unnoticed for long periods of time. The homeowner may periodically look in the crawl space for water or the source of an odor and not see any water. That does not mean in the last six months while the crawl space went un-inspected water did not come and go several times. Keep in mind the conditions described in the last paragraph and now add scum and rodent feces floating on top of the water inside the crawl space. It becomes extremely hard to reason that the old way of maintaining a crawl space is working just fine.

False water table, what’s that?

A false water table can be the culprit of why your crawl space keeps getting water in it no matter how waterproofed the foundation is. A false water table is the retention of excess water by none virgin soil. That means when a home is built there is a hole dug in the ground for the foundation, whether a basement or crawl space. After the foundation is in the dirt is filled back in around the outside of the foundation walls, this is called backfill. The backfilled dirt is not as compact as the undisturbed virgin soil that surrounds it; ultimately the backfill holds more water. The water in the backfill sets against the foundation and the virgin soil. Water will find its own level, so as the pressure builds around the foundation it naturally starts to spread itself out. If you picture a bowl that is 6” deep with 3” of water in it, the water is not on one side or only at the edges. It is evenly distributed across the entire bowl.  As long as water is poured into the bowl the water will keep rising toward the top. This is what happens around the home and that is why it is critical to have the proper elements in place to protect your foundation. The virgin soil and evaporation will slowly reduce the water levels around the foundation but meanwhile the concrete foundation takes a beating.

I think it’s time to take a look at the research.

Is this where it gets complicated? No, its common sense really it’s just too bad that it took so long to be common. In order to figure out the solution one has to first identify the problem. So what is the real problem? Is it the standing water or the bugs? Maybe it’s the vents or is it the smell? The answer lye’s with what’s common with all of these symptoms. I’m not trying to confuse you with riddles, I want you to see that what seems like the problems are only symptoms and if you only address the symptoms you will not fix the problem. Lets take the bugs for example, to many people go to the grocery store and buy those bug bombs and let them off in the crawl space thinking “that’ll fix ‘em”. Well it probably does but they also just let off a half dozen cans of poison in their home. One home I inspected had Stick-Ups on the heat ducts to help the smell, it didn’t work though. If you pump the water out it will just come back, if you seal off your vents the moisture from the dirt will be trapped inside the home. If you only address each symptom you will get either a new semi full time hobby, make the other symptoms worse or actually create new problems. What all of these symptoms have in common is moisture or humidity. Insects, wood rot, mold, fungus, mildew, odor, cold floors and high heating cost are all the result of attempts or the byproduct of the attempt to control moisture. Now that we know this and agree that moisture is the problem we can now properly address it.

Four ways moisture enters the crawl space.

There are four main ways that moisture enters a common crawl space. The first and most obvious way is through the open dirt. As the ground outside the home is saturate with rain the moisture level in a crawl space raises rapidly. The warmer the air the more moisture it can hold. This means the most devastating months are the warm ones. This excess moisture condensates to cooler surfaces like water lines and the foundation walls. It also settles on the ground when the temperature falls, gets absorbed by the wood structure and any insulation that is installed in the crawl space. The building code in nearly every area of the country requires a vapor retarder with a perm rating of less that 1 to be installed on the floor of the crawl space. There are no requirements to how it should be installed, just that it must be.  This leaves a wide gap in the final performance of a vapor retarder. If anyone wants to find recommendations on how to install; there are many opinions on the internet. It is not really a mystery if you know what it is suppose to do. For the sake of conversation, let’s say we go down to the local hardware and buy some plastic and install it in the crawl space according to the local building code. The problem of moisture entering the crawl space from the ground is fixed! Right? Well not entirely, any gaps in the plastic (at the seams, around supports and where it meets the wall) will still allow moisture to escape. It will continue to allow this to happen until the air, the wood structure and the concrete foundation are saturated. To make it more complicated, when the temperature falls in the evening the excess moisture leaves the air and settles like dew on top of the plastic only to evaporate again when the warmer air returns. Then the next time your plumber has to go down there to fix a frozen water line the plastic will probably not be in the same place as it was when you installed it causing your efforts to be wasted. The other common issue is when the crawl space floods water gets on top of the plastic and can not be absorbed by the earth. This of course leaves a breading ground for bacteria and mosquitoes while introducing a new long lasting supply of moisture.

The second way moisture gets in the crawl space is through the open foundation vents. This is a huge area of confusion

and there is a lot of very technical information out there on how the foundation vents impact the crawl space. You will read about relative humidity and vapor pressure. How the high and low pressure weather fronts impact the crawl space and how the vents being open or closed makes no difference. You will also read how the temperature affects these different technical conditions. On a quick side note, when the reports say it makes no difference if the vents are open or closed, they are referring to manually sliding the vent to the closed position. If you’re like me you will read these reports a few times just to feel comfortable with what it is saying. Some of the research is bent and formed to justify their own product line they want to sell you. I will give you a good example; the article titled “CRAWL SPACE MOISTURE CONTROL - A Fundamental Misunderstanding” written by Peter Carpenter explains, in quite a bit of detail, why you need mechanical ventilation to the outdoors in order to control moisture in the crawl space. To me this theory is more based on selling a product line than correcting a problem. The bottom line of the article is to install foundation vents with electric fans in them which are controlled by a humidistat. By doing this the idea is the fans in the vents will come on when the humidity near the vent rises past a desirable level. Essentially the fans are meant to keep the warm moist air out by turning on when the outside moisture levels rise. Think of a vertical wind tunnel with a fan at the bottom and a balloon falling down the tunnel toward the fan. Each time the balloon gets close to the fan it will turn on and blow the balloon away. This theory has solid standing IF the crawl space vents were an absolute requirement. It is, in my opinion, a compromise between actually fixing the problem and doing nothing. If you read between the lines the article clearly states that the problem is the foundation vents allowing moist outside air to enter the crawl space. Their solution is to create a negative pressure in the crawl space, (take air from the home, pull it into the crawl space and then discharge it outdoors) in order to actively keep the moisture at bay using the same vehicle that causes the problem in the first place. It is my own deduction that the air is pulled from the home; there is no other obvious source of air that will replace the air being blown out of the crawl space by the electric fans. This theory is put into practice by Humidex, E-Z Breathe and other power ventilation manufactures. Here are the flaws to this theory as I see it; the process they describe works against the natural air flow of the home (which is up not down), it does not address the other conditions in a crawl space such as odor (when the fan is off), condensation, air quality, energy loss, cold floors or frozen water lines. Like I said it is a compromise, and it only attempts to address one source of moisture. It also potentially increases the energy loss in your home by pumping your conditioned (heat or cooled) air into the crawl space and then outside.

The hot topic is whether a homeowner can close the foundation vents in the crawl space. Not just sliding the vent closed but completely removing any way for air to move into the crawl space from the outdoors all year long. In other words permanently close the vents. You can find the answers in the International Residential Code - 2006 IRC 408.3 and subsections. Like any subject, to find the correct answer you have to ask the question correctly. If someone asked “do I need my vents?” the answers would range greatly. Some would say yes, some no and some would need more information. If you asked “do I need ventilation in my crawl space” you would likely get the same mixture of answers. The two questions are, however, completely different and with different answers. Like any new ideas there is going to be a clash of correct and incorrect answers. Most of the incorrect answers will be a result of ignorance on this subject; ignorance is defined as a lack of knowledge. While our own lack of knowledge brings us to asking the question we find the blind leading the blind. My intention is not to be critical but to encourage everyone to do their own research.  If you’re not sure where to start I would be glad to point you in the right direction.

The third way moisture enters the crawl space is through the foundation wall. Concrete block foundations have a much bigger problem with this than a poured concrete foundation wall. Concrete block has two hollow cores in each block. These hollow cores have a history of filling with water as the ground get saturated during a rain, by leaving the garden hose on or by a leaking hose bib. The concrete block is very porous and will allow water to pass through it. Once in the core of the block the water will level out until it finds a compromise in a motor joint or in the block itself. At this point the water moves freely into the crawl space. If the water does not find a compromise it will slowly migrate through the block to the driest side of the foundation; either back into the dirt outside or into the air of the crawl space. Since the water came in from the outside it is not likely that it will return there unless the ground dries out extremely fast. With a poured wall foundation the weaknesses are poorly patched rod holes and foundation cracks. Both of these issues will allow water to move freely into the crawl space. If the home has a problem with a false water table the water in the crawl space can also come up through the dirt floor.

The fourth way moisture enters the crawl space is by way of small cracks that are located where the wood structure meets the foundation. This is not as serious as the other three but worth mentioning. The rim joist and sill plate should be inspected for large gaps and poor workmanship. The issues in this area are generally easy to correct.

Crawl space ventilation- Yes, you need it!

Crawl space ventilation is the most critical part of handling the crawl space moisture problem. It is not the only part, but the one that will affect the quality of air that is in your home. You have to get this part right or the current situation will likely worsen. A home, a cottage or any building that is closed up without air circulation will smell stale. It is not an uncommon practice to “air out” a summer cottage that has been closed up for the winter. The process of airing out is simply circulating the air inside the cottage; exchanging the old stale air with new fresh air. The more air that is exchanged the fresher the smell. This air exchange is also required for a crawl space. Really, that is what the foundation vents are supposed to do. When you look at the current condition of the crawl space, most often it will need a steady supply of fresh air. We already went through the negative issues with exchanging the crawl space air with the outdoor air. So, how does the crawl space get fresh air if bringing in outside air is bad? First, I want to make sure my view on this subject is clear. I am a huge advocate of bringing in fresh air from the outside into the home. Open the windows when the temperature is comfortable, air out your home regularly. Fresh outdoor air is great for your health. How does this help or hurt the conditions in the crawl space. Well, the most important thing you can do to properly assess and make good decisions about air exchanges in your home is to stop separating your home into different parts when you are considering an air circulation plan. Since the time your home was built the crawl space was not considered part of the home, its time to change that! Whether you like it or not a large amount of the air in the crawl space makes its way into your home. Every tactic we have used in the past has failed to impact this particular problem. The odor you smell, the condensation on the windows and the allergy symptoms all remind us that there is something going on in the crawl space. Simply put, we experience at least some of the conditions from the crawl space inside our home. The more serious the condition the more motivated we are to getting it fixed. Living with the more serious condition from the crawl space making its way into the home, the last thing anyone wants is to circulate that air freely into the home. That would make it much, much worse. Right?! This probably is the most successful “pitch” the companies that sell the mechanical vents can use. Their mantra can be “don’t let the crawl space air into your home”. If the goal is to allow the crawl space to exist as

is, then of course that would be a good plan, provided you are willing to use the conditioned air in your home that you paid for to fuel the plan and are willing to accept the other flaws. There is a better way, I want to give you some more information first so that you fully understand the decision you are making.

The natural air flow of any home is up. Air is pulled from the lowest level and introduced throughout the home. The vehicle for this flow is heat. Of course everyone that graduated middle school knows that heat rises and cold falls. As the heat rises it is replaced by cooler air, this is what creates the natural air flow of the home. Whether you are heating the home in the winter or fighting it off in the summer the heat will always dictate the circulation direction. The natural air flow, also called the stack effect, will continue to move the air in your home toward the roof unless you allocate sufficient resources to change it. In my opinion, using mechanical vents to change this natural condition is an attempt to reinvent the wheel. It will not stop the warm air from rising, it is not a sufficient resource to change the natural air flow in the home and it will leave the homeowner with a failed concept that was backed by a great sales pitch, while still owning the problem. Granted the problems will be lessened and less frequent but nonetheless - not extinct.

Cold floors, energy loss and insulation

This is another huge topic throughout the United States and Canada. Cold floors and energy loss are directly related to properly insulating your home and the crawl space. However, insulation alone is not the answer. Here is what I tell my customers that call me for advice and to help them understand the problem of cold floors and heat loss. If you build a shed in the back yard and insulate the walls, floor and ceiling with, let’s say R-30 and the outside temperature is 0 degrees. Is the shed warm or cold? The answers I get are split - 40% say warm and the other 60% say cold. For the 40% that say warm, I help them understand why their answer is incorrect. Insulation does not make a shed or home for that matter warm, heat does. The job of the insulation is to retain the heat. Of the 60% that answered correctly I only have to ask one more question; what would make it warm? This follow up question definitely exposes the ones that guessed but nonetheless they understand what the missing link to their problem is. If you take the heat source out of a bedroom and closed the door, that bedroom would be cold. It is no different with the crawl space, but with a crawl space the home will usually have open vents. Even if the vents in the foundation are mechanically closed it will make little difference in the temperature of the crawl space. My analogy for this- would you install those same vents in the walls of your living area and expect them to hold out the cold if you closed them? Of course not, that is not part of there design. The other part of this equation is the characteristics of warm and cold air. Heat rises and cold falls, this fact directly works against keeping the floors warm in the winter. As the home is heated the warm air rises to the ceiling where it is picked

up by the return air ducts (in most homes) and sent back to the furnace to be reheated and delivered again. Once the heat is delivered to the living area of the home and the heat begins to rise (away from the floor) colder air replaces it at the floor. So once the cycle of rising warm air starts your floors get the cold air. Now add to this the fact that the crawl space (just inches below your feet) has no heat source and is much colder than the living area. We must now recognize two other characteristic of cold to fully understand the impact; cold absorbs heat and the lack of heat is cold (not the other way around).  This is a major disadvantage to heating a home and the only reason the furnace has to continually work to warm the home during the winter months. Floors over a basement do not have these cold floor problems and that is because the basement is heated. The insulation in the crawl space ceiling does little to protect the floors from the heat absorbing cold. However if you took the insulation out and left the crawl space as is, your floors would be even colder. There is a fix to all of these problems and it is probably less expensive than you may think.

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