Crawl Space Air Quality

Why Do I Care About Crawl Space Air Quality?
If you care about how much you pay for heating and cooling, then you care about your crawl space. If you care about your home rotting, or mold, allergies, or asthma, then you care about your crawl space. If you care about the comfort of your home, cold floors, drafts, and how your home smells, then you care about your crawl space.

If you care how long the paint lasts on your house, about doors and windows sticking, about hardwood floors buckling, and about carpets going moldy, then you care about your crawl space. If you care about your resale value, then you care about your crawl space. You can't get away from it.

The house is one building. It operates as a system. You can't have one part of the building that is sick, and another part that is healthy. You can't rationalize that you never go in the crawl space so you aren't affected by it. Why? Simple. Air mixing. Crawl space air is bad. It is damp. It is cold in the winter. It is full of mold spores. It smells. And this crawl space air is in your building envelope.

It gets upstairs in one of two ways - the stack effect (explained below), or the HVAC system through your ducts. You are breathing it. In fact we know that from one third to one half of the air that you breathe on the first floor of your house came up from your crawl space. And this goes for basements too, for people that have them.

This fact has been proven by studies. If someone spray paints something in your crawl space, do you think you'd smell the spray paint upstairs? Of course you would. Radon gas is a naturally occurring radioactive gas that comes from the ground. For many years we have known that if you measure the radon levels in your basement or crawl space, and then measure upstairs, you will have about 1/3 the amount upstairs. Since radon only comes from the ground, this demonstrates how air moves in a house - from bottom to top.

The Stack Effect
The "stack effect" is when warm air moves upwards in a house. This happens in summer and winter. Warm air rises - because it's lighter than cold air. So when it rises, what happens? It escapes out of the upper levels of our homes. But we can't create a vacuum in our homes, so when air escapes, new air has to come in to replace the air that escaped. Where does the new air enter the house? At the lower levels. Through crawl space vents for one, and up from the earth for two.

Your house is sucking on the ground. So this idea that we put in crawl space vents, and expect that air will flow in through vents on one side of the house and out through the vents on the other side is a bunch of baloney. What actually happens is air enters the vents in the front, enters the vents in the back, and enters the vents on the sides, and then it goes UP!

I've heard the argument that on a windy day the air flows through a crawl space via the vents. The problem is you need wind everyday for this to be effective. The downside is that wind depressurizes the whole building, sucking air out of it at a greater rate than it blows it in. This causes a need for more replacement air to make up for the air that left. So we suck more air up from the vented crawl space on a windy day. Besides, even if vents made air flow through a crawl space on a windy day, you are just making warm humid air or very cold air flow through your house faster because of the wind.

Venting doesn't make sense unless the outdoor air is 70 degrees and 45% RH day and night, all year long. So it doesn't make sense. How does the air get from the crawl space up into the house (besides ducts)? Air is a very small thing. With this driving mechanism, (the suction of the house on the ground) air is drawn up through every tiny opening between the crawl space and your house. Holes around wires and pipes. Joints in floor boards, space around access hatches and through duct chases. You can seal these openings, but you can never get it perfect, so you can't stop it.