Crawlspaces – those nasty, damp, moldy spaces under your home that you dread entering. They’re one of the least understood parts of a home and the source of countless problems. In this post, I’ll review some of the worst problems and how to avoid them.
Crawlspaces often have two big issues:
- Water / moisture – leading to mold and wood rot
- Cold / drafty – leading to uncomfortable conditions and wasted energy
What is a crawlspace?
Joe Lstiburek of Building Science Corp., describes crawlspaces as “little, misunderstood basements.” I like this so I’ll start there.
A crawlspace is just a space under the house where the builder was too cheap to build a full basement. But otherwise, it’s the same. You have foundation walls that support the house above. Often, they’ll contain air conditioners or furnaces. Water pressure tanks. Oil tanks. Water pipes and, oh, lots of spiders!
What causes problems in crawlspaces?
Since crawlspaces are really just little basements, why don’t builders treat them like basements? Instead, they poke holes in the walls and install vents. They try to seal them off from the rest of the house. They don’t seem concerned when water seeps up from the ground or in through the walls. They know that people hate crawlspaces so they just figure they can hide shoddy construction in a space where nobody looks.
There is no such thing as a “small leak.” Treat any leak in your house as if you’re in a boat – if you don’t fix it, it will sink you!
Basements and crawlspaces often experience water seepage through concrete block or stone walls – the ground that they hold back is filled with moisture. The problem is, in crawlspaces, you might not see it for years, whereas in the basement, you will probably find it quickly.
The most common source of crawlspace water problems is incorrect drainage. You know – gutter downspouts that pour right onto the foundation walls.
The first thing to do is to walk the perimeter of your home and look for downspouts that look like this photo. Remember, when it rains, this downspout may be dumping thousands of gallons of water on the ground right next to your foundation. Under these conditions, it’s incredibly difficult for any wall to hold back the moisture.
When you do find a downspout problem, you’ll have to add proper extensions or a drainage system. You really want to get the water at least six feet away from the foundation, though you may be able to get away with less. But the further away you dump the water, the better.
You also want to ensure that the ground slopes down and away from the house. Over the years, it is common for landscaping and erosion to lead to ground that slopes towards the house, leading to puddles lying against the foundation. Again, this drives water into your crawlspace.
Letting water problems go unchecked is like playing with fire. Remember – overall, water causes more damage to homes than any other home problem. It can lead to mold which is a health risk. It can lead to wood rot – which can literally cause your house to collapse. And it’s very expensive to repair the damage once it’s been done. Don’t risk it – make sure you treat water issues before they cause serious problems.
Wasted Energy and Uncomfortable Conditions
In order to deal with the moisture that always seems to accumulate in basements, builders created the “vented crawlspace.”
We’ve all seen these – little metal slotted vents, the size of a cinder block, placed around the perimeter of the space. The idea here is that these are supposed to “let the water out.” It sounds like a good idea, and there are still builders and even building codes that insist that this is a good idea, but in fact, it’s really dumb on a number of levels.
Would you open the windows in your basement and leave them open year round? Probably not. You know that it would be freezing in the winter and hot and humid in the summer. So why treat a crawlspace differently? Especially when it’s directly connected to the basement? Because sometimes it works. Venting crawlspaces is like a band aid on a bigger problem. Sometimes it will help vent out the moisture. But often it doesn’t and just causes more problems. The issue is – builders simply don’t understand what’s happening.
A vented crawlspace is an incredible energy waster. That cold air moving through the space sucks the heat out of your house. It will lead to overly dry air in the winter and muggy conditions in the summer. It might freeze your pipes and steal energy from your ducts.
And so we add insulation…
More Band Aids = More Problems – Insulated Crawlspace Ceilings
Many “insulated” crawlspaces look like this one. They have more moisture problems and the white mold has eaten the paper facing on the insulation. Often, much of the insulation has fallen out and lies in the water on the floor, like in the far corner of this photo.
This approach is an unfortunate band aid used in a failed attempt to make the house warmer after leaving the windows open in the crawlspaces (i.e. the vented crawlspace). What builders don’t understand is that this causes even more problems, especially if there are pipes and other temperature sensitive items in there (like uninsulated ducts).
The insulation does it’s job by reducing the amount of heat that is lost from the living space to the crawlspace. This makes the crawlspace even colder. That’s the point. What people don’t consider is that a cold crawlspace can lead to frozen pipes, and more energy lost from ducts. A cold crawlspace also means more moisture problems because the water vapor in the air is more likely to condense into liquid water so more mold, and, if you’re unlucky, wood rot and structural damage to your home.
Attacking the Problems with Science
Now that we understand the basic problems:
- High moisture levels lead to mold and wood rot and air quality issues
- Low temperatures waste energy, cause discomfort and freeze pipes
we can approach the problems logically.
- Reduce outside sources of water through proper drainage
- Treat the crawlspace as a basement – close the windows and insulate it
- Provide air exchange with the rest of the house to flush out moisture
Step 1a: Reduce water entry through the walls
As already discussed, you want to prevent water buildup from around the foundation walls. Walk around outside, along the perimeter of your home during a heavy rain and look for any puddling around the foundation. Make note of these locations and fix the problem. It might require some grading so the ground slopes away from the house. Most of the time, the first step is to fix your gutters and downspouts.
In some extreme situations, it may require installing French drains. These help redirect water that builds up around the foundation. A friend of mine had to take another step – spray foaming the OUTSIDE of the foundation walls. This provides a mostly impenetrable water barrier so that water can’t even touch the block walls. When combined with a French drain, it will eliminate most water problems that people encounter.
Step 1b: Reduce water entry from the floor
There are some situations where even this won’t help – when the ground water table is high. This can occur from natural springs under the house or when there is simply a lot of water all around the foundation. In this case, water or water vapor can literally seep up from the floor. This is particularly noticeable when you have a dirt floor – you might see actual puddles of water on the floor that appear to come from nowhere.
It would take an entire book to adequately cover this topic. But in a nutshell, there are a couple solutions:
- Lay thick plastic over the floor, sealed air-tight to the walls and at all seams
- Water-proof the floor then pour a concrete floor in place
You have to treat the floor like the hull of a boat. Would simply laying plastic on the floor be water-tight? Not a chance. You have to seal every seam and make it sturdy enough to last.
The simplest and most cost effective solution is to use two layers of heavy duty plastic sheeting, sealed at the seams, and run up the walls. Then, spray foam the walls, sealing the plastic to the foundation. When done properly , it will be virtually impossible for water to enter the crawl space, though it may build up under the plastic to such an extent that you want to install a sump to remove it. As I said, I could write a book on this. In most cases, a sump should be installed to remove water that may build up.
Here’s a really great website all about dirt floor crawl spaces.
Step 2: Insulate the space
After you’ve fixed the water/moisture related issues, then you can treat the space like a normal basement. If you used spray foam on the walls in step one, then you’re basically done because you’ve water sealed and insulated in a single shot. If not, now’s the opportunity. Tise solution is shown in the first photo at the top of this post. Closed-cell spray foam is a great insulator and vapor retarder. By spaying foam on all exposed walls, down to the floor and over the perimeter few feet of floor, you’re well on your way to creating a comfortable, dry crawlspace.
One word of warning – in certain areas with termite problems, you need to leave some areas of the band joists exposed (there are some good illustrations here). Check with your local building official to see what’s required in your area.
Note, the above link to the Dept. of Energy, Energy Savers website, shows batt insulation used on foundation walls. This is a really stupid idea. Batt insulation is basically like a sponge for water. Once the batt insulation gets wet, it loses all it insulating capacity. You’re vastly better off using closed cell spray foam. It’s much less labor intensive and it’s a better material for the job. Don’t be scared off by the price which may seem like several times the cost of batt insulation. In the long run, the batt insulation will cost you more and won’t work as well.
The only viable option to spray foam is rigid board foam. This material costs about half of spray foam and it does an excellent job insulating and stopping moisture – when installed correctly. However, it’s labor intensive, and once you add in the cost of professional installation, you’ll probably end up spending more than if you had the space spray foamed.
Step 3: Provide Air Exchange
After you’ve dealt with moisture issues and insulated the space, you’re almost done. The last thing is to provide some air exchange. You do not want a “dead air space.” A dead air space is like a sealed box. What usually happens in dead air spaces is that moisture builds up inside and you end up with a disgusting, unhealthy mess. Like a terrarium – before you know it, it’s like a rain forest with all sorts of things growing in it and no place for the water to go.
In order to avoid dead air spaces, building code calls for some air to flush out the space. Originally, this was done by using outside air, but as we already saw, this is a stupid idea. Now that you’ve sealed up the crawlspace, that air should come from the house.
If you have a forced air heating/cooling system (like a furnace and air conditioner), you would provide a small amount of supply and return air from the ducts into the crawlspace. In theory, this air will be relatively dry and will help to flush out the moisture that would slowly build up in the crawlspace. This solution is the most energy efficient and can be extremely simple/inexpensive if you have ducts running through the crawlspace already. However, if you have a really dirty crawlspace, you may not want to mix that air with the air in your home.
You can also use an “exhaust only” fan, as described here. This is a small fan that exhausts air from the crawlspace, sucking it from the house and exhausting it outside. This can be a simpler and more “healthy” solution since crawlspaces are usually so dirty. From a practical perspective, this solution is probably the appropriate one for many homes.
You’ve now seen the fundamental problems with typical crawlspaces and learned some options for dealing with them. Please be cautious in implementing your own solutions. Your local code officials may insist on certain solutions, and if you do something that violates their rules, you may have to undo it before selling your home. You also want to be sure to implement a complete solution. The worst thing you can do is a half-assed solution, picking and choosing parts of what I’ve layed out above. If you do this, you run the risk of making matters worse. You must deal with moisture and properly insulate using a moisture tolerant insulation material. And you must avoid the dead air space. If you skip any of these steps, you’re playing Russian Roulette with the health and safety of your family.