Insulating Basement Crawl Spaces

Spray foam on crawlspace walls

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:

  1. Water / moisture – leading to mold and wood rot
  2. 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.

The Water Problem

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

Insulation in crawlspace ceilings can be a very bad idea

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.

So why do people keep doing stupid things? Because they don’t understand the basics. There’s a few building codes that require ventilated crawlspaces, so they cover their asses by blindly following code so that when your house gets moldy and rotten, they can point to the code and say “see – they made me do it that way!”

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.

  1. Reduce outside sources of water through proper drainage
  2. Treat the crawlspace as a basement – close the windows and insulate it
  3. 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.

Crawlspace walls after foaming the exterior foundation

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.

Concluding Remarks

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.




21 thoughts on “Insulating Basement Crawl Spaces

  1. Under your house the ground is more or less 50 degrees. It gets WARMER the lower you go. You insulate the SIDES. Keep water away from foundation. Heat RISES. Therefore, do NOT INSULATE FLOOR JOISTS. Your crawl space will help cool in the Summer and heat in the Winter. Builders are in business to make money and will use every level of bullshit to make money. Keep it DRY. NUMBER ONE.

  2. Thanks for the tips! The crawl spaces in my house causes quite a draft, so I need to have them insulated. The information about batt insulation will help me save quite a bit of money. One of my friends told me that he has a lot of batt insulation that I can use for my crawl spaces, so I was thinking of taking him up on his offer. Since it gets wet really easily and loses all of its insulating capacity, perhaps it would be better to use a different kind of insulation. You said that I would be better off paying for spray foam insulation. What makes it a better material for the job? I can see why it would be labor intensive, but it would help to have more details about why it’s better than other insulation materials.

    • Do you have a heating system in there? A lot of times basements or other areas containing old heaters/boilers will be very warm because of all the heat lost through the jacket of the system.
      If you have ductwork going through there from your heating system, often they throw off a lot of heat too. That’s why most ductwork should be insulated so you’re not heating spaces that don’t need it.

  3. What about a dehumidifier in the encapsulated crawl space? This seems to a popular push in the humid south. Also, regarding the foam board to insulate the crawlspace walls, I am concerned regarding their removal if a termite problems needs treated ie. damage to cinder block wall and rigid board requiring new board and labor. Am I overreacting? Lastly, I am considering adding some more lights to the crawlspace. If crawlspace is sealed, would this add enough heat to forego insulating the walls of the crawlspace? (would still have dehumidifier there) There will not be any insulation in floor joists as we are having moisture and beginning of mold problems. Location is North Central Alabama. Some pretty cold winter days at times.Thank you! Enjoyed reading your site!

    • In my area, we’re only worried about termites and wood. Never seen issues with them with regards to cinder blocks. However, I understand there are different varieties in each region, so if that’s a concern, I’d consult a local professional.

      Insulating the space (from an energy perspective) is much preferred. Cinder block walls, being in direct contact with moist soil, can suck a tremendous amount of heat out of the space, far more than a few light bulbs. Additionally, the foam board (properly applied and sealed) will dramatically reduce the amount of moisture that gets into the space meaning the dehumidifier will run a lot less. Dehumidifiers are a huge energy hog so the less you have to use them the better.

      As a general rule, building scientists consider passive measures, like insulation of this sort, to be much better than active ones that require constant energy, like dehumidifiers. Active devices are also prone to failure, and require maintenance. The best solution is one that: a) does the job; b) has minimal maintenance requirements; c) remains working for future owners of the house.


      • Thank you for the reply. Sorry didn’t mean termites that eat cinder block but that they would do mud tunnel up the cinder block to the wood. 🙂 There would be 6 inches below the sill that would be left clear for termite inspections. The question was prompted because I had a termite guy say treating is easier because they would just open the plastic and do their treatment. I was thinking they had to get down to the ground somehow. Anyhow…..the encapsulation people say to insulate so I was trying to find a balance. So, insulate it is and not stress about those termites…I’ll keep up the termite repair bond!

        I am interested about what you said about passive measures. Right now we have a couple of really wet areas up on floor joists, where uninsulated HVAC duct connected to floor registers in the house. I do have a couple of neighbors who had moisture and mold in their crawl spaces and encapsulated and then conditioned the space with air from the house and an exit fan out one of the crawl space vents. My heater guy says that works best in fall and summer when using the HVAC a lot but doesn’t work as well in Fall and Spring when HVAC use is low thus the dehumidifier. However, I do like the idea of less moving parts the less to break and fix. I wonder, with time, when things get good and dry, if it is possible to move to more passive measures.

        I also read your article on attic moisture. I appreciate the information which will help me to be a smarter homeowner and hopefully smarter when we move on and downsize.

      • I see now. Thanks for explaining that.

        For the moisture problems from the ducts – a possible solution to that is insulating those ducts connections. Are the encapsulation insulators using board foam or spray foam. You mentioned board foam before, but spray foam is faster and more effective because it gets in all the nooks and crannies. They can also spray foam on the duct connections and floor registers which would pretty much eliminate the condensation formation / moisture problems from that source.

        Your HVAC guy is on the right track. What people often do for encapsulated crawl spaces is to now treat them like part of the home, so they put a small supply and return from the main system so that you get air flow through the crawl space which flushes out the moisture. I hadn’t heard of leaving a vent open to the outside however. I would be very hesitant to do that because that vent would let moisture in as well as out when the fan wasn’t blowing. In the humid seasons, this would be really bad and the dehumidifier would be trying to dehumidify the outdoors – a losing proposition! I believe the best best would be completely encapsulating the crawl space, just like you would a finished basement. Provide an air supply from the central system into the space to flush the air. Add a dehumidifier if necessary to maintain a low humidity.

        Here’s a link to a set of resources talking about insulated crawlspaces. You sound pretty well informed, so I think you’d get a lot from browsing a couple of the articles.

      • Thank you! All the options and expense considerations can be a bit overwhelming. Your advise to do it right, (pay now or pay later), and not be too complicated is appreciated. Regards.

    • Not a simple answer. It could work well or it could invite rot. The devil’s in the details.
      The key thing with any solution is to reduce the chance that the joists stay warmer than the dew point of the air under all conditions. Consider the worst, most common solution – paper faced batts stapled to the joists with the insulation facing up towards the house. The paper provides a vapor retarder for the moisture in the crawl space and the fiberglass insulates the bottom of the joists from the warmth of the house. What happens is that the bottom of the joists are essentially at the temperature of the crawl space, say 40F in the winter. However, the house living space might be 70F with 50% humidity. When that warm air cools and comes in contact with the joists, or the building paper at 52F or cooler, it condenses to water.
      On the other hand, if you put an inch of pink or blue-board insulation, you get about R-5. Maybe you put R-19 fiberglass in the joists. So you have about 75% of the insulation above the blue board so the surface of the paper that comes in contact with the blue board will be at about 48F, so you still run the risk of moisture buildup under these conditions. If you use two inches of insulation board and R-19 in the cavity, then you’ll get about 66% of the R-value in the fiberglass and 33% of the R-value in the blue board. If my calculations are correct, now the paper will be at about 50F.
      As you can see, the best thing would be to exclude the fiberglass between the joists and just attach the board insulation to the bottom of the joists. Now the warmth from the house keeps the joist cavity warm enough to minimize the chance of having any condensation while the blue-board keeps the heat in the house and the cold and moisture from the crawl space out.

      • I put insulation between the joists and then nail blue to joists what do you think tony

      • That sounds like a good approach. The blue board will greatly reduce moisture from the crawlspace moving up into the living space and protect the joists from that moisture too. Most people just staple fiberglass batts to the joists and then the paper rots and the fiberglass falls out. You’re way ahead of the game doing it the way you did.

    • I’m afraid I’m not quite sure what you’re asking. Many people run ducts through crawl spaces. When you do this, it’s best to have insulated ductwork and a sealed and insulated crawlspace.
      If the crawlspace is left vented to the outdoors, this can lead to cold feet, drafts and energy loss from the ducts. But sometimes you don’t have the option. In that case some sort of floor insulation is beneficial but the specifics depend on your home, climate and a variety of factors.

  4. Building science is fascinating to me. I get excited thinking about re-insulating my house when I hopefully re-side it next year. (Most people get excited by finishes, so I guess I’m abnormal.) It’s especially fascinating when I think of how back in 1965 when my house was built, there was no spray foam, rigid foam or furnaces that could vent with plastic drain pipe. It’s amazing how far things have progressed in 50 years from energy sieve buildings to really tight buildings. Now the challenge is to get builders and codes up to speed, but it seems like it’s quite the uphill battle.

    I stumbled upon this site when I was searching for something relating to thermostats the other day, so it was an interesting find.

    Are you any relation to Naoto Inoue of Solar Market in Maine?

    • We’re definitely on the same wavelength. I use my house as a test case for many things because I want to see first hand how things work before recommending them, whenever possible.
      No relation to the other Inoue. Our family is very small in the U.S. – most of the extended family is native Japanese.

  5. I’m thinking of doing about a 200 square foot addition on the back of my house and am planning on doing a conditioned crawlspace under it. I first saw it on the “Frigid Floor” episode of Holmes Inspection and started reading about it and decided it was probably the way to go for a crawl space. When I was growing up my parents had a room addition built on a concrete block crawlspace back in 1984 and the contractor used the vents in the foundation. The floor was always freezing in the winter and that room never really got cool in the summer even with the A/C on. Plus my parents never knew when to open or close the vents. I see that Lowe’s now has solar powered foundation vents that close at about 40 degrees and open at about 70 degrees. So that would have helped that situation. But from what I’ve read the best idea is to make the crawlspace part of the living space and do it properly to avoid water issues.

    • Glad Holmes has talked about them. Crawl spaces are highly misunderstood by many builders, leading to miserable conditions for homeowners. Every properly insulated crawlspace I’ve seen has been warm and dry and, as you note, the rooms above have been much more comfortable.

      • It was a really educational episode. It seems that a lot of builders lack knowledge on a lot of “new” concepts (that aren’t really all that new). My real job is IT, but sometimes I find I know more than people in the construction industry just by reading Fine Homebuilding and watching educational shows like Mike Holmes’ stuff or This Old House. But that’s not the way it should be. I might be working in construction if I could deal with the elements better. Buildings are complicated and lack of education can make a huge difference as you’ve shown with your attic posts in particular. Glad I found this site!

      • I know exactly what you mean. That’s how I got into the building science consulting business. It all started innocently when I wanted a geothermal system installed and dove into that subject. One snafu after another with contractors led me to realize that consumers need independent advocates to help guide them through the complicated and confusing processes involved in making homes comfortable, efficient and safe.

        Hope to see you around here more, sharing your learnings too.

Leave a Reply to Janet Cancel reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s