Image Credit: Quality First Basement Systems An underground roof diverts water percolating through the soil and directs it to a location away from a building's foundation. This illustration comes from "Details for a Dry Foundation" by William Rose (Fine Homebuilding, August/September 1997).
Image Credit: Fine Homebuilding Some people call them French drains, and some people call them curtain drains. Whatever you call them, they can be a good way to intercept the flow of water before it reaches your basement.
Image Credit: Fine Homebuilding It's often easier to install an interior French drain than it is to excavate down to the footings on the exterior side of your basement. [Photo credit: Rock Solid Waterproofing]
Image Credit: Rock Solid Waterproofing Once an interior French drain has been installed, the concrete slab can be patched to hide the drain. [Photo credit: Great Lakes Waterproofing]
Image Credit: Great Lakes Waterproofing This detail drawing shows one approach to installing an interior footing drain and above-slab rigid foam insulation.
Image Credit: Green Building Advisor
A hundred years ago, homes had cellars, not basements. The typical cellar has stone-and-mortar walls and a dirt floor. Such a cellar is cool and humid, so it’s the perfect place to store carrots and potatoes. If a cellar floor got wet during the spring thaw, no one cared. After all, it’s not as if anyone was playing ping pong down there.
These days, however, most homeowners expect basements to stay dry. During the 1930s and 1940s, as basements gradually replaced cellars, construction specifications for residential foundations improved. Poured concrete walls replaced stone walls; concrete slabs replaced dirt floors. Some builders even included footing drains.
But the results of these efforts were uneven. Millions of Americans are still living in homes with damp basements.
New construction tips
This article will focus on ways to fix a damp basement so that it’s dry enough to use as indoor living space. Before tackling that topic, though, I’ll briefly outline the steps used during a new construction project to keep the basement dry.
Remember: fixing a wet basement can cost tens of thousands of dollars. It will never be easier to get the details right than when a home is first built; spending a few hundred extra dollars now can save thousands later.
If you are building a new home, and you want a dry basement, you should specify:
- Wide roof overhangs to keep rain away from the foundation;
- Gutters at the roof eaves; these gutters should be connected to conductor pipes that convey the roof water far from the house (either to daylight or a dry well);
- A 4-inch-thick layer of crushed stone under the basement slab as a capillary break; the crushed stone layer needs to be vented through the roof to help control radon;
- A layer of horizontal rigid foam on top of the crushed stone to insulate the slab from the cold soil below;
- A layer of polyethylene above the rigid foam (directly under the concrete slab) to act as a vapor barrier;
- At least one 4-inch-diameter drain pipe running through the footing, to connect the crushed stone layer under your basement slab with the exterior footing drain;
- A capillary break (for example, an asphaltic dampproofing compound, UGL DryLok, or elastomeric paint) between the top of the concrete footing and the foundation wall;
- A ring of perforated drain pipe on the outside of the footing, surrounded by crushed stone and wrapped with filter fabric to make a “burrito,” drained to daylight, to a distant drywell, or to an interior sump;
- An application of dampproofing compound or waterproofing compound on the exterior side of the concrete foundation walls;
- A layer of dimple-mat drainage board installed on the exterior side of the foundation walls; failing that, the foundation should be backfilled with coarse, free-draining material like crushed stone, topped with an 8-inch layer of dirt (ideally, dirt with a high clay content);
- A bead of sealant to seal the crack between the basement slab and the basement walls;
- Closed-cell foam sill seal between the top of the foundation walls and the mudsill, to reduce air leakage and to act as a capillary break.
Unless you are planning to have a basement bedroom, it’s best to eliminate basement windows, since windows can provide paths for water leaks.
That wet basement smell
If your basement is damp, it’s probably because some of the measures advised for new construction are missing. For example, many older homes have no footing drain or polyethylene under the slab.
How do you know that your basement is damp? There are lots of signs. For example, when you put a cardboard box on the floor, the cardboard disintegrates. Maybe you noticed that mold is growing under a carpet. Maybe the walls feel damp.
Another clue is the distinctive odor of mold.
Mold is associated with high humidity. The humidity in your basement may be coming from the soil — water can leak through cracks in a concrete wall, or moisture may be constantly evaporating form the interior side of the vapor-permeable and always-damp concrete — or the humidity may be condensing from the air inside your home.
During the winter, condensation occurs near the top of the basement walls, since those areas are coldest. During the summer, on the other hand, the coldest concrete surfaces are the slab and the lower sections of the foundation walls — so that’s where the condensation happens.
There are two reasons you might want to install a continuous layer of rigid foam insulation under a basement slab. First of all, the insulation slows heat loss during the winter. More importantly, perhaps, the insulation warms up the slab during the summer, reducing the chance of condensation. That means that your basement won’t smell moldy, and your slab will be warm enough to allow you to install carpeting. (If you install carpeting on an uninsulated slab, there is a high likelihood that you have established conditions for a mold farm.)
First things first
So, you know that your basement is damp, and you want to fix it. What’s the first thing to do?
Many homeowners guess — wrongly — that the first step is to install a dehumidifier. Actually, installing a dehumidifier is an action of last resort. After all, installing a dehumidifier condemns you to a lifetime of high energy bills.
Cover dirt floors with polyethylene. Although this step may be obvious, it still belongs at the top of the list. If a portion of your basement has a dirt floor, the first step is to cover the dirt with a layer of polyethylene that is at least 6 mils thick. If your budget is tight, weigh down the polyethylene with stones or a few bricks. If your budget can handle the expense, pour a concrete slab over the polyethylene.
If you are planning to install a slab, it usually makes sense to dig out some of the soil first, to gain a few inches of ceiling height. Then install a layer of crushed stone, a horizontal layer of rigid insulation (plus some vertical insulation at the perimeter), a layer of polyethylene, and a new concrete slab.
Check your roof gutters. If there are no gutters at the eaves of your roof, it’s time to install them. Non-perforated conductor pipes should convey the roof water to a location far from your house — to daylight or a dry well at least 10 feet from your foundation. Make sure that your gutter pipes aren’t connected to your footing drains.
If your roof already has gutters, get out your ladder and clean them.
If you don’t want to install roof gutters because of concerns over ice damage, you can install so-called “in-ground” gutters instead. For more information on in-ground gutters, see Ground Gutters and Inground Gutters.
Adjust the grade around the outside of your house. If you’re lucky, the finish grade around your foundation slopes away from the house in all directions. If your house is on a sloping site, the water flowing downhill toward your house should (ideally) be intercepted by a swale that directs rain far from your foundation.
That’s the way things should be. But if you have a wet basement, there’s a good chance that the grade around your house isn’t like that.
Maybe some of the soil near the foundation has settled, creating a low spot. Maybe rainwater pools in your yard. Maybe you have a concrete patio with a reverse slope that directs water toward your house.
Perhaps you’ve known about these problems for years, but you haven’t had the strength to tackle them. Well, it’s time to tackle them.
If you’re lucky, fixing your grading problems will take no more than a wheelbarrow, a shovel, and a rake. If you’re unlucky, you’ll need to demolish your concrete steps, your concrete patio, and your concrete walkways, and you’ll need to hire an excavator and a bulldozer.
If rainwater or melting snow arrives in your yard from just one direction — usually, from uphill — it may be worth the effort to intercept the water with a curtain drain. For more information on curtain drains, see Stormwater Management.
If you need to significantly change the grade around your house, it’s worth the time and effort to install an underground roof. Click here for more information on underground roofs.
Stop and catch your breath
Once you’ve finished all the work described above, your basement may be much dryer than it was before. In fact, you may have solved your problem.
How can you tell? There’s no easy way. You probably have to wait a year and see if anything changes. If your basement is now dry, you can congratulate yourself. If it’s still damp, you have to continue going down the list of possible fixes.
At this point, it makes sense to try a few measures which, although questionable, are relatively inexpensive and therefore worth trying. I’m talking about installing a so-called waterproof coating on the interior of your concrete walls. Brand names for these products include Thoroseal, UGL Drylok, and Xypex.
There’s no doubt that these products work, and they can definitely turn a damp basement wall into a dry one. They just can’t solve every problem. If the hydrostatic pressure on the exterior side of your basement wall is high enough, none of these products will be able to resist the pressure of the entering water. The bottom line is, these products are worth a try.
Perforated drain pipe
If the waterproof coating wasn’t enough to solve your problem, repairs will begin to get expensive. You’ll have to install some perforated drain pipe. You can either excavate down to the footings on the exterior of your house, and install the perforated pipe there, or you can use a concrete saw to cut through your basement slab to create a French drain and sump on the interior side of your basement walls.
If you hired an excavator to adjust the grade around your house, I hope you had this thought: “As long as there is an excavator on site, maybe I should go ahead and excavate down to the footings.” Once the footings are exposed, you can evaluate the condition of the footing drains (if any exist) or install new footing drains if needed. Then you can install all of the measures that your original builder forgot: dampproofing compound (or waterproofing compound), a dimple mat or free-draining backfill, and a clay cap.
If porches, walkways, and patios make you cringe at the thought of excavating down to your footings on the exterior of your house, you’ll probably prefer Plan B. To install an interior French drain, you need to cut a trench about 8 inches wide and at least 8 inches deep at the perimeter of your basement slab, near the wall. Put some crushed stone in the trench, and some perforated 4-inch pipe, followed by more crushed stone (see Image #4, below). The pipe can be installed level or slightly sloped, and should lead to a sump installed in a corner of the basement. Then install a sump pump and connect the pump’s discharge pipe to a distant drywell, to daylight far from the house, or (if permitted by your local municipality) to your sewer drain. Once the French drain is installed, the concrete slab can be patched (see Image #5, below).
Before patching the concrete, you may want to install a tee in the 4-inch pipe, and connect the tee to a solid 4-inch riser pipe that extends through the roof. This riser pipe, along with a properly sized inline exhaust fan installed in the attic, are the essential components of an active radon mitigation system. Such a system depressurizes the air under a basement slab, and provides an important side benefit: it helps keep your basement dry. For more information, see All About Radon.
Sometimes, of course, you still need a dehumidifier
The work described above is daunting. If your budget doesn’t allow you to adopt some of the expensive remedies described here, you may have to compromise.
Maybe you’ll have to accept a little dampness every April. Maybe you’ll have to give up your dream of a subterranean home theater. And maybe you’ll decide to plug in a dehumidifier after all. The dehumidifier may not solve any problems, but it will lessen the symptoms. For more information on dehumidifiers, see All About Dehumidifiers.
How to tell if your basement wall is too wet to insulate
Maybe you are ready to insulate your basement walls, but you just aren’t sure if they are dry enough for you to proceed. Is there any way to test them?
The traditional test is simple: tape a piece of polyethylene measuring about 12 inches square to your concrete wall or concrete slab. (If you’re testing your walls, it’s good to test them in multiple locations.) After 24 hours, if you see water drops under the plastic, that indicates that your walls are too wet to insulate. If you see water drops on the interior side of the plastic, it means that your indoor air is humid, and the concrete is cold. (Fortunately, that problem won’t prevent you from insulating. After all, a layer of rigid foam will eliminate the cold surface. And the high indoor humidity can be addressed by air sealing your home’s thermal envelope, or — in a pinch — with a dehumidifier.)
For more information on insulating basement walls, see How to Insulate a Basement Wall.
Insulating an existing basement slab
If you intend to finish your basement, and your basement slab lacks sub-slab insulation, you’ll need to install some rigid foam above your slab. Otherwise, the slab will be damp during the summer.
The usual technique is to install 1 or 2 inches of XPS or EPS foam insulation on top of the existing concrete, followed by a layer of plywood that is fastened through the foam to the concrete with TapCon fasteners. (If you are still worried that your slab may sometimes be damp, you might want to install a layer of dimple mat under the foam.) When installing this layer of foam, it’s important to make the installation as airtight as possible, to make it impossible for any humid interior air to contact the concrete. Seal the edges of each piece of foam insulation with a high-quality European tape, with caulk, or with canned spray foam.
If you don’t want to lose the height required for rigid foam, you could try installing a dimpled subfloor product like Delta-FL. (Note that some similar products, notably DRIcore, have mixed reviews from some builders. For more information on this topic, see comment #6 from Mike Guertin, below.) While Delta-FL is worth considering, especially when the basement ceiling is already low, it’s not the preferred solution. If you plan to install finish flooring in your basement, it’s always better to install rigid foam insulation on the slab than to proceed without any floor insulation.
For more information on insulating existing basement slabs, see:
Is it worth the expense?
Converting a damp basement into dry living space isn’t easy or cheap. But if your house is cramped and your lot is small, it may cost less to transform your basement to living space than it would to build an addition.
And if you are building a new house, don’t skimp on drainage and waterproofing details. The work will never be easier or cheaper.
Martin Holladay’s previous blog: “Air Sealing an Attic.”