How to Insulate Your Basement

New products and a better understanding of building science can help you insulate, waterproof, and improve almost any underground space.

Increased comfort, high indoor air quality, HVAC savings, and moisture control are attractive benefits to finishing basements. In fact, the U.S. Department of Energy reports that homeowners can save up to $390 on electrical costs per year with a finished basement (in homes where electricity is used for heating). And while the return on investment does take time, insulation upgrades actually offer a quicker payback than window replacements.

“We find the comfort factor is the biggest component for homeowners,” confirms Kevin Colwell, president of the Newton, Mass.-based building envelope solutions firm, BE RETROFIT. “Insulating the walls is counter-intuitive to some homeowners in an attempt to solve this problem, but the results never disappoint.”

Concrete, Heat and Moisture Control

A below-grade space can be bright, with good air quality, but you must take special steps to construct it with controlling moisture in mind. This basement was finished with the Owens Corning Basement Finishing System. Source

Non-insulated foundation walls cause so much heat loss that Jason Todd, a training manager with GreenHomes America, an Irvine, Calif.-based national builder specializing in home performance retrofits, calls them the home’s biggest Achilles’ heel.

However, before entering into any kind of discussion about basement wall insulation strategies and systems, he says, you must first address moisture control.

“The moisture drive through a concrete or stone foundation will always be toward the interior spaces for the home, so the wall assembly must control the natural inward drive of moisture, or it is doomed to fail,” explains Colwell.

So when builders begin considering insulation options, issues of water management, drainage, air barrier system, and water vapor barriers must factor significantly into the equation.

For example, a lumber-framed wall insulated with fiberglass batts and finished with drywall will only work effectively if a vapor barrier has been applied to the foundation wall’s exterior. This vapor barrier could be a cement sealant or rigid foam and drainage mat, says Colwell.

The dynamics of insulation and moisture control are very sophisticated building science issues, which must be addressed with skill and expertise.

“Throughout the home, a basic understanding of the physics of air, heat and moisture flow is essential,” confirms Todd. “Understanding vapor flow and condensation potential are also very important. For basement retrofits, the details to best improve that space depends on multiple factors including climate, temperature, moisture loads, and materials.”

The Right Insulation for the Basement Type

Before delving into the specifics of how to detail an insulated basement system for moisture control, let’s quickly discuss some of the common insulating materials and approaches to various basement types.

While there are literally hundreds of products on the market, the five basic categories are: foam board, spray polyurethane foam (SPF) insulation, fiberglass, cellulose, rockwool. In some cases, a hybrid of systems can also be effective, such as Owens Corning’s EnergyComplete.

John B. Smith, an engineer at the Johns Manville Technical Center in Denver, describes the following 5 insulating scenarios. 

1. Monolithic concrete. Rigid polyiso or extruded polystyrene (XPS) foam board can be used on interior walls to insulate, with SPF also applied to air seal and insulate the rim joists.  A less costly option could be taped foam board. Another alternative is a “hybrid” combination of taped foam board and batt insulation.
2. Finished basement. Ideally, exterior insulation should be installed before backfilling walls. However, if permanent features such as walks, driveways or patios make excavating the basement wall too costly, then foam board should run down several feet along the foundation’s outside wall. This board should then continue horizontally out from the vertical walls for 3’ to 5’. Incidentally, this is the same approach for frost-protected shallow foundations.
3. Unfinished basement. Similar to monolithic concrete, a combination of foam board and spray foam is recommended, or taped foam board to air seal and insulate the basement walls.
4. Double brick wall with rubble fill or concrete block. Closed-cell SPF is the easiest way to provide interior insulation and air sealing. 
5. Laid-up stone foundation. Smith recommends the same approach as the double brick, rubble-filled wall, while Socolean suggests waterproofing and insulating from the exterior with a vapor-retarding membrane placed throughout the floor.

Spray foam can accomplish multiple basement finishing goals at once, Colwell explains. In some installations, it can serve as the air, water, moisture, and thermal control surface for the wall. When applied in bulk quantities, however, it should not be left exposed. It should be protected with a thermal/fire barrier.

“Depending on the detailing, the spray foam can act as a drainage plane as well as an air barrier,” adds Todd. “This is an approach that is done in crawl spaces.”

One less orthodox insulation strategy for special scenarios, the California builder explains, is to use foil-faced polyisocyanurate applied to the top half of a poured or block concrete wall. “It has a high R-value per inch, and is tolerant of moisture, but is not a very finished look and may or may not fulfill local fire codes.”

Why half the wall? Because this insulation type is manufactured with a vapor barrier surface, it should not be used for the entire wall. It could trap moisture in that scenario.

Assuming that a basement before finishing has no cracks, and has been installed with good drainage on the exterior and a damp-proof barrier on the vertical concrete wall, spray foam (SPF) offers a fast, efficient way to insulate. It’s sprayed on the interior concrete, followed by wall framing, which is steel in this case, and covered with moisture-resistant drywall. Illustration: buildingscience.com

Moisture Management

Any water that gets through the concrete walls must be managed. Unless exterior wall waterproofing is flawless, walls should have an “escape route” for condensation and other water infiltration. Dennis Socolean, CEO, Rinnovo Group, Danville, Calif., suggests that as long as insulation is installed at least 1” off the wall, and a drainage system is utilized at the bottom of the “dead” space, then any type of wall insulation will work.

To begin with the big picture, however, builders must address water management by ensuring that all gutters are intact, and install a sump pump if the water table is high. The contractor must make sure that the foundation drainage system is working properly.

Foundations create a complex moisture flow, which must be well understood in order to detail the building envelope properly. For example, says Smith, “the foundation wall needs to be warm enough to keep moisture from condensing, or the humidity of the basement air needs to be lowered. Air containing moisture can also move through the foundation, so the insulation system needs to control airflow, reduce the potential for condensation and tolerate water.”

Whereas builders have traditionally used batt insulation directly against the concrete foundation, and covered it with wallboard, this is risky, if the foundation walls have not been installed and waterproofed carefully. Rather, as Socolean explains, “the best approach may be water-resistant insulation—usually foam—installed up against the concrete foundation wall while wallboard is used as an interior installation.”

Similarly, Smith recommends foam insulation or a hybrid system with foam against the foundation. “With hybrid systems, unfaced fibrous products (such as fiberglass) should be used to permit the insulation system to dry to the inside,” he adds.

Meanwhile, Colwell points out that the most important aspect of this installation is to ensure continuity with the above-grade wall system air and insulation barrier.

Consequently, when using SPF, he explains that the band joist and box ends must be insulated and connected to the subfloor of the first story.

“Using high R-value rigid insulation board—poly-isocyanurate—is another good solution and works well on poured concrete with a flat surface,” he says. “Using rigid board adds a second air sealing step. A kit system polyurethane foam air sealant should also be used to air seal the rigid insulation board to the first story subfloor in order to create the necessary air barrier and thermal barrier continuity.”

Todd explains that there’s no silver bullet when it comes to basement insulation. Every project must be evaluated based upon its own specific variables such as climate, temperature, moisture loads, wall materials and whether the foundation can be excavated and insulated from the outside. Ultimately, the goal is to reduce energy consumption and heat loss, at same time improving indoor comfort.

Barbara Horwitz-Bennett is a seasoned business reporter, writer, and editor specializing in the building and construction industry. As a regularly contributing editor to Green Builder magazine, she has deep knowledge of building materials and systems.

FAQs

How “green” is foam insulating board?

From a strictly front-end perspective, the materials in foam board are not as “green” as cellulose or arguably, fiberglass (which is made from sand). Foam board, whether polyisocyanurate or polystyrene, requires a lot of fossil fuel to manufacture.

“Polyisocyanurate and expanded polystyrene tend to use less harmful blowing agents than other foam boards,” notes Todd, “and open-cell spray foams tend to be water-based, which is good.”

As Colwell points out, foam board can make a stronger argument for sustainability from the perspective of how much energy it will save over its lifespan. The occupied stage is where about 94% of a home’s overall environmental impact occurs, so foam’s high performance carries a lot of weight.

But more hard data on life cycle is needed. Here’s a perfect example of a product that could gain from third-party life-cycle assessment and research on lifespan, with the  long-term goal of  “closing the loop.”

Will a dehumidifier be enough to keep my finished basement dry?

Probably not. A typical home humidifier might remove 30 pints of water in a day. That’s a little less than four gallons. If your basement walls are dripping from condensation or the floor tends to get wet after rainfall, you could be looking at 50 or 100 gallons of wicking water at regular intervals. 

Instead of trying to treat the symptom, focus on reducing and controlling the amount of water entering the basement before you consider finishing it as a living space.

What kind of drywall should I use on my basement walls?

Drywall comes in special moisture-resistant varieties, including “greenboard” or green drywall and PURPLE drywall, which is only manufactured by National Gypsum. Either is superior for basement applications to the standard white drywall. If the basement is at high risk of flooding or regular high humidity, you may also want to consider cementitious products such as James Hardie’s smooth panels, or even galvanized metal or other modernistic finish materials.

What is the best way to insulate basement walls?

The 5 basic categories of below-grade insulation include foam board, spray polyurethane foam (SPF) insulation, fiberglass, cellulose, rockwool. In some cases, a hybrid of systems can also be effective, such as fiberglass and spray foam used together.

Is it important to insulate your finished basement?

Yes. Insulating below grade increases comfort, and provides HVAC savings and moisture control. The U.S. Department of Energy (DOE) reports that homeowners can save up to $390 on electrical costs per year with a finished basement. And while the return on investment does take time, insulation upgrades actually offer a quicker payback than window replacements.

What exterior insulation should I use for my basement?

Ideally, exterior insulation should be installed before backfilling walls. However, if permanent features such as walks, driveways or patios make excavating the basement wall too costly, then foam board should run down several feet along the foundation’s outside wall and then continue horizontally out from the vertical walls for 3’ to 5’.

Editor’s Note: This article was originally published on April 14, 2014. It has been updated and republished with new information to reflect changes in technology and products.