New products and a better understanding of building science can help you insulate, waterproof and improve almost any underground space.
By Barbara Horwitz-Bennett
WHILE IT'S NO PICNIC TREADING AROUND the living room with cold feet, many homeowners still have cold feet when it comes to basement insulation upgrades.
As is common, that first cost issue can often be a deterrent—however, builders are quick to point out that increased comfort, HVAC savings and moisture control are attractive benefits. In fact, the U.S. Department of Energy, at EnergySavers.gov, reports that homeowners can save up to $390 on electrical costs per year. 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.”
In fact, 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 it the home’s biggest Achilles’ heel.
However, before entering into any kind of discussion about insulation strategies and systems, the first issue to address is 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 and air and 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 if a vapor barrier is applied to the wall’s exterior, says Colwell.
On the other hand, if porous concrete is capped with a vapor-impermeable material, moisture will ultimately be trapped in unwanted places, according to Todd.
The upshot is that 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 is also very important. For basement retrofits, the details to best improve that space depend on multiple factors including climate, temperature, moisture loads and materials.”
Before delving into the specifics of how to detail an insulated basement system for moisture control, what follows is a brief overview of the different 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 and rockwool. In some cases, a hybrid of systems can also be effective, such Owens Corning’s Energy Complete.
In terms of the different basement types, John B. Smith, P.E., global platform leader, environmental construction, Johns Manville Technical Center, Denver recommends the following:
Monolithic concrete. If polyiso or extruded polystyrene (XPS) foam board is selected, then SPF should be used to air seal and insulate the rim joists, although a less costly option could be taped foam board. Another alternative is a combination of taped foam board and batt insulation.
Finished basement. Ideally, exterior insulation should be installed. 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 for 3’ to 5’. Incidentally, this is the same approach for frost-protected shallow foundations.
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.
Double brick wall with rubble fill or concrete block. Closed-cell SPF is the easiest way to provide interior insulation and air sealing. However, 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.
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.
Bringing out an important point about SPF, Colwell explains that the product can serve as the air, water, moisture and thermal control surface for the wall, and when applied in bulk quantities, should be a covered with a 15-minute 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.”
Offering another insulation strategy, the California builder explains that foil-faced polyicocyanurate applied to the top half of a poured or block concrete can be a fast and effective approach, depending on how it is attached. “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.”
Because this insulation type is manufactured with a vapor barrier surface, it should not be used for the entire wall to avoid trapping in moisture.
As mentioned, moisture control–to prevent the unhealthy growth of mold and mildew–is a huge issue when it comes to proper basement insulation.
For starters, builders must address water management by ensuring that all gutters are intact, consider the use of a sump pump if the water table is high and make sure that the foundation drainage system is working properly.
In fact, 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 air flow, reduce the potential for condensation and tolerate water.”
Whereas builders have traditionally used batt insulation and covered it with a wallboard, this is no longer acceptable. Rather, as Socolean explains, “scientific evidence has shown the best approach to 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 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 sub-floor in order to create the necessary air barrier and thermal barrier continuity.”
Foam Board: How Green is it?
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.”
For now, in the absence of standardized LCA data rating the eco-friendliness of foam board, Smith advises specifiers to compare products based on assorted variables such as recycled content, embodied energy and installation method.
At the end of the day, Todd explains that there’s no silver bullet when it comes to basement insulation, so 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.