Insulation Design


Insulation is a critical element for achieving energy efficiency in buildings by serving as a thermal barrier. HVAC systems are responsible for 50% to 70% of total energy usage in the average American home.4 Within a two-story home, heat loss through the basement ranges from 15% to 30%, while the attic accounts for 25%.3 Proper insulation not only minimizes drafts and hot/cold spots but also improves the indoor environment by preventing the infiltration of outdoor pollutants and allergens. Additionally, it offers fire resistance, moisture control, and soundproofing.

By installing R-10 insulation, homeowners can reduce basement heat loss by up to 70%.3 Insulation is applied along the entire perimeter of the basement walls, extending from the base of the exterior siding down to the foundation footing, leaving one foot of the foundation exposed. This is done to prevent moisture buildup and allow for proper drainage. The foundation footing, a concrete base supporting the foundation walls, is typically wider than the walls. This broader width ensures an even distribution of the building’s weight over the soil. In a mild winter climate, a house with an area of 1,800 square feet with R-10 insulation can save you $50 to $60 per month, while the installation cost is only $300 to $600. Heat losses due to infiltration, are generally double those of cooling and two to three times higher than conduction losses, can be substantially reduced through effective insulation practices and air sealing.


There are two main types of insulation: continuous and cavity insulation. Continuous insulation is placed continuously alongside structural members, creating a seamless barrier that eliminates gaps and reduces thermal bridging, which occurs when heat is transferred through building materials with low insulation values. It is often referred to as “insulation outboard of the sheathing.” This type of insulation is typically made of foam board, such as rigid foam or insulation boards, and can be manufactured from various materials, including polystyrene (either extruded or expanded), polyisocyanurate, or polyurethane. With its superior insulating properties, continuous insulation can help reduce energy consumption and costs.

Cavity insulation is installed within wall cavities and placed between structural members, such as studs, joists, and beams, to fill the empty space and reduce heat loss. This type of insulation comes in either batts or rolls and is usually made of fiberglass or mineral wool. While cavity insulation is a more cost-effective option compared to continuous insulation, it is more susceptible to heat transfer through framing components. Therefore, in situations where thermal bridging is a concern, such as with exterior walls or roofs, continuous insulation may be the better option. Ultimately, the best insulation option depends on various factors, including the building’s design, climate, and budget.

Fiberglass insulation is a popular choice for home insulation. Although it has a relatively low R-value compared to other insulation types, it is affordable, readily available, and easy to install. On the other hand, mineral wool insulation, such as ROCKWOOL, is made from natural rock materials and is an eco-friendly option. It has a comparable R-value to fiberglass but offers superior resistance to fire and moisture, and is denser, making it more effective at reducing sound transmission. Rigid fiberboard insulation, made of flexible fibers like fiberglass, bonded together with a binder, is often used in commercial and industrial settings. It is a good thermal insulator and can be used for wall insulation and soundproofing.

To accurately calculate heat loss in a basement, it is crucial to consider the three different sections of the walls: the aboveground portion, the belowground section that is above the frost line, and the portion that lies below the frost line. While the temperature of aboveground walls is primarily affected by the outside air temperature, which can vary with season, time of day, weather conditions, and geographic location, belowground walls are subject to the highly variable temperature of the soil, which is influenced by several factors, including soil condition, moisture content, depth below grade, and the presence of snow on the ground. This complexity makes assessing heat loss in the basement a more challenging task than for aboveground walls.

Design Considerations

Insulation is a critical factor in maintaining a comfortable temperature in a home, and the type and thickness required depend on various factors, including climate, slab depth, and construction materials. The recommended insulation R-value varies depending on factors such as climate zone and local building codes. For example, in Climate Zones 4 and 5, which include colder areas like the northern U.S. states, the U.S. Department of Energy recommends R-13 to R-15 insulation. In even colder regions like Climate Zones 6 and 7, higher levels of insulation, such as R-20 to R-25, may be necessary.

Concrete blocks with cores promote vertical convection, and an 8-inch concrete wall without insulation has a relatively low thermal resistance, usually between R-1.11 to R-1.49 when accounting for air films on the inside and outside of the block. To improve its thermal performance, it’s recommended to add insulation to the wall. Insulating a slab below a 2-foot depth with an R-value higher than 20 is not necessary since the heat will bypass the insulation. When insulating an 8-foot wall, earth coupling can make insulating the entire wall with R-5 equivalent to insulating halfway down with R-10, requiring less excavation and insulation.

Using 2-inch R-8 beadboard insulation on a 20′ x 30′ basement can result in a 93% reduction in heat loss, while opting for a 3.5-inch R-11 fiberglass batting can achieve a slightly higher 95% reduction in heat loss. In colder climates, it’s recommended to use R-30 insulation that is 4 to 6 inches thick. Adding a reflective barrier to the wall enhances insulation by 16%. To further optimize results, introducing an air space between the reflective barrier and the wall can increase the R-value by as much as 50%.

Basement Insulation

There are several advantages to attaching insulation to the exterior of basement walls. It is less expensive since it can be exposed, and no fire retardant is required. A proper installation also prevents any major air leak at the sill wall. Additionally, the thermal mass remains available to heat or cool the house, and the foundation is protected from thermal stress. Interior insulation can prevent frost heaves from developing since the heat of the house warms up the soil. Frost heaves are problematic for block and stone foundations that cannot resist lateral forces, and spalling can occur. However, exterior foam insulation and backfilling with clean granular fill can prevent cracks caused by freezing and thawing.

Preventing an unheated basement from falling below the freezing point is important to prevent freezing pipes. Heat is added by underground furnaces and boilers. Water heaters and laundry equipment, a source of heat, can also be found in the basement. The temperature of the basement should be halfway between the outside and inside temperatures. Moisture control is important, so mold or bacteria do not grow. In all but arid climates, a vapor barrier is installed on the warm side of the wall. The barrier is usually a thin sheet of PET plastic. It prevents moisture from entering the wall from inside the basement, where it reduces the R-value of insulation and reflective barriers. The insulating material should not degrade after long-term exposure to the elements.

Calculating Heat Loss

Efficient insulation planning relies on accurately calculating heat loss to maintain optimal temperatures. When utilizing programs like REScheck to assess heat loss through a basement wall, at least 50% of the wall must be below ground level. The wall area should exclude windows and doors, and the basement floor area should have the thickness of the exterior wall subtracted. An approximate method for calculating heat loss through a basement is given by the equation: qem = EF x U x A, where EF is a correction factor for a belowground wall and A is the area of the basement walls. Heat loss coefficients for aboveground basement walls are given as U-factors and belowground as F-factors [Btu / (hr-ft-F)].

For the foundation, the heat loss is conducted away from the center out to the perimeter of the slab according to: Qsc = F x P, where Qsc represents the slab edge transmission heat loss, P denotes the perimeter of the slab edge in linear feet, and F represents the transmission heat loss per linear foot of the slab edge. To calculate the F-factor, factors including insulation type, thickness, R-value, and climate zone must be considered. The International Energy Conservation Code (IECC) provides tables that show the minimum R-value for foundations based on the climate zone.


  1. Norton, Paul. “Types of Insulation”. U.S. Department of Energy
  2. Berner, Mike. “Common Types of Insulation, and When to Use Them”
  3. “Basement Heat Loss Guide”
  4. Insulation Fact Sheet. Department of Energy. DOE/CE-0180. 2008
  5. Bobenhausen, William. “Simplified Design of HVAC Systems” Scholarly & Professional. Wiley. 1994