Insulation Design

Insulation helps to create a thermal barrier in buildings, reducing the transfer of heat between the interior and exterior. Insulation helps keep the interior cool in hot weather, reducing the need for excess air conditioning, and retains heat indoors during cold weather, lowering the need for excessive heating, thus lowering energy consumption. Heating and cooling are responsible for a significant amount of energy consumption in the average American home, accounting for 50 to 70% of total usage.4 Within a two-story home, heat loss through the basement and attic can be significant, with the basement accounting for 15% – 30% of total heat loss and the attic 25%.3

Design for Energy Efficiency

Proper insulation can help maintain a comfortable and consistent temperature throughout the building, reducing drafts and cold spots. Insulation can also improve the health of a building’s indoor environment by reducing the ingress of outdoor pollutants and allergens. In addition to these benefits, insulation can also provide fire resistance, moisture control, and soundproofing. Many jurisdictions require specific levels of insulation in new construction or building renovations to ensure these benefits are realized.

Homeowners can reduce basement heat loss by up to 70% by implementing R-10 perimeter insulation.3 The insulation is placed around the perimeter of the house, starting from the bottom of the exterior siding all the way down to the foundation footing. The foundation footing is the concrete base that supports the foundation walls and is typically wider than the walls themselves to distribute the weight of the building evenly over the soil. In this type of insulation installation, one foot of the foundation is left exposed, which means the insulation does not extend all the way down to the bottom of the foundation. This is done to prevent moisture buildup and allow for proper drainage.

This insulation can result in substantial savings, particularly in mild winter climates, where an 1800 sqft house with R-10 insulation can save $50 – $60 per month. The cost of installing R-10 perimeter insulation typically ranges from $300 to $600, with a payback period of 5 – 10 years. Heating infiltration losses, which are typically double those of cooling and two to three times higher than those from conduction, can be significantly reduced by proper insulation. It should be noted that underground walls are assumed to have no infiltration, and aboveground walls require air sealing to reduce heat loss due to infiltration.


Effective insulation is a critical element for achieving energy efficiency in buildings. 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 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 similar R-value to fiberglass but is a better fire retardant and moisture-resistant. Mineral wool insulation is denser than fiberglass, 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 extends 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.

While concrete blocks with cores promote vertical convection, 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. 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 still move around 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.

Covering a 20′ x 30′ basement with a 2-inch beadboard insulation with an R-value of 8 can result in a heat loss reduction of 93%, whereas using a 3.5-inch fiberglass batting with an R-value of 11 can reduce heat loss to 95%. In colder climates, insulation with an R-value of 30 (4″ – 6″ thickness) is typically needed. To further increase the R-value, a reflective barrier added to the wall can improve the insulation by 16%. For even better results, incorporating a reflective air space between the barrier and the wall can boost the R-value by up to 50%.

Belowground Insulation

When it comes to exterior belowground insulation, there are several advantages to attaching insulation to the exterior. It is less expensive since it can be exposed, and no fire retardant is required. A proper installation also prevents the 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, foam insulation and backfilling with clean granular fill can prevent cracks caused by freezing and thawing.

Avoiding the possibility that an unheated basement falls 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, where it condenses and reduces the R-values of insulation and reflective barriers. The insulating material should not degrade after long-term exposure to the elements.

Calculating Heat Loss and Insulation Requirements

Calculating heat loss is crucial to determine the insulation required to maintain an optimal temperature. If you use a program (i.e., REScheck) to calculate the loss through a basement wall, at least 50% of the wall must be below ground. Basement walls that enclose heated rooms are part of the building envelope. The wall area should exclude windows and doors. To achieve greater accuracy, the thickness of the exterior wall should be subtracted from the basement floor area. 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
  • Qsc = slab edge transmission heat loss
  • P = the perimeter of the slab edge in linear feet
  • F = transmission heat loss per linear foot of slab edge
To determine the F-factor, a number of variables must be taken into consideration, including the R-value of the insulation, the thickness of the insulation, and the type of insulation. The climate zone in which the home is located is also a factor in determining the appropriate insulation values. The International Energy Conservation Code (IECC) provides tables that show the minimum R-values for foundations based on the climate zone.

In summary, proper insulation and moisture control are critical to maintaining a comfortable and healthy living environment in the basement. Insulation materials and thicknesses should be selected based on the climate zone and the desired level of energy efficiency. By properly insulating the foundation, homeowners can reduce their energy bills, increase the comfort of their living spaces, and minimize the risk of moisture and mold issues.


  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. Insulation Fact Sheet. Department of Energy. DOE/CE-0180
  6. Bobenhausen, William. “Simplified Design of HVAC Systems” Scholarly & Professional. Wiley. 1994