August 20, 2010

Abstract: 

The following report is an excerpt from the 2010 Building Science Corporation Industry Team Building America Annual Report. Many concerns, including the rising cost of energy, climate change concerns, and demands for increased comfort, have led to the desire for increased insulation levels in many new and existing buildings. Building codes are improving to require higher levels of thermal control than ever before for new construction. This report considers a number of promising foundation and basement insulation strategies that can meet the requirement for better thermal control in colder climates while enhancing moisture control, health, and comfort. For more information see Popular Topics/Foundations and Slabs.

A. Introduction

Many concerns, including the rising cost of energy, climate change concerns, and demands for increased comfort, have lead to the desire for increased insulation levels in many new and existing buildings. Building codes are improving to require higher levels of thermal control than ever before for new construction. This report considers a number of promising foundation and basement insulation strategies that can meet the requirement for better thermal control in colder climates while enhancing moisture control, health, and comfort.

The 2009 IRC (Table N1102.1) and 2009 IECC (Table 402.27) require basements in DOE climate zones four and greater to require a continuous layer of R10 insulation or R13 in a framed wall. High R basements, for cold climates, in this report are walls that approach or exceed a true R‐value of R20. In a warmer climate, that does not require basement insulation, high‐R may be considered less.

Basements are stereotypically cool, damp, musty smelling areas of the building that were historically unfinished, unoccupied and used mostly as storage. More and more often, people are finishing their basements to increase the living environment and frequently the basement is transformed into a media room, bedroom, or extra living room. These new environments require greater control of both heat and moisture to provide a healthy living environment with minimal risk to equipment and finishes.

A successful foundation will perform the following tasks

  • Hold the building up
  • Resist soil pressures
  • Keep the groundwater out
  • Keep the soil gas out
  • Keep the water vapor out
  • Allow any water vapor in the wall to leave
  • Keep the heat in during the winter
  • Keep the heat out during the summer

Basement failures occur often due to flooding, or condensation, both of which may result in mould or dust mite problems. However, building physics and extensive field experience has shown that the majority of all basement moisture and comfort issues can be avoided by proper design and material selection.

This study compares over a dozen basement and foundation enclosure designs including historical
construction strategies, code minimum construction and highly insulated construction. This report
demonstrates through computer based simulations and field experience, differences in energy consumption, thermal control, and moisture related issues. This study is an extension of the previous Building America study of High R wall assemblies (Straube and Smegal 2009), to continue to improve the overall building enclosure and achieve greater energy savings.

1. OBJECTIVE

The goal of this research is to find durable, cost effective basement insulation system that can be included with other enclosure details to help reduce whole house energy use by 70%.This report will compare a variety of basement and foundation insulating strategies and present their advantages and disadvantages according to several comparison criteria.

2. SCOPE

This study is limited to basement and foundation systems for cold climates. A previous study was conducted for wall systems and further studies should be conducted to address roofs and attics. In general, only cold climates are considered in this report since enclosures in cold climates benefit the greatest from a highly insulated building enclosure, but important conclusions can also be drawn for other climate zones.

3. APPROACH

The quantitative analysis for each wall system is based on a three‐dimensional energy modeling program and a one‐dimensional dynamic heat and moisture (hygrothermal) model. Minneapolis, MN in IECC climate Zone 6 was used as the representative cold climate for most of the modeling, because of cold winter weather and fairly warm and humid summer months.

B. Analysis

1. WALL ASSEMBLIES REVIEWED

Because there are a number of variables for each possible wall system depending on the local practices, climate, and architect or general contractor preferences, an attempt was made to choose the most common wall systems and make notes about other alternatives during analysis. This list of chosen systems is explained in more detail in the analysis section for each wall system.

  • Case 1 : Un‐insulated Basement
  • Case 2 : Code minimum R10 continuous insulation with poly
  • Case 3 : 3.5 inches fiberglass batt in 2x4 SPF wood framed wall with poly
  • Case 4 : 1 inch XPS + 3.5 inches fiberglass batt in 2x4 SPF wood framed wall
  • Case 5 : 2 inches XPS + 2 inches polyisocyanurate with R10 under slab
  • Case 6 : 3.5 inches 2.0 cc pcf spray foam with R10 under slab
  • Case 7 : 6 inches 0.5 oc pcf spray foam with R10 under slab
  • Case 8 : 2 inches XPS + 3.5 inches fiberglass batt in 2x4 SPF wood framed wall with R10 under slab
  • Case 9 : 2 inches polyisocyanurate +3.5 inches cellulose in 2x4 SPF wood framed wall with R10 under slab
  • Case 10 : 6 inches 0.5 oc pcf spray foam in offset 2”x4” SPF wood framed cavity with R10 under slab
  • Case 11 : 4 inches XPS on exterior of basement with R10 under slab
  • Case 12 : 4 inches XPS in centre of foundation wall with R10 under slab
  • Case 13 : ICF wall with 4” XPS and R10 under slab
  • Case 14 : 2 inches XPS + 5.5 inches fiberglass batt in 2”x6” SPF wood framed wall with R10 under slab

2. ANALYSIS CRITERIA

A comparison matrix will be used to quantitatively compare all of the different basement insulation strategies. A value between 1 (poor performance) and 5 (excellent performance) will be assigned, upon review of the analysis, to each of the comparison criteria for each wall. An empty comparison matrix is shown below in Table 1 as an example.

Table 1: Criteria comparison matrix

The criteria scores will be summed for each insulation strategy, and the walls with the highest scores are the preferred options assuming all of the comparison criteria are weighted equally. It is also possible to weight the different comparison criteria asymmetrically depending on the circumstances surrounding a particular wall design. The weightings for each wall will fall between 1 (least important) and 5 (most important). The weighting is multiplied by the comparison criteria score and added to other weighted values. An example of the weighted conclusion matrix will be shown in the conclusions section of this report.

One of the benefits of using a comparison matrix is that it allows a quantitative comparison when some of the criteria, such as cost may be poorly defined or highly variable. For example, even though the exact costs of different insulations may be uncertain, fiberglass batt insulation is always less expensive than low density (0.5 pcf) spray foam which is less expensive than high density (2.0 pcf) spray foam, so these systems can be ranked accordingly regardless of the actual costs.

Each of the criteria are described in detail below.

2.1 Thermal Control and Heat Flow Analysis

The Heat flow and energy analysis of each basement system was conducted with Basecalc, developed by
Canmet ENERGY and based on the National Research Council of Canada’s Mitalas method. Mitalas used
mainframe computers to perform finite‐element analyses of a large number of basements and analyzed the results to produce a series of basement heat‐loss factors, which were then published as a reference (Mitalas 1983).

A user can apply the Mitalas method by using the correct heat‐loss factors from the published tables and perform a series of calculations to predict heat and energy losses. Basecalc incorporates the finite‐element approach Mitalas used to generate the heat‐loss factors. During this study an analysis spreadsheet model was constructed using the Mitalas method and comparisons of the results between the analysis spreadsheet and Basecalc have been conducted.

The Basecalc software is a relatively simple menu driven program that has many options for construction strategies, insulation placement and site conditions (Figure 1).

Figure 1: Screen capture showing inputs for Basecalc

Some assumptions were made for all of the Basecalc analysis to ensure comparison was possible between resulting simulations. The energy calculated is only for these specific cases, and modifying any of the variables may change the resulting energy requirements. These assumptions are listed below:

  • All simulations were run for Minneapolis/St. Paul MN, data included in Basecalc
  • Basement interior height ‐ distance from top of slab to top of foundation wall 2.44 m (8 ft)
  • Depth (below grade foundation) – distance from top of slab to surface of ground, 2.13 m (7 ft)
  • Width ‐ exterior of structural wall to exterior of structural wall, 10 m (32.8 ft)
  • Length – exterior of structural wall to exterior of structural wall, 15 m ( 49.2 ft)
  • Basement wall area – 118 m2 (1270 ft2)
  • Basement floor slab area – 140 m2 (1506 ft2)
  • Basement perimeter – 48.5 m2 (159 ft2)

In Basecalc, the rim joist is not considered, (but this was analyzed in past research, Straube and Smegal 2009), but thermal bridging across the top of the foundation wall is considered depending on above grade wall construction. For example, one of the most common thermal bridges in typical residential construction is the exterior above grade brick cladding sitting on the outside edge of the foundation wall.

Figure 2: Typical construction thermal bridging through foundation and brick cladding

This thermal bridging can be taken into account in Basecalc. For all simulations in this study, the above grade cladding was assumed to be non‐brick veneer. In typical construction with brick veneer, there is a significant thermal bridge between the interior and exterior when the brick cladding is installed on the exterior edge of the concrete foundation wall. It was assumed that there was no significant thermal bridge at the top of the foundation wall.

All of the Basecalc results are presented in units of MBtus. For clarification 1 MBtu and its equivalent energy in other common units of measure are shown in Table 2.

Table 2: Conversion of 1 MBtu to other common energy units

Million Btu's
(MBtu's)
1
Btu's1,000,000
Therms10
Megajoules1,057
Kilowatt hours293.6

A common way to explain energy savings to homeowners is often in dollars saved since the value of a dollar is well known and can be compared to other design decisions. Unfortunately, prices vary considerably across the continent for heating energy, and also vary depending on the technology used for heating, whether it be electricity, natural gas, oil, etc. For analysis purposes, if cost comparisons are used it will always be for electric heating at 15 cents per kilowatt hour ($44/MBtu). As a comparison, natural gas at $1.50/therm burnt in a 90% efficient furnace costs $16.70/MBtu. The cost of energy is likely to rise, even though the rate of increase is unknown, so dollar savings are likely to be higher in the future.

2.1.1. Building Code Requirements

According to the 2009 IECC in climate zones 4 or higher, the building code requires a minimum of R10 continuous insulation (e.g. fiberglass roll batt) or R13 discontinuous (e.g. framed wall with R13 fiberglass batt) unless it is an unconditioned basement and the floor overhead is insulated in accordance with IRC Sections N1102.1 and N1102.2.6. Adding this required amount of insulation makes a significant difference from an energy perspective as shown in Figure 3, but may not adequately address the comfort, moisture and health concerns that occur in basements. Case 1 in this study is an un‐insulated basement as many such cases can be found in existing buildings, and Cases 2 and 3 are typical of code minimum basements built in many cold climates.

An initial analysis was conducted to determine the effects of different amounts of insulation and strategies on the total heat loss prior to analyzing the various wall systems. Figure 3 shows the improvements in annual energy loss by insulating the full height of the basement wall with different insulation values compared to an un‐insulated basement. The most significant improvement is achieved by adding the first R5, which shows that adding any insulation could help with energy losses. Increasing the insulation to R10 which is the code minimum as a continuous insulation results in a predicted energy savings of 31.2 MBtus (savings of $1372/year based on $0.15/kWhr or $44/). The energy savings should be considered when determining the cost of adding insulation, and whether or not it is cost effective.

The basement wall has an area of approximately 1270 ft2 and R5 foam insulation costs approximately 50‐75¢/sf plus installation. Using R10 rigid foam insulation over the entire basement in this case would cost in the range of $1270 to $1905, and would save a predicted $1372/year.

Figure 3 also shows the predicted energy savings if the slab is insulated with R10 below the slab. In the uninsulated case there is an improvement of Heating Season Energy Loss of 1.3 MBtus, and in the R20 insulated wall comparison the improvement is slightly improved with underslab insulation at 1.5 MBtus. However, the most important aspects of the underslab insulation are not shown on this graph. Comfort levels and moisture related issues including dampness and musty odors, and storage of moisture sensitive materials on the floor will decrease if underslab insulation is used. In some cases when radiant floor heating is used, R20 or greater underslab insulation is necessary to reduce the heat loss to the ground. . .

Download complete report here.