The Warming Trend of Insulating Basements – Truths and Myths

The Concrete Foundations Association explains the most important information for understanding the requirements for insulating concrete foundations and the options that may be pursued.

Two-inch (R-10) extruded polystyrene rigid insulation board with structural wall connectors prepared for an insulated sandwich wall configuration.
Two-inch (R-10) extruded polystyrene rigid insulation board with structural wall connectors prepared for an insulated sandwich wall configuration.

Question: In a conversation with a builder last week, I was asked how I plan to provide a foundation that meets the minimum requirements for insulation. My initial response was “Why?”, since we’ve never had to provide insulation in the past. This builder is now making it our responsibility for the foundation package to be fully compliant thermally as well as structurally. I have heard about a poured concrete residential basement wall where foam panels are placed in the forms on the inside wall before the concrete is poured. Can you tell me more? – Foundation Contractor (Missouri).


Answer: This question will be asked with increasing frequency throughout markets where living space extends below grade. A great option for affordable, secure, durable, storm-resistant and quiet, below grade living space is also known for its constant climate condition. In addition, many mechanical engineers savvy in their understanding of the advantages for this space, will design a heat pump system with re-circulation intakes to take advantage of the constant year-round condition of the finished basement.

The discussion of how and why to insulate a basement always begins with what is required. Your answer begins with what is the current requirement for your state and in Missouri, that is quite interesting. According to the ICC website (www.iccsafe.org), Missouri is still implementing the 2000 IRC1 but has upgraded to the 2012 IRC2. In 2000, the IRC gave no indication of insulation requirements in Chapter 4 for foundation design. However, in Chapter 11 – Energy Efficiency, the following statement was made:

N1102.1.5 Basement walls. When the basement is a conditioned space, the basement walls shall be insulated in accordance with Table N1102.1. When the basement is not a conditioned space, either the basement walls or the ceilings separating the basement from conditioned space shall be insulated in accordance with Table N1102.1. When insulating basement walls, the required R-value shall be applied from the top of the basement wall to a depth of 10 feet (3048mm) below grade or to the top of the basement floor, whichever is less.

Table N1102.1 consists of a table organized according to the number of average Heating Degree Days (HDD) for the location and assigns the amount of R-value required ranging from 0 to 19. For Missouri, the HDD range is typically from 4,700 to 4,900 for Kansas City, Springfield and St. Louis, requiring either R-8 or R-9 insulation. Typical insulation systems for cast-in-place forming provide two-inch rigid insulation (R-7 to 10) or insulated concrete forming systems with two, two-inch rigid insulation layers (R-14 to 20).

The other requirement in Missouri right now is the 2012 IECC. This document requires basement walls to be insulated in the same general language as found in the IRC reference above but it has gone away from climate zones related to HDD and instead uses a map related to eight different climate zones. Missouri consists of roughly the northernmost three tiers of counties in Zone 5 and the rest in Zone 4. According to the Table R402.1.1, this requires basement walls to be insulated to an R-10/13 for Zone 4 or R-15/19 for Zone 5. The first number in each set is for continuous rigid insulation and the second number is for cavity insulation. Thus the direction is given for the vast majority of today’s basement market.

The number one myth associated with this requirement is that insulating basements is difficult, expensive and ineffective. The question you ask refers to a very common method represented by several different systems in the marketplace today. In fact, today’s concrete foundation contractor can be very well equipped to offer a full variety of insulation options to suit the builder, home owner, site conditions or any other variable that may place limits on the construction of the home. One quick look at the methods for insulating today’s modern foundation is all it takes to determine how effective this decision can be on both the aesthetics and the performance of the quality living space that has been added to the home.

Insulation choices are largely based on the position of the insulation in relation to the structural concrete wall. Continuous rigid insulation is the most common method for any initial construction to meet the requirements and it does so by meeting the lower R-value requirements. The decisions for the contractor include insulation located on the exterior or interior of the foundation wall; the insulation located within the foundation wall—also referred to as a sandwich wall; and the insulation containing the foundation wall—as in ICF construction. The systems in use today are not at all like the methods that have been used for decades to place insulation against a layer of solid concrete.

No longer is insulation glued or otherwise stuck to a wall nor is it placed between pieces of wooden furring or studs (this would be considered cavity and require an R-13). Today, rigid insulation that does not absorb moisture and does not break apart is set into the wall forms prior to the placement of any concrete. High-tech form tie modifications firmly secure the insulation against either the inside or the outside wall form while concrete is placed into the remaining cavity. The resulting wall has a structural thickness and reinforcement as required for the structural design along with energy code compliant insulation mechanically secured with the structural wall. These systems also provide easy attachment of common wall treatments such as drywall, siding and masonry to the high-density composite plates securing the insulation to the wall. Furthermore, they maintain equal effectiveness for waterproofing as today’s modern spray-applied and sheet membrane systems are effectively applied around the foundation whether they are applied to an exterior insulation or direct to the concrete wall.

Finally, the decision can be made to provide continuous insulation within the concrete. This method also maintains the R-10 value required for your market but results in exposed concrete on both the inside and outside. Structural, composite wall ties firmly hold the insulation in the middle of the formed wall. This separates the concrete into two structural layers that work together with the insulation layer to easily create the required load capacity for the weight of the house and the earth forces applied to the sides. The system goes one step further to provide the aesthetics of modern concrete treatments while maintaining current methods for waterproofing.

Final Thoughts

Rather than defining a difficult condition that the builder must achieve, today’s modern insulation systems define cost-effective and unequaled performance opportunities to enhance new living spaces for the home owner. For more information on these methods of insulating basements and other features of foundations, visit the Toolbox at the Concrete Foundations Association website, www.cfawalls.org.

Want to know more? Contact CFA Executive Director, Jim Baty at 866-232-9255 or by email at jbaty@cfawalls.org. The CFA is a national association with the mission to support the cast-in-place contractor as the voice and recognized authority for the residential concrete industry.

References:
1. 2000 International Residential Code For One- and Two-Family Dwellings published by the International Code Council, Inc., 4051 West Flossmoor Road, Country Club Hills, IL 60478-5795 | Phone 1-888-422-7233 | www.iccsafe.org

2. 2012 International Energy Conservation Code published by the International Code Council, Inc., 4051 West Flossmoor Road, Country Club Hills, IL 60478-5795 | Phone 1-888-422-7233 | www.iccsafe.org

 

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