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Selection of Insulation Systems for Exterior Masonry (Mass) Walls

By Xuaco Pascal

MASS WALLS IN COMMERCIAL CONSTRUCTION

Wall designs using continuous insulation on the exterior have also been described as the “perfect wall.”2 This is becausethese wall systems are very efficient and can be used confidently in any climate zone. Not only does moving insulation to the exterior improve efficiency, it also facilitates flexibility in design, addresses thermal bridges and shifts the dew point in a wall to help avoid condensation and associated moisture issues.

One common wall system that uses exterior insulation is the masonry wall system (also referred to as a mass wall system) which uses poured concrete or blocks as the main structure. With masonry use representing a significant portion of commercial construction, designers are challenged with how best to meet energy efficiency targets, control air or moisture infiltration and address vapor diffusion while keeping project costs in check. One cost-effective and high-performance solution is the use of closed-cell spray polyurethane foam (ccSPF) insulation
on the exterior of masonry walls. When sprayed on a masonry wall exterior, ccSPF’s multi-functionality has the unique ability to simplify the overall wall design, essentially eliminating the need for additional trades and scheduling complexities, while delivering excellent insulating value, air barrier performance, secondary moisture protection, and vapor control.

ENERGY USE IN COMMERCIAL BUILDINGS

Commercial buildings represent a significant amount of energy use and associated carbon emissions. According to the Department of Energy (DOE), the “average commercial building wastes approximately 30% of its energy,” often due to gross inefficiencies in the building envelope.

The DOE also states that “it’s often possible to reduce energy use by 10% with little or no cost.”3 This typically involves an improvement of insulation levels and air sealing. Statistics such as this highlight the commercial sector as an area where energy efficiency improvements can have a significant impact on overall energy consumption and emissions. As a result, more stringent policy and code requirements that drive the use of improved insulation levels and air sealing have been enacted.

One such policy is the Energy Independence and Security Act of 2007 which has driven the use of enhanced insulation systems in federal government buildings. Energy reduction policies and a focus on sustainability goals have contributed to energy consumption by the U.S. government being at its lowest level since at least 1975.4 More broadly across the nation, significant enhancements have been made to building envelope insulation requirements over the last decade.

INSULATION REQUIREMENTS

With concerns over carbon emissions, energy consumption and overall efficiency of buildings, there has been a substantial shift towards designs that call for increased R-Value requirements5 (more insulation), tighter air sealing of buildings and greater moisture control. For commercial buildings, the minimum insulation levels (R-Value) required by code vary by climate zone. They are determined by the applicable requirements established by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards or the commercial provisions of the International Energy Conservation Code (IECC).

Generally, in non-residential applications, ASHRAE standard 90.1 requirements are followed since these offer flexibility with three equivalent paths for code compliance (prescriptive, performance and cost). Table 1 lists the ASHRAE 90.1 (2010) prescriptive insulation requirements for non-residential construction.

table 1INSULATION’S ROLE IN IMPROVING ENERGY EFFICIENCY

In any building, long-term energy efficiency is most affected by the building envelope. With a typical service life of 60+ years, a well designed, energy-efficient shell contributes to lower operating costs, better interior conditions and the overall sustainability of that building. As a result, this has driven the use of enhanced insulation levels as a means for achieving better building envelope performance in new buildings and for renovating existing inefficient buildings.

In addition to enhanced insulation, code requirements (ASHRAE 90.1 and IECC) have mandated the use of continuous insulation (CI) on the exterior of buildings. Installing insulation on the exterior of a wall system can address many issues that result in inefficiencies and problems such as thermal bridges, thermal bypasses and air leakage, to name a few. For mass walls in particular, installing the insulation on the exterior optimizes the thermal mass of the wall which can improve overall thermal performance. Note that the total prescriptive R-Value5 requirements in Table 1 are lower for mass walls than framed wall (wood or steel) construction in recognition of this performance advantage.

MASS WALLS

Masonry walls, whether concrete or block, are often referred to as “mass walls” as mentioned previously. The use of masonry adds mass and resulting thermal heat capacity to the wall system. Heat capacity is defined as the amount of heat necessary to raise the temperature of a given mass by 1 degree F. The heat capacity of masonry walls enables the masonry layer to absorb, store and release heat. By storing heat in the masonry layer, the wall system helps buffer external temperature fluctuations and can better regulate interior temperatures.

The use of mass wall construction has been in place for centuries and found in early adobe and stone structures. Early systems required very thick walls to utilize the heat capacity and maintain consistent interior temperatures. Although strong, aesthetic and durable, the cost, weight and material requirements make very thick wall systems impractical today.

However, advancements in materials such as light concrete and concrete masonry units (CMU) have made mass wall systems much more practical. Combined with the use of exterior insulation and HVAC equipment, mass wall systems are much more cost-effective, while maintaining excellent efficiency and durability across all climate zones and temperature extremes. These systems benefit from the heat capacity of the concrete layer and are desirable due to their ability to dampen interior temperature fluctuations and ability to limit peak loads on HVAC equipment. The structural and thermal benefits make mass walls very attractive for use in commercial construction and are commonly found in a wide array of building types including education, health care, retail, lodging, restaurant, office and government.

INSULATING MASS WALLS

Insulating mass walls from the exterior ensures the best overall performance. However, this configuration exposes the insulation layer to exterior elements such as moisture and wind. It requires an insulation system that can withstand this level of exposure. Most commonly used are closed-cell spray polyurethane foam insulation (ccSPF), extruded polystyrene (XPS), expanded polystyrene (EPS), polyisocyanurate (PIR) or mineral wool insulation boards. Table 2 lists some relevant performance criteria for each of these insulation materials.

table 2REQUIREMENTS FOR COMPLETE PROTECTION

To be effective, these insulation systems must provide the following performance attributes: thermal barrier, moisture barrier, air barrier, and vapor control layer. Closed-cell spray polyurethane foam insulation is an ideal choice because it provides all of this functionality in a single material layer. Although the board insulation materials noted in Table 2 can withstand weather exposure, they provide less air and moisture protection for the wall system (versus ccSPF) and thus require additional layers or detailing to adequately protect the building. When evaluating materials, these additional layers should be considered as part of the insulation system since they directly impact the complexity of the overall wall design, assembly scheduling, and overall system costs.

COST OF INSULATING MASS WALLS

With the advancement of materials and lighter concrete, the cost of mass walls has become more competitive relative to framed walls. Wall system costs are highly dependent on specific material selection, labor, regional and project requirements. However, for a specific project wall design, general cost comparisons can be made regarding the insulation system selected using data for national averages and standard labor rates. One such database is RSMeans, a construction cost database that is updated annually. By factoring in the combined costs associated with each of the components that comprise an insulation system under consideration, designers, owners and general contractors can make more informed decisions regarding product selection and project requirements.

CCSPF: A COST-EFFECTIVE, HIGH-PERFORMANCE INSULATING SYSTEM

While block or concrete walls are sturdy and durable, they can be prone to moisture intrusion and air infiltration that may reduce their effectiveness and compromise durability. Adequate protection requires insulation systems that can be installed as a continuous layer, are resistant to moisture and air infiltration and can be easily attached to the exterior of the building.

As an insulation material, ccSPF provides superior insulating performance at a given thickness, is inherently moisture resistant, typically serves as an air barrier at just 1” of thickness6 and provides vapor barrier performance at 2” thickness.7 The multi-functionality of ccSPF makes it ideally suited for use as an exterior insulating system that provides a comprehensive building protection layer. It adheres well to the mass wall, provides seamless monolithic protection and expands to seal penetrations and other irregularities that are inevitable in wall assemblies. In addition to its performance attributes, designers and general contractors often find that ccSPF is a cost-effective solution that can simplify trade scheduling, improve speed of installation and increase reliability compared to alternative insulation systems.

As shown in Table 2, XPS, EPS, PIR and mineral wool board products may not deliver the moisture and air infiltration protection that architects and builders require. Consequently, these systems require additional layers or detailing to provide sufficient moisture, air and vapor control to adequately protect the building. Although these extra layers result in additional costs associated with the insulating system, they are often treated discretely rather than included in a total insulation system cost comparison.

A SYSTEM APPROACH TO INSULATION COST COMPARISONS

Chart 1 breaks down the relative individual costs associated with each insulation system option based on meeting mid-climate Zone-5 insulation requirements (R-12). Closed-cell spray polyurethane foam was considered along with XPS, EPS, PIR and mineral wool board insulations. For the purpose of this cost comparison, only the insulation, air barrier and moisture control measures were included since the cost for the remaining components are neutral relative to established wall system design.

In this comparison, the average of a sheet membrane system and fluid applied bituminous layer was used because they are the most commonly applied water/air barrier layers for protection of mass wall systems. Using the RSMeans database, the additional cost associated with installing these layers, based on national averages, was determined to be $2.91 and $2.60 per square foot respectively. When you include the cost of these additional layers plus sheathing tape costs to seal board joints, ccSPF becomes the clear choice as the external insulation system for mass wall designs. See the Insulation System Cost References at the end of this paper for additional cost data.

When one takes a system approach to comparing insulation costs, ccSPF becomes the preferred system for mass wall design and construction. By looking at insulation system costs holistically, a more accurate comparison can be drawn than by looking at the components alone. Although individual project costs will vary by location, the data indicates that choosing a ccSPF insulating system for mass walls can result in significant costs savings in excess of 30% relative to board insulations that often require additional air and water infiltration control layers. In addition to these significant savings, contractor feedback reveals that one of the greatest benefits when installing a ccSPF system is the simplicity and time savings associated with using one material, one labor pool and one time around the building. By coordinating a single trade with the cladding installation, significant project efficiencies can be realized that can often outweigh the initial material cost savings alone.

CASE STUDY: MASS WALL CCSPF SYSTEM EXCELS AT HIGH SCHOOL

Informed design and engineering firms have utilized high performance mass wall systems with ccSPF for decades. One such example is the 138,000 sq ft renovation and expansion of Merrimack High School in Penacook, NH.

To meet the project’s energy savings criteria, the school district worked in conjunction with the spray foam contractor (FOAM-TECH™), the design firm (Banwell Architects) and associated engineering firms to select a high performance wall system using ccSPF on the exterior.

The design qualified Merrimack High School as the first project in New Hampshire to meet the performance criteria of the Collaborative for High Performance Schools program. Not only did the exterior ccSPF system meet the program’s energy efficiency requirements, its superior air tightness and efficiency facilitated a 25% reduction in HVAC system sizing at a significant project cost savings of $6.85 per sq ft (total project savings exceeded $940,000).8 This associated cost savings resulted in an immediate payback that far exceeded expectations.

Average cost sq ft bar chartCONCLUSION

Mass wall systems are durable, can easily be constructed and finished with multiple veneer claddings for a range of aesthetic and architectural detailing. They can be professionally finished with a variety of brick, stone, stucco or metal claddings offering a versatile, low-maintenance and cost-effective building design. Mass walls are desirable because they offer the added benefit of heat capacity which buffers temperature fluctuations and may limit peak HVAC loads. This improves occupant comfort and can lower energy consumption, making them attractive for use in many commercial designs.

When designing or specifying a masonry/mass wall, one key component is the insulation system which ultimately impacts the overall wall system performance. With its multi-functionality, ccSPF provides an all-in-one insulation system that meets exterior wall design criteria, offers superior performance and simplifies the construction process resulting in a cost-effective, yet high-performance system. By selecting ccSPF as the insulation system; designers, owners and general contractors can simplify their designs. This can lead to significant advantages associated with overall system cost reductions, improved project scheduling efficiencies and fewer trades when compared to other insulation materials.

For more from Honeywell, visit
www.honeywell-blowingagents.com.

Sources:

1. Based on national averages determined using RSMeans database for each material at a nominal thickness required to achieve a thermal performance of R-12. Because building code requirements can differ by region, potential material/labor costs and associated savings can vary. It is important to follow building codes and standards for your respective region.

2. “Perfect Wall”- Building Science Corporation Insight-001, July-2010.

3. Department of Energy: http://www.energystar.gov/buildings/about-us/how-can-we-help-you/improve-building-and-plant-performance/improve-energy-use-commercial.

4. U.S. Energy Information Administration: http://www.eia.gov/todayinenergy/detail.cfm?id=19851.

5. R-Values vary by insulation type. Check your seller’s fact sheet for specific R-Values when comparing insulations.

6. When a minimum of 1” is applied, closed-cell SPF qualifies as an air barrier according to ASTM E-2178 which is the test used by the Air Barrier Association of America (ABAA) to define an air barrier.

7. Two inches of closed-cell SPF qualifies as a 1 perm vapor retarder according to ASTM E-96. Perm ratings vary by manufacturer; please consult manufacturer literature.

8. Determined by Banwell Architects and GWR Engineering project assessment. $6.85/sq ft is the difference between HVAC savings minus insulation package costs. Project total was 138,000 sq ft (addition + renovation) resulting in total project savings of approx. $946,000

Insulation System Cost References (based on RSMeans national averages):

1. Transition detailing: Window flashing and expansion joint detailing for ccSPF ($0.43).

2. Seam (sheathing) tape: Tape to seal joints between insulation boards: ($0.21).

3.Water/Air barrier: Average cost for sheet adhesive membrane ($2.91) or modified bitumen fluid applied ($2.60) waterproofing systems. ($2.91+$2.60/2= $2.76). Sheet adhesive membrane is defined as PE/PVC sheet + bitumen adhesive backing.

4. Insulation costs: For min. R-12 (installed cost):

i. ccSPF 2” (R12.5)-$2.39

ii. XPS 2.5” (R12.5) -$1.87

iii. EPS 3” (R12)-$1.76

iv. PIR 2” (R12.5)-$1.62

v. Mineral (rock) wool 3” (R12)-$2.52

ABOUT THE AUTHOR

Xuaco Pascual is a Global Market Manager for honeywell with the responsibility for Spray Foam Insulation applications. Xuaco holds a BS in Mechanical Engineering from Old Dominion  University and brings over 15 years experience with construction building envelope solutions  focused on energy, moisture, and sustainability practices. Prior to joining Honeywell in 2008, Xuaco held several business development and technical roles at DuPont, has been involved in the launch of over 50 new products and services, managed material research projects at NASA, and is a U.S. Navy Submarine Service veteran. Xuaco has several patents in construction, composites, and consumer applications. He has a diverse background in product development, training, mitigation, quality issues, and construction remediation.