{"id":3082,"date":"2013-10-02T08:44:43","date_gmt":"2013-10-02T12:44:43","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=3082"},"modified":"2017-01-20T10:00:42","modified_gmt":"2017-01-20T15:00:42","slug":"detailing-masonry-and-frame-walls-with-continuous-insulation-and-air-barriers","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/detailing-masonry-and-frame-walls-with-continuous-insulation-and-air-barriers\/","title":{"rendered":"Detailing masonry and frame walls with continuous insulation and air barriers"},"content":{"rendered":"
\"Photo<\/a>
Photo courtesy Sto Corp.<\/em><\/figcaption><\/figure>\n

by John Chamberlin, MBA<\/strong>
\nAccording to the U.S. Department of Energy (DOE), buildings account for 39 percent of total energy use in the United States.1<\/sup><\/a> Efforts to reduce this consumption is reflected in recent changes to building practices and codes.<\/p>\n

For example, the DOE has mandated all states update their commercial building codes to meet or exceed American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2010, Energy Standard for Buildings Except Low-rise Residential Buildings<\/em>, by October. Two key requirements of ASHRAE 90.1-2010 are continuous insulation (ci) and a continuous air barrier.<\/p>\n

ASHRAE 90.1 defines continuous insulation as \u201cinsulation that is continuous across all structural members without thermal bridges other than fasteners and service openings.\u201d Meanwhile, continuous air barriers are defined as \u201cthe combination of interconnected materials, assemblies and sealed joints and components of the building envelope that minimize air leakage into or out of the building envelope.\u201d<\/p>\n

With these requirements and definitions in place, compliance with ASHRAE 90.1-2010 should be easy. However, this may not be the case.<\/p>\n

\"This<\/a>
This map shows the U.S. Climate Zones based on 2009 International Energy Conservation Code (IECC).<\/span>Data courtesy Building Energy Codes Resource Center, Pacific Northwest National Laboratory, U.S. Department of Energy<\/em><\/figcaption><\/figure>Why choose continuous insulation and air barriers?<\/strong>
\nThe biggest problem with many insulated buildings involves thermal bridging. These occur when poor thermal insulator materials meet, creating the path of least resistance for heat to pass through.<\/p>\n

An easy example of thermal bridging would be studs in a building\u2019s wall. Even though these studs typically have fiberglass batt insulation between them, the insulation does nothing to reduce the transfer of heat through the stud. Insulation materials include a nominal R-value, which is a measure of the material\u2019s ability to retard heat flow. In theory, a wall assembly constructed with an insulating material of a certain R-value would have a minimum of that material\u2019s R-value as an assembly. However, due to thermal bridging, the assembly\u2019s R-value may be much lower than the R-value of the insulating material itself. This has led to the concept of an assembly\u2019s \u2018effective\u2019 R-value versus its \u2018nominal\u2019 one.<\/p>\n

<\/a>According to studies conducted by the Oak Ridge National Laboratory (ORNL), thermal bridging in metal frame construction reduces the insulating performance of a wall assembly by 40 to 60 percent.2<\/sup><\/a> For example, a 152-mm (6-in.) metal stud wall assembly including R-19 fiberglass batt insulation may only have an effective R-value between R-8 and R-11. Heating systems for buildings are often designed based on the assembly\u2019s nominal R-value, so in many cases unplanned for energy may be expended to heat a structure\u2019s interior. This is where continuous insulation comes into play.<\/p>\n

ASHRAE now requires minimum continuous insulation R-values for buildings based on the climate zones in which those buildings reside. The International Energy Conservation Code<\/em> (IECC<\/em>) has identified eight unique climate zones throughout the United States (Figure 1). These zones are determined based on the region\u2019s average temperature, humidity level, and moisture level.<\/p>\n

Continuous insulation on an assembly\u2019s exterior, either by itself or in conjunction with interior insulation, is the most economical and efficient way of achieving highly effective R-values. Since ASHRAE 90.1 requires continuous insulation be exempt of thermal bridges, its nominal R-value should be much closer to the effective R-value of the assembly.<\/p>\n

\"This<\/a>
This shows a sample detail of a wall assembly featuring a fluid-applied air barrier with continuous insulation under masonry veneer on steel stud backup.<\/span> Images courtesy Sto Corp<\/em>.<\/figcaption><\/figure>\n

Another major cause of energy consumption is air infiltration and exfiltration. Two of the major air pressures on buildings causing infiltration and exfiltration are wind pressure and stack pressure.<\/p>\n

Wind pressure on buildings may have a serious effect on energy and moisture-related air leakage. Since wind pressure is not uniform across a building face, the higher up on a building, the higher the pressure is likely to be, especially as the building rises above objects that might restrict the wind\u2019s flow. Wind pressure pressurizes a building positively on the side it is hitting, but as the wind goes around the corner of the building, it speeds up. This causes the pressure to decrease, and change from positive to negative. These changes in pressure can affect how water moves around the building, especially at corners where water may enter the wall if there is an opening at a joint in the cladding. This phenomenon is known as \u2018wind wash\u2019\u2014its effects can be avoided as long as the air barrier is intact at the corners.<\/p>\n

Stack pressure occurs when there is a difference in atmospheric pressure at the top and bottom of a building. This difference is the result of a variance in temperature at the top and bottom, causing varying weights of the columns of air inside the building compared to outside. In climates where the interior is heated, stack pressure may cause air infiltration at the bottom of a building and exfiltration at the top. In climates where the interior is cooled, exfiltration may occur at the bottom and infiltration at the top. Stack effect can be controlled by designing airtight vestibules, closing off openings (i.e.<\/em> mechanical penetrations) between floors, and sealing vertical shafts within the building.<\/p>\n

Air barrier systems help control air infiltration and exfiltration in buildings. These systems accomplish this by sealing joints, penetrations, and openings to create an airtight assembly. This, combined with venting and compartmentalizing, allows pressure to equalize between the interior and exterior of a structure. Without these differentials, air infiltration and exfiltration is restricted.<\/p>\n

Air barrier systems should meet three key criteria:<\/p>\n

    \n
  1. They should be continuous to not allow opportunities for air leakage.<\/li>\n
  2. They should be structural, or in other words, permanently secured to the supporting structure. (Air barriers must be able to withstand wind pressure, stack pressure, and any pressure caused by mechanical effects, and ultimately transferred to the structure. If not a permanent part of the structure, air barriers may tear or displace under stress.)<\/li>\n
  3. Air barriers must be durable. (They are typically installed behind a building\u2019s cladding and may require removal of the cladding for any type of maintenance or repair. For this purpose, a highly durable air barrier will always be preferred so maintenance can be avoided.)<\/li>\n<\/ol>\n

    Benefits of continuous insulation and air barriers<\/strong>
    \nA study conducted by Morrison Hershfield, \u201cEnergy Conservation Benefits of Air Barriers,\u201d focused on the inclusion of both continuous exterior insulation and a continuous fluid-applied air barrier. Using 3D modeling, a prototype medium three-story office building was proposed for Dallas, Seattle, and Toronto climates. Baseline case buildings were set up to meet the minimum requirements of ASHRAE 90.1-2007 for newly constructed buildings.<\/p>\n

    \"The<\/a>
    The installation of fluid-applied air\/moisture barrier using spray equipment over sheathing.<\/span><\/figcaption><\/figure>\n

    According to the report:<\/p>\n

    <\/a>Heating, cooling, lighting, and interior equipment energy consumption were modeled for each load case at 10-minute intervals and the results are summarized on a monthly usage basis in units of kilowatt hours (kWh). This data was then used to calculate energy savings over the base case, and an annual carbon equivalent was calculated based on the annual heating and cooling costs<\/span>.3<\/sup><\/a><\/p>\n

    The modeling determined that in terms of energy efficiency, the inclusion of a continuous air barrier actually had a greater impact than the continuous insulation. Annual energy cost savings ranged from $5000 to $19,000, compared to the baseline building which included continuous insulation by itself. Also, there was a diminishing return in energy cost savings as the R-value of the continuous insulation was increased.<\/p>\n

    The challenges with continuous insulation<\/strong>
    \nThe difficulty that arises with continuous insulation is ironically similar to the phenomenon that makes it necessary in the first place. The definition of continuous insulation explains it must be \u201ccontinuous across all structural members without thermal bridges other than fasteners and service openings.\u201d<\/p>\n

    In this case, \u2018fasteners\u2019 refers to materials such as nails and screws. What this definition does not account for are common details such as ties and shelf-angles in masonry construction or clips and z-grits (i.e.<\/em> horizontal structural member providing lateral support to the wall panel) in non-masonry cladding assemblies.<\/p>\n

    In a recent paper prepared by RDH Building Engineering Ltd., entitled \u201cThermal Bridging of Masonry Veneer Claddings and Energy Code Compliance,\u201d 3D-thermal modeling was used to determine the effects these types of details might have on the effective R-value of a wall assembly including continuous insulation. This value would change based on the actual construction assemblies and structure size. According to RDH:<\/p>\n

    <\/a>metal cladding support connections occupying less than 0.5 percent and even less than 0.05 percent of the wall\u2019s surface area can have a profound impact on effective R-values (i.e.<\/em> anywhere from 10 percent to greater than 50 percent)<\/span>.4<\/sup><\/a><\/p>\n

    Using masonry construction as an example, RDH goes on to explain different types of masonry ties (e.g.<\/em> steel versus fiber) and various backup materials (e.g.<\/em> wood stud, concrete, or steel backup) also have an impact on the degree of thermal bridging that may take place.<\/p>\n

    \"Continuity<\/a>
    Continuity of the air barrier is maintained at a transition from sheathing to foundation through the use of a flexible transition membrane embedded in a fluid-applied air\/moisture barrier.<\/span><\/figcaption><\/figure>\n

    Due to their low insulative value, steel masonry ties may reduce the insulation effectiveness by five to eight percent, depending on the type of steel and backup materials. Even more impactful, direct attached masonry shelf angles may reduce the effective R-value by 40 to 55 percent in conjunction with typical exterior insulation thicknesses and steel masonry ties.<\/p>\n

    <\/a>Another study by Morrison Hershfield, \u201cThermal Performance of Building Envelope Details for Mid- and High-rise Buildings,\u201d looked at 40 common building design envelope details and again used 3D modeling to determine the effect thermal bridging might have on the overall thermal efficiency of buildings including continuous exterior insulation.5<\/sup><\/a><\/p>\n

    This study noted the addition of more materials with higher insulating values did not greatly improve the overall thermal performance of the wall assembly so long as these thermal bridges existed. It also points out quite clearly that even in buildings including continuous exterior insulation, all thermal bridges must be considered to determine a structure\u2019s overall thermal efficiency.<\/p>\n

    Continuous air barrier challenges<\/strong>
    \nThere are numerous products that qualify as air barriers including:<\/p>\n