August 31, 2016
by John Edgar
Insulation manufacturers have devised numerous ways to improve the thermal performance of their products. Adding carbon or alumina particles to expanded polystyrene (EPS) increases infrared reflectance, and hence boosts R-value. For buildings that need heat stopped during the day and released at night, there are phase-change materials. More exotic products, like vacuum insulated panels (VIPs) and aerogels, boast great R-values for difficult-to-insulate locations. Materials using recycled or renewable products will appeal to those chasing points under the green rating programs.
Unfortunately, none of this matters when these high-performance products are installed ineffectively. It is old news batt insulation in steel studs loses half its R-value. Steel studs themselves act like ‘radiator fins’ to the exterior—they are a prime example of a thermal bridge, along with shelf angles and structural connections.
New research shows that thermal bridging through insulation is a much more serious problem than previously suspected. Continuous insulation (ci) outbound of the studs is the accepted solution. Code-writers have taken note and change is coming. For example, in the International Energy Conservation Code (IECC), Table C402.1.2 provides the total U-value of the opaque wall area including continuous insulation. It also references the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings. If ASHRAE 90.1 is modified to include linear transmittance, as described in this article, then there will be significant change in the way buildings are designed.
In some ways, the new battlefront in construction will be energy conservation versus gravity. Thermal bridging needs to be eliminated, but something has to hold the cladding on the building. In all methods of construction, the cladding dead load has to be secured to the structure through the ‘continuous’ insulation.
What holds the insulation in place? It could be adhesive, as is the case for exterior insulation and finish systems (EIFS). However, screws with washers are often used to secure the insulation to the supporting structure. Typically, for thermal energy calculations, the diameter of the screw shaft can be compared to the insulation area, yielding a thermal bridge ratio of about 0.05 percent—an apparently insignificant amount. In fact, a thermally conductive shaft transmits heat from an area many times larger than its diameter (Figure 1).
Thermal bridging through fasteners cools the temperature inside by radiating heat out of the wall. If the fastener tip is below the dewpoint, the result may be condensation and potential corrosion—not good for long-term durability and resilience. Fortunately, there are now inventive collections of fasteners that have reduced thermal conductivity and are corrosion-resistant.
With the energy modeling discussed in this article, the starting point in the calculation will be a ‘clear wall’—that is, an area of opaque wall devoid of major thermal bridges. The ‘clear wall’ value is determined by guarded hot-box calculations. Insulation fasteners are included as part of the ‘clear wall,’ so their effect is thus included in the base calculation. Essentially, there is no need to model every one. However, if a particular design is running close to the acceptable mark, something as trivial as fasteners may require examination.
Calculating thermal performance
The way the building envelope’s thermal performance is calculated has changed, thanks to more powerful computers and an amazing degree of sophisticated modeling. The same computer simulations that model heat shield performance during spacecraft re-entry also work, for example, to determine heat loss through a floor slab.
Until recently, the parallel approach to heat loss calculations was used. As with the slab penetration shown in Figure 2, the energy flow through the wall and slab were calculated based on the elevation area. The heat flows were added based on the ratio of wall to slab area. Any lateral flow of energy was ignored.
The reality is energy moves in all directions and a cold uninsulated slab will draw heat not only from the slab, but also from the interior air and wall assembly above and below (Figure 3). Therefore, the insulation in the wall is rendered less effective because heat flows around it and out the slab. The problem has been how to accurately calculate the energy end run.
In July 2011, engineering firm Morrison Hershfield Ltd. presented a paper to ASHRAE showing how advanced computer modeling could accurately predict heat flow through thermal bridging. (The firm’s “Thermal Performance of Building Envelope Details for Mid- and High-Rise Buildings (1365-RP)” was presented to the American Society of Heating, Refrigeration, and Air-conditioning Engineers’ Technical Committee 4.4 on July 6, 2011). This research evaluated thermal performance data of 40 common building envelope details for mid- and high-rise construction. The modeling was validated by 29 guarded hot-box measurements. The intention was to modify the energy calculation methods described in ASHRAE 90.1.
The paper introduces a simpler, but accurate, way to calculate the effect of thermal bridging. Rather than calculate the cross-sectional areas of building elements and work out heat loss per area, the modeler only has to measure the length of the thermal bridge (e.g. the length of the deck or perimeter of a window), or identify point sources (e.g. a penetrating structural beam). Each element has a thermal transmittance value already determined by the research. Linear transmittance is measured in W/(m•K) and point flow is measured in W/K.
The opaque wall, without any thermal bridging, is the ‘clear wall’ for which the thermal transmittance value has been predetermined and validated. It is measured in W/(m2•K). For an insulated stud wall, all components—including the air films, studs, batts sheathing, insulation, cladding, and fasteners—are included.
To calculate the thermal bridging effect of a cantilevered balcony, for example, the length of the balcony at the wall interface is multiplied by the linear transmittance value. This number is then divided by the area of the wall, before then adding the increased thermal transmittance to the ‘clear wall’ value. If a whole elevation is calculated, then the lengths of all balconies, windows, doors, shelf angles, and other linear penetrations are added. Point penetrations are also calculated and added. Once all the values are added to the ‘clear wall’ value, the result should be the total effective assembly thermal transmittance.
The complete method is described in Morrison Hershfield’s Building Envelope Thermal Bridging Guide, Version 1.1. (Building Envelope Thermal Bridging Guide, Version 1.1, was prepared by Morrison Hershfield and published by BC Hydro Power Smart in 2016.) The guide is based on research done for ASHRAE, plus additional research funded by a number of industry partners such as the EIFS Council of Canada. (This comes from Table 1 of the 2014 “Thermal and Whole Building Energy Performance of Exterior Insulated Finishing Systems Assemblies,” Report No. 5130962.00, by Morrison Hershfield Ltd. Visit www.eifscouncil.org.) The method, compared to previous data manipulations, is elegantly simple. The result, compared with what is allowed in a local building code, may be surprising.
In fact, the real fun of determining the effective thermal transmittance of a proposed building begins when one realizes it does not comply with the code. That dilemma will increase as different code bodies start stretching the codes to meet the demands for more thermally efficient buildings. It is at that point where designing and building to eliminate thermal bridging will get serious.
Comparing methods of construction
Comparing brick and EIFS details illustrate how different elements can affect the thermal transmittance of a wall. Constructions similar to the brick veneer wall assembly supported by a steel shelf angle are common (Figure 4). Exterior insulation is used, often 50 mm (2 in.) for a nominal R-8 to R-10 value. The thickness and subsequent R-value are limited by the size and projection of the shelf angle.
The insulation is not continuous, of course, because of the projecting shelf angle. If the steel stud cavity has batt insulation, then a nominal R-12 for a 92-mm (3 5/8-in.) stud space is added. Considering the insulation alone, this could be a nominal R-22 wall.
Until recently, the thickness of the steel shelf angle has been considered a minor thermal bridge through the insulation, because the cross-sectional thickness of the steel shelf is relatively small compared to the area of the wall. However, the Morrison Hershfield research has shown this thermal bridging effect can reduce the effective R-value of the ‘clear wall’ by more than 30 percent. Since many variables affect this calculation, practitioners who want to work out the linear transmittance for their design should refer to the Building Envelope Thermal Bridging Guide.
The effective thermal performance of this design can be improved with more energy-efficient details. For example, moving the shelf angle out from the wall using clip assemblies (e.g. knife plates, hollow structural steel [HSS] sections, or overlapping angles) reduces the area of thermal bridging. Continuous insulation can be installed between the shelf angle and the wall. Optimized clip design, spacing, and materials can improve thermal performance to 15 percent clear wall reduction range. Using proprietary insulated connections can make further improvements with reductions of only seven percent from the ‘clear wall’ value. (“Masonry Veneer Support Details: Thermal Bridging,” was presented by RDH Building Engineering Ltd.’s Michael Wilson, M.Eng, Graham Finch, MASc, P.Eng., and James Higgins Dipl.T, at the 12th Canadian Masonry Symposium, held in Vancouver in June 2013.)
An additional consideration for brick veneer is the choice of brick ties. In terms of cross-sectional area of the brick tie as a ratio to the gross wall area, the brick tie might appear insignificant in terms of its thermal bridging. A paper published in 2013 suggests this assumption would be a mistake. Based on tie material and design, the effective R-value reduction of the exterior insulation can be from “… five to almost 30 percent…” (“Thermal Bridging of Masonry Veneer Claddings & Energy Code Compliance” was presented by the same authors at the same conference as in Note 4.)
The brick wall in Figure 4—from sheathing to the face of the brick, including 50 mm of continuous insulation—could have a dimension of approximately 180 mm (7 in.) with an approximate effective R-18 value (versus a nominal R-22) if all the design improvements are made to the details. Again—designers should make their own calculations based on the BETB Guide.
EIFS offer an alternative method of construction with a number of thermal advantages over the brick veneer wall with regard to this specific criterion. Since EIFS is adhesively fastened, there is no thermal bridging by fasteners. EIFS is also self-supporting, so there is no requirement for structural support to carry the dead load.
The intrinsic advantage of EIFS is it provides great thermal efficiency without the mass and with thinner wall dimensions. The assembly illustrated in Figure 6 with 100 mm (4 in.) of insulation and R-12 batts in the cavity will have an effective R-value of 24.6 Greater thicknesses of insulation are possible to meet requirements in colder zones.
However, EIFS also have limitations that must be taken into account. For example, EIFS wall assemblies are drained from the sheathing to the exterior. (It is important to note the Morrison Hershfield calculations for the EIFS clear wall included a geometrically defined drainage cavity (GDDC).) As with brick, a through cladding flashing is installed to drain penetrating moisture to the exterior (Figure 6). Good construction practice recommends the flashing drain beyond the cladding below. Both methods of construction would benefit from thermally broken flashing.
The Morrison Hershfield study done for the EIFS Council of Canada points out that terminations at windows should be carefully considered. The position of windows in the wall and how they align with the EIFS insulation can affect the assembly’s overall performance. Detailing the window inbound of the insulation, supported on the wall framing, has a linear transmittance of 0.35 W/m•K. Wrapping the rough opening with insulation or moving the window so the thermal break is in alignment with the insulation will make a significant performance improvement to 0.2 W/m•K.
EIFS terminated at a cantilevered concrete deck will not stop the heat loss through the slab. Adding thermal breaks in the slab aligned with the insulation will have an impact on the wall performance, as was shown in Figure 5. The report shows the linear transmittance improvement from an uninsulated floor slab (i.e. 1.0 W/m•K) to one with a thermal break (i.e. 0.35 W/m•K) is significant. This would be true for all claddings.
Just as advances in computer modeling now allow a more complete understanding of building science details, so, too, the advancing computer analysis of global warming trends continue to clarify the big picture. As data accumulates, the question of “What’s happening?” is being answered with greater accuracy, and the questions “What can we do?” and “What must we do?” are beginning to merge.
Late last year, 175 countries signed the Paris Agreement, committing to take action on climate change. Keeping global temperature rise below 1.5 C (2.7 F) was accepted as “an aspirational goal.” This is a stretch, but how does the industry get there?
In the world of energy consumption, buildings are a leading energy-user. To meet international commitments to reduce greenhouse gas (GHG) emissions, the focus must be on the built environment. Effective continuous insulation, working hand-in-hand with air leakage reduction, is the primary building prerequisite. (Airtight construction is a code requirement, and necessary for any insulated wall to function as designed. The information in this article assumes airtight construction.) This will produce the best energy reduction for the most effective cost. When the envelope is tight and insulated, other building systems may be addressed.
A study conducted by McKinsey & Company compares the effectiveness of implementing an energy strategy with the cost of undertaking that strategy. If a methodology is effective in reducing GHG emissions and saves money, it is an obvious place to start. In general terms, conserving or reducing energy use tends to be more cost-effective than developing new carbon-free sources. Specifically, insulating buildings, especially retrofits, ranks highly in the global greenhouse gas abatement cost curve. (The 2009 report, “Pathways to a Low-Carbon Economy, Version 2 of the Global Greenhouse Gas Abatement Cost Curve,” in online at www.mckinsey.com/business-functions/sustainability-and-resource-productivity/our-insights/pathways-to-a-low-carbon-economy.)
There are some recent outstanding examples of using insulation creatively to improve performance
of buildings. In New York City, the Knickerbocker apartment building is a multi-family residential building that meets Passive House standards. It is distinctive for the sculptured wall designed
to both shade and reflect light into the units. This project is designed to approach net-zero energy consumption (Figure 7).
North of the border in New Brunswick, the iconic Algonquin Hotel used to close down every fall because it was uninsulated. In a recent retrofit, the exterior was completely wrapped in an airtight blanket assembly designed to look exactly like the original Tudor stucco design. The hotel now operates year-round (Figure 8).
There are many good reasons to improve the thermal efficiency of buildings. From a sound business perspective, the modest increase in capital cost has long-term benefits of reducing operational expenses. An effectively insulated building has more protection from rising energy costs. The U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) program and other initiatives are producing ratings for buildings—having a building that carries a high performance rating is an excellent marketing tool for reaching energy-conscious clients. Finally, reducing greenhouse gas emissions is an international objective of the Paris Agreement.
As the nations of the world come together to save the planet, it is humbling for design/construction professionals to remember a significant part of this mission involves energy-efficient buildings, improved insulation, and the elimination of thermal bridging—all goals within our reach.
John Edgar is president of John R. S. Edgar Consulting Inc., and past chair of the Exterior Insulation and Finish Systems (EIFS) Council of Canada. He has held positions including technical manager, building science at Sto Corp., member of the Standing Committee on Environmental Separation (part 5) of the National Building Code of Canada (NBC), and chair of ULC S716 Task Group for EIFS. He can be reached at firstname.lastname@example.org.
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