Improving continuous insulation

by Katie Daniel | November 14, 2016 10:01 am

By John R. S. Edgar
Insulation manufacturers have devised numerous ways to improve the thermal performance of their products, from adding specialized particles to polystyrene to refining vacuum insulated panels (VIPs) and phase-change materials to aerogels. Unfortunately, none of this matters when these high-performance products are installed ineffectively. Batt insulation between steel studs loses half its R-value. Steel studs themselves act like ‘radiator fins’ to the exterior—along with shelf angles and structural connections, they are a prime example of a thermal bridge.

New research, using computer modelling, has shown thermal bridging through insulation is a far more serious problem than previously suspected. The accepted solution is continuous insulation (ci) outbound of the studs. Canadian code-writers have taken note—the National Energy Code for Buildings’ (NECB’s) focus has shifted to the building envelope’s passive performance, limiting reliance on powered mechanical systems in the alternative path options.

In some ways, the new design battlefront will be about energy conservation versus gravity. In all methods of construction, the cladding dead load must be secured to the structure through the ‘continuous’ insulation. If thermal bridging is eliminated, what will hold the cladding to the structure? Currently, stainless steel channels, fibreglass clips, and composite assemblies with built-in thermal breaks are being rolled out to support the cladding.

Over and above the structural penetrations, what will hold the continuous insulation in place? With some types of insulation, adhesives do the job. With others, screws with washers are required to secure the insulation to the supporting structure. Typically, in thermal energy calculations, the screw shaft diameter is compared to the insulation area, yielding a thermal bridge ratio of about 0.05 per cent—a seemingly insignificant amount. In fact, a thermally conductive screw shaft transmits heat from an area many times larger than its diameter (Figure 1).

Thermal bridging through every fastener cools the interior temperature by radiating heat out of the wall. If the temperature of a fastener tip is below the dewpoint, the result may be condensation and potential corrosion—not good for long-term durability and resilience. To combat these drawbacks, there are now inventively designed fasteners that have reduced thermal conductivity and are corrosion-resistant. Fortunately, designers no longer need to deal with bridge-by-bridge calculations. Much simpler and more accurate methods have been developed to design code-compliant building enclosures.

With the energy modelling discussed in this article, the starting point in the calculation is a ‘clear wall’—that is, an area of opaque wall devoid of major thermal bridges. The ‘clear wall’ value is determined by guarded hotbox calculations. Insulation fasteners are included as part of the ‘clear wall,’ so their effect is included in the base calculation. Essentially, there is no need to model every fastener. However, if a particular design is running close to the unacceptable mark, something as trivial as fasteners may require examination.

Calculating thermal performance
The way a building envelope’s thermal performance is calculated has changed, thanks to more powerful computers and an amazing degree of sophisticated modelling. The same computer simulations that model heat shield performance during spacecraft reentry, for example, are also used to determine heat loss through a floor slab.

Prior to advanced computer simulations, the parallel approach to heat loss calculations was used. For a slab penetration, the energy flow through both wall and slab was calculated based on the elevation area (Figure 2). The wall and slab energy flows were added together, based on the ratio of wall-to-slab area. Any lateral flow of energy was ignored.

The reality is that energy moves in all directions and a cold, uninsulated slab will draw heat not only from the slab, but also from interior air and the wall assembly above and below the slab (Figure 3). Wall insulation is rendered less effective because heat flows around the insulation and out the slab. The problem has been how to calculate the energy end run accurately.

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In July 2011, Toronto-based engineering firm Morrison Hershfield Ltd. presented a paper to the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) showing how advanced computer modelling 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 ASHRAE 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 modelling was validated by 29 guarded hotbox measurements. The goal of this research project was to modify the energy calculation methods described in ASHRAE 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings.

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, modellers only have 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, or ‘clear wall’ as described above, for which the thermal transmittance value has been predetermined and validated, is measured in W/(m²•K). As an example, all components of an insulated stud wall—air films, studs, batts, sheathing, insulation, cladding, and fasteners—are included.

Thus, to calculate the thermal bridging effect of a cantilevered balcony, 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 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 (BETB) Guide (Version 1.1), (The BETB Guide was published by BC Hydro Power Smart earlier this year.) which is based on research done for ASHRAE, plus additional research funded by a number of industry partners, including the Exterior Insulation and Finish Systems (EIFS) Council of Canada. (See 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[1].) 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 to most design/construction professionals.

In fact, the real fun of determining the effective thermal transmittance of a proposed building begins when one realizes it does not comply with local codes. That dilemma will increase as different code bodies start stretching their codes to meet the demand for more thermally efficient buildings. At that point, designing and building to eliminate thermal bridging will get serious.

Comparing methods of construction
How can different components affect the overall thermal transmittance of a wall assembly? A comparison of brick and EIFS wall sections at a floor line illustrate how the variables can change design decisions.

Brick veneer wall assemblies supported by steel shelf angles are a common wall construction (Figure 4). Exterior insulation, often 50 mm (2 in.), has been used to produce a nominal RSI 1.4 to 1.76 (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 RSI 2.1 (R-12) for a 92 mm (3 ⁵/8 in.) stud space is added. Considering the insulation alone, this could be a nominal RSI 3.9 (R-22) wall.

Until recently, the thickness of the steel shelf angle was 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 per cent. Since many variables affect this calculation, practitioners who want to work out the linear transmittance for their design should refer to the BETB 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 (ci) can be installed between the shelf angle and the wall. Optimized clip design, spacing, and materials can improve thermal performance to a 15 per cent reduction of the ‘clear wall’ value. Using proprietary insulated connections will also bring the thermal transmittance closer to the ‘clear wall’ value. (The paper, “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 12^th Canadian Masonry Symposium, held in Vancouver in June 2013.)

An additional consideration for brick veneer is the choice of brick ties. Once again, the cross-sectional area of the tie as a ratio of the gross wall area might appear insignificant in terms of its thermal bridging. A paper published in 2013 suggests this assumption would be a mistake and that, based on tie material and design, the effective R-value reduction of the exterior insulation could be from five to almost 30 per cent. (“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 outer face, with 50 mm (2 in.) of continuous insulation—could have a dimension of approximately 180 mm (7 in.) with an approximate effective RSI 3.2 (R-18) value (versus a nominal RSI 3.9 [R-22]). To achieve this effective R-value, all the design improvements would have to be made to the details. This would meet the 2015 NECB requirements for the lower mainland of British Columbia. Other regions would have to increase the continuous insulation value. Again—designers should make their own calculations based on the BETB Guide.

EIFS offers an alternative method of construction with a number of thermal advantages over the brick veneer wall. Since the assembly is adhesively secured, there is no thermal bridging through fasteners. EIFS is also self-supporting, so there is no need for structural support to carry the dead load.

The intrinsic advantage of EIFS is it provides great thermal efficiency with less mass and thinner wall dimensions. The assembly illustrated in Figure 5  with 100 mm (4 in.) of insulation and R-12 batts in the cavity will have an effective R-value of 24. Greater thicknesses of insulation are possible to meet requirements in colder zones.

All claddings have limitations. For example, both EIFS and brick wall assemblies are drained from the sheathing to the exterior. A through-cladding flashing is installed for this purpose (Figure 4 and Figure 5). Good construction practice, not to mention code, requires this flashing to drain beyond the cladding below. Both methods of construction will benefit from thermally broken flashing.

The Morrison Hershfield study done for the EIFS Council of Canada stresses that design at windows’ terminations 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, significantly improves performance to 0.2 W/m•K. (It is important to note the Morrison Hershfield calculations for the EIFS ‘clear wall’ included a geometrically defined drainage cavity [GDDC].)

EIFS terminated at a cantilevered concrete deck cannot stop heat loss through the slab. However, adding thermal breaks in the slab and aligning them with the continuous insulation will have an impact on wall performance, as shown in Figure 6. The study 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.

Implementation
Just as advances in computer modelling have led to a new level of building science savvy, so too have the advancing computer analyses of global warming trends continued 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. (For some thought-provoking comparisons on the total energy increase resulting from a mere 2-C [3.6-F] temperature rise, visit www.skepticalscience.com/4-Hiroshima-bombs-worth-of-heat-per-second.html[2].)

Late last year, Canada was one of 175 countries to sign 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.” In North America, buildings are a leading energy-user and hence a primary target for energy conservation measures. Effective continuous insulation, working hand in hand with air leakage reduction, is the primary 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. (The 2009 report, “Pathways to a Low-Carbon Economy, Version 2 of the Global Greenhouse Gas Abatement Cost Curve,” is online at www.mckinsey.com/business-functions/sustainability-and-resource-productivity/our-insights/pathways-to-a-low-carbon-economy[3].) If a methodology is effective in reducing greenhouse gas (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, the act of insulating buildings, especially retrofits, ranks high in the global GHG abatement cost curve.

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Among recent Canadian examples of using insulation creatively to improve performance, the Algonquin Hotel (St. Andrews, N.B.) is a stand-out. The iconic century-old 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 7).

There are many good reasons to improve the thermal efficiency of buildings. From a business perspective, the modest increase in capital cost has a long-term benefit of reducing operational expenses. An effectively insulated building has more protection from rising energy costs.

International commitment to action is now being translated into change for the construction industry. The Canadian government has signed a $40-million contract with National Research Council (NRC) to rewrite building codes to meet energy conservation goals. The Ontario government has introduced the Ontario Climate Change Action Plan to tackle greenhouse gas reduction. Electric cars attract the media spotlight, but enforcement of new building and energy codes is going steadily forward. Already announced, the Ontario energy code, SB-10, requires a 13 per cent energy performance improvement.

If this level of improvement seems hard to achieve, then it is important to watch out for the Ontario Climate Change Action Plan, which will target an 80 per cent reduction in GHG emissions by 2050. Further, design professionals can look forward to building commissioning and labelling similar to that done in Europe. To start the process, proposed Ontario Bill 135 amends the Energy Statute Act to require reporting of greenhouse gas emissions. The public will know which buildings perform well and which do not. Inevitably, that information will affect leasing and resale values. Cities like Vancouver, Calgary, and Toronto are working hard at the municipal level to create ‘stretch codes’ because, like Canada’s commitment to the Paris Agreement, the industry will have to stretch to meet them. For example, the Toronto Green Standard requirements will be 15 per cent tighter than the province’s SB-10.

When the nations of the world agree to do something to save the planet, and a significant part of that ‘something’ concerns the built environment, our industry becomes a vital participant in the agreement. We are on the front lines of GHG emission reduction, and we can make change happen.

John Edgar is president of John R. S. Edgar Consulting Inc., and past-chair of the Exterior Insulation and Finish System (EIFS) Council of Canada. He is convener of ISO TC163 SC03 TG09, preparing three new international standards for EIFS. Edgar has held positions including technical director at Sto Canada, member of the National Building Code of Canada (NBC) Standing Committee on Environmental Separation (part 5), and chair of Underwriters Laboratories of Canada S716 Task Group for EIFS. He can be reached at john@johnrsedgar.com[5].

Source URL: https://www.constructioncanada.net/improving-continuous-insulation/

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