July 11, 2016
By Paul Nutcher, CSI, CDT
In northern climates, architectural elements—such as cantilevered beams, concrete balconies, roof penetrations, parapets and canopies, spandrel glass, and other ornamental features often are limited due to compliance with local energy codes. Executing these design elements can create thermal bridges that extend beyond the insulation systems within the building envelope. The resulting temperature differences can cause condensation buildup in exterior systems, in addition to significant loss of energy performance for the whole building. Both issues can hamper efforts at designing and constructing buildings to meet the owner’s expectations while meeting energy codes—or pursuing green building rating programs.
Thermal bridging can have a very large impact from a cost and compliance standpoint. Heat flows determine the building’s heating and cooling system capacity, long-term energy costs, and compliance with energy codes, and energy performance ‘overlay’ programs like Leadership in Energy and Environmental Design (LEED) Canada.
How building materials are arranged in the envelope determines the surface temperatures, moisture development, long-term durability, and the potential for mould growth within outward facing building systems. Much has been done in recent decades to address inaccuracies in HVAC sizing because previous generations of energy modelling did not account for interface details, which can have a significant impact on the overall wall area. More accurate thermal performance measurements will have to include typical envelope details such as wall/wall (corners), wall/roof, wall/floor, wall/door, and wall/window connections. This is in addition to measuring the thermal performance of the ‘clear wall’ area, insulation layers, and the structural elements.
Thermal break coatings 101
Due to new product releases and newly available baseline energy performance data (“Studies in Thermal Bridging,” page 4), construction details are now available for insulating steel and concrete penetrations. These types of enhancements can give project teams new freedom for creative design, including designs for net-zero energy buildings that can also address condensation resistance and energy efficiency of the building envelope.
Fluid-applied insulated coatings can assist project teams with condensation reduction and whole building energy efficiency when applied to structural building elements—or thermal bridges—bypassing the insulated portions of the building envelope. Essentially, the acrylic insulation coatings stop the process of heat convection via a steel beam; these coatings have been infused with insulating fillers in order to produce a low-conductivity material that can be applied while in its fluid form. Fillers can be ceramic or glass spheres, which can provide thermal conductivity in the 70 to 100 mW/mK range. Newer fillers, such as aerogel particles, can offer a thermal conductivity as low as 35 mW/mK.
The use of these materials comes from a case where the beneficial attributes of a product developed for one use were repurposed to serve another. This insulative coating was created in 2010 to provide ‘bump protection’ (or ‘safe-to-touch’ applications) to protect human skin-to-hot pipe contact within certain workplace environments such as along steam pipelines and in boiler rooms. Aerogel, an insulative particle, was introduced into acrylic coatings for this purpose. Testing proved there was beneficial thermal performance on extremely hot surfaces. (Previously, Aerogel had only been used as a ‘dry’ particle, chiefly for application on deep-sea pipelines, thermal insulating blankets, and day lighting panels.)
According to Greg Pope, co-author of Aerogel Fluid Applied Coatings Solution for Thermal Bridging for Design Community, interest for architectural use grew quickly after the thermal performance test results were discovered.
“The low thermal conductivity of 12 mw/mk make aerogel 50 per cent more efficient than still air,” said Pope, who works with coatings consultants, Righter Group. “Once incorporated into a coating at very low film thickness, the aerogel coating at 60 to 150 mils [i.e. 1.5 to 3.8 mm] reduced heat transfer by 47 to 51 per cent.”
To put this in context, this allows a human hand to touch for 10 seconds a 150-C (300-F) metal pipe that has been insulated with a fluid-applied thermal break coating without any burn to the skin of his hand. The aerogel-infused coating will insulate the pipe, lowering the surface temperature of the coating to approximately 38 C (100 F).
Understanding the architectural applications
Architects designing with cantilevered beams, concrete balconies, roof penetrations, canopies, spandrel glass, and ornamental architectural features must address the cold-to-hot issue. At these moments in the design, moisture formation has to be taken into consideration or problems can occur. Otherwise, the creative approach would have to be scaled back.
For architectural applications, the thermal performance of fluid-applied acrylic insulation coating was recently documented by a Morrison Hershfield report commissioned by a manufacturer. The findings of the engineering firm’s thermal analysis of the products were very promising for a variety of common construction details, including:
The testing by Morrison Hershfield included three coating thicknesses—1.5 mm (60 mil), 3 mm (120 mil), and 13 mm (½ in.)—using a conductivity of 0.0356 W/m·KL (0.0202 BTU/hr·ft·F). The thermal analysis was completed with 3D heat transfer software.
Unlike structural thermal break products, insulated coatings do not require additional engineering to determine whether they will support the load. Insulated coatings can also be more cost-effective by comparison to the steel fabrication and installation of structural thermal breaks and pads, which also require expensive bushings and washers. The aerogel-based coatings can be shop- or field-applied and can be utilized in both new construction or retrofits. However, the benefits of design flexibility with both the steel and the concrete elements can be realized, along with simplified specifications and scheduling.
Thermal coatings work by providing thermal conductivity levels at 35 mW/mk compared to 260 mW/mk for thermal-break pads. The thermal insulating coating prevents the surface temperature from lowering below the dewpoint so moisture does not condense—even without a physical break in the beam.
Overall, design professionals see the potential for cost reductions compared to structural products with aerogel coatings and the prospect of simpler specifications. Manufacturers have drawings and specifications with construction details available for keeping interiors warmer and avoiding condensation and mould formation within the building envelope.
LEED and energy code compliance considerations
The LEED energy efficiency credits referencing ASHRAE 90.1-2010 will gain the most contributions with thermally broken designs. Further, the new design approach indirectly plays a role in assisting with Indoor Environment Quality (EQ) Credit 5, Thermal Comfort.
The overall energy benchmark ASHRAE 90.1-2010 is used by project teams to meet energy codes and voluntary green building programs. Generally, the Canadian and U.S. versions of LEED are harmonious. (There are some options to follow Canadian codes, but they are not required.) A great reference for guidance on building envelope thermal bridging (BETB) was published initially by BC Hydro and recently updated in BETA 1.1 in April 2016. Additional construction details were added and the new version focuses only on thermal performance data; sections related to cost benefit analysis and market transformation were removed in BETA 1.1.
However, some mid-rise multi-family projects at four or more storeys in height could be covered by the LEED for Homes Rating System: Multi-family Mid-rise. In LEED NC, healthcare projects must develop a moisture control plan per EQ Credit 3, Construction Indoor Air Quality Management Plan.
The continuous insulation requirements in energy codes are enhanced by thermal break design details. Often, much attention is paid to upgrading HVAC and lighting systems with increased R-values in the building envelope. Fluid-applied thermal coatings have the potential to create additional energy efficient upgrades to the exterior facing systems.
The latest version of ASHRAE 90.1 and the 2011 National Energy Code for Buildings (NECB) require increased energy performance design details. Building energy performance upgrades, therefore, must include more efficient HVAC, lighting, and insulation systems, but these are not always enough while maintaining the building owner’s desires for unique esthetics and flexible design requirements. ASHRAE 90.1-2010 calls for the additional step of continuous insulation (ci) throughout the entire building envelope. The NECB is based on U-values only, and there is no prescriptive insulation requirements. Once all these requirements have been met, thermal break coatings can further assist project teams with the potential for gaining additional energy performance to comply with the new energy codes.
In the past, it was difficult to comprehensively account for all thermal bridging in U-value calculations—in fact, it was also thought to be a relatively minor impact. Now, there is more of a mainstream awareness the effect is significant; further, there are better tools/data to comprehensively address it. Codes and standards are in the process of evolving to better estimate U-values by more holistically accounting for thermal bridging.
British Columbia, Ontario, Alberta, Manitoba, and Nova Scotia have adopted NECB, but the first two provinces have alternative compliance paths that include compliance paths to ASHRAE 90.1. However, in Ontario, there are supplement requirements outlined by SB-10, Energy Efficiency Supplement, that supersede the ASHRAE U-value requirements with much more stringent values. There is also so-called ‘stretch codes’ in the works that could mean bylaws requiring buildings to exceed both NECB and ASHRAE 90.1.
Condensation and IAQ requirements
The health benefits of a building with a reduced potential for mould growth in wall systems are desirable for building owners who want to avoid litigation due to sick building syndrome. The warmer interior temperature of the structural members of the building can be improved by liquid-applied thermal coatings.
The forthcoming LEED for New Construction (NC) v4 will address the thermal comfort of occupants in its Indoor Environment Quality (EQ) Credit 5, Thermal Comfort. The relevant standard is ASHRAE 55-2010, Thermal Comfort Conditions for Human Occupancy, with errata or local equivalent. Without thermal bridging within the building envelope, project teams that design thermally broken balconies can meet the requirement of LEED v4 Credit 5.
This technology is not only driven by performance and need, but has evolved to water-based technology as well as low- or zero-volatile organic compounds (VOCs), meeting LEED requirements for all design parameters.
The compatibility of air and vapour barriers, along with sealants and cementitious and intumescent fireproofing, is key to all designers for building envelope code requirements. In other words, the inadvertent development of an aerogel-filled thermal break coating is a workable solution to a nagging design problem.
Interior and exterior areas where thermal bridging is a concern
For fenestration within interior rooms where the relative humidity (RH) can reach high levels in the cold winter months and thermal bridging has not been mitigated, ice buildup on windows can present moisture issue to the materials and systems surrounding the opening. As the ice melts, the moisture can migrate to the drywall and damage the interior walls, as well as provide moisture for mould growth.
To mitigate the surface temperature of the window frames, an aerogel-insulated coating can be applied; case studies have shown the temperatures from an infrared (IR) camera can be raised significantly to above the dewpoint to greatly reduce and potentially eliminate condensation. For example, an existing aluminum window frame within a high-RH room with an outside temperature of −2 C (28 F) will go from a spot surface temperature reading of 8.4 to 20.3 C (47.1 to 68.5 F) once 2.8 mm (110 mils) of the aerogel coating has been applied.
Another common area within the interior of the building affected by condensation is where the walls and ceilings meet at a corner. Often, mould is noticeable on the ceiling in these areas.
At the intersection of the interior and exterior on multi-family construction, balconies are a common source of heat lost and condensation issues. The balcony slab extends to the outside air, exposing the entire slab to exterior temperatures via thermal bridging. The intersection of the low- and high-energy temperatures heating and cooling energy throughout the building is often lost, with any exterior systems in contact or nearby absorbing more moisture from condensation than they can withstand.
Dangers of continuous condensation cycles
Structural steel beams, their hardware, or their welds can weaken when they remain wet from moisture infiltration. This can lead to costly building code violations, mitigation projects, or structural failure (especially when leaking exterior systems exacerbate the issue compounded with heavy snow loads and chlorine rock salt applied for years on roof pavers). (These cases are rare, but have happened. For an example, see the forensic engineering investigation into the Algo Centre Mall Collapse in Elliot Lake, Ont.)
Insulated fluid-applied coatings were first used on boilers and pipelines where preventing steel corrosion under insulation (CUI) was of vital concern. Corrosion can attack the jacketing, insulation hardware, or the underlying steel. The National Board of Boiler and Pressure Vessel Inspectors (NB) is a great resource for further information on steel corrosion under insulation.
These coatings are durable and corrosion-resistant, bonding to the substrate to greatly reduce issues associated with CUI. Once moisture and thermal protection exterior systems have been installed, the property installed insulated coating will provide peace of mind to the project team, building owners, and occupants for decades to come.
For insulating the building envelope, fluid-applied insulated coatings cannot replace dry insulation methods. Nevertheless, they can help achieve optimal energy efficient performance of the building envelope. Corrosion resistance is achieved with a primer applied to the substrate prior to applying the coating. Often, a finish coat is also applied for esthetic or other reasons. (The use of zinc-rich primers is not generally recommended when in-service temperatures exceed 49 C [120 F]. For more, see NACE SP0198, Control of Corrosion Under Thermal Insulation and Fireproofing Materials).
|STUDIES IN THERMAL BRIDGING|
|Over the past decade, the work of building scientists or building engineers began to address ‘whole-wall’ R-value estimations, comparing them with simplified ‘centre-of-cavity’ and ‘clear wall’ R-values. This work started on the residential side and has now progressed to research on commercial mid-rise and high-rise construction.
How heat is transferred within a building envelope was determined by Toronto-based engineering firm Morrison Hershfield in American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) Research Project 1365-RP, which initiated a catalog of thermal performance data for 40 common building details for mid-rise and high-rise construction.* Published in 2011, the goals of the project were to:
The team of engineers at Morrison Hershfield employed heat transfer software, with the resulting models calibrated and validated against measured and analytical solutions. International Organization for Standardization (ISO) standards for glazing were used, along with a guarded hot box test measurement. They assessed 40 construction details common to construction methods in North America; while there was some focus on glazing, the highest priority was on the details with thermal bridges in 3D.
The research project was initiated when ASHRAE 90.1-2007, Energy Standard for Buildings Except Low-rise Residential Buildings, was the most ubiquitous standard applied in U.S. energy codes and in the then-current LEED 2009 program’s Energy & Atmosphere (EA) prerequisite and credits for determining whole building energy performance. However, ASHRAE 90.1-2007 largely avoided the thermal bridging of outside assemblies, according to Mark Lawton, P.Eng., FEC, of Morison Hershfield’s Vancouver office. Crediting the work of his colleagues Patrick Roppel, M.A.Sc., P.Eng., and Neil Norris, M.A.Sc., EIT, Lawton says the standard did not address avoiding potential improvements to building envelope assemblies beyond its continuous insulation (ci) prescriptive compliance path.
The Morison Hershfield team applied a European method to streamline assessing various assemblies by looking at their heat flow with and without the thermal bridge for a linear transmittance measurement. In this respect, they were North American pioneers for taking into account construction details not previously considered in earlier energy modelling programs.
Since the ASHRAE project, more has been developed for industry professionals who have been tasked with designing energy efficient building envelopes—especially in situations where lateral heat flow is affecting thermal performance of the assembly via linear transmittance (or thermal bridging). In August 2014, Morrison Hershfield and BC Hydro published Building Envelope Thermal Bridging Guide: Analysis, Applications, and Insights, which serves as a guide for designers and specifiers as they confront mitigation of thermal bridging and reducing energy consumption in buildings.
The guide addresses several issues challenging project teams today and addresses those challenges by:
Paul Nutcher, CSI, CDT, is the president of Green Apple Group LLC, a marketing, technical, and sustainability consulting firm. A LEED Green Associate, he has more than a dozen years of building industry experience
as a specifications and technical writer, educator, and consultant to product manufacturers and design/construction professionals. Nutcher has served in leadership roles with CSI, the U.S. Green Building Council (USGBC), and the American Institute of Architects (AIA). He can be reached at email@example.com.
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