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Integrated design measures at the top and bottom of a building are required as structural engineers meet the industry’s increasing need to reduce operational energy losses and monetary costs, mitigate harmful environmental effects, and safeguard the public’s health. Designs that incorporate insulative or even less conductive material in the load path provide measurable improvements in building energy and air quality performance. Breaking a highly conductive energy loss path through the roof and foundation structure with simple insulative materials, designed for load using common mechanics, can have a significant effect on operational energy savings that offsets the embodied energy of the insulation used.
This third part of a series on thermal bridging in basic building design outlines example strategies for mitigating damages to the rated thermal assembly at foundation or base levels, and roofs. The previous two articles on thermal breaks published in the November 2024 and December 2024 issues of STRUCTURE serve as a reference with respect to various material conductance properties, thermal break layout effectiveness; and design process and protocols implementations.
Foundations and Bases
Insulating foundations and base level floors is critical in achieving high performance buildings. Available locations for insulation layers are limited.
Frost-Protected Shallow Foundations (FPSF) can have the added benefit of limiting the excavation and fill work and reducing reinforced concrete volumes. Comparative LCA studies have indicated FPSFs can reduce embodied carbon impacts as well as initial monetary costs. Following the code design requirements of SEI/ASCE-32 and emphasizing overall appropriate drainage have resulted in time-tested well-functioning structural designs in commercial and industrial facilities.
Subgrade Insulation Types
While a necessary part of foundation and slab design involves analysis using the mechanical properties of underlying insulation, the use of Environmental Product Declaration (EPD) or similar product data would also be part and parcel of comparing the effects of various options. Just like soils are essentially a spring, so too are insulations, with pressure and deformation characteristics. (Though common spread footings waste concrete by being of uniform thickness, it does allow them to behave as effectively rigid in distributing loads uniformly to the subgrade.)
Plastic-based foam insulations have been commonly used in recent times. Although these petroleum products offer improved thermal performance over earlier use of cork and cinders below slabs and treated hardwood column bearing blocks for example in refrigerated structures, they are also subject to long-term performance degradation from moisture infiltration. Mineral wool and stone-based products such as FoamGlas and RockWool offer durable performance with R-value of approximately 4.0 per inch and moderate compressive resistance. These products can be energy-intensive to produce, and product EPDs are established to evaluate GWP impacts.
However, the increasing availability of mineral-based glass insulation offers materials with lower embodied carbon from heightened use of recycled materials and lowered waste, in addition to enhanced durability due to its effective impermeability. Also, as energy codes and certification programs may now require the entire slab area to be insulated, simplified insulation placement is availed by recycled glass insulation aggregates that also have enhanced durability due to their effective impermeability. The foamed glass products are available as lightweight aggregate or in board form with a range of R-values of 0.9 – 1.7 per inch. Their use shows significant reductions in embodied carbon impacts over foam board insulation.
Column Bearing Blocks
Column bearing blocks, from treated hardwood and to high-strength foams, have been long used in cold-storage buildings to address thermal bridges that waste energy and could lead to frost heave. Two conditions of note are where:
The foundation wall is insulated on the inside and columns pierce the sub-slab insulation.
The envelope is above grade such as in podium structures and the columns break the through the soffit.
Column bearing blocks are acutely effective in enhancing the building’s modelled thermal performance with compressive strengths of 300 to over 2000 psi at 2% deformations; and R-values of 1.5 to 2.5 per inch. Column bearing blocks also serve well as thermal breaks for roof penetration for fall protection and dunnage framing.
Foundation Walls and Exteriors
The base insulation required by energy codes will be in at least one of three locations, the “inside,” the “outside,” or “the middle” around a building’s perimeter. The insulation on the exterior continues up to the facade wall and is usually an uninterrupted envelope that does not require primary structural measures. (The durability and protection of exterior insulation at a building base is a related, but different study however.) Where the insulation is on the inside of a basement or foundation wall and requires a transition under the exterior wall to connect with the envelope or insulate the wall’s base, a structural design incorporating insulative material is often required.
Rigid Polyurethane Foam Column Bearing Block and Mineral Wool “Inside” Insulation
Use of materials at wall base that are less conductive than concrete, but durable, is an effective design method to follow. Moisture protected Autoclaved Aerated Concrete (AAC) or hemp-based block, compression-rated insulations including graphite polystyrene (GPS) that greatly reduces water absorption, treated wood, or proprietary insulated “sill” products are examples of materials in-service.
Insulation in the middle of a foundation is a similar construction to concrete sandwich wall panels. Proprietary systems in the market use low conductance form ties to maintain foam insulation panels to not float about on a vertical pour and allow for the foundation wall insulation to align with the exterior wall envelope above and obviate transitional insulated sill elements required for “inside” conditions and additional insulation protective measures for “outside” arrangements.
Roof Penetrations
The thermal bridges encountered in roof penetrations are similar to those for beams or facades as they present intermittent violations of the envelope. In addition to energy efficiency reductions, the penetration points can generate condensation, especially above closed ceilings or in high-performance enclosure buildings, if adequate thermal break measures in the structural load path are not provided. Thermal break connections can use available product assembly specifications with the less conductive layer in alignment with the roof insulation to achieve greater efficiency. Even without this alignment, an improvement of 10% at the penetration point has been determined possible.
The obvious steel post framing for equipment dunnage, screen walls, fall protection anchors, and solar energy racks are common thermal breaks to be attenuated. Note that carbon steel screws for mechanically fastened roofs protruding into the building are also an operational energy waste factor. Recent studies indicate that such roofing fasteners can result in effective R-value losses in the range of 4-16%; projects looking to address that loss have considered ballasted roof systems and using Structural Insulated Panels (SIPs) above the above a conventional metal roof deck and connected to the primary framing only to reduce the aggregate area of fasteners through the envelope.
While this three-part series has shown that thermal break designs engender integrated, interdisciplinary work, the articles have also provided the basis and background for structural engineers to invoke the lead in meeting energy codes; attaining enhanced, durable building performance; and actualizing sustainable measures. In addition to evaluating operational energy improvements, thermal break options can be differentiated with regard to embodied energy as part of the structure’s Life Cycle Analysis. These assessments can be verified by specifying EPDs, equivalent industry, or product information to be submitted. The structural engineer’s roles and responsibilities can be preserved and surely enhanced by reducing the wasted energy pollution. ■
About the Author
Russ Miller-Johnson, PE, SE, is a Principal Emeritus with Engineering Ventures, PC in Burlington, VT (russmj@engineeringventures.com). He serves on the Thermal Bridging Working Group of the ASCE SEI Sustainability Committee.