This study aims at assessing the changes happening within the residential construction industry with respect to walls. There are three major goals of this study. The first is to assess the attributes demanded by builders in single family wall products and systems. The second is to assess product usage and substitution in single-family walls. The third aim is to assess the move to component building in residential walls.
A mail out survey was sent to single-family homebuilders in the US, one randomly drawn list of builders plus a list of the top 100 builders in the country. The survey covered builders concerns, attributes demanded in walls, and products and systems used for walls.
Results indicated that energy codes were the top concern of builders. Interestingly, very few builders were concerned with engineered wood or prefabricated systems availability, but lumber availability was considered a constraint by some firms, especially the large ones.
With respect to walls attributes it is clear that the most important attribute of a wall is straightness and square. However, the next three most important attributes are related to on-site issues; speed of assembly, easy to handle, and low on-site waste. This was especially true for large builders. Cost factored in as moderately important with installed cost finishing ahead of material cost.
With respect to walls systems it was found that over 40% of builders have tried prefabricated wood walls. This was strongest in the North. Large builders also were high users of prefabricated wood walls. Prefabricated exterior walls were more common than prefabricated interior walls. Many builders, especially those in the West, used site-built steel for interior walls. In fact, it would appear that of the prefabricated wood interior walls and site-built steel are substitutes.
Labour availability is an equal, if not greater, factor than product availability in the competition among building products and systems for residential construction today. Further, demographic forecasts show labour availability decreasing into the future. At the same time the consolidation of residential building firms is giving rise to more automation and off-site building. For these reasons, it is safe to assume that prefabricated building will only increase into the future. Therefore, it is imperative that the wood products industry defines how the competitive advantage their products have always had in the residential construction industry can be adapted and maintained in an era of prefabricated construction.
Transformative Technologies Program identifierSeries Energy Efficiency of Advanced Building Systems
The largest source of energy consumption and greenhouse gas emissions in Canada and around the world is buildings. As a consequence, building designers are encouraged to adopt designs that reduce operational energy, through both increasingly stringent energy codes and voluntary green building programs that go beyond code requirements. Among structural building materials, wood has by far the lowest heat conductivity. As a result it is typically easier to meet certain insulation targets (e.g., thermal transmission and effective thermal resistance) with wood-based wall systems when following current construction practices. Good envelopes greatly contribute to energy efficient buildings. However, there are many factors in addition to building envelope insulation levels that affect the operational energy of a building. This study aims to provide designers with information which will assist them to choose energy efficient exterior wall systems by providing energy consumption estimates for an archetypal 6-storey residential building. Comparisons were made among several exterior wall systems including light wood-framing, cross-laminated timber (CLT), steel-stud framing, and window walls, for a range of structural systems including structural steel, light wood-frame, CLT, heavy timber, and concrete. The opaque exterior wall assemblies targeted meeting the minimum thermal requirements based on the National Energy Code of Canada for Buildings (NECB. NRC 2011). A 3-D method was used to calculate effective R-values of these exterior walls by taking into account all thermal bridging, in comparison with a parallel-path flow method in compliance with the NECB. Three glazing ratios, including 30%, 50%, and 70%, and two efficiency levels for Heating, Ventilation, & Air Conditioning (HVAC) systems, termed basic HVAC and advanced HVAC, were also assessed. Whole-building energy consumption was simulated using EnergyPlus. Four climates, from Zone 4 to Zone 7, with cities of Vancouver, Toronto, Ottawa, and Edmonton to represent each climate, were selected in this study. The energy assessment was conducted by Morrison Hershfield.
A comparison of operational energy consumption among these different exterior wall systems for this archetypal 6-storey building has shown that accounting for thermal bridging is critically important for improving thermal performance of building envelopes. Wood-based systems including light wood-frame walls, CLT, and wood-framed infill walls in concrete structures have inherently lower thermal bridging compared with other systems, such as steel-frame walls in steel and concrete structures, or window walls in concrete or timber structures. Conclusions are provided for specific climates and cities in Section 4.2. General conclusions and highlights are summarized as follows:
Building envelope influences only the energy required for space conditioning. The space heating energy consumption ranged between 28% and 49% of the entire building energy consumption, when the basic HVAC type was used, for the four cities assessed in this study. An efficient HVAC system would further reduce the proportion of space heating energy consumption. The rest of the energy is used for hot water and electrical appliances etc.
Compared to the NECB-compliant calculation, the 3-D method showed a greatly reduced effective R-value of the opaque wall assemblies due to thermal bridging. Steel-stud wall assemblies showed much larger reductions in effective R-values than wood-based wall assemblies.
Wood-based walls in a light wood-frame building, or a CLT building, would improve building energy efficiency, with total energy savings ranging from 3% to 9%, compared to a concrete building with steel-stud walls, depending on the HVAC type and the glazing ratio, when the 3-D method was used for calculating thermal resistance. The energy savings were higher in colder climates, such as Toronto, Ottawa, and Edmonton, than in Vancouver.
The use of wood-frame infill wall in concrete structure improved the whole building energy efficiency by up to 6% depending on the climate, relative to the use of steel-stud infill walls, under the same HVAC (basic or efficient type) and glazing ratio (30% or 50%).
Concrete structures typically have much higher glazing ratios than wood buildings. The wood-framed building, with exterior-insulated walls meeting the thermal insulation requirements and at a glazing ratio of 30%, showed whole-building energy savings of about 13-18%, compared to a concrete structure with window walls at a glazing ratio of 70%.
Simply adding insulation (e.g., exterior insulation) in a building envelope while ignoring thermal bridging is not the most effective way to improve building energy efficiency.
The thermal bridging at window transitions greatly reduced the effective R-values of the opaque walls and consequently the whole-building energy efficiency. The higher the glazing ratio was, the larger the impact would be. Window wall with a high glazing ratio would further reduce building energy efficiency, compared with regular windows.
The energy efficiency of the HVAC system used in a building had the largest impact on the whole-building energy efficiency, compared to the impacts caused by exterior wall systems, glazing ratios, or thermal bridging at various details.
The energy efficiency measures studied in this report delivered higher energy savings in colder climates, such as Montreal, than in warmer climates, such as Vancouver.
It is recommended that future effort be put into further developing tools for practitioners to account for thermal bridging more conveniently.
Work reported in this study was carried out with the key objective of evaluating and providing recommendations on potential improvement of connection systems typically used in prefabricated wood wall panels. Several visits were made to major prefabricated wall panels and modular houses in Quebec to provide a better understanding of the typical connection systems being used in the prefabricated wall assemblies and to identify issues of concern.
Results of eighteen (18) racking tests and nine (9) bending tests on full-size 2.44 by 2.44 m walls composed of two segments (1.22 by 2.44 m) attached with three (3) different types of connection configurations are presented. Several wall-to-foundation attachments were also investigated; including bolted, nailed, and fully anchored walls. Monotonic and cyclic racking tests were performed according to relevant ASTM standards. Bending wall tests were carried out according to a proposed protocol based on the calculation of the wind pressure corresponding to five hurricane categories from 112 to 257 km/h. Bending wall tests were carried out using an airbag system to simulate the inward wind-pressure with three (3) types of attachments to the foundation.
Results reveal that the racking load carrying capacity of wall assemblies subjected to either monotonic or cyclic loading was not strongly affected by the type of central connection configuration used to joint the two wall segments. In addition, for all types of inter-segment connections tested under monotonic or cyclic loading, wall assemblies with hold-down anchors were nearly three times stronger than those nailed to the base. They showed 80% higher stiffness and dissipated five to seven times more energy before failure. Moreover, the type of loading seems to have some influence on the maximum load carrying capacity and to a less extent on stiffness of prefabricated wall assemblies, regardless of the type of inter-segment and wall-to-foundation attachments. As for out-of-plane loads, the tested wall assemblies resisted wind-pressure beyond 4.3 kPa corresponding to 232 km/h sustained wind speed equivalent to Category 4 hurricane. Their strength was controlled by the strength of the studs rather than the type of the connections used.
In addition, results on static and monotonic tests carried out on small size connections used in full-size racking and bending tests are also given. Such information is necessary for the finite element model being developed to predict the performance of prefabricated wall panels subjected to bending and racking forces, with special focus on the interface and anchorage behaviour for performance optimization.
Recommendations are given on how to improve the connection systems used in the assembly of wall panels. The information generated in this study provides data for comparative quantitative analysis of conventional and engineered wall assemblies and is expected to serve the development of design methodology for lateral load resisting systems of prefabricated houses.
In order to alleviate the high costs and inflexibility associated with full-scale fire tests which are commonly employed to predict the fire resistance of wood-frame building assemblies, the North American wood industry has identified the need for computer models to calculate fire resistance. The two-dimensional computer model presented in this report simulates fire tests for gypsum-board/wood-stud wall assemblies. In such assemblies, the gypsum board acts mainly as a protective membrane and it provides much of an assembly's fire resistance. The primary advantage of the two-dimensional computer model as opposed to its one-dimensional predecessor is that, in addition to predictions of heat transfer through gypsum board attached to wood studs, it also predicts heat transfer through the wall's cavity. Thermophysical properties of both gypsum and wood were reviewed before being incorporated in the model. Along with the added dimension mentioned earlier, other interesting features of this model include its consideration of linear expansion (contraction) of gypsum and the considerable effect of charring of wood which ultimately affects all of the latter's thermophysical properties. Simulation results obtained using the model are remarkably accurate when compared to full- and small-scale fire tests. The aforementioned tests were performed at the National Fire Laboratory (NFL) to provide data for the validation of the model. A brief overview of the sensitivity analyses which were performed on the model is also included in this report.