A single-family wood-frame house in the Lower Mainland of British Columbia built to the German Passivhaus (Passive House) standard was monitored to investigate its thermal performance and durability in this mild climate. Two double-stud walls, south- and north-facing, were instrumented during construction to measure moisture and thermal performance. A limited amount of thermal modelling was conducted to compare with the field measurements.
Monitoring over the past 20 months showed that:
The double-stud walls, south- and north-facing, were both performing well in terms of durability. The moisture content (MC) measured at the bottom of the studs was in general below 15% after the construction was completed. The MC of the south-facing wall dropped from an initial 20%, measured during construction, to about 11% after construction was completed. During the same period of time, the MC of the north-facing wall fell from about 19% to 15%; the slightly higher MC in this wall compared to that in the south-facing wall was a result of lower amounts of solar gain in this orientation.
The relative humidity (RH) measured on the interior side of the medium-density fibreboard (MDF) exterior sheathing in the south-facing wall ranged from 70% to 80%, and occasionally up to 90% during the winter. Being typical of exterior sheathing conditions without exterior insulation in this mild climate, the corresponding RH ranged from 80% up to 100% in the north-facing wall in the winter, indicating potential vapour condensation at this critical location.
Based on vapour pressure analysis, no steep vapour pressure gradients between any specific layers were found in these two walls, indicating the overall vapour permeable nature and good drying performance of the wall design. This could be partially attributed to the use of plywood as structural sheathing located between the double-stud walls as the air barrier and vapour retarding layer, and using MDF as the exterior sheathing.
In the south-facing wall, the vapour pressure analysis showed a vapour drive in the summer from the exterior layers towards the interior layer, primarily due to high temperature outside. The exterior sheathing should have good drying potential if wetting occurred. On the other hand, the partial vapour pressures were largely consistent across the north-facing wall in the winter, not showing a strong vapour drive from interior to exterior in this mild climate. The exterior sheathing would have poor drying performance if wetting occurred in this location.
The simulated temperature distributions based on THERM 6.3 simulations were generally in good agreement with the measured temperatures across the walls, indicating that the thermal simulation was reasonably accurate. The effective R-value of the double-stud walls of this passive house was calculated to be approximately R-50 (hržft2žF/Btu) or RSI-8.8 (m2K)/W) (i.e. with a thermal transmission coefficient of 0.114 W/m2žK).
The use of heat flux sensors was not successful in this work, probably due to improper sensor calibration or in-situ installation. Its use needs further exploration to measure heat flow in building envelopes in order to validate calculated effective thermal insulation.
Movement in structures due to environmental condition changes and loads must be considered in design. Temperature changes will cause movement in concrete, steel and masonry structures. For wood materials, movement is primarily related to shrinkage or swelling caused by moisture loss or gain when the moisture content is below 28% (wood fiber saturation point). Other movement in wood structures may also include: settlement (bedding-in movement) due to closing of gaps between members and deformation due to compression loads, including instantaneous elastic deformation and creep. Differential movement can occur where wood frame is connected to rigid components such as masonry cladding, concrete elevator shafts, mechanical services and plumbing, and where mixed wood products such as lumber, timbers, and engineered wood products are used.
Evidence from long-term wood frame construction practices shows that for typical light frame construction up to three storeys high, differential movement can be relatively easily accommodated such as through specifying “S-Dry” lumber. However, differential movement over the height of wood-frame buildings becomes a very important consideration for taller buildings due to its cumulative effect. The APEGBC Technical and Practice Bulletin provides general design guidance and recommends the use of engineered wood products and dimension lumber with 12% moisture content for floor joists to reduce and accommodate differential movement in 5 and 6-storey wood frame buildings. Examples of differential movement concerns and solutions in wood-frame buildings can also be found in the Best Practice Guide published by the Canadian Mortgage and Housing Corporation and the Building Enclosure Design Guide –Wood Frame Multi-Unit Residential Buildings published by the Homeowner Protection Office of BC Housing.
This document illustrates the causes and other basic information related to vertical movement in wood platform frame buildings and recommendations on material handling and construction sequencing to protect wood from rain and reduce the vertical movement.
Most buildings are designed to accommodate a certain range of movement. In design, it is important for designers to identify locations where potential differential movement could affect structural integrity and serviceability, predict the amount of differential movement and develop proper detailing to accommodate it. To allow non-structural materials to be appropriately constructed, an estimate of anticipated differential movement should be provided in the design drawings.
Simply specifying wood materials with lower MC at time of delivery does not guarantee that the wood will not get wet on construction sites and will deliver lower shrinkage amounts as anticipated. It is therefore important to ensure that wood does not experience unexpected wetting during storage, transportation and construction. Good construction sequencing also plays an important role in reducing wetting, the consequent wood shrinkage and other moisture-related issues.
Existing documents such as the APEGBC Technical and Practice Bulletin on 5- and 6-Storey Wood Frame Residential Building Projects, the Best Practice Guide published by the Canadian Mortgage and Housing Corporation (CMHC), the Building Enclosure Design Guide –Wood Frame Multi-Unit Residential Buildings published by the BC Housing- Homeowner Protection Office (HPO) provide general design guidance on how to reduce and accommodate differential movement in platform frame construction.
It is not possible or practical to precisely predict the vertical movement of wood structures due to the many factors involved in construction. It is, however, possible to obtain a good estimate of the vertical movement to avoid structural, serviceability, and building envelope problems over the life of the structure.
Typically “S-Dry” and “S-Grn” lumber will continue to lose moisture during storage, transportation and construction as the wood is kept away from liquid water sources and adapts to different atmospheric conditions. For the purpose of shrinkage prediction, it is usually customary to assume an initial moisture content (MC) of 28% for “S-Green” lumber and 19% for “S-Dry” lumber. “KD” lumber is assumed to have an initial MC of 15% in this series of fact sheets.
Different from solid sawn wood products, Engineered Wood Products (EWP) are usually manufactured with MC levels close to or even lower than the equilibrium moisture content (EMC) in service. Plywood, Oriented Strand Board (OSB), Laminated Veneer Lumber (LVL), Laminated Strand Lumber (LSL), and Parallel Strand Lumber (PSL) are usually manufactured at MC levels ranging from 6% to 12%. Engineered wood I-joists are made using kiln dried lumber (usually with moisture content below 15%) or structural composite lumber (such as LVL) flanges and plywood or OSB webs, therefore they are usually drier and have lower shrinkage than typical “S-Dry” lumber floor joists. Glued-laminated timbers (Glulam) are manufactured at MC levels from 11% to 15%, so are the recently-developed Cross-laminated Timbers (CLT). For all these products, low shrinkage can be achieved and sometimes small amounts of swelling can be expected in service if their MC at manufacturing is lower than the service EMC. In order to fully benefit from using these dried products including “S-Dry” lumber and EWP products, care must be taken to prevent them from wetting such as by rain during shipment, storage and construction. EWPs may also have lower shrinkage coefficients than solid wood due to the adhesives used during manufacturing and the more mixed grain orientations in the products, including the use of cross-lamination of veneers (plywood) or lumber (CLT). The APEGBC Technical and Practice Bulletin emphasizes the use of EWP and dimension lumber with 12% moisture content for the critical horizontal members to reduce differential movement in 5 and 6-storey wood frame buildings.