A High Performance Envelope with Conventional Framing
Dimensional lumber, engineered wood products and careful insulating deliver a wall system that outperforms its peers.
A few weeks ago, the lumber supplier delivered studs, floor joists, beams, sheathing, and other material to the VISION House Sussex jobsite. This lumber has been on quite the journey. Coming from British Columbia, Canada and Oregon, you could say that this lumber is well-traveled. A year ago, the studs were timber, about 25 mature trees in a managed forest outside of Vancouver.
They were big trees, averaging 80 feet tall and 2 feet in diameter, planted around the time of the Civil Rights Act in 1964. Sustainbly farmed, most are Spruce Pine Fir (SPF), a general classification of softwood trees that have similar characteristics, and are abundant in North America. SPF is strong, lightweight, and, like a pre-shrunk cotton shirt, undergoes a kiln drying process that removes most of the lumber’s moisture content.
Lumber has been delivered to the homesite and the framers begin their first day by installing the sill plate and setting the LVL beams.
Moisture Control Matters
Wood, alive or dead, interacts with its surroundings. The fibers expand when humidity levels are high and shrink when humidity levels are low. This seasonal expanding and contracting of the material impacts the frame, marginally. Especially for the first few years of the home, audible pops in the walls, floor squeaks, and/or hairline drywall cracks at room corners are expected to happen, to a varying degree. Installing a dehumidifier in the basement and maintaining an indoor humidity level of 30-50% is one of the best maintenance practices a homeowner can do.
In cold climates like ours, here in Wisconsin, indoor humidity control in the winter is exceptionally important. Moisture in the air clings to cold surfaces. In your home, the coldest surface on the interior of the home are the windows. High indoor humidity levels in the winter can lead to interior window condensation. When the water droplets drip down the window and onto the window ledge, they can begin to rot away at the wooden sills.
As temperatures outside continue to fall, the indoor humidity must also fall. For example, an outdoor temperature of 30 degrees Fahrenheit requires indoor humidity levels to be below 40%, however, for an outdoor temperature of 0 degrees Fahrenheit, indoor humidity levels should not exceed 30%.
It is a delicate balance between prioritizing the integrity of the home and the comfortability of the occupants. At Tim O’Brien Homes, we do not recommend humidifiers to increase indoor humidity levels in the wintertime. The energy recovery ventilator (ERV) installed standard in our homes reapplies most of the humidity of outgoing air to incoming air, preventing the need for a humidifier.
Another reason why we do not recommend humidifiers is because our warranty team finds far more often than not that indoor humidity levels are too high in the wintertime, largely because homeowners are not adequately running indoor ventilation equipment (bath fans, cooktop vents).
The Frame: From the Bottom Up
The first piece of wood applied to the foundation is the sill plate. The bottommost member of the framing, the sill plate is 2x6 lumber that is pressure treated. Also referred to as green-treated, the pressure treatment process consists of forcefully injecting wood-preservatives into the wood fibers, protecting the lumber from insects, moisture, and wood rot caused by fungal decay.
The sill plate is sourced from Southern Yellow Pine, a strong and dense species that holds fasteners well. This is important since foundation anchors submerged 4-inches into the top of the foundation wall and fastened atop the sill plate, to hold the framing firm to the foundation.
The sill plate is applied over the sill sealer atop the foundation wall.
To further protect the basement from drafts, moisture, and insects, a thin strip of foam is placed between the foundation wall and sill plate. Called a sill sealer, this irregular foam fills crevices between the rough surface of the foundation wall and the smooth surface of the sill plate.
Although the sill plate is pressure treated, the sill sealer acts as an additional capillary break, inhibiting the transfer of moisture from the concrete to the wood. During the insulation process, this topmost section of the basement will be coated with a layer of expanding spray foam, to further beef up this thin section of the exterior wall system.
LVL Spans Carry the Load
Structural beams are required for large spans of flooring. Laminated veneer lumber is an engineered wood product that can shoulder a significant amount of weight.
Freshly debarked Douglas Fir timber from the Pacific Northwest is rapidly spun and peeled like a potato to create one long, thin sheet of wood–a veneer. Looking like an unrolled toilet paper roll, this long sheet is then cut into sections, coated with an adhesive, and perpendicularly stacked atop the next–laminating.
Once the desired thickness has been achieved, the stack is compressed to produce an incredibly strong and dense composite of wood. LVLs can be up to 5 times stronger than conventional lumber beams of the same size.
For large spans, steel columns transfer the weight to square footings in the center of the basement. Most of the structure is transferred to the foundation walls, but for the center of the home, the weight is transferred to the LVLs, steel columns, and column footing pads. LVL beams are not pressure treated, so when set inside a foundation wall pocket, they are wrapped in an impermeable, rubberized asphalt membrane to prevent the transfer of moisture from the concrete.
The framers have set the LVL beams and begin installing engineered floor joists atop them.
Referred to as I-joists due to their resemblance of a capital “I,” engineered floor joists offer numerous benefits. Costing marginally more than solid lumber joists, I-joists are lighter, less prone to warping at long spans, minimize floor squeaks, and are easier to install.
An I-joist is composed of a sheet of OSB, known as the web, sandwich between two pieces of thin lumber, known as flanges. This assembly, resembling a steel beam, provides remarkable resistance to joist rotating.
The framers set an exterior wall with their lift as the lumber supplier rolls the prefabricated trusses off of the flatbed truck.
The Right Insulating Armor
About 35% of heat loss in a home occurs at the exterior walls. A common approach to increasing building energy efficiency is to increase the exterior wall depth and add more insulation. Although this works, it only works to a degree.
Wood is a natural insulator, however, it is more conductive to heat transfer than insulation. To prevent exterior studs from acting as superhighways for heat transfer, we wrap exterior walls with 1-inch rigid foam (R-5).
A cross section view of the VISION House Sussex exterior wall.
Studs in the wall are a path of least resistance for heat. The rigid foam acts as a warm jacket and hinders thermal bridging. The specific product we use, Neopor GPS, is made of graphite polystyrene, a breathable insulation material that prevents mold, rot, and subsequent structural damage.
The unique product uses 30% less material than other rigid foam insulation of the same R-value and its insulation properties increase as the temperature decreases. Termed double sheathing, it adds initial cost, but adds immense long-term value to the home.
A weekend warrior can replace their cabinets or flooring, but getting the building envelope right should be the biggest priority of any builder producing a value-driven home.
The building envelope of our home is conventional, aside from the rigid foam. Utilizing the rigid foam adds structural bracing to the wall and increases its R-value, negating the need for a 2x6 wall. We frame 16-inches on-center, sheath the walls with 7/16-inch oriented strand board (OSB), and utilize Tyvek as our weather resistant barrier.
Siding and masonry will protect the home from harsh weather, but they are not waterproof. Acting as a weather resistant barrier, Tyvek is a continuous drainage plane that exists behind the siding and stone.
By design, it is not an air barrier, or a vapor barrier. Tyvek prevents liquid from seeping into the building, stops air from moving into the exterior wall, and quickly removes water vapor should it enter the wall system. Being an air or vapor barrier would trap those elements within the wall space.
Tyvek allows movement out of the wall system, but not into it. Similar to the vapor retarder installed during the installation process, we want to hinder air flow out of the building, not completely eliminate its movement in our cold climate. In cold climates, eliminating the movement of vapor through an exterior wall system can trap it within, thus causing structural damage.
Tyvek HomeWrap viewed with 350x magnification. Image credit: https://www.dupont.com/tyvek-weatherization/perforated-wrap.html
The Tyvek membrane is made of randomly laid, compressed fibers that give it remarkable resistance to water, bacteria, and abrasion. The millions of pores created by the compressed fibers promote the movement of water vapor in one direction: out of the home.
For vinyl siding we utilize Tyvek Homewrap, which has a flat, smooth surface. For HardiePlank siding, we utilize Tyvek DrainWrap, for enhanced drainage and drying of the moisture absorbing cladding.
Trusses with Insulation “Rolls”
As opposed to traditional solid rafters, we use prefabricated trusses to save time, material, and labor onsite. Since their inception in the 1960s, prefabricated roof trusses have been common practice for supporting a roof load. The trusses are designed to account for live loads such as snow, roofers, and equipment and dead loads including the weight of the roof deck and shingles.
A comparison of conventional roof trusses and raised heel trusses. Image credit: http://www.home2blog.com/2015/02/energy-trusses.html
Conventional roof trusses have sharp corners at their ends, known as the heel. During the insulation phase, roughly 18-inches of loose insulation is blown on top of the attic-ceiling.
However, the heels of conventional trusses are not 18-inches tall. This small amount of space for insulation produces cold spots at the tops of exterior walls where the trusses rest, which can make rooms uncomfortable and decrease energy-efficiency. Raised heel trusses alleviate the issue.
To further fortify this vulnerable section of the building envelope, we roll a standard batt into a cinnamon roll and stuff it above the top plate between each truss. This roll of insulation also gets sprayed foamed. I will discuss this when explaining our thorough insulation process, a personal favorite.
Another standard practice we exceed are truss tie downs. To ensure that the roof doesn’t lift off of the home during a high wind event, and to resist tension, metal straps are used as industry standard to secure trusses to the frame.
Although they are small, they are exposed metal. In the wintertime, they can produce cold spots and condensation at the top edges of exterior walls. To prevent this, we utilize structural lag screws. By burying the length of the screw within the truss and top plate, heat loss and moisture is less inclined to collect on the surface of the metal.
Ice Dam Preparedness and Prevention
The melting and subsequent freezing of accumulated snow on a roof can cause havoc above the gutters. image credit: Summit Point Roofing
This home takes into account the conditions that form ice dams. Easily identified by the presence of icicles, ice dams form when heat inside the home leaks into the attic and warms the immediate underside of the roof. At truss ends, this causes the bottommost layer of snow to melt, only to be frozen at the gutter when exposed to freezing temperatures. Snowmelt then works its way up the roof and eventually underneath the shingles.
As a precaution, we install Ice and Water Shield, an adhesive asphalt membrane, 3 feet up along the gutterline prior to shingle install, hinders water intrusion through roofing nails due to the material’s sealing properties.
Raised heel trusses also reduce the risk of ice dams by allowing more insulation above the exterior wall, thus minimizing heat transfer to the underside of the roof deck. The roll of insulation coated in spray foam also helps.
A closer look at the raised heel truss utilized in the home.
Next on the agenda is rough mechanicals. This general milestone includes plumbing; heating, ventilation, and air condition (HVAC); and electrical work. Between framing and rough mechanicals are two crucial activities: roofing and window/exterior door installation. These two activities allow the mechanical Trades to complete their work shielded from wind, rain, and snow. After rough mechanicals, insulation and siding will seal the home for good.