6 ways to build a better thermal envelope for your home
Photo courtesy of Covestro.
Your home wears a coat that keeps you warm inside. High-performance homes are using high-tech solutions wall systems that are like a mountain climber's parka that keeps the energy inside even in the harshest conditions.
On house, that warm coat is called the thermal envelope. But what does that matter for your home? An airtight thermal envelope translates to lower utility bills and greater comfort. A leaky thermal envelope leads to excessive energy use and hot and cold spots in your home.
The U.S. Department of Energy estimates that about 40 percent of the energy used in buildings is associated with heating, ventilation, and air conditioning.
Does it cost more to put a high-tech coat on your home? It may cost a little more to add insulation and air sealing to meet green building standards such as Zero Energy Ready Home, LEED for Homes or Passive House.
But keep in mind that construction represents only 15 to 20 percent of a family’s cost of owning a house over 20 years. Other things like maintenance, insurance and energy consumption dwarf the construction cost during that time.
The thermal envelope is not the same as the building envelope, although in some cases such as windows, they can be the same thing. The thermal envelope is also known as the heat flow control layer that controls the movement of air through a home's structure. The building envelope includes items such as the roof and cladding that may not impact energy efficiency very much if at all.
Here's a look at some of the latest approaches to building wall systems for a high-performance thermal envelope.
Advanced wood frame construction
Traditional wood frame or “stick built” homes are still the most common way homes are constructed today. These homes are built with pieces of lumber that are cut and nailed together into walls and roof trusses. The frame is sheathed with wood on the exterior and drywall on the interior. The exterior of the homes is usually finished with brick, clapboard, vinyl siding or something similar.
New advanced construction techniques reduce the amount of lumber used, allowing for more insulation in the home while not significantly changing the familiar building methods. Advanced techniques include:
- Designing homes on 2-foot modules to make the best use of common sheet sizes and reduce waste and labor.
- Spacing wall studs up to 24 inches on-Spacing floor joists and roof rafters up to 24 inches on-center.
- Using two-stud corner framing and inexpensive drywall clips or scrap lumber for drywall backing instead of studs.
- Eliminating headers in non-load-bearing walls.
- Using in-line framing in which floor, wall, and roof framing members are vertically in line with one another and loads are transferred directly downward.
- Using single lumber headers and top plates when appropriate.
Known as advanced framing or optimum value engineering (OVE), these advanced techniques can result in lower material and labor costs and improved energy performance for the building. While the various techniques can be applied as a whole package, many specific techniques can be used independently, depending on the needs of the project.
Insulated concrete forms
Insulated concrete forms (ICFs) are rigid plastic foam forms that hold concrete in place during curing and remain in place to serve as thermal insulation for concrete walls. The foam blocks or planks are lightweight and result in durable, energy- efficient construction. Because of their benefits, ICFs are desirable in above-grade applications as well as foundations.
The forms, made of expanded polystyrene (EPS) foam insulation, are either pre-assembled interlocking blocks or knock- down panels connected with plastic ties. The stay-in-place forms not only provide a continuous insulation and sound barrier, but also a solid backing with a continuous fastening stud for drywall on the inside, and lap siding, EIFS, stucco or brick and stone on the outside.
ICFs allow contractors to construct concrete walls without a significant investment in reusable wood and metal forms. Because ICFs fit together easily and remain in place after concrete is poured, they can simplify and speed up construction. ICFs increase the temperature range for pouring concrete to below freezing (freezing inhibits proper curing) by insulating the concrete until it is fully cured. ICFs can also result in stronger walls than standard cast-in-place concrete due to more constant, predictable cure during all seasons.
Structural insulated panels
Structural insulated panels are systems of rigid expanded polystyrene foam sandwiched between panels of oriented strand board (OSB). SIPs are fabricated off-site, come in thicknesses from 4 to 12 inches, and are fairly interchangeable with wood frame construction.
Little dimensional framing lumber is used, although windows and doorways greater than 5 feet across require headers and beams are required to support longer roof spans.
Advantages of SIPs include a very high effective R-value (thermal resistance), excellent soundproofing performance and rapid on-site installation.
SIP construction can be directly competitive with frame construction, especially when compared to other high- performance R-30 wall systems.
SIPs are capable of supporting typical loads for bearing walls, intermediate floors and roofs. They can provide both the primary structure and the envelope or they can be used with other structural systems such as post-and-beam construction to provide exterior envelope and insulation.
Panelized wall systems
Panelized wall systems speed up construction time, reduce waste, and result in a tight building envelope.
Covestro has developed PUReWall system that combines several layers into one panel.
The design replaces traditional exterior sheathing and replaces it with a combination of polyisocyanurate (polyiso) continuous insulation on the exterior and spray polyurethane foam (SPF) in the wall cavity, both of which are installed at a panelization facility.
The wall was developed to overcome shortcomings in current wall construction technology that won't meet future build codes. Covestro said PUReWall meets thermal performance requirements in all climate zones and allows for high-quality building consistency in an uncontrolled field environment.
BASF developed launched the HP+ Wall System that features advanced wall-framing technology and leverages the BASF BEYOND.High Performance approach, which combines innovative chemistry and applied building science.
The cost savings from reduced lumber use allows builders to incorporate once-too-costly polyurethane insulation, a component of the HP+ Wall System, into their construction. By offering consultative services alongside its products, BASF helps builders identify cost-shifting opportunities so they can incorporate upgraded insulation solutions that have long been cost-prohibitive.
The HP+ Wall System combines three BASF products:
- Neopor, a graphite- enhanced and rigid thermal foam insulation that helps mitigate the risk of moisture inside of wall cavities
- WALLTITE HP+, a closed-cell, spray-applied polyurethane for superior insulation, strength and energy efficiency
- ENERSHIELD, an air and water resistant resilient coating that creates a seamless barrier of protection to above grade wall substrates.
This combination of layers in single integrated wall system delivers exceptional control of heat, air and moisture and increased structural integrity.
The design capacity of the HP+ Wall System is up to 135 percent greater than that of a wall built with standard construction methods, offering builders the opportunity to reduce lumber content by up to 25 percent and eliminate the need for plywood or OSB sheathing.
Extended plate and beam
The Extended Plate & Beam system sandwiches rigid foam between the sheathing and the studs in a wood frame wall. The studs are still 2x4s but the top and bottom plates are 2x6. The platesare flush on the interior side but extend past the studs on the exterior. In that gap, there's room for up to 2 inches of foam insulation. Then, wood sheathing is installed on the exterior side just like any other wood frame wall. Light cladding materials such as vinyl siding don't require additional fasteners. Standard window and door materials for 6-inch walls can be used.The EP&B wall system is composed of familiar wall materials but in a different configuration.
The assembly was designed to meet or exceed Energy Code prescriptive insulation requirements for all U.S. climate zones. The EPB wall assembly offers air, water and thermal management and an R-value of 23.
Read more about thermal envelope options for your home.