If you’re building a new home, chances are you’re going to want it to be as energy-efficient as possible. There are a multitude of elements you’re going to have to consider, including your plumbing and energy needs. But in this article, I’m going to get back to the basics of building and talk about energy-efficient wall construction.
I’m also going to talk about R-Values. In case you’re wondering, these relate to thermal resistance and the performance of insulation. The higher the R-value, the better its thermal performance will be.
Because you can’t consider wall construction materials separately from the insulation materials used with them, I’m going to kick off with insulation and R-values. Then I’ll talk about the most popular building materials used for wall construction and how energy efficient each one is. I will also cover recommended insulation types for different wall construction methods.
Basics of Insulation
You probably know that proper insulation will make your home more comfortable and reduce your heating and cooling costs.
It does this by counteracting three basic mechanisms of heat transfer:
- Conduction – heat moving through materials
- Convection – heat circulating through liquids and gases, with light, warm air rising and cool, dense air sinking
- Radiation – heat traveling in a straight line, heating any solid matter in its path that absorbs energy
Most insulation materials slow conductive heat flow (and to a lesser extent, convective heat flow) through materials. Reflective insulation and radiant barriers reduce radiant heat gain.
If you don’t have decent insulation, heat will flow from warmer to cooler areas in your home. For instance, from your heated living room to your garage, basement, or attic. In summer, the heat will flow from outside into the rooms you want to keep cool.
But you don’t want to over-insulate your house. Otherwise, moisture can get trapped in the insulation. The idea is to tightly seal the interior of your home to keep it cool in summer and warm in winter.
Good insulation and air sealing will also mean that you can cut down on expensive and not-so-environmentally-friendly heating, ventilation, and air conditioning (HVAC) systems and embrace the advantages of energy efficiency.
The R in R-value stands for thermal resistance, and the R-value of insulation measures how well it will resist heat flow. It is always shown with a number (R-00) which indicates the level of its insulating power. Simplistically, the higher the R-number, the more money you will save on heating and cooling bills.
Similar products from two companies may or may not have the same R-values. Also, different types of insulation may have the same R-values.
Energy Saver has useful information about insulation R-values and how they work. It’s all quite technical, but it’s worth getting your head around the basic concept.
Ultimately, the R-value will be determined by the type, thickness, and density of an insulation product. It might also depend on temperature, moisture, and aging.
In general terms, the thicker the insulation in your walls, the higher the R-value will be. The exception is loose-fill insulation.
But you don’t have to calculate R-values yourself. The Federal Trade Commission’s rules state that manufacturers must determine and disclose the R-value of all insulation products. They are also expected to reveal thickness, coverage area per package, and so on. For loose-fill insulation, the R-value is determined at settled density.
Types of insulation is such a vast subject that it merits its own article. Fiberglass, rock wool, cellulose and spray foam are just a few of the options available. And what you ultimately choose will depend on a number of different factors. For example, certain types of insulation work better for different methods of wall construction. Different areas of the country will have different codes and requirements. And everyone has their own personal goals they’re aiming for with any construction project. Dive into our Ultimate Guide to Insulation here (link).
Building Materials For Energy-Efficient Wall Construction
The energy efficiency of building materials used for wall construction is dependent largely on the insulation materials used within the walls. But, as I have already pointed out, different insulation products suit different wall types.
Another factor that Energy Saver points out is that in most climates, you will save energy (and money) by installing a combination of both insulated sheathing and cavity insulation in walls. They also recommend products that provide combined structural support and insulation, like SIPs, and masonry products like ICFs.
Concrete masonry units (CMUs) are manufactured in standard sizes and are commonly used for building walls. They generally have two or three hollow spaces called cells, that are separated by sections of concrete called webs.
Some cinder blocks look like concrete blocks, but they are not as strong and are not suitable for house wall construction.
When we build the walls of houses with concrete blocks, we lay the blocks in mortar so that each block course bonds with the next one. The mortar we use is a mixture of cement, sand, and water, sometimes with the addition of a little lime. Quantities are very specific, and a relatively high-strength mix is used for load-bearing and exterior walls of houses.
The most common options for finishing exterior concrete block walls are stucco and plaster, both of which are cement mixes.
People have been building homes with concrete blocks for a very long time. Concrete-block walls are cost-effective, durable, and fire-resistant. But they aren’t great when it comes to thermal insulation.
The R-value of an 8-inch concrete block wall without insulation is between R-1.1 and R-2.5. Insulating the cavity will increase the R-value radically.
Additionally, concrete has a high environmental impact, because lots of resources are required for the materials and transport costs are high. The fact that it is made under high temperature and pressure conditions adds to the environmental impact.
Insulating Concrete Block Walls
Options for insulating concrete block walls include injection foam, spray foam, loose-fill products, and foam boards. The product you use will determine the R-value of the finished wall.
Alternatively, you can use concrete blocks that incorporate insulation.
For instance, there is a specialist concrete block insulating system (CBIS) that incorporates specially designed concrete masonry units and individually molded inserts that provide insulation. Some have a decorative facade.
There are also inserts for standard-size concrete blocks.
I don’t have personal experience using these blocks and cannot compare prices or R-values. But the manufacturer maintains that a wall system built using this system will have much higher R-values than conventional masonry. The products have also been code-authorized for use in grouted reinforced masonry construction.
Concrete block insulating units (CBIUs) are another special type of concrete block that many contractors use to provide R-values of up to R-22.
They are also cost-effective because they are manufactured with a finished face. This reduces the need for drywall cladding or a paint finish inside the house. They are also said to be more environmentally friendly than other concrete blocks.
Bricks are smaller than blocks, but just as suitable for house wall construction. They are laid the same way, using mortar so that each course bonds with the next.
The most common bricks are either solid or have three holes, in which case they are known as cored bricks.
Companies manufacture bricks from clay or concrete. There are more options when building with clay brick, including facing bricks that have a nicely finished surface that doesn’t have to be covered with a cement stucco or plaster finish before you paint it. Clay is also more environmentally friendly than concrete.
Like concrete block walls, you can insulate brick walls with various products including foam that is sprayed into the cavity. The size of the cavity and product used will determine the R-value of the walls.
The ubiquitous American timber-frame house is built from wood. But this doesn’t mean that the wall cladding is necessarily wooden. You also have a choice in terms of the insulation material that is sandwiched between the inside and outside cladding materials.
The first specification for wood walls is the timber that you use for the wood-frame, or more specifically, the studs. The common choice is between 2×4-inch or 2×6-inch.
Wood has an R-value of about 2.5 per inch. But when you compare a 2×4 with a 2×6 inch framework, there is no difference in terms of the way the walls are constructed. Ultimately, the insulation qualities will determine energy efficiency.
2×4-inch wood studs are the traditional standard.
A rule of thumb from Energy Saver is that you can install cavity cellulose or fibrous insulation that is up to R-15 in 2×4 walls. The R-value can be higher if you use foam or other advanced insulation systems.
2×6-inch studs are considered an upgrade to 2×4 studs, but they are also more expensive. They are heavier, and you will need considerably more insulation in terms of volume. Because the cavity space is bigger, it means you get a house with more insulation. You will also need about double the number of wall plates.
The recommendation from Energy Saver is to install insulation up to R-21 in 2×6 walls.
The question is, is the extra cost worth it? The answer: It depends on how cold it gets in your part of the world, and how long it stays cold. If you live in Florida as I do, or in any other warm-climate area, you don’t need extra insulation in your walls to make the house energy-efficient.
Possible Payback on 2×6 vs 2×4
Scientists from the University of Illinois have devised a method for measuring insulation requirements. Known as the “degree-day method,” it is based on data supplied by the National Weather Service.
A heating degree day is the number of degrees (not days) that the average temperature in one day is below 65°F (18°C). When your interior is this temperature, you don’t need heating or cooling. But they reckon that if you have more than 5,750 degree days in a year, it will make economic sense to think about using 2×6 exterior walls.
Fort Myers, where our first net-zero solar home renovation is located, has an average of 99 days every year where the temperature is above 90°F (32.2°C). It also ranks in the upper half of the 50 hottest cities in the U.S. So, if I decide to build a wood-frame house in this area, I’ll be sticking to 2×4 studs that can accommodate more than enough insulation to keep a house cool.
Exterior Walls & Cladding
Most timber frame houses are built with two layers on the outside. The first is usually either plywood, the traditional sheathing used for wood-frame houses, or oriented strand board (OSB). OSB is a very dense, engineered wood intended for structural projects. It withstands moisture and most weather conditions.
Both materials offer a good balance between energy-efficiency, structural performance, and affordability. They are also easy to insulate for high R-values.
Cheaper than plywood, OSB has a textured finish, so it is usually finished with some sort of cladding or siding, rather than paint. This includes timber, PVC, vinyl, and fiber cement planks or panels. OSB can also be plastered or rendered with stucco. The same options apply to plywood.
The R-values of OSB and plywood are very similar. For example, both have an R-value of 0.62 for inch-thick boards. Similarly, cladding doesn’t add much insulation to wall systems, so it’s not going to have an impact on the R-value of your walls. An exception is insulated vinyl siding.
Generally, it’s the insulation between the plywood or OSB layer and the internal drywall (usually gypsum board) that matters.
There are several types of insulation that are suitable for wood-frame houses, including:
- Fiberglass that is installed in batts or blown in. This is the least expensive option.
- Cellulose that is blown into the cavity.
- Foam, which is also blown into the cavity, but expands after you spray it.
Although contractors don’t always insulate interior walls, there is no doubt that insulating interior walls has advantages. It is also essential for walls between the house and garage as well as other attached areas.
Insulating internal walls will:
- Dampen sound (although it won’t be soundproof)
- Improve moisture control
- Improve energy efficiency
Optimum value engineering (OVE) is a wall-construction method that uses wood, but only where it is most effective. This reduces cost and saves space for insulation.
Its name is a clue to the fact that buildings are engineered. The amount of wood specified must be determined to be structurally sound using both field and laboratory tests. It is also essential that builders have experience using this method.
R-values will be determined by the insulation used in the walls.
Insulated concrete forms (ICF) are relatively new but are popular with contractors who recognize the value of qualifying for energy credits and other ratings. Estimates are that houses built with ICF vs. regular wood-framing only cost about 3% more.
Exterior walls comprise two layers of extruded plastic foam board or expanded polystyrene (EPS), one that is inside the house, and the other outside. These create a type of formwork for a steel-reinforced concrete center that is between two and six inches thick.
Typically, because thermal bridging doesn’t happen, ICF walls have an R-value of 22 or higher.
Houses built with ICFs are extremely strong and well-insulated. They will typically exceed the code requirements for areas that are prone to tornadoes or hurricanes, like Florida.
Prefabricated 3D structural concrete insulated panels (SCIPs) were invented by Victor P. Weisman, a California man who secured a world patent in 1967.
Manufactured from 100 percent recycled material, they meet LEED Platinum requirements. The structure consists of a core of rigid, super-insulated expanded polystyrene sandwiched between two engineered sheets of 11-gauge steel-welded wire mesh.
Better still, SCIPs are said to be one of the strongest building materials available for wall construction. And they meet a whole bunch of codes, including those set by the American National Standards Institute.
Panels are delivered to the building site with the foam and galvanized wire mesh tied together with steel wire trusses. They are then coated on-site with shotcrete, a very strong concrete mix that is sprayed onto both sides.
A common method used for in-situ swimming pools, shotcrete is much lighter in weight than concrete and is said to reduce the weight of concrete buildings by 50 percent. This makes them popular in areas where earthquakes and high winds are common, including New Zealand. This could make them suitable for areas like Florida, which experiences frequent hurricanes.
The panels have been dubbed a “disaster-resistant building material” because houses built with SCIPs are known to have survived various natural disasters including hurricanes, floods, earthquakes, tsunamis… and fire!
SCIPs are environmentally friendly and known to reduce HVAC costs by keeping thermal loss at a minimum, making them energy-efficient. Construction is simple and not labor-intensive.
Companies that manufacture SCIPs offer an optional polystyrene core that improves the R-value of the panels by 10-12 percent. A polyiso core is also available. Any core is, of course, the insulation that will ultimately determine the energy efficiency of the panels.
A 2.5-inch thick SCIP with a polystyrene core will have an R-value of about 11. A polyiso core of the same thickness will achieve R-21.
Structural insulated panels (SIPs) are sustainable, energy-efficient, and durable panels that can be used for the framework of houses. SIPs provide a high-performance material that is strong and relatively inexpensive.
Panels may be made of plywood or OSB that is laminated to a 4- to 8-inch-thick foam board. They may also be made with an insulating foam core that is sandwiched between two sheets of OSB.
Construction is considerably faster than conventional wood-frame methods and OVE construction. It is also a relatively simple method that leaves little opportunity for mistakes, resulting in high-quality construction.
A 5½-inch thick polyurethane SIP panel has an R-value of 42. That’s a similar rating to SCIPs.
Steel frames are a relatively new method for wall construction. Apart from the studs, steel framing uses the same principles as wood-frame construction, including sheathing, insulation, and cladding (see Wood, above).
Steel has zero R-value. So the R-value of completed walls will be similar, but a bit lower, since wood has an R-value of about 2.5 per inch.
Steel frame houses are estimated to cost about 15 percent more than traditional timber-frame houses.
There’s a surprisingly wide choice of wall construction methods out there, some of which incorporate insulation to make them more energy-efficient. More traditional building methods rely on the addition of insulating material to the core or cavity of the wall to make them energy efficient.
Ultimately, when assessing which type of wall will work best for your energy-efficient structure, you need to weigh the benefits of the materials in terms of ease and speed of use as well as cost. When insulation is an added material, you need to weigh the benefits, including the R-values of the different types as well as which are best for your building method and climate.
It’s a tough call for anyone who doesn’t have construction experience. Hopefully, this article will help you get your head around the various issues, though. At very least, it’s useful know what questions to ask your contractor before you start building.