Having recently completed our first net-zero solar home renovation, I know how important home insulation is. While it’s only one piece of the energy-efficiency puzzle, it’s an important one. So, in our ongoing quest to enable all homeowners to achieve the goals of sustainable, affordable net-zero homes, I have compiled the Ultimate Guide to Home Insulation.
Whether you are building a home from scratch or renovating an existing house, good insulation isn’t only going to up your comfort levels by keeping you cool in summer and warm in winter. It’s also going to help you to reduce energy costs and minimize harmful greenhouse gases.
But there isn’t a single solution for everyone. Every situation is different, and it’s vital that you consider all the options in terms of local climate, construction methods and materials used to build or renovate your home, and which elements of your home you are going to insulate.
Table of Contents
- Insulation Basics
- Where to Use Insulation in Your Home
- Types of Insulation
- Less Popular Insulation Types
- Last Thoughts
Heat moves from areas of high to low temperature–often upwards, but in all other directions as well. Warm, low-density air moves outside during winter and inside in summer, while cool, dense air tends to leak out of our homes once it sinks to the floor.
Insulation helps to prevent the movement of air. And because air moves in all directions, we need to install insulation in our roofs, ceilings, walls, and floors to stop air from getting inside or moving outside.
We can measure the effectiveness of insulation by its thermal resistance to heat transfer (or R-value). In general, the higher the R-value, the better the insulation. (I talked more about R-values when we explored the different types of energy-efficient wall construction.)
But if you live in a tropical region, you aren’t going to need the same degree of thermal resistance as you will in a polar region or even a temperate area.
To maximize the energy efficiency of your home, the Department of Energy’s Energy Saver advises that you need to insulate from the roof of your house right down to the foundations:
- In the roof above ceilings with cold spaces
- In attic spaces
- Between the rafters and studs of the roof and outside walls
- In short knee walls, either between the internal and external skins of brickwork/blockwork or between the studs
- Around attic access doors
- In joist spaces below attics
- In all exterior walls, including foundation walls and walls in heated basements
- In all floor spaces, including:
- Vented crawl spaces
- Unheated garages
- Floors that are cantilevered above and away from exterior walls
- Slab floors built on the ground
- Basement floors built below the ground
If you want your insulation to be as effective as possible, it is also important to control air leakage and moisture. So you should also caulk and seal around all external doors and all windows.
But once you’re ready to insulate, which types of insulation should you use?
There are many different types of insulation for different applications. The type you choose for the different areas of your home will depend largely on what your home is built of.
For example, the insulation for a concrete block wall will not be the same as the insulation you use for structural insulated panels (SIPs). Similarly, some types of insulation are more effective for flat roofs vs. high, pitched roofs and cathedral ceilings that follow the line of the pitch.
Another factor to consider is that insulation is available in various forms. These include batts and rolls, rigid foam and foam board, loose-fill material that can be blown into wall cavities, foam that is sprayed into place, and rigid fibrous or fiber insulation.
Insulation facings, often used to secure insulation material against some surfaces, can also be used to form a radiant, vapor, or air barrier. For example, aluminum foil can provide an excellent insulating radiant barrier in ceiling spaces.
Additionally, some wall types use materials that incorporate insulation, including insulating concrete blocks and concrete forms, as well as structural insulated panels.
It is common practice to use different insulation in walls and ceilings. For example, as the Department of Energy Efficiency and Renewable Energy points out in an educational document, Elements of an Energy-Efficient House, a typical New York house might have R-11 insulation in its exterior walls and R-19 in the ceiling. But other similar houses could have wall insulation ranging from R-20 to R-30 and ceiling insulation from R-50 to R-70. It’s e variable.
State and local building codes incorporate minimum insulation requirements, but these standards are not guaranteed to get you an energy-efficient home. To maximize energy efficiency, you must consider how the insulation you use will interact with other building components.
The choice of insulation materials is remarkably varied. The Department of Energy’s Energy Saver website lists a variety of insulation materials that are used in the U.S. and gives an R-value per inch for some of them.
I have singled out the ones that are most commonly used today:
Mineral (Stone) Wool
Mineral wool, stone wool and rock wool are all different names for the same thing. There’s also slag wool. Both types are man-made insulation materials:
- Rock wool is made from natural materials including basalt, a common volcanic rock, and diabase (dolerite)
- Slag wool is made from waste blast furnace slag that forms on the surface of molten metal
Available in blanket form, batts, and rolls, as well as in the form of loose-fill insulation, mineral wool contains, on average, 75 percent post-industrial recycled content, making it eco-friendly.
Mineral wool is typically used in blanket form to insulate walls, floors, and ceilings. It is used in a loose-fill form in existing wall cavities and other hard-to-reach areas where it can be blown in. Mineral wool is also used to insulate ducts in unconditioned space.
It has an R-value that is about 22 to 37 percent higher than fiberglass (see below).
Rockwool, a trade name, is manufactured by a Danish company that makes rock wool mineral fibers that are used for insulation. The company has been licensed to produce stone wool in Denmark, Sweden, Norway, and Germany since 1935.
The North American subsidiary of Rockwool was originally called Roxul, but in 2018 was renamed Rockwool. They are not the only U.S. company to make rock wool, but they are the world’s largest stone wool manufacturer.
Also known as glass wool, fiberglass has a lower R-value than rock wool, but it tends to be more cost-effective and versatile at lower weights. Made from a mixture of natural glass and between 20 and 30 percent recycled materials, it also has a lower compressive strength than Rockwool.
Generally more affordable than mineral wool insulation, fiberglass is also available in blanket form and loose-fill options.
Typical R-values for high-density batts range from R-15 to R-38, while medium-density batts have an R-value of 13, and low-density batts, R-11. Loose-fill fiberglass insulation is also available.
Since we are focusing on net-zero energy and sustainable options, I need to mention that it takes about 10 times more energy to produce fiberglass versus cellulose insulation, which we will look at next. So, even though fiberglass insulation incorporates recycled materials, it doesn’t quite hit the target when it comes to a green solution.
Nevertheless, you can use fiberglass insulation in blanket form for walls, floors, and ceilings, and loose-fill can be blown into wall cavities and other difficult-to-reach areas. The material is also available in rigid forms which, like mineral wool, can be used to insulate ducts.
Just remember that individual products made with different materials may vary in terms of their R-value. Installation methods and the spaces insulated can also affect R-values.
For example, high-density fiberglass batts for a 2×4 inch stick- or stud-frame wall, have an R-15 value, while high-density batts for a 2×6-inch wall offer R-21. You can also get high-density fiberglass batts with an R-30 for 8.5-inch spaces, and R-38 batts for 12-inch spaces.
Typically made from recycled paper (primarily newsprint), cellulose insulation has a recycled content of between 82 and 85 percent–so it’s also eco-friendly. When builders use it in walls and cathedral ceilings, it must be densely packed.
Loose-fill cellulose can be sprayed damp or blown dry into wall cavities and open attic installations.
While it is considered one of the safest insulation types for our homes, there are potential problems that you need to consider. Cellulose insulation can settle and reduce its mass by up to 25 percent if it isn’t properly installed. This means that it can reduce its thickness within a few months of being installed.
Manufacturers often use borate blended with ammonium sulfate for insect and fire resistance.
Other Natural Fibers
While some cellulose insulation materials are treated with chemicals, other natural fiber insulation types are available, including cotton, wool, straw, and hemp. The R-value of each is similar, but cotton is the most eco-friendly natural fiber material because of its 85 percent recycled content.
Other types of natural fibers include sheep’s wool, straw, and hemp.
Wool batts for 2×4 inch and 2×6 inch stud-framed walls have an R-value of R-13 and R-19. According to Energy Saver, sheep’s wool is generally treated with borate to make it fire, mold, and pest-resistant.
The use of straw goes back to straw bale construction, which was very popular 150 years ago. Today, manufacturers fuse straw into boards for insulation and for sound-proofing interiors.
Hemp is an effective and eco-friendly insulation material, but it is not yet widely used in the U.S. It has an R-value of about 3.5 per inch of thickness and can provide an R-value of 19.25 when 5.5-inch batts are installed between 3.5-inch studs.
Ideal for ceilings and some floor systems, reflective systems comprise materials that include plastic film, foil-faced kraft paper, polyethylene bubble sheets, and cardboard of various types.
Generally fitted between joists, rafters, beams, and wood-frame studs, it is the one type of insulation that is universally regarded as suitable for DIY installation.
Various insulation products are made from foam that is manufactured in the form of rigid boards, laminated panels, small foam beads, and either closed- or open-cell spray foam.
Closed-cell foam is made up of high-density cells that are filled with a gas that causes the foam to expand into open spaces around it. Closed-cell spray foam insulation is one of the most efficient insulating materials available today, boasting R-values consistently higher than 6.0 per inch.
Open-cell foam isn’t as dense and it contains air, producing a spongy texture and a lower R-value than closed-cell foam.
Even though some foam products are available as rigid boards, today the most popular method is for foam to be sprayed in place. Generally, spray-in-place insulation is more expensive than blanket types, but both will achieve R-values of R-3 to R-4 per inch of thickness. Rigid foam insulation has a slightly higher average R-value of between R-4 and R-5 per inch of thickness.
Four foam insulation materials are commonly used today: polystyrene, polyisocyanurate, polyurethane, and cementitious foam. We have also included the less popular phenolic foam because it has extremely high R-values.
Ideal for insulating concrete block walls, insulating concrete forms (ICFs), and structural insulating panels (SIPs), polystyrene is manufactured in the form of foam boards and loose-fill beads.
Molded expanded polystyrene (MEPS) is often used for foam boards, SIPs, and ICFs. It is also used in foam bead form to insulate hollow wall cavities and concrete blocks. These are very light but not easy to control because the little beads fly everywhere.
Two other types are used for polystyrene insulation products:
- Expanded polystyrene (EPS), which is made of small beads that are fused and commonly made into blocks.
- Extruded polystyrene (XPS), which is melted before being pressed into sheets to produce foam board insulation. It is also used for SIPs and ICFs.
Foam boards have the highest R-value of all polystyrene foam insulation products, but numbers depend on the density of the material. The R-value of XPS insulation often drops because of thermal drift, when air replaces the low-conductivity gas that escapes from the polystyrene.
Polyisocyanurate, also known as polyiso, is a closed-cell foam that contains a low-conductivity gas that is free from hydrochlorofluorocarbon (HCFC), a harmful greenhouse gas.
Polyisocyanurate insulation is available as rigid foam boards, in liquid form, and as sprayed foam.
Generally, when it is foamed-in-place, polyisocyanurate insulation is less expensive. It also performs better than polyiso boards because the liquid closed-cell foam molds itself to the adjacent surfaces. Nevertheless, installing plastic or foil facings can help to stabilize the R-value of rigid panels.
Sometimes, this liquid closed-cell foam is injected between two skins of wood under high pressure to form SIPs for walls and ceilings. This type of polyiso board is more expensive, but also offers more water-vapor diffusion and fire-resistance than EPS, and it provides 30 to 40 percent better insulation for any specified thickness.
If thermal drift occurs, it usually happens within the first two years after manufacture. Tests show, however, that if boards are faced with metal foil, the R-value doesn’t change for at least 10 years.
A foam insulation material similar to polyiso, polyurethane is available as open- and closed-cell products, as a liquid sprayed foam and rigid foam board. It also contains a low-conductivity non-HCFC gas that tends to escape during the first two years after manufacture, resulting in thermal drift and a deterioration in the R-value.
Air is used as the blowing agent for low-density open-cell polyurethane foam.
Plastic and foil facings can help to stabilize the R-value, and reflective foil can be used to create an additional radiant barrier.
Spray- or foamed-in-place applications are generally more cost-effective than installing foam boards, and they perform better because the liquid foam molds to adjacent surfaces.
Like polyiso, polyurethane foam can be injected between wood skins to create wall and ceiling panels, with the same improved insulation and cost results.
This is a cement-based foam that may be sprayed or foamed-in-place. It is nontoxic and nonflammable and costs about the same as polyurethane foam.
Cementitious foam is made from minerals like magnesium oxide that are extracted from seawater. Airkrete, a U.S. cementitious spray foam product, contains magnesium silicate. When it’s sprayed it has a consistency that is similar to foamy shaving cream.
Phenol-formaldehyde (phenolic) foam used to be very popular. It uses air as its foaming agent but tends to shrink up to about 2 percent once it has cured.
Nowadays, it is only available as a foamed-in-place insulation material.
|R-Values for Some Popular Insulation Types|
|Insulation Material||Insulation Type||R-Values per inch of thickness|
|Fiberglass||Loose||2.2 – 2.9|
|Batts||2.9 – 3.8|
|Mineral (stone) wool||Loose||2.2 – 3.3|
|Batts||3.3 – 4.2|
|Cellulose||Loose||3.1 – 3.8|
|Natural fibers (cotton)||Batts||3.0 – 3.7|
|Polyisocyanurate||Foam||5.6 – 8.0|
|Polyurethane||Foam||5.6 – 8.0|
|Cementitious foam||Foam||2.0 – 3.9|
|Phenolic foam||Foam||4.4 – 8.2|
Source: Department of Energy
Polyethylene terephthalate (PET)
Another material that is becoming increasingly popular for insulation is polyethylene terephthalate (PET). It is a highly effective, eco-friendly material made from recycled clear plastic soft drink bottles.
Thermal PET insulation is available in various foam configurations, including rolls and panels, some of which have an extruded polystyrene core. The thermal resistance of PET insulation is excellent, and its R-values are quite high.
There are several types of insulation that were used decades ago that we rarely use nowadays. Some are often found in older buildings, though, so it’s not a bad idea to know what they are, especially if you are planning to upgrade insulation during a renovation.
Commonly used for attic insulation before 1950, vermiculite and perlite comprise very small, lightweight pellets that are used for loose-fill insulation. Sometimes they were mixed with cement to create a lightweight concrete that wasn’t as heat-conductive as pure vermiculite or perlite.
Some vermiculite insulation materials contain trace amounts of asbestos, so it’s best to leave it in place if it was used to insulate your old attic.
Urea-formaldehyde (UF) foam insulation was quite common during the 1970s and early 1980s. It uses compressed air as its foaming agent and doesn’t expand like polyurethane insulation.
When it comes to insulating your home, there are many choices in terms of performance and R-values, materials, and types.
Ultimately, the idea is to reduce your home energy use as cost-effectively as you can. Then, you can meet your reduced energy requirements by using the best possible on-site renewable energy systems. Even if you aren’t ready or able to aim for a net-zero scenario yet, you can make changes that will point you in the right direction.
There is no doubt that installing insulation during construction is considerably more cost-effective than retrofitting it once your house is complete. However, if you are renovating a house like I did when I moved to Florida, you won’t have a choice. Insulation will probably cost you a bit more than it would for new home construction, but it must be high on your list of priorities.
It’s important to insulate many different areas of your home. If you install a combination of insulated sheathing and cavity insulation in walls, ceilings, and under your floors, you will save energy and money.
You can also reduce energy leaks through exterior walls by taping the joints of outside sheathing and by caulking and sealing the walls.
Remember that you can install cellulose and fibrous insulation products up to R-15 in 2×4 inch cavity walls and up to R-21 in 2×6 inch cavity walls. If you use foam insulation or other more advanced systems for insulation, you will achieve even higher R-values.
It’s a lot of information! But, at the end of the day, your aim is to keep your house cool in hot climates and warm in cold climates, reducing both your energy usage and expenses.