Construction Materials-Cavity Insulation

Heat wants to move from someplace hot to someplace cold.  It’s desire to reach equilibrium is one of the principles of the second law of thermodynamics.  We have many methods and materials we use in construction to try to slow this movement.  It’s expensive to condition a space and we want to hold on that space conditioning for as long as possible.  One way we try to slow heat loss or gain is to prevent the wind from blowing through the home.  Another is to shade the sun from beating through a window on a hot, sunny day (in some climates at certain times of the year, the sun can be a blessing).  Insulation is one of the big ones we use to provide comfort to homeowners.

Cavity insulation is a product that provides thermal resistance that is placed between the framing members of the structure, typically on exterior walls.  I’ve worked on a few older homes that had old newspapers stuffed in the walls, I’ve also found old clothing used as cavity insulation.  More often than not, the oldest homes still standing originally had no insulation in outside walls.  To maintain comfort, homeowners used more energy, energy used to be cheap.  Past generations of homeowners were also more tolerant of temperature differences inside the home.  If a room was cool, they just put on another layer of clothing.  As a kid during the summer, when the house was too hot, we went outside to play.  We spend more time inside our homes today, homeowners have higher expectations with regards to comfort.

What is an R-Value?

Before we get going discussing the different types of insulation used in wall cavities, we should first talk about how insulation values are expressed.  According to Wikipedia, “the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat.”  The higher the R-value number, the slower the movement of heat. 

R-value is not the original metric we used to show this resistance to heat, U-value was first used.  Back to Wikipedia, “Thermal transmittance is the rate of transfer of heat through matter.  The thermal transmittance of a material (such as insulation or concrete) or an assembly (such as a wall or window) is expressed as a U-value.  The concept of thermal transmittance is closely related to that of thermal resistance.  The thermal resistance of a structure is the reciprocal of its thermal transmittance.”  So, in other words, U-value is the inverse of the R-value.  A code built; 2 x 6 wall might have a U-value of .05.  To convert U-value to R-value, the equation of 1/U-value = R-value is used.  1/U-.05=R-20.  The lower the U-value, the higher the insulation value.  U-value, the original way resistance to heat flow was expressed, was confusing to many people when the metric was first developed, having a lower number meaning a higher resistance to heat flow, more insulation, was counter intuitive, so the change was made to make the higher number equal more insulation, the change to R-value was made.

Types of Cavity Insulations

Most cavity insulations used are the fluffy and fibrous types.  They are lower cost, quick and easy to install, and provide a decent resistance to heat flow.  They include fiberglass batts, mineral wool batts and blown cellulose.  There are other, less common options such as hemp batts made from the fibers of the hemp plant, an insulation made from used blue jeans, and even actual wool insulation from sheep.  One drawback to fibrous insulations, they provide little to no air control.  The air control layer will always need to come from another product when using fiber insulation products.

Fiberglass is the most commonly used insulation product in my very cold climate, the opening photo is a fiberglass insulation installation on a project I was involved with from a couple years age.  This type of insulation has been around for decades, is easy to install and newer versions tend to be less irritating to the skin and lungs.  R-values for fiberglass range from around R-3 to more than R-4 in the denser options.  It is available in many sizes, ranging from 3.5-inch to 11.25-inch-thick batts.  There are also many width options to accommodate 16-inch on center framing, 24-inch on center framing, as well as different sizes when using metal studs as framing.  Fiberglass insulation is also available with a factory installed paper facing.  These kraft faced batts provide the required vapor retarder for certain climates, though the facing does not provide much in the way of air control.

Mineral wool batts are my go-to cavity insulation, specifically the Rockwool brand.  The product is manufactured by heating basalt, which is a rock, and steel slag, a biproduct of the steel industry and spinning the materials into a fiber.  The fibers are naturally water resistant, heat and burn resistant and bugs and rodents do not care to tunnel through the insulation.  The biggest reason I like the product is the density.  Mineral wool will fill a cavity without needing to be “fluffed”, one of the more important factors when installing any insulation is the quality of the install, more on that in a bit.  R-values are slightly higher than fiberglass, R-15 2×4 cavity and R-23 2×6 cavity insulations are common though lower cost R-13 and R-21 versions are also available.  Air control layers and, in colder climates, vapor control layers are required when using all fibrous insulations.

Rockwool’s R-30 cavity batt installed at the Barndominium project.

Blown cellulose is arguably the most “green” of all the insulations used in wall cavities.  The process involves using a netting or other retaining material fastened to the wall studs, poking a hole through that material, then blowing insulation into the cavity by using a special insulation blower able to achieve a specific density.  Density needs to be at least 3.5 lbs./ft³ to prevent settling at the top of the wall.  If this settling occurs, a cold spot and moisture issue can cause building durability issues.  I have yet to find an installer in my area that is comfortable installing blown cellulose in wall cavities.  This lack of experience in working with dense packing walls is not widespread, many areas of the country and across the world commonly use this fibrous product as cavity insulation.  R-values are in the R-3.6 to R-3.8 range.  Costs fall between fiberglass, which is one of the cheapest insulations, and the other options.  The installation can result in a lot of dust being blown into the air, for that reason breathing protection is needed.

Another cavity insulation option are the spray foams, open cell and closed cell. 

Open cell polyurethane spray foam is a low-density two-part, chemically made insulation, typically having a density between .5 and 1.4 lbs/ft³, that is sprayed into the wall cavity.  The product expands to fill any voids, typically expanding past the wall framing.  The installer then comes through after the foam has cured and shaves off the excess foam flush to the wall studs.  Open cell spray foams are more porous and can more easily move water and water vapor through the insulation than the closed cell foam option, because of this, they require some type of vapor retarder on the warm in winter side of the wall assembly in cooler climates.  Open cell spray foams do provide some air sealing benefits, though not as much as the closed cell option.  R-values are around 4 per inch.  I have only seen a few homes in my very cold climate use open cell spray foam.  Cost is less than closed cell spray foam.

Closed cell polyurethane spray foam is very similar to open cell in that it is a two-part, chemically made expanding insulation product.  One big difference is its density, typically 1.5 to 2.4 lbs./ft³.  At this density, the insulation produces significant increases in rigidity for the structure.  R-values also increase over closed cell, R-6 to R-6.8 per inch though the R-value will slightly decrease over time.  Another advantage is closed cell foam can used as an effective air barrier and, if installed in a thick enough layer, will also become a vapor retarder.  Installation typically does not completely fill the wall cavity and thus does not need to be trimmed to accommodate the wall finish.  Closed cell spray foam is the most expensive of the common products used as cavity insulation and is typically installed only by trained insulation contractors.  Another negative is it tends to not be good for the environment.  Both open and closed cell spray foam applications will require full personal protection gear, breathing, eye and the need to wear a protective suit.

Closed cell spray foam used to prevent air from moving into a vented attic and providing additional R-value at the eave of a roof.  This area and rim joists are two areas where I regularly use closed cell spray foam.

How much is required?

The amount of cavity insulation required to meet code is specified in the energy code section of the International Residential Code book, specifically section R402.1.3 of the 2021 IRC.

The values we are looking for are in the wood framed wall R-value column highlighted in yellow.  The first number refers to the cavity insulation R-value requirement, the second number is for any required continuous exterior insulation.  So, climate zones 6, 7 and 8 require R-20 cavity insulation plus R-5 continuous insulation, or R-13 cavity plus R-10 continuous, or you can just install R-20 continuous.   The table shows the requirements for the different climate zones, Again, these values are in the 2021 IRC, local codes may have different requirements.

Insulation Grading

How well insulation is installed in a wall cavity, or any area will greatly affect the performance of the product.   A good example of this is insulation that does not fully contact all six sides of a cavity, the top of the bottom plate, the bottom of the top plate, both side of the framing lumber, the exterior sheathing and inside wall finish.  Small gaps or not being in complete contact with these surfaces can result in air movements inside that cavity that can strip the resistance to heat flow, resulting in loss of performance and the possibility of moisture accumulation due to cold temperature surfaces condensing moisture.

There is a grading system for rating insulation installations, a HERS rater and others will use this system to assess the quality of the wall cavity and other areas of insulation installations.  A grade I installation is the best, in the case of fibrous cavity insulation, there can be no more than 2% compression no more than 2% in voids, no voids are allowed to extend from the interior to the exterior.  In other words, there can be only very minor installation defects.  A grade II evaluation can have not more than 15% compression and no more than 15% voids with none extending from the interior to the exterior.  A grade III install is considered low quality with 2%-5% of the insulation missing or voids extending from the interior to the exterior.  The rating system is subjective and is performed by visual inspection.

Cavity insulation is one of the ways we try to control the movement of heat.  In addition to cavity, there is the option, and in some areas, requirement to also add a layer of continuous exterior insulations which we talked about in the How Much is Required section.  The keys to cavity insulation, pick the type and thickness that best fits your needs, and then install it well.  Once the wall coverings are in place, it becomes difficult and expensive to make changes.


One Reply to “Construction Materials-Cavity Insulation”

  1. Great article on the common insulations. It would be great to hear more about the uncommon ones, maybe in a future blog post. ( the ones I know of are Blown-in fiberglass, wood fiber batts, sheep’s wool, recycled denim batts, hemp fiber insulation, and my favorites, straw, and hempcrete.)

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