Construction Design-The Anatomy of a Well-Built Wall

I’m a fan of mineral wool insulation, specifically the Rockwool brand.  Over the next few months, I’ll be writing a series of blog posts discussing tips, tricks and the tools used to work with this insulation made from rocks and steel slag a byproduct of the steel industry.  Before we get into those topics, I want to discuss the Rockwool products I use, the building science behind the how these products work in a wall and the characteristics of the insulation, all of which can create a well-built wall assembly.

Characteristics of Rockwool Insulation

#1-Rockwool does not burn.  I live and work in a rural, heavily wooded area in northern Minnesota, of course, Minnesota is known for its very cold and often snowy winters.  It’s also an area that can experience wildfires from time to time.  Fires of course can also start inside a structure.  Knowing I can choose an insulation that will not burn is a big plus when designing assemblies.

#2-Rockwool is vapor open.  Often, we concentrate on how to keep moisture out of our buildings, but how the building dries is just as important.  When designing an assembly, whether that be a wall, roof or floor system, one of the first consideration I make is how can the assembly dry.  Rockwool is vapor open, meaning water vapor can pass through the insulation.

#3-Rockwool is hydrophobic.  I know the term sounds terrible, but if you’re a building product, being hydrophobic is a good thing.  It means that moisture doesn’t create problems, Rockwool doesn’t care if it’s wet or dry.  It won’t mold or mildew.  Most water will simply roll off.  Any insulation that does become saturated, the water will eventually drain or evaporate out without any lasting affects to the insulation.  I have experienced the draining and drying of ComfortBatt cavity insulation firsthand.

#4-Bugs don’t particularly care for it.  Rodents and burrowing insects don’t like moving through mineral wool.  I know I wouldn’t want to try to tunnel through a bunch of rocks and steel slag, especially using just my fingers and teeth.

#5-An environment of comfort.  The main purpose of an insulation is to slow the movement of heat.  That’s true whether you live in a hot or cold climate.  One of the tricks to thermal comfort is to slow temperature swings inside the building, a good layer of a quality insulation is one way to achieve that.  Comfort can also go beyond thermal; sound can also be a metric of comfort.  Listening to your neighbor’s air conditioner or the traffic on a busy street can result in an uncomfortable environment.  Rockwool insulation can help with both.

#6-Lowering your energy costs.  One of the biggest factors affecting a home’s operating cost is heat loss or gain through the building materials.  The job of all insulation is to resist heat flow, it costs money to condition an environment, we want that investment to last for as long as possible.  Rockwool is no different in that regard as any of the other types of insulation, they are all designed to slow heat loss or gain, it is the other characteristics I listed above that makes Rockwool stand out.

The Science

So, how can we improve the typical wall assembly?  Most walls built in residential construction are going to have several common materials.  Cladding, some sort of water resistive barrier, sheathing, structural framing, cavity insulation and an interior wall covering such as drywall.  There may or may not be a vapor retarder also included.  Many of these building materials are moisture sensitive.  They can become wet by both bulk water leaks from the exterior and also building deficiencies in the air barrier.  A way to improve the typical wall assembly is improvements in the water management strategy on the exterior, constructing a tight envelope, and the addition of continuous exterior insulation.

Let’s talk a little why we would want to install continuous exterior insulation and the building science behind the assembly in both hot and cold climates.  First, the reduction in heat loss or gain through the wall.  Walls that are built with cavity only insulation lose heat faster through the structural wood framing.  This conductive heat loss (or gain) is called a thermal bridge.  It’s the result of wood (or metal) framing having a higher conductivity rate, heat passes through the wood easier than through the insulation.  Before I get into this next section, you’ll need a definition.  The weighted average R-value is defined as the average insulation value of an entire building section, in the case of a wall, this may include cladding, sheathing, framing, insulation, interior wall finish materials and windows, it can even include the air film against the inside and outside of the assembly.  If a traditional 2 x 6 wall is insulated to R-20 using cavity insulation only, the weighted average of the wall’s R-value after factoring in the lower R-value of the wood framing is closer to R-16.  Add in windows, and the weighted average will be much lower.  A way to boost a wall’s total R-value and reduce any thermal bridging losses or gains is by installing a layer of continuous exterior insulation.

Thermal Bridging

Another way continuous insulation can improve a wall is by maintaining the insulating performance of the cavity insulation.  By keeping the sheathing warmer during the winter, there are less air currents that form inside a wall cavity.  These air currents form when cold wall sheathing cools pockets of air inside the insulated cavities of the wall.  This cool air wants to drop.  As we move towards the warmer side of the wall cavity, the warm air wants to rise.  This forms what is called a convective loop, the movement of air inside the insulated cavity strips some of the insulation performance away, lowering the cavity insulation’s R-value.  This happens when the cavity insulation is poorly installed, with gaps between the insulation, interior wall coverings and exterior sheathing products.  Convective loops are directly related to the difference in temperature between inside and outside, called the “delta tee” along with the height of the wall.  The bigger the delta tee, and/or taller the wall the greater the convective loop.  These convection loops are reduced when the sheathing is warmed and when cavity insulation completely fills the cavity.  Rockwool can accomplish both.

Poorly installed fiberglass insulation

Added durability is another plus. In cold climates, exterior sheathing on a home without exterior insulation will become cold.  If it’s cold enough, the sheathing will become a condensing surface, any moisture that comes in contact with the sheathing will make the sheathing damp.  I’ve seen both liquid and frozen moisture inside wall cavities.  Over the course of a heating season, moisture is able to accumulate, damage to the moisture sensitive wood products can occur.

Where does the moisture come from?  It’s most commonly transmitted by air currents through deficiencies in the building’s air control layer.  Moisture can also move through building materials by way of diffusion, though at a much slower rate than by way of air currents.  We can reduce the risk of this “condensing surface” and the damage caused by water a few different ways.  Making the structure more airtight is one way, controlling the humidity level inside the building by using dehumidification is a second.  A better way is to warm the surface of the sheathing above the condensing temperature.  We do this by installing enough exterior insulation.

Moisture on exterior sheathing caused by an air leak.

An important factor when adding exterior insulation, there is a relationship between any cavity insulation and continuous exterior insulation, this relationship is needed to assure the wall sheathing temperature stays warm enough, so the sheathing does not become a condensing surface.  The ratio is dependent on the amount of cavity insulation and the expected intensity of the winter.  This “intensity” is expressed in the building community as the climate zone, the higher the number, the colder the climate.  The more cavity insulation present in a wall, and the colder the climate, the more exterior insulation is required.  The chart shows the ratios.  My area, climate zone 7 requires a 45% ratio of exterior insulation compared to 55% of cavity insulation.  R-20 cavity insulation will require a minimum of R-15 continuous exterior insulation.  This is a common 2 x 6 framed wall assembly.

Climate Zone Map for the US

The Rockwool products I use

I’ve used a few different Rockwool products, with varying thicknesses and R-values.  ComfortBatt is the product I’ve used most often.  This is the typical cavity insulation, it comes in sizes to fit 16” and 24” wood framed structures, it’s also available in slightly wider widths for use with steel stud framing.  The batts are 48” long and available in many different R-values, from R-13, which is a 3.5” thick product, all the way up to the 9.25” R-38.  What I like most about ComfortBatt is its density.  You can stand a single batt on end without it falling or slumping over.  This insulation density allows for a very high installation grade.  I’ll talk a little more about insulation installation grading in a future post.  The insulation cuts easily with a serrated blade and can be easily shaped to fit around electrical boxes, plumbing pipes and other objects found inside wall cavities.  The photo is from our recent barndominium project, we stacked ComfortBatt inside walls that had framing six feet on center to a height of 18 feet.  I wouldn’t have tried to insulate that structure with any other product.

I’ve also used Rockwools Safe’n’Sound insulation, which is used for, as you guessed, sound control and is typically installed on interior walls and between floors.  Because Rockwool doesn’t burn, Safe’n’Sound can also help with fire.  This insulation also comes in various widths, both for wood and steel framing, but only two thicknesses, 3-inch and 6-inch.  Safe’n’Sound has no listed R-value, it’s not rated as an insulation for heat loss, it’s used only for fire and sound control.  It feels and installs just like ComfortBatt.

One of the most impressive product displays I’ve seen is Rockwools sound tunnel they display at the International Builders Show.  It’s basically a hallway with walls and ceiling insulated using Rockwools Safe’n’Sound.  A trip through the tunnel with the noise of a very busy trade show going on around, the noise reduction is impressive.  I’ve used the Safe’n’Sound product with success on both walls and between floors.

Rockwool’s Safe”n”Sound sound tunnel at the International Builder’s Show in 2019

The last product from Rockwool I’ve used is ComfortBoard.  This is a rigid insulation product that comes in sheets and thicknesses of various sizes.  I’ve most often used the product for continuous exterior insulation, but we also used it for interior insulation on the barndominium project.

There are a couple different densities of this insulation, they are listed from Rockwool as ComfortBoard 80 and 110.  The numbers indicate the insulations density, 80 is 8 lbs/ft³ and 110 is 11 lbs/ft³.  The ComfortBoard 80 is more common for the residential market.  Thicknesses range from 1-inch, R-4.2, all the way up to 5-inch, R-21.  Common sizes range from 2’ x 4’, 4’ x 6’ and 4’ x 8’.  Because of the weight of the product, the 4’ x 8’ sheet thickness tops out at three inches.  The four and five thick versions are only available in the 2’ x 4’ sizes.  One advantage ComfortBoard has over foam rigid insulations, it doesn’t shrink.  It’s best to install foam insulations in a couple layers staggering the seams because that type of insulation can shrink in both width and length.  With ComfortBoard, one layer is all you need.  I will be writing a future post on working with ComfortBoard, especially the tricks and tips of installing as a continuous layer of exterior insulation.  Stay tuned for that blog post.

There are a lot of different ways to build a really good wall assembly.  They all share four common traits, they are good at managing moisture, both from the interior and exterior, they all have the ability to dry if they do become wet, they are well air sealed, and all have better than code minimums of insulation, the insulation strategy can include exterior insulation, which is my preference.

This post contains a lot of information about products manufactured by Rockwool, I wouldn’t be promoting their insulation if I didn’t think they were the best fit for my projects.  Have a question about Rockwool?  If I can’t answer it, I’ll put you in touch with someone who can.  Stay tuned, more to come on working with Rockwool insulation.

Leave a Reply

Your email address will not be published. Required fields are marked *