Answering the question: What makes insulation effective?

In the countless conversations I’ve had about insulation over the past 20 years, I am often asked about which insulation is the cheapest, or which is the “best” in one regard or another. R-value is another common topic. But rarely am I asked about which insulation is the most effective. Maybe that’s because we don’t know what we don’t know. If that is the case, then this article will help create a framework to understand insulation.

There is more to effectiveness than just insulation R-value, or air tightness.

To understand what makes insulation effective, we need to look at it as a “system” rather than a product.

For any insulation system to work, it needs to control energy flow through the building envelope, as well as moisture flow. And then ideally, if the walls or roof get wet somehow, it needs to allow for these systems to dry out.

With this understanding, the first thing we need to look at are the challenges presented by the local climate and how they influence energy flow through our buildings. What works in Miami, Florida will be different from what will work in Phoenix, Arizona or Ashland, Wisconsin.

Here in Northern Wisconsin, we have a cold-to-very-cold climate that is heating dominated. This means that the primary energy flows through our homes are from the inside to the outside. Heat flows from warm to cold. Water vapor flows from areas of higher concentration to lower concentration, and also from warm to cold normally. Consequently, during our heating seasons, water vapor wants to flow from inside the home to the exterior.

The other thing we need to control is air flow through our building envelope. This depends on the three components of an insulation system: conduction, convection and water vapor diffusion retarders. If any of these components goes unaddressed, problems are sure to show up very soon.

So how do we evaluate an insulation system’s ability to address these concerns? Let’s take them one by one.

Component 1: Conduction, or heat flow.

To control heat flow, we need something that can reduce the amount of conductive heat loss through the wall, roof, floors and foundation of the house. All insulations we normally use in a house insulate against heat loss by trapping air within the insulation. When that happens, the air itself is what actually insulates. It works well as an insulation because it’s a poor conductor of heat. The approximate maximum R-value we can achieve with insulations that trap plain ordinary air is R-4.2 per inch of thickness.

There are insulations that achieve a higher R-value per inch of thickness, like Extruded Polystyrene foam. This is the typical blue, pink, yellow or green foam board you can buy at your local lumber yard. Because this foam has an insulating gas in the tiny pores of the foam, it can typically achieve an R-value per inch of 5.

The largest challenge to overcome with any insulation, though, is not the R-value per inch limits, but the ways that the heat in our homes wants to sneak around this insulation.

This brings us to the notion of a system. The fact is, no insulation is a complete solution in and of itself. It works only as well as the system of which it’s a part. For example, fiber insulation alone does not have the ability to stop air flow through itself. That is why we must use a polyethylene plastic sheet over the insulation to stop air from freely blowing through the insulation.

Component 2: Air flow, or convection

Just because R-21-rated insulation is installed in a wall does not mean that it is actually providing R-21 performance. We have to trap the air within that fiber insulation before it can provide the R-value it is capable of supplying.

The ability to trap air is the primary role of what has traditionally been referred to as a “vapor barrier.” The most important thing provided by this vapor barrier is not stopping water vapor, but stopping air from flowing through the wall and ceiling assemblies. If we do not stop the air from flowing through the wall assemblies, then the fiber insulation is only filtering the air and is no longer insulating. You may have heard insulation contractors joke about “filter glass” insulation. That’s what they’re referring to.

So, is spray foam the “magic bullet” that solves the air flow problem? No.

Spray foam insulation must be installed as part of a system as well or it will not work effectively. Every home has multiple joints and connections that cannot be effectively sealed by spray foam. Failure to address them will wipe out any performance gains from using a spray foam.

An example: Every home has framing joints that cannot be sealed with foam. Gaps from 1/16” to ½” in width typically have to be addressed with something else. In most cases, the best product for this is a good flexible caulking like an elastomeric sealant. It will fill in and seal these framing joints. Even as the wood flexes and moves seasonally, an elastomeric sealant will bend, stretch and squeeze along with it, maintaining its seal.

Air leaks are a major source of heat loss for most homes. Air can transport a significant amount of heat and water vapor. Because of this, it’s essential that an insulation system work to inhibit air leaks. As warm, moist air flows through the walls or roof of a home, it will at some point hit colder surfaces. Moisture in the air cannot stay suspended as the air cools and the air molecule shrinks. Imagine the warm air as a sponge that has significant capacity to soak up water. When the air cools, it’s as if the temperature is squeezing the sponge and forcing the water out. This is an issue for wall and roof systems, not because water vapor causes problems, but because the liquid that’s produced as it cools can cause mold and fungus growth. Fungus (rot) eats lignin, the part of the wood that gives it strength. Over time and repeated wetting cycles, the structure can be damaged and destroyed simply as a result of moist air being transported through leaks in the walls and roof.

Component 3: Water vapor diffusion

If we have the R-value right and a continuous air seal in place, then addressing water vapor diffusion is very easy. In fact, it’s the simplest part of the system to have in place. We can achieve a vapor diffusion retarder through the use of paints, a polyethylene sheet, coated papers, or even some insulation materials. If we have a continuous air barrier, then it is not critically important to have a continuous vapor retarder. We just want to slow down the diffusion process somewhat to limit the amount of moisture content that builds up within the wall and roof materials. The challenge with completely shutting off all vapor diffusion is that we then eliminate any ability for the assembly to dry to the inside during the summer months if a bad rainstorm were to cause a wall or roof to leak.

To address this changing need for a vapor diffusion retarder, we prefer to use “smart” vapor retarders like Certainteed Membrain or Intello Plus. Both products have directional vapor permeance, which restricts vapor flow into the wall. However, if the wall gets wet, then the pores open up to facilitate drying as rapidly as possible. This gives a structure the most resilience we can possibly plan into it.

The Answer to “Effectiveness”

We started out by asking which insulation is the most effective. Knowing now that we need to address the three components of a solution – heat flow, air flow and water vapor diffusion – we can see that insulation can only do its job as part of a system. That requires choosing not just the right insulation product, but complementary sealants and vapor retardants as well. Choose the right system and install it properly, and you’ll not only achieve your energy efficiency goals, you’ll keep the structure safe from the forces of nature that would seek to destroy it. That’s how we define “effective.”