When it comes to insulation, R-value is often the first metric people consider. While this measurement is crucial, it doesn’t really tell the whole story. When discussing insulation there is a better method to assessing the performance of an insulation product. The effective R-value, which tells how well an insulation material performs in real-world conditions, is what truly determines energy efficiency and comfort.Â
In this article, we’ll explore why effective R-value matters more than nominal R-value, and how, as a result, cellulose insulation often outperforms fiberglass in practical applications, despite having similar nominal R-values on paper.
Understanding R-Value: Nominal vs. Effective
What is R-Value?
Short for Resistance Value, R-value is a measure of a product’s thermal resistance, or how well an insulation product resists the flow of heat or cold through it.
R-Value is determined by a laboratory test in which an insulation material is sandwiched between a cool and a warm surface. The ability of the material to resist temperature changes results in an R-Value for that material. The higher the R-value, the better the insulation should perform—at least in theory. Of course, depending on the insulation type (fibrous or foam plastic), the maximum R-value may vary.Â
For example, fibrous insulations like fiberglass and cellulose have what appear to be lower R-values than foam plastic insulations. This is because fibrous insulations use air trapping to insulate. As a result, fibrous insulations cannot have an R-value that exceeds R-4 per inch, as air only has thermal resistance up to 4 per inch (or R-4). If a fibrous insulation is shown to have an R-value per inch that is higher than that of air, be wary! Chances are that product either doesn’t perform as it should, or the manufacturer is being dishonest in their marketing of the product. A fibrous insulation that generally performs well should usually come close to air’s per inch resistance value without exceeding it.
Nominal R-Values of Fiberglass and Cellulose
The nominal R-values for two common fibrous insulation types is as follows:
- Fiberglass insulation typically has an R-value of around R-2.9 to R-3.8 per inch for batt insulation and anywhere between R-2.2 to R-2.9 for loose insulation.
- Cellulose insulation generally has an R-value between R-3.1 to R-3.8 per inch for loose and sprayed insulation.
Of course, R-values vary depending on the insulation brand or manufacturer, as well as the application method used. But, at a glance, these values suggest both materials offer comparable thermal resistance, and perform similarly. However, real-world conditions can actually change their performance significantly. That’s where measuring the effective R-value comes in.
What is Effective R-Value?
A machine in a laboratory gives a relative number (nominal R-value) that can be used to compare products, but a laboratory R-Value does not provide the entire story about the effectiveness of those products.
Insulation is subjected to a wide range of temperature conditions in a building. Effective R-value refers to how well an insulation material performs when installed in actual building conditions. Factors such as air infiltration, proper installation, insulation density, and settling/gaps all impact an insulation’s true thermal resistance over time.Â
To best assess how an insulation will perform, we must consider its nominal R-value alongside its effective R-value to see the bigger, long-term picture.Â
Why Cellulose Insulation Outperforms Fiberglass Insulation
While fiberglass and cellulose may have similar nominal R-values, cellulose consistently provides better real-world insulation performance as a result of its effective R-value. Here’s why:
Cellulose Prevents Air Infiltration Better than Fiberglass
Due to its lower installed density, fiberglass allows significant air movement, which reduces its effective R-value. Even small air gaps around fiberglass insulation can lead to heat loss and drafts.
In contrast, cellulose insulation is denser and better at filling gaps, reducing air infiltration and improving energy efficiency. Studies have shown that cellulose can reduce air infiltration by up to 30-40% compared to fiberglass. Studies also show that in otherwise identical builds, the cellulose insulated build performs better overall than the fiberglass insulated build.
For a demonstration of this comparison in action, see the video linked below:Â
Cellulose is Less Prone to Installation Error
In real-world construction, wires, pipes, electrical boxes, and other obstructions exist in the wall cavities of a building. Fiberglass batts rely on proper installation in order to achieve peak performance. If an insulation installer doesn’t take the time to meticulously cut around every obstacle in a wall cavity and ensure an airtight fit, then fiberglass batts are prone to having small gaps or air pockets in areas of the wall. This can lead to heat loss over time as air is able to bypass the insulation.
Because blown-in cellulose fills cavities completely, it prevents gaps and air pockets that are common with fiberglass batts. This allows the home to maintain consistent indoor temperatures, preventing heat loss and helping save energy.
Cellulose is More Environmentally Friendly
If you’re looking for a sustainable insulation option, cellulose is the clear winner. Most cellulose is made from up to 85% recycled paper, with Nu-Wool Premium Cellulose Insulation being made with 86% recycled paper content. Fiberglass, on the other hand, is produced from energy-intensive glass fibers. It also requires the consistent use of natural resources, whereas cellulose is made from already existing paper products that have been recycled for a new purpose. Plus, cellulose is a carbon-storing material due to being made from recycled paper.Â
Additionally, cellulose insulation requires less energy to manufacture. In fact, manufacturing cellulose insulation uses 10 times less energy than what it takes to manufacture fiberglass. This results in cellulose producing less carbon emissions.
Another added benefit to cellulose insulation is that it’s a zero waste insulation product. When installed, any excess cellulose can be vacuumed up to be reused for other job sites, ensuring no product is wasted. And, given that it is made from paper, any excess in the manufacturing process can be recycled right back into the lines. The same cannot be said of fiberglass, in which wasted product is usually sent to landfills.Â
Cellulose Performs Better Over Time
When it comes to longevity, cellulose also beats fiberglass in this area. When installed properly, cellulose insulation will last the entire lifetime of the structure without needing to be replaced. The same cannot be said of fiberglass, which often requires retrofitting over time in an attempt to preserve its effectiveness.
The Bottom Line: Effective R-Value Matters Most
When choosing insulation, don’t just look at the listed R-value on the label. Consider and research how the material performs in real-world conditions.
For homeowners looking to maximize energy efficiency and comfort, especially long-term, cellulose insulation is often the better investment. While fiberglass may be cheaper in many cases, you kind of get what you pay for. Cellulose in most cases is very competitively priced and won’t break the bank in exchange for better overall performance.Â
If you’re considering upgrading your insulation, consult with a professional, certified insulation contractor to determine the best material for your home. By choosing insulation with a high effective R-value, you’ll enjoy lower heating & cooling bills, a more comfortable home, and better long-term performance.
