Getting Attic Insulation Right

Online forums tend to offer half-truths about how to properly insulate an attic. Here’s a factual update from years of research and expert insight.

This article takes a deep dive into the insulation category, looking at myths, research and best practices.

If you’re expecting to have your questions about attic insulation answered in online public forums, prepare for disappointment. Social sites such as Reddit publish regular commentary about the best (and worst) practices for installing and adding attic insulation.

Some of the advice is good. But much of it exaggerates or repeats myths and misconceptions, particularly about cellulose insulation. 

To set the record straight, I reviewed several years of building research and also asked for insights from tech experts at three large manufacturing firms—each specializing in different insulation materials--mineral wool, fiberglass or cellulose. I’ve organized the outcome into three buckets:

1. Mismatched Materials. I’ll respond to some common questions about how and when to layer in different types of insulation in attics, in a section called “The Alchemy of R-Values.”
2. Settling Rumors. Negative statements about cellulose, arguably the most “organic” of insulation materials, will be put to rest in “Settling the Dust Rumor.”
3. Will it Burn? Let’s set the record straight on the fire safety and performance of different blow-in materials, in a section called “Flame Factor.”

After you reach a certain maximum depth of various insulation materials, they no longer improve performance, in part due to compression. The benchmarks shown at right are ENERGY STAR targets, above and beyond most building codes.

The Alchemy of R-Values

Homeowners often ask whether different types of insulation can be combined effectively in attics. For example, should loose fill cellulose be “stacked” over fiberglass batts? What about over mineral wool? If the batt installation is not perfect, will the blown in material fill in the cracks and improve the overall R-value of the attic enough to warrant the extra cost?

The short answer is yes, but the amount of blow-in has a limit, as I’ll explain: First, here’s Laura Woodford, VP of marketing at Greenfiber cellulose.

“There is not an issue at all with mixing Greenfiber cellulose with other types of insulation,” she says. “You can refer to this blog on blowing [our product] over fiberglass for more details.”

By “mixing,” of course, she means layering, not literally mixing the two together. In the referenced article, Greenfiber’s team asserts that: “By adding a few inches of blow-in cellulose insulation on top of fiberglass insulation, you will make the home significantly less vulnerable to uncontrolled air infiltration traveling between the attic and living space.”

I’d like to zero in on two details from Greenfiber’s overview of this practice. 

First, they make clear that you should not install the cellulose wet, nor should you add it to insulation that is wet at the time. It’s also important not to overinsulate, in an attempt to improve R-values. I’ll explain why shortly.

Antoine Habellion, technical director for Rockwool North America (a maker of mineral wool insulation), offers the same caveat about stacking different types of product. 

“If a denser product is installed over a less dense product,” he says, “the latter may compress, resulting in a loss of performance.” Mineral wool, it should be noted, is somewhat denser and heavier than either loose pack cellulose or blown-in fiberglass.

Francis Babineau, technical advisor for Johns Manville insulation, maker of fiberglass products, issues a similar caveat related to insulation density. His advice is even more specific. Layering should follow a clear hierarchy.

“When it comes to layering attic insulation materials, you want to be sure not to accidentally compress the products,” he says. “I would only put lighter density materials on top of heavier materials.  Blown-in fiberglass could go over either fiberglass batts or cellulose.  Cellulose could probably go over fiberglass batts, but not over fiberglass loose-fill. Fiberglass batts should really only be added over other batts.  If existing batts are not perfect, new blown-in insulation will help fill in any gaps.”

So what’s the last word on layering? Take a look at the Energy Conservation Code climate map for insulation below. If you’re adding to fiberglass batts to hit these benchmarks, make sure the batts are of the higher density variety, typically 2.0 to 3.0 PCF (pounds per cubic foot). This exceeds the density of all of the most common loose fill materials, as indicated by the Comparison Table below.

Experts say less dense materials should stack on denser one. High density fiberglass batts (not shown in table) have a higher density than any of these and can form the bottom layer without excessive compression.

As a hypothetical case study, consider an attic that currently has high density R-19 batts installed between rafter joists, located in the Northeast. You want to increase the R-value to R-30, raising it by R-11.

Assuming an R-value of about 3.1 per inch for cellulose, you would add about 3.5 inches on top of the R-19 batts without undue compression. With loose fill fiberglass (about R-2.7/ inch), you would add 4 inches, and with mineral wool (R-4/inch), you would add about 2.75 inches to achieve optimal R-value. 

 In every scenario the overall performance of the ceiling should hit the desired R-value. 

Recommended R-values for homes by region. Source: IECC

Settling the Dust Rumor

One of the “memes” that tends to occur in social forums is the belief that cellulose will settle, turn to dust, and age too rapidly. Both case studies and company experts say these criticisms are ungrounded.

Laura Woodford asserts that Greenfiber cellulose “does not break down. It lasts for the lifetime of the home, and we have a lifetime warranty (details here) to guarantee the quality of our product, if installed properly.”

Fiberglass and mineral wool do have some physical advantages over cellulose. They’re made from “inert” materials with the exception of a small amount of binder. But cellulose has the edge when it comes to sustainability and in some cases, installation “forgiveness.” It’s a product that finds its way into the smallest cracks and crannies, and is quite dense. That density may be where it gets its “dust” reputation. Another source may be the misinterpretation of building science research.

For example, I read this technical report on the properties of cellulose under various temperatures and humidity. 

Researchers found that cellulose insulation does not appear to suffer significant loss of mass at temperatures less than 210°C, a set point way above what you would find in any residential attic. I’ve never heard of an attic exceeding about 66°C, or 150°F, even in a desert setting. Also, they discovered that borate additives improve the response of cellulose under high temperatures, extending its durability. 

The report notes that: “During the thermal stress of the cellulose insulation in the temperature range from 50°C to 550°C, three regions of its mass loss occur. The first phase, characterized by temperatures below 120°C, is caused by drying. The second phase, the main one, occurs in the temperature range from 250°C to 380°C. During this phase, the basic component of cellulose insulation –cellulose – decomposes. The third phase (about 450 °C) can be described as the reaction of carbonaceous residue with oxygen in the air. This phase is not clearly bounded.” 

As for other concerns about cellulose, such as vulnerability to insects, companies including Greenfiber offer borate infused products. “We just launched a new product, Sanctuary Defense, Woodford says, “that is treated with an EPA registered treatment that kills 25 different types of pests when they come in contact with the insulation.” 

Will it Burn?

Let’s begin with some basic statistics: About 5-7 percent of house fires start in the attic. Of these, a large percentage can be traced to old knob and tube wiring or faulty HVAC equipment. In other words, it’s not the insulation causing fires. It’s whether or not the insulation has some resistance to system failures. All categories of blown-in insulation have been made safer and more fire resistant in recent years, in tandem with better electrical and HVAC safeguards.

Only a small percentage of fires actually start in the home’s attic and often old wiring is the cause.

“The amount of binder or other potentially combustible materials added to fiberglass insulations are usually in small enough quantities that they don't impact the fire performance of the products,” notes Babineau. “This can be shown just by looking at product data sheets for fiberglass insulations, which are often listed as noncombustible and/or as materials with very low flame spread and smoke developed when tested to standard industry fire tests.  This fact sheet talks about insulation and fire safety.”

Here are a few fire-ready improvements related to all three types of loose-fill insulation:

• Modern fiberglass insulation uses non-toxic fire-retardants and resin binders that reduce air gaps and slow flame spread, and it melts rather than burns at high temperatures. 
• Mineral wool, naturally fireproof due to its rock-based composition, has become even more effective with denser, more uniform fibers that enhance its fire-blocking ability. 
• Cellulose insulation, once prone to flammability, is now treated with up to 20% borate fire retardants and tightly packed to create an oxygen-poor layer that chars rather than ignites. Improvements in attic mechanical systems and wiring have also enhanced fire safety. Modern electrical wiring uses fire-resistant coatings and often includes GFCI protection to prevent sparks, while outdated knob-and-tube systems and older PVC coatings have been phased out due to fire hazards. HVAC systems now feature fire-rated ducts, intumescent seals, and automatic shutoff switches that prevent blowers from spreading smoke or flames. 

And finally, building codes have evolved, requiring fire-resistant conduits and barriers in attics to slow fire spread and improve safety. In short, although they vary in their natural and enhanced fire resistance, all blown-in insulation types now meet Class A fire ratings.

The “Perfect” Attic

Over the years, building scientists (and contractors) have learned a lot about how to do attic insulation better. But along the way, a debate has been raging about whether vented or unvented attics are best, whether spray foam should be applied on the underside of roof decks, and whether so called “hybrid” systems of insulating are the best. 

Those are big questions for future articles, but before you tackle your next attic insulation job, here are a couple of other things to consider.

Know Before You Blow. When you apply loose fill insulation, you make future lighting or duct changes a little harder and messier.  With batts, the insulation material can simply be lifted or rolled back without much fuss, to make way for new recessed can or a wifi damper. 

There is a workaround, however. If you know an upgrade will happen later in a certain part of the home, why not pre-emptively install a junction box above the depth of the blow-in product. You can’t bury a junction box, but as long as you can access it and it has a cover, it should make your inspector happy.

Code requires that junction boxes in attics remain accessible. If a blown-in product is added, it’s wise to install junctions where future electrical work is likely to take place. Image source: Greenattic.com

Convection Inflections to Consider. Do you live in a zone that flipflops wildly between hot and cold temperatures such as Salt Lake City? If so, you may want to read this older article from Oak Ridge National Laboratory, titled “Thermal Performance of Fiberglass and Cellulose Attic Insulations.”

In it, researchers found that one type of loose-fill fiberglass insulation showed dramatically worse performance under large temperature differences of 70°F to 76°F. Their effective R-value was about 35 to 50% less than at small temperature differences.” 

Why? Warm air from indoors was rushing up through the equivalent of the ceiling (this was a lab test) and out. 

The good news? “The additional heat flow, attributed to natural convection, was effectively eliminated by applying a covering of fiberglass batts or a combination of a polyethylene film and fiberglass blankets. No significant convection was found either with fiberglass batts or with one type of loose-fill cellulose.” 

In other words, by simply increasing the R-value of the loose fill, and/or also applying an air barrier, they were able to arrest the unwanted performance loss.