For high-performance structures relatively high R-values for insulation (at least when compared to current building code requirements) are required from the foundation all the way up to the attic (e.g. Passive House or The Pretty Good House).

After some initial research and product pricing, we knew we were going to predominantly use Roxul (with its recent name change, it’s now known as Rockwool) for our insulation needs. But after realizing blown-in rock wool wasn’t available (at least at the time anyway — presumably this will change in the future), and that batts didn’t make much sense for this application (too costly, and they’re considered more difficult to install properly), we knew we wanted some kind of blown-in insulation. The main options, currently, are fiberglass and cellulose.

Fiberglass vs. Cellulose

Best Attic Insulation

We wanted to avoid foam as much as possible throughout the build, both because of its  environmental impact and the fire risk associated with its use, so we didn’t consider spray foam as a real potential option.

After evaluating blown-in fiberglass and cellulose, we decided that cellulose made the most sense for us.

The next decision was to figure out how much, meaning how many inches did we want to blow into the attic. Our first builder was going to do R-49, which is the current code minimum standard here in Illinois. At the time, even before things went horribly wrong with this builder, this felt like too little. I had read stories about other Passive House projects using significantly more, but many of these were in even colder climates than ours (we’re in climate Zone 5 here in the suburbs of Chicago).

How much do I need?

How much insulation is enough?

We decided that rather than settle on a hard R-value as our goal, we would just do a solid two feet of cellulose since we would be doing the installation of the material ourselves (less out near the 12″ raised heel trusses on the north and south sides of the house). There wasn’t a significantly greater cost in materials to go from an R-49 (just under 15″) to the approximately 24″ we blew into the attic.

After doing a little research, and speaking with a Passive House consultant and a local general contractor who consulted with us on various issues as they arose, the consensus seemed to be that attic insulation was an easy, relatively inexpensive place to sneak in more R-value, which is particularly beneficial in our predominantly cold weather climate (the ceiling/attic is where a significant amount of conditioned air wants to escape in the winter anyway). The blown-in cellulose, like the Rockwool, also has some nice sound deadening qualities as an additional benefit.

How much insulation do you need for Passive House?

The cellulose brand in our local Home Depot is GreenFiber, so that was the product we ended up using. Their product is DIY friendly, even allowing homeowners to rent machines for the actual installation:

We started out with 200 bags delivered to the job site. We assumed we were going to need more (the GreenFiber insulation calculator suggested we would need 250 bags to reach 2′ throughout the attic), but thought it might be easier to estimate a final total once the first 200 bags were installed.

The boys, who helped us with various grunt work chores throughout the project, were nice enough to return and help us bring the bags of insulation indoors the night before we started the installation in the attic. We set up a bucket brigade between the driveway and the kitchen, so it went pretty quickly.

On the day of installation, getting everything set up and started was fairly straightforward. Apart from a loose hose connection at the machine, which a small strip of Tescon Vana tape rectified, we had no issues with the blower. While my wife fed the bags of cellulose into the blower, I was up in the attic directing it into place.

The first couple of hours were actually kind of fun, but getting a consistent two feet of insulation throughout the attic was time consuming and eventually mind-numbingly boring. The first 12″-18″ weren’t so bad, it was having to wait in each section of the attic for that last foot or so to be blown in place that it began to feel like real drudgery.

It also didn’t help that I had a fever and a cold on the day of installation, so being up in the attic surrounded and covered in dust didn’t improve my mood. The process, although very DIY friendly, does require patience and a willingness to cover up — eyes, mouth, and nose — for adequate protection against all the dust floating around.

The day before blowing in the cellulose I went through the attic and marked my goal of 24″ of insulation on various roof trusses so I would have a good visual goal to shoot for. In fact, had I known just how dusty and challenging visibility was going to be during the blowing process, I would have marked every single roof truss at the 24″ level to make the job a little easier.

We didn’t have much in the way in terms of obstacles from various services, other than a few pipe vents for plumbing and radon, along with a small amount of electrical conduit for solar on the roof and a single light in the attic (we kept the majority of all services in our ceiling service core and our walls). This made for a fairly straightforward installation of the cellulose.

Thankfully I was able to keep the cellulose out of the insulation chutes, instead slowly piling it up just below each chute. The siding guys already had most of the soffits installed (this was the end of October, 2017 last year), including a channel for air flow for our “vented roof” assembly, so any cellulose that found its way into the chutes and down into the soffits would’ve been a real pain to remove (I’ll have a separate post later about the siding installation, including the many details of our rain screen and 4″ of Rockwool on the exterior side of the Zip sheathing).

The bench next to the attic access hatch ended up working out really well, and I was very thankful it was in place.

By the end of the first day it was clear we didn’t have enough cellulose to finish the whole attic. We started with 200 bags, but we finished up the second day at just under 300 bags total (288 was the final number of bags installed, so a little more than the 250 recommended by the GreenFiber calculator). What we didn’t use we were able to return to Home Depot for a refund.

Apart from the north and south sides of the attic around the raised heel trusses, we had a solid 24″ throughout the attic, in fact, a little more in the center of the attic where it was easiest to pile it up and let it accumulate (closer to 28-30″ in some areas). This probably explains, too, the additional 38 bags we used that exceeded the initial estimate by the GreenFiber calculator.

On a side note, there was also some concern about the weight of the cellulose on the Intello (our ceiling air barrier), but in the end, even where the cellulose was at its deepest, there was thankfully very little sagging evident in the Intello. Even if it had been worse, the 1×4’s were in place to help support the Intello and the cellulose for the long term (the 1×4’s were spaced roughly 16″ apart between the 2×6’s of the service core).

It’s worth keeping in mind that the cellulose will settle a bit, especially during the first few months. This is obviously very important when it comes to establishing what depth you’re initially going to blow in and your expectations about long-term R-value after settling has occurred (something to consider before signing a contract if you’re going to be hiring someone to do the work — both parties should agree and understand what the final R-value will be before the work commences).

I was back up in the attic recently as I finished up painting the master bedroom and closet. Since I already had drop cloths down, I thought I should take what will hopefully be one last look at the attic.

On average, the cellulose looks like it has settled about 2-4 inches below its original depth, depending on where I looked.

Even with this settling, the attic probably still comes in close to R-70 on average —significantly less out at the north and south ends of the roof with the raised heel trusses, but a little more in spots towards the middle of the attic where some red lines are still hidden below the cellulose.

Just under or over R-70 in the attic is in tune with both the Pretty Good House and Passive House metrics for attic insulation for my climate region (Zone 5 here in the suburbs of Chicago).

While I was up in the attic I also noted that there was no evidence of any water or moisture damage on the OSB roof sheathing, or any indication of wind washing of the cellulose, so the attic seems to be performing as designed, which is a great relief.