Is Blown-In Cellulose Better Than Fiberglass in Cold Climates?

If you've been researching insulation you've probably run into the "cellulose vs fiberglass" debate. And if you've spent more than about 15 minutes researching insulation, you've probably discovered that contractors have some surprisingly strong opinions about this stuff. So let's skip the bullshit. For most Midwest homes, I would choose blown-in cellulose over fiberglass. Not because fiberglass doesn't insulate, it does. Not because cellulose has some magical ability to stop winter from existing, it doesn't. I prefer cellulose because it is much better suited to dealing with the way houses are actually built. And houses are messy.

Fiberglass Looks Better on Paper Than It Does in a Wall. Fiberglass batts are manufactured to fit standard wall cavities and that's great if your wall is a perfectly straight and perfectly sized rectangle with absolutely nothing inside of it. Unfortunately your house was not built in a laboratory. It was built in the bloody real world. A real wall has electrical boxes, wiring, plumbing, blocking, corner bucks, headers, windows, doors, and framing that isn't always as straight or consistent as the plans would have you believe. Fiberglass has to be cut around all of this. And that's where the problem begins. If the batt is too small, you've got a gap. If it's too large, you've got compression. If a wire runs across the cavity, you either cut the batt around it or push the insulation behind or over it. If you have an electrical box, you've got another problem to solve. Every one of those details creates an opportunity for the insulation to be installed poorly. Like very poorly. And remember that R-value is not a "magic force field". If you have an R-21 batt with a gap beside it, the wall doesn't get to average the gap out and say, "close enough". Instead, heat takes the path of least resistance.

A New House Has the Same Problems.

This is where I think the standard argument for fiberglass falls apart. People will say, "fiberglass is great in new construction because the walls are open". The walls are open. But they're also full of all the same stuff that makes fiberglass difficult to install. You still have electrical, and plumbing, and framing, and blocking, and corners, and windows, and doors, and headers, and odd sized cavities, and all the other wonderful little complications that humans have figured out how to put inside walls. I spelled that all awkward on purpose, mainly because I'm a bad person.

The advantage of new construction is actually that you have the opportunity to do something better. You can install the electrical and plumbing first, then install blown insulation around them. Instead of cutting insulation to fit the house, you cut the house to fit the insulation (I wrote that for my own amusement). All jokes aside, you can make the insulation conform to the house. That brings a pretty significant advantage.

Compression Is a Real Problem.

This is especially important with fiberglass batts. Fiberglass gets its insulating ability partly from the air trapped between its fibers. Compress the batt and you change its thickness and density. If you've got a batt designed to fill a cavity and you compress it to make room for wiring, you no longer have the same thickness of insulation that was tested for its nominal R-value. This is why you don't want electrical wiring simply smashing a fiberglass batt flat against the wall. If the solution is to put the electrical on top of the fiberglass, you've created another set of problems. CONDENSATION

The better solution is to design the installation so the insulation can completely surround the wiring and other penetrations. That's one of the reasons blown-in cellulose makes so much sense. Cellulose Doesn't Need the Wall to Be Perfect. Cellulose is made primarily from recycled paper fiber and treated for fire and pest resistance. When it's blown into a cavity, the fibers can work their way around wires, plumbing, boxes and framing. That's the big advantage. It fills the space instead of requiring the space to fit the insulation.

In existing walls, dense-pack cellulose is particularly useful because you can fill cavities without tearing the entire wall apart. In new construction, you can install the mechanicals first and then fill the cavities. That gives you a much better opportunity to create a continuous layer of insulation. And continuity is one of the most important things in an insulation system.

What About Attic Insulation?

For Midwest attic insulation, I'd also lean toward blown-in cellulose. I'd give it roughly 75/25 odds over blown fiberglass for a typical residential attic. Both can work. But cellulose provides a dense, continuous layer and is relatively easy to install around obstructions and penetrations. Where cellulose takes the real lead is when it's stabilized Stabilized Cellulose against wind-washing, which is a real problem in large open attics. The important part, regardless of the material, is getting the attic properly air sealed Air Sealing before adding insulation. I'm inundating you with links with hopes that you keep reading.

What About Sound?

Cellulose wins this one in my opinion. Dense cellulose is excellent at reducing sound transmission through walls and floors. Fiberglass can help with sound control too, but if I were specifically trying to make a wall quieter, I'd rather have cellulose filling the cavity. I'd put the odds around 65/35 in favor of cellulose. And honestly, this is one of those benefits homeowners don't think about until they realize they can hear somebody having a conversation three rooms away. At that point, sound insulation suddenly becomes very interesting.

What About Cost?

This is where fiberglass has its strongest argument (and its a great argument). Fiberglass is cheap and that's a legitimate advantage. For a contractor trying to build a house as cheaply as possible, fiberglass batts are hard to beat on material cost. But material cost isn't the same thing as operating cost. You also have to consider the actual performance of the finished wall. Saving a few bands on insulation doesn't look like a great deal if you spent the next 80 years heating the neighborhood.

So Which One Is Better?

If you force me to put money on it, here's where I land for a typical Midwest house:

| Application | My Pick | Rough Odds |

| ----------------------------- | ---------------------------- | ------------|

| Existing wall retrofit | Dense-pack cellulose | 90/10 |

| New-construction walls | Blown cellulose | 80/20 |

| Attic insulation | Blown cellulose | 75/25 |

| Irregular cavities | Cellulose | 90/10 |

| Sound control | Cellulose | 65/35 |

| Lowest material cost | Fiberglass | 80/20 |

These aren't laboratory probabilities. They're my practical estimate of which system I'd rather bet on when the goal is a better-performing Midwest home. And there's an important distinction here. I'm not saying fiberglass is incapable of performing well. I'm saying I don't think "it performs well when installed perfectly" is a particularly compelling argument when we're talking about actual houses. If you have an insulation system that requires every piece to be cut perfectly around every obstruction, then the quality of the finished system depends heavily on how perfectly somebody did all that cutting. Blown insulation has a fundamental advantage because it can conform to the building.

But Don't Forget the Rest of the Building.

You can have the best insulation in the world and still have a lousy house. A good building envelope needs:

* Continuous insulation

* Proper air sealing

* Moisture control

* Attention to thermal bridging

* Properly installed windows and doors

* Appropriate attic and roof assemblies

* Proper foundation insulation

* Adequate mechanical ventilation