Does Furnace Filter Thickness Matter? Here's What You Need to Know

I’ve spoken with many people that are dead set on using the thickest air filters that they can use in their home’s furnace system. This usually involves modification of their furnace’s filter rack to accommodate a new, thicker air filter. 4-inch filters are common, but some people go even bigger!

But are these larger, thicker air filters even necessary? And does it make a difference in your home’s air and comfort level when using a thick air filter in your furnace? I’ll discuss these things below.

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What is filter thickness, anyway?

First, I need to clarify what I mean when I say ‘filter thickness’. I am talking about the depth of the entire air filter, including the frame.

Air filter size — Air filter dimensions. That last number is the filter’s thickness. This filter is 2 inches thick.

Another thing I am going to talk about is an air filter’s filtration media, which is usually a blue or white fiber material contained in the air filter.

A thicker air filter will (usually) have more filtration media than a thinner air filter. This is because the thicker air filter has longer pleats in its filtration media. The pleats are the folds in the air filter. The longer the pleats, the more filtration media in the air filter.

But why am I making such a big deal about the amount of filtration media in air filters? It turns out that the amount of filtration media is the key advantage that thicker air filters have over thinner ones.

Is a thicker air filter better than a thinner one?

Usually, but not always.

First of all, you should use the type of filter that your furnace’s filter slot is designed for. So if your furnace has a slot for a 2-inch filter, then you should use 2-inch filters.

Secondly, a thicker air filter isn’t the be-all-end-all for your HVAC airflow issues. If your furnace’s air filter is restricting airflow, then you might be using an air filter that restricts too much airflow. Airflow restriction from air filters is usually (but not always) due to thin air filters with a high MERV rating.

However, all else being equal, a thicker air filter will outperform a thinner air filter in the following ways:

More efficient air filtration. A thicker air filter will contain more filtration media, so particles in the air will have a higher chance of being captured and filtered out.
Better airflow. A thicker air filter will provide better airflow because the air going through the filter is dispersed through a larger surface area of the filtration media. It’s like if you were to blow air through a straw versus a pipe. It’s much easier to move air through a pipe because its hole is much larger than the straw’s. It’s the same with air filters—except instead of having a larger hole, the thicker filter has a larger filtration media.
Longer lifespan. A thicker air filter will generally last longer than a thinner air filter because it has more filtration media. All the particles captured in the air filter will be spread out over a larger area in the thicker filter. The filtration media in a thicker air filter won’t clog up as quickly as a thinner air filter, and you won’t need to change a thicker air filter as often.
More rigidity. A thicker air filter will generally have a stronger frame and be more rigid than a thinner air filter. This reduces the risk of the air filter collapsing in the HVAC system.

What air filter thickness is best?

So, with all these points about thicker air filters outperforming thinner ones, does that mean you should just go out and buy 4-inch media air filters? Not really.

Most home furnace systems don’t require the performance of a thick media air filter. In fact, these thicker filters are usually reserved for commercial and industrial applications where air filtration is critical.

You should use the thickness of air filter that your HVAC system was designed for. That means if your system has a 1-inch air filter slot, you should be using 1-inch air filters.

Oh, and please don’t try to force-fit an air filter into a slot that it wasn’t designed for. That’s going to cause more problems than it will solve.

Is it okay to use a thinner furnace filter in a thicker slot?

You can use a thinner filter in a thicker slot, but you should avoid it if at all possible. It’s okay to use a thinner filter in a larger slot temporarily until you get a properly sized filter. Again, you should only do this as a temporary measure.

For example, if you only have a 1-inch filter, but your furnace has a 2-inch filter slot, then you can tape the 1-inch filter in the slot so it stays in place. You don’t want the filter to move around from the airflow when your furnace starts up because the filter may become dislodged and collapse into your blower.

However, do not, under any circumstances, use an air filter with a smaller length or width than what your furnace’s filter slot is designed for. The air filter will just fall out of its slot.

Can I modify my filter slot to accommodate thicker filters?

Some people retrofit their system’s air filter slot to use thicker air filters. While I wouldn’t go out and say that everyone should do this, for most people, retrofitting their air filter slot to use thicker air filters is not necessary.

Oftentimes, people look to upgrade to a thicker filter size to fix airflow problems when they are using strong MERV-11 or MERV-13 filters. But many home HVAC systems were not designed to be running with high MERV filters in the first place!

Unless you or someone in your household has specific needs, a filter rated higher than MERV-8 is unnecessary for most people.

Key takeaways

When deciding whether to use a thicker air filter in your furnace, it all boils down to the surface area of the filtration media in the air filter. The more surface area, the more advantages you gain—better filtration, more airflow, and longer lifespan.

You can sometimes gain the benefits from a thicker air filter by just going with an air filter with more pleats. Having more pleats means that there is more filtration media, and is almost just as good as having a thicker air filter.

So next time you go shopping for furnace air filters, take a look at the pleats in the air filter and compare them to other air filters with the same thickness. You’ll be surprised at how much the pleats can vary between air filters with the same MERV rating from different brands!

Are you following my air filter blog series?

Air filters are a critical component of your HVAC system. I’ve written a ton of blogs answering questions that people have about air filters. You can check them out by clicking the links below.