Can You Mix Different Types Of Insulation?[Esp. Blown-In]

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When looking for new insulation material, the common choice regarding your current insulation is to remove it. However, the cost of removal might be a problem for some. So, is there a solution to this problem? Is it possible to mix two types of insulation to save on costs? If that's what you're wondering, we've researched the issue for you! 

Mixing insulation types is standard practice. Opting to go this route instead of removing old insulation is cost-effective and time-saving. However, keep the insulation type's weight in mind. For example, you wouldn't want to lay mineral wool over fiberglass. Blown-in types - like fiberglass over cellulose or vice versa - should work with no problem.

So, you can mix insulation types if you don't want to spend extra having the old layer removed. Now, it's time to consider other factors. For example, what varieties of insulation can lay on top of another? In addition, does mixing them increase the R-value? These are some topics we cover in-depth further ahead.

Different types of an insulation, Can You Mix Different Types Of Insulation?[Esp. Blown-In]

What Is Hybrid Insulation?

What you're attempting to do is something known as hybrid insulation. Though, most won't call it that term. Hybrid insulation refers to mixing spray foam with conventional insulation material - like fiberglass. Still, the goal is the same.

Various scattered blocks of insulation model house

Most will opt to go with hybrid insulation because it saves on cost. So, if you've wanted to use energy-efficient spray foam - but don't have the budget to cover an entire cavity - you can mix two types. On walls, the process is simple: 

  1. There are no accommodations necessary. So, you or professionals will prepare the walls as customary. 
  2. Depending on your climate, professionals will spray 1/2-inch to 2-inch thick spray foam into the wall cavities. They will not fill it.
  3. After the spray foam cures, they'll finish filling the cavity with an insulation material of choice - usually fiberglass because it's cheap. 
  4. Little to no insulation trimming will be necessary. For materials like fiberglass, it's essential to avoid compressing them. 

Alternatives include materials like foam panel insulation. However, instead of laying the board over the spray foam, they will install the boards first and spray over them. 

What About Blown-In Insulation?

vWorker spraying blown fiberglass insulation between attic trusses

Blown-in insulation will come in three types - Rockwool, cellulose, and fiberglass. In general, mixing the different types to save on costs should be possible. However, the one type you should be cautious with is Rockwool. 

Rockwool is a rock-based mineral fiber. More specifically, it's made of basalt and molten slag. These two materials are why Rockwool is heavier and stiffer than fiberglass and cellulose. So, if you have Rockwool as the base, laying cellulose or fiberglass on top should work. 

But, why would the weight matter regarding which type can lay on top of the other? As some homeowners suggest, you want to avoid compressing a lighter material. When you put a heavier fiber - like Rockwool - over fiberglass or cellulose, it will squeeze the bottom layer. Consequently, you'll lose the overall R-value, but the R-value per inch goes up. 

Is that a negative side-effect? Some would suggest compression makes the insulation material useless. In other cases, light compression won't make much of a difference as long as you make up for it by using an insulation type with a higher R-value. 

You might have to consult a professional on what to do if you want to use heavier material. Otherwise, you can mix fiberglass and cellulose since they're light. It's a practice some refer to as capping.

The Benefits of Hybrid Insulation

So, why would you choose to go with mixing insulation materials other than cost-efficiency? As you might already know, there isn't one-size-fits-all insulation that covers all bases. Some won't be able to dampen sound as efficiently. 

Others are air-permeable. So, if you want all the benefits each type of insulation offers, you can mix the two! 

For example, the three insulation materials that people will purchase for excellent sound dampening and relatively low price are mineral wool, cellulose, and fiberglass. The problem all three suffer is air-permeability. 

On the other hand, spray foam insulation does not offer sound dampening as good as the three mentioned. Additionally, filling wall cavities or attics with spray foam can get pricey. 

If you mix the two, you combine all the benefits of the materials. Meaning, you won't spend as much compared to filling a cavity in with spray foam completely. Additionally, you add fire resistance and better sound control to the area. 

It also helps by making installation time quicker. 

Can You Put Insulation on Top of Insulation?

Stack of thermal insulation material in roll is wrapped with cellophane foil

It's possible to put insulation on top of existing insulation as long as you meet two conditions: 

  1. The top layer has a higher R-value than the bottom layer to make up for compression.
  2. Avoid using faced insulation on top. Insulation batts with a vapor barrier will trap moisture. 
  3. Avoid flattening or squishing the bottom layer. 

If you're not comfortable installing the insulation right away, you can also seek guidance from a professional before carrying out the job. 

However, there are some instances where you wouldn't want to lay new insulation over old. It all boils down to the condition of the old insulation. Does it have water damage? Did you have problems with pests? 

When Should You Replace The Old Layer?

In general, old insulation will still carry some R-value. But, if there's damage present, you might have to play a guessing game about how efficient the material will be. 

The two culprits that will determine if it's best to remove the old insulation are pests and water damage. 

Pests are a problem because of the dropping and urine they leave behind. In most cases, rodents will build nests using insulation - they mainly like to nest in cellulose and fiberglass. The urine and droppings they leave behind are hazardous. So, if this describes your situation, insulation removal is necessary. 

Water Damage

The second culprit is water damage. Though, that depends on how quickly you've caught the problem. Fiberglass won't absorb water. However, moisture can fill the air pockets between the fibers - essentially making the insulation material lose its effectiveness.

If you've dried it in time, it should work as usual. However, if you didn't dry it in time, you might notice a smell coming from the material. Meaning mold has likely grown inside. This situation will require you to remove all of the infected areas. 

The same thought process goes for cellulose. Cellulose is made of recycled paper material. So, it might be difficult to restore it to good condition once it gets wet. Mold won't grow directly on cellulose. 

However, if it stays wet long enough, it will promote mold growth in the area it covers. Meaning, if it's against wood or drywall, mold will infest it. If the area of damage is small enough, you can remove the affected area and dry it. Extensive damage is a different case. 

Water that damages a substantial portion will require removal. This way, it won't promote mold growth.

Does Doubling Insulation Double the R-Value?

The answer to this question depends on what you're trying to get out of the insulation material. As mentioned, when you compress the insulation, it will lose its overall R-value. However, the R-value per inch goes up.

Now, let's apply this to a situation. If you use two batts with a rating of R-19 - and you stack them together - it should get a combined R-value of R-38. But, this is assuming it retains the same thickness.

If one of them is compressed, it will lose R-value. So, you won't get the expected R-38 value. To get the R-value you desire, it's recommended to go by the compressed values of the insulation.

The example they give is simple. If you fill a 2"x6" cavity with two 3.5" thick batts with an R-value of 13, the insulation will lose 1.5 inches of thickness. Accordingly, each batt will have a reduced total R-value of 10.9. 

So, the combined R-value will be R-22 instead of R-26.

How Thick Should My Insulation Be?

Man insulating the attic with rock wool

In general, the thickness you need will depend on the type of insulation you're planning to install. As mentioned, R-value for the material is measured at full thickness. If it's compressed, it loses R-value. So, you won't get the R-value you'd expect. 

It also depends on the area that you're going to fill. For example, walls will need an R-value anywhere from R-13 to R-21. Attics will need an R-value anywhere from R-30 to R-60 - depending on your climate zone. 

As some suggest, 11 inches of fiberglass or mineral wool has an R-value of R-30 - giving it an R-value of 2.7-2.8 per inch. To meet the wall's required R-value, you'll need 5 to 6 inches of fiberglass or mineral wool.

Final Takeaway

You don't always have to go the expensive route if you want adequate insulation. As we've learned, mixing different types of insulation can work. However, you'll want to avoid compression. But, if you can't, you'll need to go by compressed R-values instead of full-thickness R-values. We hope you found the information above helpful!

Before you go, do you have other concerns about compression? Does mineral wool suffer the same consequences as fiberglass when compressed? To learn more, check out our post - Can You Compress Mineral Wool Insulation?

Are you questioning whether to top up or replace the existing insulation? We can offer some guidance. If you'd like to learn more, check out our post - How Long Does Insulation Last And When To Replace It?

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