Is Rockwool Insulation Waterproof? What you need to know!

Rockwool insulation, or mineral wool, is very commonly used for insulating homes and buildings, be it for thermal or acoustic insulation. But it’s also extremely important to take into account how this insulation material behaves when exposed to water, mainly if it gets wet, if this might ruin it, if it’s still as effective while wet, and what the long-term implications of moisture exposure are, and more.

In this article, I will be going over the different characteristics of Rockwool insulation, if it’s waterproof or not, what moisture does to it, if you can simply let it dry or if it’s ruined once wet, etc, and I will also compare it to other similar materials such as Rockwool, Spray foam, Cellulose, etc., and a lot more.

So, without any further ado, let’s get started!

Is Rockwool Insulation Waterproof?

Rockwool insulation is not waterproof since it’s permeable, which means that water and air, and therefore vapor as well, can get through it without any issues. If Rockwool gets saturated with water, it becomes a much less effective insulator, but letting it dry will restore its insulation capabilities.

Although I will go into much more detail in the coming sections, it’s worth mentioning that Rockwool insulation, or mineral wool as it’s actually called (Rockwool is just the brand name), won’t be harmed by getting wet temporarily and that letting it dry out completely will restore all of its insulation capabilities, be it thermal or acoustic insulation.

What is Rockwool Insulation?

Also known as mineral wool, Rockwool is made out of rock fibers (volcanic rock and slag) that are spun and then given a specific shape (batt, pipe, and board forms).

Rockwool is a great sound absorber since it’s thick and has a lot of air pockets inside of it, and it’s also worth mentioning that you can get different densities and thicknesses depending on your needs, all of which affect how well it works in terms of thermal and acoustic insulation.

It’s not impermeable to water or vapor, and although it doesn’t absorb water like other types of woo dol, if it gets wet it won’t be able to insulate as well as when it’s dry (this applies to both thermal and sound insulation). Luckily, since it’s made of rock, once it dries it’s as good as new.

What happens when Rockwool Insulation gets wet?

Rockwool insulation won’t be ruined by coming into contact with water, even if it’s soaking wet, and letting it dry completely will get it back to its original state and it will be as effective as the first day.

However, other insulating materials, such as glass wool, cellulose, etc., are made from organic matter which can promote mold growth. So, let’s see how water affects Rockwool in the long term as well as in the short term.

Is Mold an Issue with Rockwool?

As I just mentioned, mold can very easily grow in the fiberglass batts you put in your home since it’s made from organic matter.

As you can imagine, being in a room filled with wet fiberglass batts that are riddled with mold poses some health risks to anyone living there.

Luckily, Rockwool doesn’t have this problem because it’s made of stone (inorganic matter) and no mold can grow in it no matter how wet it gets.

It stops being as effective as long as it’s wet

As soon as Rockwool insulation gets wet, and this also applies to other types of insulation, it loses most of its insulation properties, both thermal and acoustic, and as long as it’s wet, your home will lose heat, and sound will also be able to get in or out.

This means that as long as you let the insulation dry out, which might actually take a lot of time since it’s generally in the middle of the walls, or in the floors or ceiling, everything should return to normal as long as the added weight didn’t make the insulation settle.

If it did, then there will be an uninsulated gap on top of the fiberglass insulation where sound and heat can get through.

How does Rockwool insulation handle humidity and vapors?

Rockwool insulation itself is permeable to both air and water, which means that any type of vapor can get through it unhindered, which is why a dedicated vapor barrier is needed.

Other Alternatives

Out of all the insulation materials out there, be it Rockwool, Fiberglass, Cellulose, Spray Foam, etc., Rockwool, or mineral Wool, is by far the best one for the reasons I laid out previously, but also because it’s safer (less flammable, non-toxic, etc.).

Not only that, but Rockwool generally has, although very slightly, higher R and STC ratings than any of the other solutions.

Here are all the reasons that Rockwool is superior to other insulation materials:

  1. It’s made from inorganic matter which means that mold can’t grow in it even if it gets wet.
  2. Although it loses its insulating capabilities when wet (just like fiberglass), once it dries it works flawlessly again.
  3. It can withstand up to 1000°C whereas Fiberglass Insulation only handles up to 500°C before melting and combusting.
  4. It’s not toxic to the respiratory system, eyes, or skin.
  5. Really good insulation performance.

I would recommend going for Rockwool, or mineral wool, over the other types of insulation whenever possible since it costs roughly the same, is less flammable (mineral wool withstands much higher temperatures), mold can’t grow in it, and it isn’t dangerous or toxic for your respiratory system and won’t irritate the eyes.

A quick note about spray foam and cellulose: Both materials have a lot of added chemicals that make them fire retardants, but they still become extremely flammable after being exposed to sufficient heat for a period of time, whereas Rockwool and Fiberglass are much better in this regard (Rockwool especially).


As I think I made clear in this article, Rockwool is not waterproof in the sense that water and moisture can easily get through it or even saturate it which greatly lowers its performance.

Luckily, as soon as it’s allowed to dry, it’s as good as new and does not need to replace. Plus, mold can’t grow in it, which is another pro.

I hope this was useful!

Last Updated on August 30, 2022 by Facundo

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