Plaster is a tried and true material used on the interior surfaces of a building such as walls. Insulation is commonly installed behind the walls of buildings. But there are a range of different types of insulation. In this article I will cover whether you can put plaster directly over insulation, and whether it’s a good idea.
Plaster can not be applied on blanket insulation, loose fill insulation, reflective insulation, and sprayed foam insulation. But, it can be applied to concrete block insulation, rigid foam/foam board, fiber board insulation, and structurally insulated panels (SIPs).
The reason you should not apply plaster to some of these types of insulation is that they don’t have a flat surface to apply the plaster to, therefore, after you apply it, it will easily break apart from the wall. But, some types of insulation have a surface that plaster can be applied to, and is recommended. Below, I will explain the types of insulation that are used, if it’s a good idea to apply plaster to them, step by step instructions about how to apply a plaster to insulation, and what tools you will need.
Can You Put Plaster over Insulation? (How To)
There are 9 different types of insulation, and each of them are quite different from each other. But, overall, here is whether you can put plaster over insulation.
In general, plaster can be applied to insulation. But, only to fiber board insulation, structurally insulated panels (SIPs), rigid foam/foam board, concrete block insulation. It can not be installed on loose fill insulation, reflective, insulation, blanket insulation, and sprayed foam insulation.
Here’s a table that shows each of the different types of insulation that are used according to Energy.gov, and whether you can apply plaster to them:
|
Type of insulation |
Can you apply plaster to it |
|
Blanket |
No |
|
Concrete block insulation |
Yes |
|
Rigid foam/foam board |
Yes |
|
Insulating concrete forms |
Yes |
|
Loose fill |
No |
|
Reflective system |
No |
|
Fiber board |
Yes |
|
Sprayed foam |
No |
|
Structural insulated panels |
Yes |
As you can see from the table it’s about 50/50 whether it’s possible to install plaster over the type of insulation you choose.
Should You Put Plaster over Insulation?
Insulation is a unique type of material, therefore, and is installed where it needs to ideally last a long time. So, here’s a rundown and explanation about whether it’s a good idea to install plaster over insulation.
Copyright protected content owner: ReadyToDIY.com and was initially posted on December 4, 2022.
In general, you should apply plaster over insulation. But, it’s not possible to install plaster over all types of insulation. The types of insulation which you can apply plaster to are fiber board insulation, rigid foam/foam board, structurally insulated panels (SIPs), concrete block insulation.
The main consideration when installing plaster over insulation is if you will be using fiberglass mesh. It’s common to use fiberglass mesh when installing plaster because it will make the end surface much flatter, and stronger.
As a result, the plaster is far less likely to crack, and break apart. In general, plaster lasts a long time, and makes the plaster stronger, and more resistant to wear and tear.
What Tools Do You Need to Put Plaster over Insulation
There are a few tools that you may not have, which are required to apply plaster, and to apply plaster over insulation. Here’s a list of everything you will need to plaster over insulation.
Generally, you need a trowel, hawk (hand board), bucket, mixer, water brush, fiberglass mesh, hand brush, plaster, and water. A mixer is not absolutely required but saves considerable labor compared to mixing the plaster by hand.
These are the minimum tools you will need. Here is what each tool is used for:
- Bucket – Putting the plaster into, and mixing the plaster
- Trowel – Applying the plaster to the insulation
- Hawk (also called a hand board) – Held in one hand to hold the plaster
- Mixer – Mixing the plaster
- Water brush – Used last to smooth out any imperfections
- Hand brush – sweeping the floor after you have finished
- Fiberglass mesh – recommended but not required
What Preparations Do You Need to Put Plaster over Insulation
Before applying plaster to insulation there can be a few steps to do to prepare the surface, but it depends on the type of insulation it is. Here’s an overview of what to do to prepare the surface, and after that, I will explain what types of plaster it needs to be done on.
As a general rule, the surface should be free of dust, and should be roughed up using sand paper. But, only foam board (rigid foam), insulating concrete forms, fiber board, and structurally isolated panels should be roughed up using sandpaper.
Other types of insulation, roughing up the surface is not required. The reason is they already have a rough surface. Therefore, they only need to be free of dust.
Doing so makes the plaster stick better to the wall. Removing dust can be done using a soft bristled brush, or a wire brush.
Depending on how strong the material is. For example, on concrete block insulation you can use a wire brush.
How to Put Plaster over Insulation
Plastering is quite a simple skill to learn, but it’s important to do it right so that it lasts as long as it should, and the end result is very smooth and flat. Below is how to put plaster over insulation according to professional plasterers.
In general, you mix the plaster and put some on to a hawk (hand board). After that you trowel it onto the wall using a trowel, spread it out. Fiberglass mesh is recommended, and if you are using it, first apply a base coat of plaster and then use the trowel to smooth out the plaster over the mesh.
The process to apply plaster over insulation is the same one used on virtually all surfaces on the interior of a home. Drywall is the most common type of material used on the walls of the inside of a home. Therefore, you can choose to either install plaster over the insulation directly, or put up drywall first.
The steps involved are:
1. Getting all the tools and materials ready
2. Mixing the plaster in a bucket
3. Apply it to a hawk (hand board) using your trowel
4. Transferring it from the hawk to the wall using a trowel
5. Smoothing it out using a trowel
6. Apply fiberglass mesh, optional but recommended
7. Smoothing out the plaster on top of the fiberglass mesh
8. Smoothing out any imperfections using a water brush (similar to a large paint brush)
The amount of plaster you use does vary. Most fiberglass mesh installers will state on their packaging how thick to make the plaster underneath the sheet of fiberglass. However, some people skip using fiberglass altogether.
As a general rule, plaster can be applied over insulation. But, it’s only possible on fiber board insulation, rigid foam/foam board, and structurally insulated panels (SIPs), and concrete block insulation. It’s not possible to plaster blanket insulation, or sprayed foam insulation.
Copyright article owner is ReadyToDiy.com for this article. This post was first published on December 4, 2022.
Also, it’s not possible to apply plaster to reflective insulation. If you try to apply plaster to these types of insulation it won’t adhere to it, and will break off very easily. Therefore, it’s not recommended to apply plaster to the types of insulation that will not work.
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ReadyToDIY is the owner of this article. This post was published on December 4, 2022.
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