Foam Board Over OSB Sheathing (Is It Acceptable?)

Foam board has many uses, but when it comes to construction, it is used to enhance insulation. If you live in an area that experiences very low temperatures, insulation should be an essential factor to consider when erecting the walls.

OSB and foam board are key materials when insulating a home. And today, I will be answering whether it’s acceptable to use foam board over OSB sheathing while sharing valuable tips with anyone interested in starting this project.

Foam insulation

Can You Use Foam Board Over OSB Sheathing?

Even though OSB sheathing offers weather resistance when used in a building, its efforts can be complemented by adding an insulating foam board.

This is a common practice that’s popular amongst DIY enthusiasts and professional builders. Therefore, if you are interested in installing a foam board over OSB sheathing, I can confirm that this is acceptable.

But before you do so, you may want to check out some of the tips shared below, which will ensure your project is successful.

Tips on Installing Foam Board Over OSB Sheathing

There is nothing wrong with placing foam board over OSB sheathing. However, you first need to figure out the thickness of the foam board you want to install.

You will get better insulation from a thicker foam board, preferably one measuring 2 inches thick. However, this will increase the size of the wall. So, you will have to make adjustments for door and window jambs.

Secondly, after installing foam board over OSB sheathing, you shouldn’t install siding directly next to it. It’s best to leave some space to avoid moisture problems.

As you may already know, some rainwater usually enters the siding, especially when it’s windy. This can also be avoided by installing a house wrap such as Tyvek over the foam board. This way, you will know that your foam board and OSB are protected from moisture.

Another pro tip you should remember is that foam board shouldn’t be installed over OSB sheathing that has been damaged by moisture. This will be a recipe for disaster.

Before starting this project, carefully inspect for moisture damage on the OSB or stud cavities. You should also check if your siding is leaking water. If you spot any moisture problem, replace it first and then install foam board over OSB.

Will the Expansion and Contraction of OSB Affect the Foam Board?

As OSB expands and contracts, it will affect the foam board. As we all know, foam boards don’t do well in high-stress areas. Therefore, this can lead to damage.

However, you need to ask yourself why the OSB would expand or contract. That will probably occur due to exposure to moisture and that’s why I advised you earlier to inspect for leaks and moisture problems.

Also, with Tyvek wrapped on the foam board, there will be no entry point for moisture. So, your OSB won’t have a reason to expand or contract.

Can OSB Offer Structural Support to Foam Board?

This is another common reason why people prefer to use foam board over OSB sheathing. Foam board, on its own, may be good at improving insulation, but it can’t stand on its own. That’s where OSB sheathing comes in.

With some OSB at the back, it will offer structural strength to the foam board. As a result, your foam board will provide you with insulation for the longest time possible.

If you are worried that the foam board won’t have enough support when glued to the studs, you can reinforce the structural support by using OSB sheathing behind it.

Can You Add Spray Foam Between Foam Board and OSB Sheathing?

If you are keen on improving your home’s insulation, after installing foam board over OSB sheathing, you may be tempted to add spray foam in between.

Among DIY folk, spray foam is a cost-effective and easy way of increasing your home’s insulation. But will it work?

I would strongly advise you to avoid adding spray foam between the foam board and OSB. Using a foam board and OSB sheathing is enough to ensure the temperatures on the outside don’t affect those on the inside and vice versa.

Spray foam heats up when it cures. On the other hand, a rigid foam board has a low melting point. When you add spray foam between the foam board and OSB, the heat can cause the foam board to liquefy or ignite into flames. Therefore, this is a pretty big risk.

Is It Possible to Put OSB Sheathing Over Foam Board?

Foam board over OSB sheathing is standard practice, but we can’t say the same for vice versa.

If you are interested in placing OSB sheathing over the foam board, no matter what your reasons are, you should know that it’s not acceptable. And no building inspector out there would let such a setup slide. Here are a couple of reasons why.

With foam board in between the studs and OSB, how will you be able to fasten the OSB sheathing firmly into the studs? When you put OSB over the foam board, you can forget about the shear strength on your walls.

When you install foam board over OSB, you don’t just create a structurally strong wall. You also create a thermal break.

Is There an Alternative to Using Foam Board Over OSB Sheathing?

Though acceptable, installing foam board over OSB sheathing requires careful planning. Therefore, you may be interested in exploring other alternatives on how you can still get insulation and the structural support of OSB.

There is a product known as R-Zip OSB by Huberwood. It is a ready-made, single-sided OSB panel that represents what you may want to achieve from installing a foam board over OSB sheathing.

Zip-R OSB offers insulation and a surface you can nail to the studs on the same board. This product eliminates the time and labor that would have been used to install foam boards over OSB sheathing.

It also offers a complete thermal break. The R-Zip OSB is worth considering if you want to install a foam board over OSB sheathing because it makes your work easier.

Should I Be Worried about Water Vapor Condensing Between Foam Board and OSB?

When it comes to wood, you should always be worried about moisture. But its impact heavily depends on the weather your area experiences.

Water vapor must be exposed to fluctuating temperatures frequently to condense between the foam board and OSB. Very few places experience such weather because, in most areas, the seasons usually last for a few months.

Therefore, you shouldn’t excessively worry about water vapor condensing between the foam board and OSB. If you can’t get this off your mind, when insulating your home, you can ensure that the space between the foam board and OSB sheathing is air-tight.

A strong adhesive can help you with that. The less room you have between these walls, the less space there will be for moisture to sit and condense.

Adding Tyvek on the foam board after installing it over OSB can also ensure that there is no water vapor between these two materials.

Tyvek, unlike other house wraps, is breathable. That is, apart from keeping water from getting in, Tyvek also allows your walls to breathe out trapped moisture.

Was this article helpful?