"Isn't spray foam bad for my roof?" We often hear this question, and other concerns like it. You may worry about spray foam as you research your insulation options upstairs. You don't want to invest in a premium upgrade that wrecks your roof. If spray foam insulation can create an airtight seal, won't that negatively impact the ventilation in the roof system?
South Central Services has insulated hundreds of new and existing homes. Building science is a huge priority for our team. We install insulation to improve a home's functionality, energy efficiency, and comfort. We would never want to install a product in your attic that would ultimately fail you. Spray foam insulation is not going to rot your roof. Still, you should be aware of some relationships between roofs, ventilation, and attic insulation.
By the end of this article, you will understand:
- Why spray foam alone won’t rot a roof
- Why roofs insulated with spray foam could fail
- How ventilation and roofs function together
No, Spray Foam Insulation Will Not Rot Your Roof.
You may have read horror stories online describing how a perfectly functioning roof was ruined after installing spray foam insulation. Similar stories about spray foam installed almost anywhere in a home can be found. How could you trust a product that has let people down?
You can think about the success of spray foam insulation in the same way as any product. Nearly anything you purchase at the store comes with a little pamphlet of instructions. You are warned if the product is misused, it will not function properly. Warnings not to microwave metal or submerge electronics in water may feel like common sense. However, those warnings are there for legal and practical protection.
Spray foam insulation is the same. It isn't a miracle product that works despite any installation circumstances. Spray foam has disclaimers and technical specifications like anything else.
When spray foam insulation is installed in a roof system correctly, the risks of failure are mitigated. When installed incorrectly, spray foam could exacerbate existing issues. Installed incorrectly, spray foam could impact roof decking and shingle life.
There are four significant variables that determine a correct spray foam installation.
- Climate zone
- Type of foam
- Foam thickness
- Foam application
If you live in a climate zone that needs a vapor barrier in the attic and open cell spray foam was installed, that's a faulty installation. If you needed a minimum of 3 inches of closed cell upstairs, but your contractor swore 2 inches was the magic number, that's a faulty installation.
If you work with a competent spray foam contractor who understands building science, you should not have to worry about rotting your roof.
The Wrong Foam Can Contribute To Roof Failure.
How do you know which type of foam your roof needs? There may only be two types of spray foam insulation, but this isn't something where you flip a coin and take your 50-50 odds. If the wrong foam could contribute to roof failure, you need to have the right foam installed.
You May Need Closed Cell Foam For Its Vapor Barrier Capabilities.
The right type of foam will depend on a number of factors. One of the biggest factors is the climate zone you live in. Our team operates within Climate Zone 5, which means that, in most cases, a vapor barrier in the roof system is ideal. The further south you travel, the less critical vapor barriers become.
Without a vapor barrier installed correctly, when it is necessary, excess moisture could build up in the roof deck. That moisture will stay in the wood and could cause warping, shingle failure, or deck failure. Overall, there could be a premature failure of your roof. Moisture problems can also cause mold and mildew.
However, these types of moisture problems can be prevented with a vapor barrier that is correctly installed. Spray foam doesn't cause moisture issues: improperly installed spray foam can exacerbate existing moisture issues.
A Roof Needs To Be Either Completely Vented Or Unvented.
Generally, most roof systems are vented. Vents are installed in the eaves, gables, or at random to encourage air exchange. This ventilation is healthy for the roof deck and the overall roof system. In fact, many shingle manufacturers will void their warranties if the roof system is not properly vented.
However, it is also possible to have a completely unvented roof. These unvented attics are created through spray foam encapsulation. Spray foam insulation delivers an airtight seal and is installed on the underside of the roof and the gables. This encapsulation blocks any air exchange, keeping the roof from experiencing any venting.
A completely unvented roof is not necessarily unhealthy as long as that encapsulation is done correctly. There should not be any air leakage through the insulation material to the roof deck. When it comes to ventilating the roof, you have to choose one lane or the other.
It is possible to have an encapsulated attic with a vented roof. This system is a bit more complicated, but when done correctly does allow for air to vent between the roof deck and the layer of spray foam insulation. None of this venting is internal because that would compromise the air-seal in the attic and promote air leakage. Instead, this allows air to flow throughout the roof.
If you want an encapsulated attic but are concerned about shingle warranties being voided, this knowledge should put encapsulation back on your list.
When Installed Correctly, Spray Foam Mitigates Moisture Risks.
Spray foam insulation is like any other product. Nothing is foolproof. When making a home upgrade, it is vital to work with a qualified professional. You wouldn't want someone who only kind of knows what they're doing to replace your roof. The same goes for whoever will be insulating your attic, replacing your toilets, or resizing your HVAC system.
There is always a little bit of margin for error in things, but you want to work with someone who knows that margin well and will not stray outside. The reality of spray foam in your roof system is that it will benefit your home when it is done correctly. An encapsulated attic supports your HVAC ductwork performance and greatly affects your comfort and energy costs.
Spray foam insulation harming your roof is not left to chance or a roll of the dice. As long as you choose a qualified contractor, you should not have to worry about moisture concerns in your roof.
The Bottom Line About Moisture Concerns When Insulating The Underside Of A Roof
When installed correctly, spray foam insulation will not cause any moisture concerns in the roof system. If installed incorrectly, spray foam insulation could contribute to existing moisture problems. It is essential to install the right foam, at the correct thickness, for the climate zone you are living in. If your contractor cannot explain why that foam product is best for your climate and how its thickness will impact your home, they are not worth working with.
A qualified contractor will be insured and stand behind their work's quality.
Now that you understand why spray foam doesn't ruin roofs, your next step is to:
- Compare attic encapsulation to more traditional attic floor insulation
- Examine the differences between open cell and closed cell spray foam
Disclaimer: While we strive to publish information accurate to building science, local building codes and standards supersede our recommendations.
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