How Spray Foam Responds to Moisture Exposure from Roof Damage
How spray foam insulation responds to moisture exposure from roof damage depends entirely on the type of foam that is installed. Closed-cell spray foam, with its dense and waterproof structure, acts as a resilient barrier. It will not absorb the water and is not damaged by the exposure; in fact, it often helps to contain the leak and make it more obvious. Open-cell spray foam, on the other hand, is soft and absorbent like a sponge. A significant roof leak can saturate it, causing it to lose its insulating value and hold a large amount of water against the roof structure, which can lead to serious long-term problems like rot and mold if not addressed quickly.
Understanding this fundamental difference is critical for any homeowner who has or is considering professional spray foam insulation for their attic. This guide will explain the science behind how each type of foam interacts with water, how to identify a leak in a spray-foamed attic, and what the proper steps are for remediation. The information is based on the practical, hands-on experience of certified insulation and building science professionals.
The Two Types of Foam: A Tale of Two Structures
The way spray foam handles a roof leak comes down to its cellular structure. The two main types of foam are polar opposites in this regard.
Closed-Cell Foam: The Waterproof Barrier
Closed-cell spray foam is composed of millions of tiny, individual plastic cells that are completely sealed off from one another. This creates a hard, rigid, and dense material. This structure makes closed-cell foam "hydrophobic," which means it repels water. It has a very low water absorption rate and is considered a waterproof material.
When a roof leak occurs in an attic insulated with closed-cell foam on the underside of the roof deck, the water will typically run over the surface of the foam or get trapped between the foam and the wood sheathing. Because the foam is also a vapor barrier and will not allow water to pass through, it can effectively contain the leak. This often has the useful side effect of making the leak much easier to spot, as the trapped water will find the lowest point and may begin to drip, clearly alerting the homeowner to a problem. The insulation itself is not damaged by this moisture exposure.
Open-Cell Foam: The Absorbent Sponge
Open-cell spray foam has a very different structure. Its cells are not fully sealed; they are interconnected, which creates a soft, flexible, and spongy texture. This structure is what gives open-cell foam its excellent sound-dampening qualities, but it also makes it "hydrophilic," meaning it will readily absorb and hold liquid water.
If a roof leak develops above open-cell foam, the foam will act just like a sponge, soaking up the water. This leads to several significant problems:
Loss of R-Value: Once the foam is saturated with water, it loses its ability to trap air, and its insulating value drops to nearly zero.
Increased Weight: Water is very heavy. A large area of saturated open-cell foam can put a great deal of stress on the drywall ceiling below it.
Prolonged Wetting of the Structure: The biggest risk is that the saturated foam can hold a large volume of water directly against the wood roof deck and rafters for a long time. This creates the perfect conditions for wood rot and mold growth to develop.
How to Identify a Roof Leak in a Spray-Foamed Attic
Finding a leak in an attic with spray foam is different from finding one in a traditional attic, where you might see a patch of wet, clumpy fiberglass.
Signs of a Leak with Closed-Cell Foam
With closed-cell foam, the leak may not show up directly below the problem area on the roof. Look for:
Water Staining: The surface of the foam may look darker or discolored in the area where water is running.
A Single Drip Point: Because the foam contains the water, you may see a consistent drip from one low point in the foam, which could be several feet away from the actual leak in the roof.
Signs of a Leak with Open-Cell Foam
Finding a leak in open-cell foam is often more direct. Look for:
Damp or Saturated Foam: The most obvious sign is that the foam will feel wet or spongy when you touch it.
Discoloration: The foam will likely be a darker color in the affected area. If the leak has been going on for some time, you may see dark stains that indicate the beginning of mold growth.
Sagging: The weight of the absorbed water can sometimes cause the foam to sag or pull away from the roof deck.
Here is a quick comparison of how the two foam types perform when exposed to a leak:
The Correct Process for Repair and Remediation
If you find a roof leak in your spray-foamed attic, it is essential to follow a clear set of steps to fix the problem correctly.
Step 1 Fix the Roof Leak First
This is the most critical rule. No insulation repair can begin until the source of the water has been stopped. This means hiring a qualified roofing contractor to find and properly repair the leak on the outside of the roof.
Step 2 Address the Insulation
Once the roof has been repaired, the approach to the insulation depends on which type of foam you have.
For Closed-Cell Foam: In most cases, the foam itself will not need any repair. The main task is to ensure that any trapped water has been drained and that the surrounding wood framing is allowed to dry out completely. A professional may use moisture meters to confirm that the wood is back to an acceptable moisture content.
For Open-Cell Foam: The response depends on the extent of the leak. For a very small, recent leak, the foam might be able to dry out on its own with the help of high-volume fans and dehumidifiers. However, for a significant or long-term leak, the saturated section of foam must be professionally cut out and removed. The exposed roof sheathing and framing must then be carefully inspected for any signs of rot or mold, treated if necessary, and allowed to dry completely before new insulation can be installed.
Bonus Tip: It is absolutely essential to ensure the wood sheathing is completely dry before installing new insulation. Sealing wet wood inside a roof assembly is a guaranteed way to cause mold and rot.
Things to Consider Before You Insulate Your Roof Deck
A thorough roof inspection is a must: Before you even consider insulating the underside of your roof deck with any kind of spray foam, you must have your roof inspected by a professional roofer. You need to be confident that your roof is in excellent condition with many years of life left in it. Insulating over a failing roof will just hide the problems and make them much more expensive to fix down the road.
Choose the Right Foam for Your Climate: In a hot climate like Fresno's, which can also experience humid conditions, closed-cell spray foam is often the safer and more resilient choice for an unvented attic. Its built-in vapor barrier properties are a major advantage.
Hire an Experienced Insulation Contractor: A qualified insulation contractor can help you choose the right foam for your project, and more importantly, they will understand the building science behind how to properly create a sealed attic system that will perform for decades without causing moisture problems.
Final Thoughts on Leaks and Insulation Performance
While no insulation can stop a roof from leaking, the type of insulation you have in your attic will determine the consequences of that leak. Closed-cell spray foam offers a high degree of resilience, protecting the home's structure and maintaining its performance even after it has been exposed to water. Open-cell foam, while an excellent insulator when it is dry, is vulnerable to water and requires immediate and proper attention if a leak occurs. The most important takeaway, however, is that the health of any insulation system begins with the health of the roof that is protecting it.
Get a Professional Insulation and Moisture Assessment
If you have a known roof leak or are concerned about the condition of your attic insulation, the best course of action is a professional inspection. An experienced insulation contractor can assess the situation and provide a clear plan for any necessary repairs or upgrades. For homeowners in the Fresno area, the team at Supreme Spray Foam – Fresno has deep expertise in diagnosing and solving complex moisture and insulation issues. They can be reached for a consultation by email at info@supremesprayfoamfresno.com or by phone at (559) 545-0800.
This article was reviewed by Emma Robinson, who contributed knowledge from her 8 years of experience in the spray foam industry. Her review helped shape this content with ideas grounded in what contractors and homeowners actually value.
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