You may have heard that spray foam insulation can trap moisture against the roof deck and rot the wood from the inside out without you even knowing it.
That is an alarming thought, especially because most homeowners would rather not discover a roofing problem after it has been hiding behind several inches of insulation.
So, will spray foam rot your roof?
Spray foam does not directly cause roof rot, but open cell and closed cell spray foam handle moisture very differently. Open cell spray foam allows moisture to move through it, which can make a roof leak easier to notice. Closed cell spray foam resists water and limits moisture movement, so a leak may remain hidden between the foam and roof deck. If that wood stays wet, rot and other damage can develop.
That does not mean open cell spray foam prevents roof problems or that closed cell spray foam always causes them. The roof’s condition, the source of the moisture, and the design of the entire attic system still matter.
However, homeowners need to understand this difference before choosing spray foam insulation for a roof. The type of foam affects how the roof deck handles moisture, how easily a leak may be detected, and how the wood can dry if it gets wet.
RetroFoam of Michigan has been insulating homes since 2002. We know homeowners want the good, the bad, and the “what happens if my roof springs a leak five years from now?” before making a decision.
Let’s look at how each type of spray foam handles moisture and what needs to happen before either one is installed against a roof deck.
• Spray foam does not directly cause roof decking to rot.
• Open cell spray foam allows moisture vapor to move through it and may allow water from a roof leak to pass through or leave visible staining.
• Closed cell spray foam limits moisture movement and resists liquid water, which can make a roof leak more difficult to detect.
• Water can collect between closed cell spray foam and the roof deck without immediately becoming visible inside the attic.
• Open cell spray foam does not guarantee that every roof leak will be noticed immediately or appear directly below its source.
• Wood rot can develop when roof decking remains wet for an extended period.
• Spray foam should not be installed over wet, rotted, or damaged roof decking.
• Roof leaks, indoor humidity, air leakage, and condensation can all introduce moisture into the roof assembly.
• The roof should be inspected before spray foam covers the underside of the decking.
• Michigan’s cold winters make air sealing, indoor humidity, and condensation control important parts of an unvented attic design.
• Roof-deck spray foam may not be the best option when the roof is leaking, damaged, or nearing replacement.
Open cell spray foam allows moisture to move through it, while closed cell spray foam is much more resistant to water and moisture movement.
That difference can affect how quickly a homeowner discovers a roof leak.
Spray foam is often discussed as if it were one material. In reality, open cell and closed cell spray foam behave very differently when water enters a roof assembly.
Open cell spray foam has a softer, less dense structure.
Because it is more vapor-permeable, moisture can move through it more easily than it can through closed cell foam.
If the roof leaks, the water may:
These warning signs can help reveal a problem before the roof deck experiences more extensive damage.
Closed cell spray foam is harder, denser, and much less permeable. It resists liquid water and limits the movement of moisture vapor.
Those qualities can be useful in certain parts of a home as part of a bigger system. Against the underside of a roof deck, however, they create an important trade-off.
If water gets through the shingles or roofing materials, it may collect between the closed cell foam and the wood. Because the foam resists water, the leak may not drip into the attic or create an obvious stain.
The water may:
If the wood remains wet long enough, it can develop mold-like growth, decay, or structural damage before the homeowner realizes there is a problem.
This does not mean closed cell spray foam creates the leak. The water is still entering through a problem in the roofing system. The concern is that closed cell foam can make that leak harder to detect.
Open cell foam offers more moisture movement and drying potential, while closed cell foam provides greater resistance to moisture movement and more R-Value per inch.
Here is a quick look at how the two materials compare.
|
Consideration |
Open Cell Spray Foam |
Closed Cell Spray Foam |
|
Structure |
Softer and less dense |
Harder and denser |
|
R-Value Per Inch |
R-3.6 to R-3.9 |
R-6 to R-7 |
|
Water Vapor |
Allows more movement |
Limits more movement |
|
Liquid Water |
Doesn’t absorb water |
Doesn’t absorb water |
|
Leak Visibility |
Water or staining may become visible |
Water may remain hidden between the foam and roof deck |
|
Space Required |
More thickness is generally needed |
More R-Value can fit into less space |
Neither foam is automatically right for every roof.
Open cell spray foam offers more drying potential and may provide earlier evidence of a leak. Closed cell spray foam provides more R-Value per inch and greater vapor resistance, but it can hide water entering from above.
For a homeowner, the decision should come down to more than the advertised R-Value. It should account for:
Neither open cell nor closed cell spray foam causes wood rot on its own.
Roof decking begins to rot when it remains wet for an extended period.
The difference is what can happen after water enters the assembly.
With open cell spray foam, the moisture can move through the foam more easily and may become visible. The roof deck also has more ability to dry inward toward the attic.
With closed cell spray foam, the water may remain concealed against the roof deck, and the wood has less ability to dry inward.
In both cases, the source of the moisture must be corrected. That could be a roof leak, indoor air reaching a cold roof deck, excessive humidity, or wood that was already wet before the foam was installed.
Spray foam cannot repair a leaking roof or reverse existing wood damage. It should only be installed after the roof deck has been inspected and confirmed to be dry and sound.
Moisture can reach the roof deck from outside the home, from inside the home, or from materials that were already wet before the insulation was installed.
Finding the source matters because each problem requires a different solution.
This is the most obvious source of roof moisture.
Rain or melting snow can enter through:
Spray foam should never be used as a substitute for roof repair. If the roof leaks, the exterior problem needs to be found and fixed.
Spraying foam beneath the leak might hide the symptoms for a while, but it does not make the water disappear. Water is annoyingly persistent like that.
Not all roof-deck moisture comes through a hole in the shingles.
Warm air inside your home contains moisture. If that air leaks into the attic and touches a cold roof deck, the moisture can condense.
It is the same reason water appears on the outside of a cold drink. The glass is not leaking. Moisture in the surrounding air is turning into liquid when it touches the cold surface.
Indoor air can reach the attic through:
Spray foam can help reduce that airflow when it creates one continuous air seal. If there are gaps, thin areas, or missed connections, moisture-carrying air may still reach the roof deck.
Everyday activities add moisture to the air.
That includes:
Michigan winters make this especially important. The colder it gets outside, the colder the roof deck becomes. If indoor humidity is too high, condensation becomes more likely when warm indoor air reaches that cold wood.
There is no single indoor humidity setting that works for every home and every winter day. The appropriate level depends on the outdoor temperature, windows, ventilation, insulation, and the way the home is built.
Sometimes the moisture problem started before the spray foam contractor arrived.
The wood could be damp because of:
Installing foam over wet wood will not solve the problem. Depending on the foam and the roof assembly, it could make it harder for the wood to dry.
Yes. Open cell spray foam allows moisture to move through it, but that does not make the roof assembly immune to leaks, condensation, or wood damage.
Open cell foam can offer more inward drying potential than closed cell foam. It may also allow water from a roof leak to become visible sooner.
However, moisture can still cause problems if:
Allowing moisture to pass through is not the same as keeping the roof dry.
Open cell foam spray foam doesn’t absorb water, which is good news, and that visible water, staining, or dampness may alert the homeowner that there is a problem. That does not guarantee the leak will appear immediately or directly beneath the damaged roofing.
No. Closed cell spray foam does not automatically cause hidden damage, but it can make water intrusion more difficult to detect if the roof develops a leak.
A dry, properly built roof assembly can perform without rotting simply because closed cell foam is present.
The concern is what happens when something goes wrong above the foam.
Shingles age. Flashing can fail. Storms can damage roofing materials. When water reaches decking covered with closed cell foam, the usual warning signs may not appear inside the attic.
The water may remain between the foam and the roof deck while the attic side of the foam looks dry. This makes the condition and remaining life of the roof especially important before installation.
If the shingles will likely need to be replaced soon, completing the roofing project first may be the smarter move.
Closed cell foam also adheres firmly to the roof deck. That can make future sheathing repairs more involved because the foam may need to be removed around the damaged section.
None of this means closed cell foam is universally wrong for roofs. It means homeowners should understand the trade-off before choosing it.
Yes. The roof and underside of the roof deck should be checked before spray foam covers the wood.
The contractor should look for:
If the contractor finds wet, soft, or damaged wood, the installation should wait until the source is identified and the necessary repairs are completed.
A moisture meter may help determine whether the wood is ready for insulation. However, the acceptable reading should be based on the foam manufacturer’s instructions and an evaluation of the wood.
There is no reason to publish one universal number and pretend every roof, foam product, and piece of wood is exactly the same.
Roof-deck spray foam may make sense when the roof is sound and the attic needs to be brought inside the home’s insulated boundary.
That could include homes with:
In these situations, insulating the roofline may be more practical than trying to separate the attic completely from the rest of the home.
The roof should still be dry and in good condition. The contractor should also have a clear plan for controlling moisture and handling the existing vents.
Insulating the attic floor may make more sense when the attic can remain vented and does not need to be part of the home’s insulated space.
An attic-floor system may be a better fit when:
Fiberglass or blown-in cellulose may be options for this type of attic.
Keep in mind that insulation and air sealing are not always the same thing. Fibrous insulation can slow heat movement, but it does not automatically stop warm indoor air from leaking into the attic.
The best choice depends on the home, not on which insulation product wins the internet popularity contest.
Spray foam should not rot a healthy roof simply because it is installed beneath the decking.
The bigger concern is what happens when moisture gets into the assembly.
Open cell spray foam allows more moisture movement, so a leak may soak through or leave visible signs. Closed cell spray foam resists water and can keep a leak hidden between the foam and the roof deck. Neither option makes roof inspections, maintenance, and moisture control unnecessary.
Before moving forward, have the roof evaluated and ask how the complete assembly will handle moisture. Make sure the contractor can explain why a particular foam is being recommended, how condensation will be controlled, where the roof can dry, and what would happen if the roof leaks in the future.
If the roof is damaged or nearing replacement, take care of that work first. If the roof is in good condition, an experienced insulation contractor can help determine whether open cell spray foam, closed cell spray foam, or attic-floor insulation makes the most sense for your Michigan home.
If you want to learn more about spray foam insulation for your attic, check out our Learning Center.
Open Cell or Closed Cell Spray Foam: Which is Best for the Attic and Roof?
Spray Foam Attic Floor or Rafters: Which is Best to Insulate?
How Thick Does Spray Foam Insulation Need to Be?
Spray foam does not directly cause roof rot.
Open cell foam allows more moisture movement and drying, while closed cell foam can conceal leaks. Wood rot develops when the roof deck remains wet.
Closed cell spray foam is more likely to hide a leak because it resists liquid water and limits moisture movement.
Water may remain between the foam and roof deck instead of dripping into the attic.
Water may pass through open cell spray foam, creating staining or another visible warning.
However, water can travel before appearing, so the visible spot may not identify the leak’s exact location.
Open cell foam is more vapor-permeable and allows more drying than closed cell foam.
Open cell spray foam doesn’t absorb or retain any moisture.
Closed cell foam does not create rot by itself.
The concern is that it can hide water entering through the roof and limit the deck’s ability to dry toward the attic. If the wood remains wet, rot can develop.
There is no universal answer.
Open cell foam provides more inward drying and may reveal leaks sooner. Closed cell foam provides more R-Value per inch and greater vapor resistance but may conceal leaks. With that being said, both create an air seal.
The complete roof assembly and Michigan’s climate should guide the choice.
No.
The source of the moisture should be found, repairs should be completed, and the wood should be dry and sound before installation.
Yes, but foam attached to the decking can make the repair more complicated.
The roofer and insulation contractor may need to coordinate the repair and replacement of the foam.
No.
Spray foam insulation is not a substitute for roofing repairs. The exterior source of the leak must be found and fixed before the roof deck is insulated.