What happens with a roof leak and spray foam?

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Some have inquired as to how a spray foamed attic responds to a roof leak. In this article we'll try to explain how spray foam reacts to a roof leak, what might be needed to be reparied and we'll do an experiement on how quickly spray foam becomes saturated and transmits water. Like many new products that are different, spray foam's use is often misunderstood and maligned, often by its traditional insulation competitors.

In this section we'd like to present a number of assumptions, facts and notes. 

Here's the situation:

  1. The attic ceiling (which is under the roof decking) has been sprayed with open-cell polyurethane foam
  2. The attic foam has a typical depth of 5.5"-6" thereby covering the roof rafter.
  3. Other open cell foams may have similar but different properties. This article discusses open-cell spray foam as opposed to closed-cell spray foam which has very low water absorption.
  4. The roof decking is made from composite board or plywood covered with tar paper or a similar water barrier.

Consider These

  1. Attic spray foam adds structural strength to the roof system particularly increasing its racking strength. In periods of high winds and rain, not only will spray foam minimize roof tear-off damage but will minimize water penetration. This video (1:35 min) demonstrates how water penetrates a roof after significant roof damage occurred.

    This follow-on video demonstrates the benefits of a particular roof sealing product.

    While polyurethane spray foam and similar open cell foams are different from adhesive and water barrier sprays the two do have water penetration similarities and will not allow short-term water penetration as this video demonstrates.
    Furthermore, a study by Alan G. Davenport Wind Engineering Group of the University of Western Ontario showed that an unvented attic performed much better than a traditional vented attic during wind-driven rain. Unvented attics exhibit greatly reduced property and water damage during severe storms.
  2. Often when roof decking is removed for repair the underlying spray foam remains intact. Since the underlying spray foam is intact, new roof decking can be reapplied normally. With new decking installed, the previous foam-to-deck barrier at the repair location is no longer present.  Therefore, the foam's added structural strength (of being physically attached to the decking) at the repair site has been lost.  However, as long as the foam is still intact in the rafter cavity then it's thermal qualities remain.
  3. Small roof leaks often take months or even years to be noticed. During this time, roof decking and (to a much lesser degree) roof rafters rot and mold develops. Under most circumstances spray foam prevents leaks from penetrating the attic and just sheds most water off the roof's deck. Water that does penetrate the roof deck and get past the foam will be from 1) a non-foamed area such as a roof penetration or 2) saturation of foam over an extended length rain storm or 3) both 1 and 2.
  4. Foam spraying an attic has a number of other distinct advantages over traditional attic insulations such as 1) a cooler attic temperature making the home's HVAC system more efficient, 2) provides for temperate attic storage area and 3) provides an exellent air barrier against ar infiltration, insect (such as tiny sugar ants) and polluants.
  5. Even when spray foam is added to an attic's ceiling, at some future time additional batt or blown attic insulation can be always added.
  6. This video demonstrates the insulation benefits of spray foam with roof snow on a northern US residence.
    Here's a short video demonstrating.

  7. High-pitched roofs or those with a 5/12 or high roof slope suffer fewer roof leaks except at roof penetrations. Generally, what's more prevalent is roof damage caused by hail and high winds. In these situations, spray foam provides additional protection from water infiltration. The leaks that do occur are often around roof penetrations.
  8. For the typical attic ceiling application open cell spray polyurethane foam is used. This foam is water permeable and will, over time, saturate and drip water. Conversely, when wet, spray foam will dry quickly.
  9. Spray foam also reduces the movement of moisture-laden air near outside walls where the moisture typically tends to condense.
  10. Spray foaming an attic precludes having to otherwise properly vent an attic. "Proper" roof ventilation means a residence will have on-going energy-related costs due to power attic vent(s) or on-going maintenance issues with solar powered attic vent(s) not mention bug and dust infiltration.
  11. Spray foam provides no food for mold or mildew.
  12. As an air barrier, spray foam inhibits the movement of air-borne moisture, spores, food for mold, pollen and other contaminants into the structure.
  13. CertainTeed brand spray foam is not considered a vapor retarder (when tested in accordance with the water penetration transmission standard ASTM E96).  If required by local code, a vapor retarder must be applied.

A Single Shingle is Torn Off - Two Example Cases
Take the situation of one single shingle (and tar paper) being torn-off by a high-wind storm accompanied by 2 hours of rain. In this example, the roof tear results in a small leak of about 1/4 gallon per hour or a 1/2 gallon water leak during the life of the rain storm. So, as a homeowner consider which of the two scenarios is better?

Case 1 - the attic ceiling has no spray foam. The water penetrates through the roof decking, begins dripping on the attic insulation and finally to the ceiling sheetrock where it collects until it seeps out through the sheetrock or through a light fixture. The homeowner immediately hears water dripping.  At this point, the attic insulation is soaked., the ceiling sheetrock is saturated and has a wet stain ring requiring repair.

Case 2- the attic ceiling has spray foam. The water penetrates through the roof decking and begins a very slow penetration of the spray foam. After two hours some small amount of water penetrates the foam and begins dripping on the attic insulation.  At this time the rain stops. The leak only slightly wets the attic insulation and causes no sheetrock damage. With the rain storm over there's no longer a water source. Following the storm, drier humidity arrives, the sun heats the roof desk and the spray foam begins to dry. No visible mold is created. Having no water in the house, the next day the homeowner notices a missing roof shingle and initiates a roof repair. His shingle repair is his only repair expense as he has no interior water damage.

Conclusion:  Attic spray foam reduces water infiltration thereby reducing water damage.

A Personal Water-Through-Spray Foam Penetration Experiment
While ASTM testing results may mean something to the technically trained, for most, these formal test results are meaningless. So, this spray foam absorption test was done.

Wet Open Cell Foam Block

A 2"x6"x6" block of CertainTeed open cell spray foam was cut from a larger block. In the larger side, the foam was cut-out leaving a 1" dug-out impression with a total excavation area of  4-5 square inches. The foam block was placed atop two straws over a paper towel to provide visual separation. Tap water was poured into the scooped cavity nearly filling it.

After about 1 hour moisture could be felt on the foam's outside edge. At about 2 hours moisture on the paper towel could be seen.  After 3 hours the paper towel had gotten partially wet. For the next several hours a rough equilibrium occurred between the slow dripping on the paper towel and the drying of the towel's large surface area.

By the next morning or about 8 hours later, no water remained in the foam's cavity although the foam was still partially wet to the touch but was not dripping. No water was seen on the counter top past the paper towel's edge. Apparently, the foam's slow water drip and the paper towel's water disipation in the low-to-moderate room humidity created its own wet-to-dry equilibrium. Within 24 hours the test foam was dry to the touch even when compressed.

Conclusion: While spray foam leaks water, it's really quite slow. In the situation where a small roof leak exists in a dry air environment, the leak might not be noticed for an extended period of time. Since applied attic spray foam is about 3 times thicker than the 2" thick experiment block above only rains lasting hours would produce water drip.

Homeowner Concern: "With a spray foamed attic an unnoticed roof leak might damage roof decking before the leak is noticed." 
Conversations on this situation with several roofers have confirmed that there is virtually no repair difference between a spray foamed attic and one without foam. Not still not convinced? Call your favorite roofer and ask.


Roof Leaks by Foam Types - Open Cell and Closed Cell

Closed cell spray foam is a vapor (water) barrier; open cell spray foam is not a water barrier and does allow water transmission over hours.

When closed cell spray foam is sprayed on the underside of roof rafters and if the roof leaks the closed cell foam will not allow the water to weep through. This can lead to rotting of the roof deck before the roof leak is discovered and repaired. Depending upon the circumstances this can be an advantage or disadvantage. For example, a property owner may not want water penetration under any circumstances and closed cell spray foam will provide an additional layer of protection. Unlike most open cell installations, tearing off roof decking will damage the rigid closed cell installation.

Since open cell foam is not a vapor barrier water will drip or weep from the roof leak through the foam to the space below. After the leak is repaired, the open cell foam can be allowed to dry and its thermal properties return unlike traditional insulations that must be replaced to maintain R-value.

Is Spray Foam Currently Specified by Builders and Architects? The short answer is yes. Consider:

  1. Leading green architects, city green programs are specifying spray foam in walls and attics. A leading Austin builder has created this attic spray foam video and calls spray foaming an attic "a perfect application".

  2. In Austin, Many of the city's Green Building Case Studies homes have spray foam such as this example residence.

In summary, if spray foam is good solution for a number of reasons. Reputable independent and professionals often recommend spray foam especially for mid-to-high-end homes and for those wanting green building status (including the LEED program) and other special need situations.