Does Spray Foam Shorten Shingle Life?

For years I’ve been asked by homeowners, “Bob, I’ve heard that spray foam will disqualify my shingle warranty, and reduce the life of my shingle roof, because of higher shingle temperatures. Is that true?”

Let me ask, “Does roof ventilation significantly lower the temperature of asphalt shingles?

I am quick to tell anyone that Mama didn’t birth a scientist. But I can read!

I will tell you what the scientists have put into print, in official publications.

Too many shingle manufacturers (is it all of them?), “... imply that unvented sheathing can get hot enough to shorten shingle life, since they provide their full warranty coverage only for shingles installed over ventilated sheathing”. (Aspenpublications.com … an energy design group).

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In all instances, changing roof color from black to white has more effect on yearly mean temperature than ventilation.

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William Rose is a veteran shingle investigator and research professor (I didn’t know such a person existed!) at the University of Illinois.

Rose wrote a paper entitled, “Measured summer values of sheathing and shingle temperatures for residential attics...”

Carl Cash & Edward Lyon wrote an article entitled, “What’s the value of ventilation”, and it was published in Professional Roofing magazine.

Go to Aspenpublications.com to read the scientific findings. Here, I’m only going to give you the punch line.

  1. Rose found that ventilation does lower shingle temperatures, while other factors, like shingle color and roof orientation have a GREATER EFFECT on shingle temperature.

    1. Ventilation lowered shingle temperature 2.7%

    2. White shingles were found to be 20% to 23% cooler than black shingles.

    3. The north slope of a roof was 8.7% cooler than the south roof slope.

2. Keep in mind, the manufacturers don’t want to honor warranties any more than other manufacturers. According to the article, two of the biggest reasons warranty claims are denied, is for improper installation, and, … “there’s a problem with the ventilation”.

In Cash and Lyon’s article, here is the quote I want you to see:

”We found roof temperature extremes do not relate directly to mean temperatures for service life. In all instances, changing roof color from black to white has more effect on yearly mean temperature than ventilation. Ventilation reduces the yearly mean temperature of a black roof system by an average 0.7 degrees Celsius and changing to white shingles reduces the yearly mean temperature by an average 1.6 degrees Celsius.”

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Here is my Take-Away: variations in geography, direction, and shingle color has a greater effect on shingle temperatures than does ventilation.

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Conclusions in the Cash and Lyons article:

  • Geographic location is the greatest influence on roof temperature. Miami vs Green Bay, Wisconsin has a 10 degree Celsius difference!

  • The second greatest influence is the direction the roof faces. South-facing roofs are hotter than north-facing roofs.

  • Barely the third greatest influence (it’s very close between #2 and #3) in roof deck temperature is thecolor of shingles. Lighter colored shingles are definitely cooler than darker colored shingles.

  • Attic ventilation reduces the average temperature of a roof deck shingle by only 0.5 degrees Celsius, (0.9 degrees Fahrenheit).

  • Roof slope (pitch or angle) has the least influence on roof deck temperatures.

Here is my Take-Away: variations in geography, direction, and shingle color has a greater effect on shingle temperatures than does ventilation.

Here’s a good question: If ventilation has such a small influence over shingle temperatures when compared to other factors, why are the manufacturers isolating it as such a vitally important component in honoring warranties?

Here are questions generated from the research:

  • Do shingle manufacturers make dark shingles? If so, they accept a 27% increase in shingle temperature.

  • Do shingle manufacturers make shingles for the south direction of a roof? If so, they accept a 14% increase in shingle temperature.

  • Yet, a 2.7%increase in shingle temperature voids the warranty?

  • How do they explain disqualifying a warranty on a white shingled roof, positioned over a spray-foamed attic, when the shingle temperate only rose an average of 2.7% when compared to the (higher) temperature of a darker-colored shingle on a vented attic?

Now you know the facts. As it relates to spray foam in your attic, spray foam will have little to no effect on the temperature of your shingles.

After all, how important is 9/10s F of a degree to you?

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