Blisters in asphalt shingles

This subject is a transfer from another threadto avoid drifting that thread.

Inspectors are free to say whatever they want about blisters after doing their own research. I researched the Mastering Roof Inspections article but only included a few sentences about blister that reflected what I found.
Mainly what I found was different opinions by people/companies with different motivations and perspectives

My understanding is that in the vast majority of cases, insurance claims alleging manufacturing defects based on blisters will be denied, and that only in extreme cases (blisters at a level approaching delamination) will blisters shorten the lifespan of an asphalt shingle roof.

If you disagree, that’s your prerogative. Here’s links to articles on blisters:

Google result: blisters asphalt shingles

Here’s the Inspectopedia page on blisters.

I’ve been reading the* Manual of Low Slope Roofing Systems* by CW Griffin and RL Fricklas. In the BUR section they spend 10 extremely technical pages on blister development, including formulas for calculating pressures inside blister voids and detailed information on how they start and propagate.

They’re very convincing, since they cite studies and offer detailed explanations. Most of what you’ll find, including the search results, talk in generalities and you have to accept or reject them based on how credible you think the author was, not whether the explanation makes sense.

Obviously BUR and asphalt shingles are two different products, but we’re talking about felt and bitumen, and the explanation in this book makes a lot of sense.

Blisters are caused by liquid water or air/moisture vapor trapped in the asphalt or on the mat before asphalt is applied, which can happen in a number of ways. When the sun heats the roof, trapped liquid water evaporates and expands to about 1500 times its original volume, creating pressure and/in a small void. In some types of low-slope membranes, the bond between bitumen and felt (mat or or substrate) is weaker than the tensile strength of the membrane and failure will happen first in adhesion around the perimeter of the void. The blister will retain its cap and grow in diameter.

If the tensile strength of the overlying membrane is weaker than the mat/bitumen bond (asphalt shingles) the pressure will cause the cap to detach and you’ll be looking at an open blister typical of asphalt shingles. The size of them blister depends on the relative strength of the adhesive VS that of the layer of asphalt that forms the cap of the blister.

That’s the short version but it’s more detail than you’ll get from most articles on blisters.

I’m in the process of writing a course on commercial roofing. Here’s what I’ve written so far on blistering in BUR, and a lot of it is relevant to asphalt shingles:


Blisters can range in size from barely detectable, to 6 inches tall and covering 50 sq. feet. A few small blisters are tolerable and although it may require some repair, the membrane will achieve a full lifespan.

Larger blisters accelerate physical and chemical degradation. A built-up with numerous large blisters is at or near the end of its useful life and will need to be replaced soon.

Blister Development

All blisters develop from voids in the mopping bitumen, either between interply felts, or between felt and the substrate. Voids usually (not always) are the result of the original application.

Causes of voids:
• Moisture in or on the felts (either top or bottom ply mopped).
• Use of felt rolls that had been stored lying down instead of standing on end. Over time rolls stored in this manner develop and oval shape.
• Uncoated portions of felt due to poor mopping or clogged holes in the bitumen-dispensing machine.
• Failure to broom out entrapped air.
• Distorted or damaged insulation board.
• Improper bitumen viscosity Often too high, but occasionally too low.
• Entrapment of foreign material (dirt, stray aggregate or windborne debris, etc.).
• Tenting or ridging caused by expansion of insulation or underlying plies.
• Unfilled voids in the insulation (damage, divots, broken corners, etc.).
• Upturned metal flanges of improperly set flashing or other components.
• Unfilled edges/corners of cant strips, tapered edge strips, or blocking.
• Side- or end-laps skipped over by the mop.
• Fishmouths or wrinkles in plies.

Blister Mechanics
When moisture finds its way into a void and is heated by the sun, pressure within the void develops that can become strong enough to break the adhesive bond around the perimeter of the void. The void has now started becoming a blister.

Moisture can enter voids from below; rigid board insulation averages about 2% moisture content, or the source may be the building interior. If activities in the interior generate large amounts of moisture vapor that is not ventilated adequately, vapor can condense on the underside of the roof deck and through diffusion, absorption, and/or capillary action… through joints in the insulation and micro-cracks in the felt/bitumen layers… it can enter a void.
Moisture can also enter voids from above; if hot mopping or bitumen spreading starts in the morning before dew has evaporated, liquid water may be sealed into the membrane.

Water-based Blisters

Figure: BUR Pressure within blister

Membrane temperatures can easily rise from 70°F to 150°F during the day.

Blisters grow at the fastest rate if liquid water is trapped in the membrane, and the process is powered by heat. If during the day the roof membrane experiences an 80°F temperature rise like that mentioned in the caption above, water trapped inside a void will evaporate, expanding to about 1500 times its original liquid volume as it turns to vapor. This expansion creates pressure that is relieved by adhesion failure around the perimeter of the void, resulting in growth of the blister.

Figure: Blister Development

Blisters may develop single interply or substrate chambers, or multiple interply chambers.

Vapor-based Blisters**
Blisters can also form without liquid water when, during hot mopping operations, air/moisture vapor becomes trapped in the bitumen. As the bitumen cools it hardens, trapping the air/moisture vapor. As it cools, the vapor will condense, pressure in the blister will drop, becoming negative, and the membrane will lie flat. As the sun again heats the membrane, the moisture will evaporate, blister internal pressure will rise, but the trapped vapor will not start to create positive pressure until the temperature of the membrane exceeds the temperature at which the vapor was originally trapped (approximately 100°F +).

Thermal Pumping**
Blisters will grow very slowly or not at all unless additional air/moisture vapor is introduced into the blister interior on a regular basis. The process that contributes to this is called “thermal pumping” and it’s similar to respiration.

As bitumen cools, negative pressure pulls additional air /water vapor into the void (inhalation). This entry can occur either through micro-cracks in the membrane, or through gaseous diffusion through the membrane or system components.

As the bitumen is heated, internal pressure rises and this condition tries to push air out of the blister (exhalation). The sun-heated bitumen eventually softens and tends to seal micro-cracks, which reduces exhalation.

When the membrane begins to cool again, contraction reopens the micro-cracks. Because the micro-cracks are sealed during part of the day, the net result is greater daily inhalation than daily exhalation, and the resulting rising internal pressure will cause blister growth to continue to grow steadily in small increments.

Size limitations
Small blisters may generate void pressures of 2psi (288psf). Assuming an aggregate-surfaced membrane that weighs 6psf, membrane weight is clearly not a hindrance to blister growth. As blisters grow larger, the weight of the membrane becomes an increasingly important factor in limiting growth. Eventually the weight of the membrane exceeds the pressure generated by the vapor in the void and growth stops. Again assuming an aggregate-surfaced membrane that weighs 6psf, the theoretical maximum blister diameter is about 8 feet.

For both types of blisters, blister growth reduces pressure and growth will not resume until additional moisture/vapor enters the void, or membrane temperature increases sufficiently.

The main question I get as a thermographor for a flat commercial roof as asked from the roofing contractor (we need to know if moisture is present beneath the substrate to determine if a tear off is required or a overlay will work)

Thanks Kenton

I am confused. The title says blisters in asphalt shingles, which made me curious because I never heard of asphalt shingle blisters, but the article is talking about BUR, which does not surprise, as BUR will blister. So what about asphalt shingle blisters?

I’ve seen it from time to time. Usually the blisters have popped. Once they have popped and leave a crater. The crater is always small, 1/16" or smaller.

Is present or to what degree, and what’s required. Good point Charlie, thanks.

Erik, in some parts of N. America, especially in the past, blisters in asphalt shingles have been fairly common.

I’m in the process of researching low-slope built-up roofs, and the information I found seemed relevant to questions about blisters in asphalt shingles, so I posted it. Built-up roofs and asphalt shingles have components (and problems) in common.

If as inspectors we can understand the characteristics of the basic materials from which roofing assemblies are made, and how common materials interact with each other under circumstances common to certain climate zones and within areas of common building practices, we improve our skills.

No moisture is allowed for a overlay must be dry or it voids any MFG warranty.

So Charlie, I know you can spot moisture in a low-slope roof using IR if the conditions are right. Are you limited by the source of the moisture, like an interior swimming pool or manufacturing facilities that release a lot of moisture into interior air (which might affect the density of the moisture deposit that would show up on your camera) and its form (vapor VS liquid)?

For determining if a overlay is possible the source of the moisture is not my concern, I only want to know if moisture is present or its dry.

Blister development is a similar process. With asphalt shingles it has to do with moisture, voids, or debris between the asphalt mat and the overlying bitumen (asphalt) layer. Over time, due to thermal influences, gasses in these voids expand into bubbles.