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Blistering – Taking Care of a Common Problem

The problem

Blistering is a problem found in all types of roofing. It is generally caused by moisture being trapped between the layers of roofing materials and expanding. The trapped water is turned to steam by the heat of the sun. As the water turns to vapor and expands, it creates the blister. As the blister expands and contracts over days and weeks, it increases in size until it becomes unsightly and a real problem. It continues to get larger until the vapor can escape.

Blistering on Monolithic Domes

It’s usually minimal because of the materials used. Nevertheless, at times blisters will occur. Each time the sun gets hot on that same spot, it increases the size of the blister as the vapors expand.

On Monolithic Domes the simple, easy answer is to poke a hole in the bottom end of the blister. That is the entire remedy for the blister in ninety-nine per cent of the cases.

The urethane foam keeps any gross water out of the dome. So the remedy for a blister on a Monolithic Dome is a very small puncture, such as one made by an ice-pick or a thin knife blade, preferably in the bottom end of the blister.

Small holes through the membrane will not affect the insulation underneath; they simply allow whatever gas is built up in the blister to leave. Blisters should be treated as soon as possible and not allow them to grow. The tension of the fabric pulls the fabric back down to conform to the dome.

This simple operation works extremely well. The blister will rapidly shrink and no longer be a factor.

In some places, sometimes, it’s difficult to reach the blisters. The Airform can be punctured by using a long-handled stick with an ice pick attached. Remember you are making a very small hole. Therefore it will take time for the air to escape and the blister to lay down.

The cause

An understanding of what causes blisters would be helpful. The problem is, we don’t know. We think it is water vapor, but how does the water vapor get there? Sometimes, it’s the inside coating, sometimes it’s water trapped there from other sources.

A few things we do know: If there’s water trapped beneath the surface of the Airform, there will eventually be a blister. Obviously, no one should spray foam against a wet surface. But a surface that appears dry may also contain small amounts of water under certain conditions.

We have built buildings in terrible winter weather conditions. It would seem impossible not to have blisters, and there are no blisters. We have built in wonderful summer conditions where no blisters should ever form, and we find blisters. It seems that excessive probing of the foam may be a problem.

We suggest that probing be done only to the extent that is necessary to determine proper thickness. Sometimes we think it may be the moisture from the actual spraying of the urethane. It is suggested that water traps be used in the air supplying the gun for spraying.

Primarily, it’s a cosmetic problem. To date, we have never experienced a time when we’ve had a leak because of a blister. In the case of older structures that do not have the Airform, the problem is increased. The coating will not shrink.

It is still a good idea to puncture the coating to stop the blister from growing, but steps sometimes need to be taken to replace the blister. This can be done by cutting the blister out, refoaming and recoating.

For older structures that have a significant number of blisters it may be necessary to completely recover the dome.