Monolithic

Chapter Six: Application - Substrates, Water and Blisters

by David B. South April 02, 3:05 p.m. The Polyurethane Foam Book

The Foam Book has been updated and is now in print. Purchase your copy of Urethane Foam: Magic Material – and the Best Kept Insulation Secret

The following is mostly about urethane and Monolithic Domes. Many of the problems are the same as for any urethane roofing. Blisters are best prevented whenever possible.

In all our years of urethane application, we’ve been plagued with one problem that just doesn’t seem to go away. This problem is not unique to the urethane foam industry; it’s a general roofing problem: Blisters!

First and foremost, blisters are caused by trapped water that the sun can vaporize and expand in a contained area. In the construction of a Monolithic Dome, that contained area is between the Airform (dome’s roofing fabric) and the materials beneath it.

An understanding of what starts blisters would help. But no one really knows. Most people suspect that water vapor initiates blister formation. If that’s the case, how does the water vapor get there? Sometimes, it’s the inside coating and other times it’s trapped there from other sources.

We do, however, know: If water becomes trapped beneath the surface of the Airform, eventually a blister will form. Obviously, no one should spray foam against a wet surface. But a surface that appears dry may also contain small amounts of water vapor.

On the other hand, we have built domes in terrible, wet winter conditions. In those circumstances, it seems impossible not to get blisters; yet blisters do not form. We have built in wonderful summer conditions during which no blisters should ever form; yet we found blisters.

It seems that excessive probing of the foam may contribute to blistering. We suggest that probing be done only when necessary to determine proper thickness.

At times, we also suspected moisture from the actual spraying of urethane as the start of a blister. It is suggested that water traps be used in the air supplying the spray gun.

So, what is the problem with blisters? Primarily, it’s a cosmetic problem. To date, we have not had a leak develop because of a blister. Fortunately, most blisters are not hard to eliminate. Simply stick an ice pick into the blister at its lowest point or at any point on the blister where water vapor can escape as its pressure increases. This works extremely well. The blister will rapidly shrink and no longer be a factor.

But some blisters may be difficult to reach. In such a case, the blister can be pierced with an ice pick attached to a long-handled stick. An accurate shot from a pellet gun or .22 short may also work. Obviously, any time any kind of propelled object is used, great care must be taken not to hurt anyone.

In the case of older structures that have coatings rather than Airforms, the blister-removing process is more complex. Unlike an Airform, most coatings cannot shrink back to their original flatness. Consequently, if a blister has formed, it and the foam it sits on must be cut out. The spot is then repaired with a concrete or plaster patch and refoamed. For older structures with a significant number of blisters, it may be necessary to completely recover the dome. For that process, we recommend metal cladding.

Table of Contents: