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Sashco Savvy Technical Bulletin:

Preventing Blisters in Chinking Material

Occasionally, blistering of a chinked surface occurs. After several years of investigation, Sashco Sealants has

uncovered most or all of the causes* of this problem and can recommend ways to prevent it.

Causes of Blisters

Basic Cause

The basic cause of any blister is a build-up of gas between the log or backing surface and the chinking. It occurs

during the early curing stages when the chinking is soft but has a skin formed on the surface. Heat, usually in

the form of direct sunlight, causes the gas to expand. As the gas expands, it pushes the chinking out, causing a

“bubble” or blister. This gas comes from either water, wood sap, or even backing material.

What circumstances create the gas?

Trapped Water Vapor

As chinking dries, it gives off water vapor. Any physical condition which traps the resulting water vapor could

cause a blister, especially if the logs and chinking are heated by direct sunlight. Here are some things that could

trap this vapor.

1. Crack or indentation in styrofoam or

polyethylene backer rod which doesn’t

go all the way through. Even tiny

spaces can trap enough vapor to

create a blister big enough to see.

2. Air bubble trapped during the

application process against

non-porous backer material.

*We at first thought the chinking itself

was to blame. But facts do not support

this. For example:

1. The same batch of product has blistered on one log and not on another.

2. It is common to find blisters on one side of a house and not on the other,

3. Blisters occur above a certain point on a wall and not below it.

4. Some blisters appear to be random on a chinking line and others are in a straight row.

5. Blisters often reappear in the same place when repaired with the same batch of chinking or even

with a different batch.

Summary of content (4 pages)

PAGE 1
Sashco Savvy Technical Bulletin: Preventing Blisters in Chinking Material Occasionally, blistering of a chinked surface occurs. After several years of investigation, Sashco Sealants has uncovered most or all of the causes* of this problem and can recommend ways to prevent it. Causes of Blisters Basic Cause The basic cause of any blister is a build-up of gas between the log or backing surface and the chinking.
PAGE 2
3. Dead-end crack in wood where moisture vapor can accumulate and expand when heated. (This, most likely, is the most wide-spread cause.) Backing Material 4. Backer rod out-gassing. Closed cell backer rod has a puncture and is squeezed by expanding logs. The gas used to make the rod forces a blister in the chinking. On day 1 the backer rod and sealant are applied in the cool part of the day.
PAGE 3
Pitch Pockets 5. Logs often have what is termed “pitch pockets.” These are areas of concentrated pitch, or sap, sometimes containing several gallons! (Pitch pockets of 50 or more gallons are known!) This sap (or its vapor) will follow cracks in logs, coming out of the log—almost anywhere. It especially exits at the knots where the sap flow was directed during the life of the tree. Sap has a volatile component and, when heated, forms a gas which causes blisters.
PAGE 4
Control the Backing Material 5. Use blunt tools to install closed cell backing materials, being careful to avoid puncturing or denting. 6. Open cell backer rod** may be used in place of closed cell backer rod. These backer rods will not generate gas and they can help dissipate any gas originating from another source. 7. When chinking over flat insulation (styrofoam, bead board or foil-faced foam board), surface cracks, holes or dents should be covered with high quality sticky mylar tape, 2” wide preferably.