Blistered paint raises moisture issue
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Q: My house was painted dark green last summer during the hottest part of the year. The south side especially began to blister. It was re-prepped and re-painted before the job even wrapped up. Within a few months, the bubbling recurred, especially on the south side, but this time the painters said it must be due to moisture INSIDE my house. (It's 70-plus years old, 1,020 square feet, single occupant, single bathroom against north side of house, no leaks or known moisture problems, no mold growth, dry basement.)
With no interior moisture testing, the paint company said their testing with a moisture sensor on the outside of the house proved their hypothesis. A painter friend re-checked with his own sensor on the outside and found no moisture in the bubbles on the exterior.
A: The initial coat blistered off due to it being applied to a very hot and moist surface. Heat blistering occurs only on uncured paint surfaces, so it is (and was) quickly recognized and repaired. Solvent entrapment or moisture blistering seem to be plaguing it today, the former a common occurrence with darker colors.
Solvent-entrapment blisters occur when one coat is applied before the prior has completely cured (assuming oil primer). A solvent blister will extend only through the top coat. If the blistering takes off the lower paint layers, primer and top coating, we need to look at moisture issues — so keep reading below.
Q: I own a 1945 post-WWII "Cape Cod" house. It has cedar plank siding. I had the house professionally painted last August. The painter cleaned, scraped loose paint, sanded and used oil-base primer on the entire house. He sprayed the first coat of paint and then back-rolled by hand.
When the job was finished, it appeared as if he had done an excellent job. We are now noticing bubbles popping up all over the place on the north side of the house. Some are half-dollar size, and others are as big as a baseball, filled with water.
Is there something faulty with our paint job, or does it have something to do with our climate or the type of house?
A: Baseball size? Impressive. I would consider this a feature; a source of drinking water in case of drought or earthquake.
Your house is passing gas! Water vapor is passing through your siding, inside-to-out. All buildings have moisture flowing out the walls in the winter, to the tune of several gallons per day.
Consider how much water gathers on aluminum single-pane windows. Multiply that by the size of all the exterior walls, combined. Then add a lot more for the ceiling. That's a lot of water! Newer houses stop the moisture before it gets to the insulation and can condense, via a vapor barrier. Older houses leak it out through the walls and siding, and it goes away harmlessly.
The physics of warm air always chasing cold air (the basis of the second law of thermodynamics) causes "moisture drive" towards the exterior, and the paint begins to bubble in the winter — not the sun "pulling moisture out" in the summer, as the common folk wisdom goes.
In the summer, with central air conditioning on, this is reversed and blistering and mildewing occurs on the inside of vinyl wallpaper.
After multiple coats of paint are applied over the years, the gaps between the siding pieces lessen and disappear. In the interim years, adding insulation (without a vapor retarder) to the exterior walls saves energy, but it also makes for cooler air on the back of the siding, increasing condensation. Lack of paint on the backside of the siding allows vapor to enter the siding with no way out. To eliminate future blistering you need only pick one:
a) Stop heating the house in the winter.
b) Leave the windows open continually.
c) Pull all the siding off, paint all the backsides and replace it.
d) Paint a vapor retarder on the interior walls — not highly recommend due to mixed results.
e) Add shims between each horizontal siding joint. These shims are pounded up between individual pieces, creating a small airspace. They are not noticeable to the untrained eye when installed. Wholesale paint suppliers can order this for you.
And yes, you will have to repaint that side of the house.
Darrell Hay answers readers' questions. Call 206-464-8514 to record your question, or e-mail dhay@seattletimes.com.