Blistering Paint: Solving the Problem
I was raised in several colonial and Victorian era homes on the New England coast and from the moment my father first taught me how to paint our house, my summers consisted of more hours behind the brush than I care to remember.
As anyone knows who has scraped and sanded paint from a house, applied primer to house and then lathered finish coats onto house, there is no more frustrating feeling than watching your hard work blister and peel off prematurely the following summer.
One of the things I see regularly during my home inspections are older homes that appear to have been painted recently but seem to be suffering from a premature blistering paint problem. Sometimes you can even poke one of the blisters with a knife and water will ooze out of the paint pocket; an alarming moment to a prospective home buyer.
When I see this, I like to survey my clients about what they feel is happening. Some of the usual suspects are: the siding was not properly scrapped and primed, the seller was using cheap paint or they painted in the rain without allowing the building to properly dry. While all of these causes are plausible, there is a more likely reason that often goes over-looked. To understand, let’s think about the difference between old houses and new houses.
Why is this happening?
For the purposes of this article, I would consider an old house any house built prior to our energy codes – think late 1960’s and earlier. These homes are not very air tight compared to our newer homes so there is generally more air moving through the walls. This is both good and bad. The drawback is heat loss and higher energy bills. The upside is the resulting dry potential – the air moving through the walls helps keep the building dry. This is one of the things that makes older homes less susceptible to rot and wood decay than modern homes.
So, when you see blistering paint on the outside of an old house, there is a pretty good chance you are witness to the effects of heat loss – air moving through the wall, carrying moisture in the form of vapor. This does not usually hurt your building but it does create expensive and frustrating on-going painting costs.
Why is this a new phenomenon?
In the old days, this was seldom a problem. The reason is twofold. First, we used to paint our buildings with oil based paint, which does not trap moisture as effectively as latex paint. Also, they used to fasten siding to homes using nails with rounded heads – back when siding was hand-nailed and not applied with a pneumatic nail gun. These old nail heads created a slight gap between boards – a place where moisture could escape. Today’s older homes have many layers of paint and the bottom edges of the siding are now sealed by successive layers of latex. The result: there is no place for that moisture to escape so it bubbles off the finish paint in blisters as moisture does what it always does – goes from concentrations of more to concentrations of less.
How can I fix this problem?
To fix this problem, start outside by cutting open the paint on bottom edge of the lap siding with a razor knife. In a few places, try using a cedar shim to open up the siding slightly to let the wall breathe. Inside the house, be mindful of relative humidity: try and keep the interior dry using kitchen fans and bath fans to vent moisture outside. Aim for indoor relative humidity below 55%. More extensive air seal-up repairs can also help. This will make the home more energy efficient as you slow heat loss through the house.
I hope this helps explain an unusual phenomenon we often see when out looking at houses. Remember: knowledge is power when looking at homes. Enjoy your house hunting.