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Exterior Paint Blistering - Why?


We purchased a 1890's era house on the Front Range of Colorado a little less than 2 years ago. We have a problem with blistering paint on the north side of our house. When we removed the blistered paint it exposed the bare wood underneath. From all the info on the internet, this would suggest that we have moisture building up from inside. The problem is that we can't figure out where the moisture might be coming from. We did install a humidifier on the downstairs gas furnace last fall (because we have incredibly dry air) but that only humidifies the downstairs. Most of the blistering is occurring on the exterior of the second floor. And most of it is happening on the exterior wall of a walk-in closet. The closet has a heating vent but we shut it off last year to conserve energy. There is virtually no blistering on that wall in the area of the upstairs bathroom, however, where we would be more likely to expect it. The sofit extends quite a distance over the portion of the wall that is blistering and we cannot find any leaks in the gutters or the roof. We're stumped! Any ideas?

Re: Exterior Paint Blistering - Why?

You could have a situation where heat is escaping out near the closet, causing moisture when meeting the cold exterior.

Another possible cause could be that the bad painting job was done over wood which wasn't completely dry at the time.

Re: Exterior Paint Blistering - Why?


An old law of physics is that temperature and moisture will try to even itself out on both sides of a permeable membrane - in this case your house walls.

Colorado is bone dry both summer and winter, however, in winter the temperature can be drastically lower outside than in. The relative humidity outside is also virtually nothing when it is down in single digits outside. Now to the equation you add moisture from showering,laundry, breathing, cooking and now a humidifier. Your moist, warm air is trying to get to the bone dry, cold exterior. It is getting as far as the back side of your siding where it condences and freezes. As the siding warms up, this moisture is drawn into the siding. Come springtime, all that accumulated wetness tries to get out NOW! When it can´t get out, the vapor pressure pushes the paint film out of the way - peeling!

Your observation about the lesser peeling outside the colset area is interesting. Why would that be so? By closing the door to the closet and turning the vent off, you have lowered the temperature and the humidity in the closet. The differential between the inside and outside has now been lessened.

The converse is often seen on exterior walls outside bathrooms: warm humid air exists in the bath. The differential is now accentuated. Peeling is more pronounced.

Modern homes are typically built with a vapor barrier behind the drywall to stop this moisture movement into the wall cavity (this also creates problems in warm, humid climates where air-conditioning is used alot, but that is another story).
New homes also have insulation to slow down the moisture and heat transfer.

The question I would ask now is does your home have any type of vapor barrier and insulation? You need to slow both heat and moisture movement into the wall cavity. You want to make sure that sudden excess moisture, such as from showers and cooking, is rapidly vented directly to the exterior (not the attic!).

Is your house on a crawl space or basement? A home of your age would typically have ballon construction with little or no barriers to block air and moisture moving right from the basement up to the attic. If so, you want vapor barriers below on grade and blocks to prevent moisture from migrating into the wall cavities.

What I would advise is that you get some expert help. One place to turn is to your local gas and electric companies. They often have no or low cost energy audits to point out what steps to take to stop energy loss. It is actually a bonus. When you have stopped or slowed the energy loss, you will also have stopped or slowed the moisture transfer. The utility companies also have a list of reputable contractors to assist in the needed remedies.

I hope this has given some insight,however, I don´t claim to be an expert, just an old painting contractor who is aware of what some of the mechanics that are occurring.

Re: Exterior Paint Blistering - Why?

Thank you for your replies.

ordjen - that all makes a lot of sense. But one point of clarification, we are seeing the most peeling on the area outside of the closet and very little outside of the bathroom. That is what has us so perplexed! We don't know if the outside closet wall has insulation or not. The previous owners popped the top on the house around 10 or 15 years ago and we have no idea what they did. We do have a crawlspace (root cellar) with part of it made into a semi-finished basement. We have noted that it sometimes feels a little damp down there but have never actually seen water or condensate anywhere. Hmm...the mystery continues...

Thanks again for you help!

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