Blisters under paint

[exploreparadise2]

I found water-filled blisters under the paint of a 1940s house. The blisters appear on three sides of the house, at both the main and upper levels. My understanding is that using both oil and latex paint is the problem. Water vapor migrating through the walls is trapped under the paint and condenses on the cold surface. I don't know which paint must be on the top surface. Your feedback on the cause is welcome.

So what's the solution? Removing the paint? Some of the wood siding boards have warped. I don't know how long ago the last paint of coat was applied.

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[Jim Katen]

Originally posted by exploreparadise2

I found water-filled blisters under the paint of a 1940s house. The blisters appear on three sides of the house, at both the main and upper levels. My understanding is that using both oil and latex paint is the problem. Water vapor migrating through the walls is trapped under the paint and condenses on the cold surface. I don't know which paint must be on the top surface. Your feedback on the cause is welcome.

So what's the solution? Removing the paint? Some of the wood siding boards have warped. I don't know how long ago the last paint of coat was applied.

That's extremely common in my area in houses from the 40s through the 60s. I sometimes see huge blisters that are holding over a cup of water.

As far as I can tell, the type of paint has nothing to do with it. My neighbor had this problem with his house and it drove him crazy. He & I experimented with all sorts of things. We'd scrape, sand, prime & repaint with latex, then again with oil. The blisters always re-appeared the next spring -- even in areas that we had sanded down to the bare wood and painted the previous fall. We had some success with driving small wedges under the laps to allow air to circulate behind the siding but we really didn't want to do that too both blistering faces of the house.

He solved the problem by selling the house & moving away. The new owner had the whole house sanded down to bare wood and professionally repainted. The next spring, the blisters reappeared.

When I see this, I tell people to plan on scraping, priming and touching up the paint each summer.

If anyone has a better idea, I have a new neighbor who'd love to hear about it.

- Jim Katen, Oregon

[Chad Fabry]

Heat wants to get to cold and it takes the water vapor along for the ride. About the only way to detail an existing structure without disturbing wall surfaces is to fill the walls with foam and turn the house into a thermos.

[hausdok]

Hi,

It happens a lot around here when they repaint in damp weather and don't ensure that the siding is absolutely dry. I only remember seeing it with oil-based top coats. Something to consider is that if you repaint a home without cleaning and priming the old paint properly, the new paint won't bond well to the old. Also, if siding is primed but left unpainted for several months, and then painted, the same thing can happen 'cuz the primer will no longer give the topcoat good "teeth."

My theory, formed without any actual scientific testing over the past decade, is that the new paint dries to a skin but doesn't bond to the surface. The trapped moisture contains some kind of bacteria that forms gas that expands and causes the blisters. It seems to happen more with claps that haven't been back-primed. Those claps are probably already high in moisture content anyway from vapor diffusion before the new paint is applied. Besides, I've often seen painters around here painting in damp weather - even in the rain. Once that gas bubble forms a blister, vapor moving into the bubble cools to dewpoint, and condenses to water. The water remains trapped under the outer layer of paint. As diffusion continues, additional water just allows the gas bubble to keep growing. Like Jim, I sometimes come along and hit them with my probe to see if they're full of water. Most of the time they are.

ONE TEAM - ONE FIGHT!!!

Mike

[belvedere]

I believe Chad has the right idea. On an older house with no vapor barrier, the warm air migrates through the wall surfaces and ends up under the paint, lifting it and forming the blisters.

[randynavarro]

How about as simple as no paint / primer on the back sides of the boards?

I too, have had bad experiences on my home with 70 years cedar claps. I did notice it was worse on the southern exposure.

I finally had to remove and replace with fiber-cement.

Don't worry, the cedar claps were in pretty bad shape anyway. Splits and cups.

[AHI in AR]

What I see around here on homes of similar age is generally limited to walls outside bathroom areas. As Chad said, moisture is the culprit. You have a wall that is trying to dry to the outside. Short of removing the interior wall surface and installing insulation with a vapor barrier, I have no easy answer. Some manufacturers make paints which they claim to have vapor barrier properties, but I can't vouch for their effectiveness.

[hausdok]

I remember my Dad using paint wedges like Jim described. I just did a search. Hell, even Dr. Joe Lstiburek recommends using them under some circumstances.

http://www.castlewholesalers.com/ALLWAY ... f-100.html

http://www.sherwin.com/pro/problem/faq/ ... sel/#q5271

http://www3.sherwin.com/pro/problem/pro ... /index.jsp

http://www.myperfectcolor.com/Allway-SW ... 412002.htm

http://www.bennette.com/pdf/whyfail.pdf

Siding wedges

http://www.fpl.fs.fed.us/documnts/finlines/knaeb95c.pdf

OT - OF!!!

M.

[Bill Kibbel]

Originally posted by hausdok

I remember my Dad using paint wedges like Jim described. I just did a search. Hell, even Dr. Joe Lstiburek recommends using them under some circumstances.

I used the siding wedges and a few concealed louver disks at the gable ends of my 1690 house. It solved the problem.

[Brandon Chew]

I'll chime in too. The paint is forming a vapor barrier on the exterior of the house. Water vapor is moving from inside the house to the outside. The wall is trying to dry from the inside to outside.

Back-priming the siding before installation and a better vapor barrier on the inside of the wall would help prevent this from happening. The wedges could help as a corrective measure by providing a way for the vapor to escape without traveling through the wood siding and pushing the paint off. Beyond that you are looking at replacing the exterior cladding with something breathable and/or reconstructing the wall assembly to get an insulation and vapor & air barrier system that is appropriate for your climate.

[house painter]

http://www.housepaintingguide.org/why-paint-blisters-and-what-to-do-about-it/ are caused by moisture, chemical incompatibility, or accelerated cure times.

If the moisture issue is resolved, Usually removing the bubbles primimg and painting will solve the issue.