Why do I have moisture in my walls?

The inside of a wall filled with entrapped moisture

I just received a question that was too complex to answer quickly, and so interesting that it deserves an entire post. Unfortunately, it represents a situation that occurs far too often.

To summarize, the question came from a reader who opened a small hole in their wall and found condensation on the vapor barrier at the inner surface of the wall. The question is, what could cause this?

Here are some more clues:

  • The moisture was observed during the winter
  • The hole was cut in a south wall
  • The home has 3″ foam board sealed to the exterior wall
  • A 2×4 framed wall was built inside this wall and insulated with Roxul
  • A 6-mil poly vapor barrier was then applied just behind the inner sheet rock
  • They did not measure high moisture towards the outer wall
  • The wall was built during a humid summer

Before delving into this more deeply, we have to discuss a little physics, but just a little, I promise. (If you’re a chemist or physicist – cut me a break. I’m going to take liberties with the definitions so that I can describe this in a way that people can actually understand.)

You’ve probably heard of the “dew point” – it’s the temperature at which dew forms. Dew is just condensation and it occurs any time the temperature drops low enough that the water vapor in the air coalesces into liquid water. It’s also relavant to the “relative humidity.” The RH is 100% at the dew point and any temperature below it. This is why the windows in your bathroom fog up – they are at a temperature below the dew point of the air.

That’s about all the science you need to understand many moisture problems. End of physics lesson.

Any time somebody tells me they have condensation, the first thing I find out is the temperature of the surface where the condensation occurs. By knowing the temperature, it tells me something about how much moisture is in the air. If the surface is cold, like a bathroom window in winter, then I expect condensation because even relatively dry air will condense on a window at 35F!

But, when somebody tells me that the condensation is occurring on the “warm side” of the wall (like in this case), all sorts of alarm bells go off. Why? Because the air has to have a LOT of moisture in it to condense at 70F. And by a lot, I mean the inside of the wall has to be like a steam bath. After all, have you ever seen condensation on you walls when it’s 70F? The ONLY time I ever see condensation under these conditions is when I boil water on my stove and the steam condenses on my cabinets. We’re talking serious humidity.

If you’ve been paying attention, you’ll wonder “why is the condensation forming on the inner surface of the wall, shouldn’t it be forming on the “cold” side?

Ah grasshopper, you have learned well…

Look at the clues. This is a south wall. What’s the south wall? It’s the sunny side of the house. So it’s possible that the heat from the sun is heating the wall so that it’s actually warmer than the inside of the house. This has the effect of “pushing” the moisture towards the colder surface, which happens to be the side of the vapor barrier facing the inside of the wall.

Note however that I’m not convinced that there aren’t moisture problems elsewhere in these walls. I would want to take moisture readings using moisture meter that can probe all the way into the wall. Then I would measure the wall on the north side of the house. Since this never sees direct sunlight, the outer surface should always be colder than the inner surface. In this case, if there was moisture, I’d expect to see it on that surface and not the inner surface.

Where’s the moisture coming from?

This is the $10,000 question. The writer said that the walls were constructed during a humid summer. It’s certainly possible that building the walls under such conditions could lead to entrapped moisture, but it would have had to be really, really, hot and humid for the moisture to be high enough to cause problems during this winter.

My guess? There is a leak that allows “bulk water” into the walls. Bulk water is the building science term for rain or other liquid water. Bad flashing allowing rain in is one possibility. Ice dams  are another. This water could be dripping in, puddling at the bottoms of the wall, then evaporating and condensing on the vapor barrier.

The giveaway for this type of problem is to stick moisture probes into the wall at various points along its height, starting at the bottom, a few inches up. If there’s a leak, this substantial, there is likely to be a puddle at the bottom so the wood wood be saturated down low. Up higher on the wall, the moisture readings would likely decrease. They could also increase closer to the source of the leak.

The other test, as I mentioned, is checking moisture readings on the cold north wall. If water was entrapped in the walls during construction, there should be high readings on that wall.

The other thing I would do is measure the south wall far away from the area where moisture was discovered. If it is a leak, most likely the water is running down through one cavity. However, it is possible that a really bad flashing job is leading to lots of leaks, so this isn’t definitive.

So testing this type of problem involves poking a bunch of holes in the wall and reading the moisture with long probes, throughout the thickness of the wall cavity.

Now comes the ugly stuff!

In situations like this, I always recommend that people locate an area of very high moisture readings in the wall then cut the wall open from the inside to examine the condition of the wall framing. Persistent moisture problems lead to rotten wood, bug infestations and mold. You really have to examine the interior of the wall with your eyes. Just remember – sheet rock is cheap, wall framing and mold remediation is really expensive. So cutting out a foot or two of sheet rock is a no brainer.

Back to basics – why is this happening?

This wall was built in a way that makes something like this almost inevitable. In building science, we call this the dreaded double vapor barrier.

Building code in many places includes a misguided provision that requires a vapor barrier on the inside of the wall. In many places, that means 6-mil poly plastic sheeting. The idea is that you don’t want moisture from the house getting into the walls and causing problems. Unfortunately, this assumes that everything is textbook perfect – that the walls are built air tight and that moisture never gets into the wall. But they get away with this often because the outer wall is usually not air-tight. Most walls are pretty leaky and just contain plywood or OSB as the outer wall sheathing. When water gets into the walls, if you’re lucky, it will evaporate and go out the outer wall before it can do substantial damage.

In this case, the home has 3″ of board foam and that was air sealed with foam to the outer wall. That 3″ of board foam, while slightly vapor permeable, is very unforgiving. You’ve used a styrofoam cup for coffee. Do you see coffe pouring out? No, of course not. On a molecular level, water vapor is much smaller and it can pass through to some extent, but you can imagine, it’s a pretty slow process. If you pour a few gallons of water in your wall, sandwiched between 3″ of foam on one side and plastic sheeting on the other, where do you think that moisture is going? Nowhere fast, that’s for sure.


While it’s impossible to fully diagnose this problem from afar, let’s assume that there’s a water leak. Water is getting into the wall and can’t get out. Let’s also assume that no other areas of the home are showing moisture problems.

The first thing would be to find and fix the leak. This can be a tricky process and is far too involved to go into details here. But I’ll give you a hint – water mostly flows downhill. Check flashing. Check for ice dams.

After that, you have to assess the extent of any moisture damage to the wood in the walls.  Hopefully, there’s no damage yet. If not, you got to fix the walls.

On a longer term, this wall construction scares me. Eventually, there will be a leak into the wall that won’t be able to get out and it won’t be discovered until some serious damage has been done. Anything done at this point short of ripping out all the poly and rebuilding the wall is a band aid. You simply should not build walls with dual vapor barriers. To do so is to doom a building to failure.

There might be some less destructive ways of dealing with the problem enough to prevent the house from self destructing. *One possibility could be to cut out horizontal strips of the bottom and tops of the wall and remove the vapor barrier. This would at least give the moisture a way to slowly leave the wall cavities AND give a way to see if there was a water leak into the wall because you’d see staining on the sheet rock. There’s no guarantee that this would work however, but if it were my house, I might try something like this so I wouldn’t have to rip open every wall!

In fact, given the problems, I would probably do this at the bottom of the wall regardless, just to inspect all my walls for water damage. Cutting out a strip, say the bottom foot of the wall, is pretty easy to repair, and would give me peace of mind that other areas aren’t rotting out.


In very cold climates, interior vapor barriers ARE called for because even a small amount of moisture moving through the walls can condense inside the wall causing problems. Don’t take anything I write here as set in stone for all climates – it definitely is not. You are responsible for consulting a local engineer, familiar with building science and best practices in your area.


6 thoughts on “Why do I have moisture in my walls?

  1. Ted, We have a similar problem with moisture inside our walls but our situation is a little different.
    We built an addition to our existing home 12 years ago by adding a 3 car garage with children’s bedrooms above. We live in Mississippi where it gets really hot in the south. For several years, our Termite inspector found high moisture levels in all of the outer walls upstairs in this addition. I looked around in the attic for a leak and saw nothing. I really just didn’t put much more thought into anymore until last summer when I moved a picture. I saw that there was mold on the wall (it could easily be cleaned away with a washcloth and had not grown into the sheetrock). The termite inspector was due back at that same time, and this time his readings were close to 100% in some of the walls.
    Last summer, we opened up the walls (every outside wall in 3 bedrooms) and saw that there was dripping condensation with some mildew (since this had been going on for a while) inside the walls. A local contractor conferred with several experts and found no leaks from the roof entering into the home. The final conclusion was that the air conditioning unit that had been put in the upstairs was too large for the space and was putting out too much cold air. They a/c guys said that this caused a high humidity level inside the home and along with the heat outside resulted in condensation inside the walls.
    We replaced the air conditioning unit, added attic fans in the attic, let the walls dry out and then put everything back together. It was truly a terrible and costly summer. We purchased our own moisture reader and checked the walls every now and then feeling satisfied that the problem had been corrected when we saw that the moisture level continued to be low. Feeling pretty good about our repair, we stopped checking the moisture level.
    It is now a year later and the termite inspector was here again. Yes, his moisture readings were almost the same as they were last year! 100% in one exterior wall and 50-60% in the others. Apparently this happens in the summer. This time I don’t even know where to start. I don’t want to “correct” this problem again only to find out that we did not get a correct diagnosis of the problem once again. Your blog was the closest to describing our issue and I am hoping you might have some insight into our situation. Any ideas?

    1. First, I have to say, I feel really badly for you and others who experience these types of situations. They’re truly heartbreaking and unnecessary.

      While what your air conditioning guy said could be partly true in that oversized air conditioners do not properly dehumidify the air, that is not what causes a high humidity inside of your walls.
      You’ve clearly done your homework, but for the benefit of others who may be reading this answer I’m going to go back to basics.

      The fundamental issue of condensation is a cold surface of staying in contact with moist air. If the surface is below the dew point off the air then condensation will occur.

      In humid hot climates like yours, the outside air contains a huge amount of water and the dew point is relatively high, so condensation occurs very easily. This is why beer cans or a cold glass will almost immediately get soaking wet and start dripping.
      In your climate, at a relatively normal temperature of 85 degrees F and a high humidity of 70%, water will start condensing inside of your walls if the sheetrock temperature is around 70 degrees. This is a very common condition and therefore it is extremely important to minimize the chance that outside humid air will come in contact with your cool walls. This is typically accomplished by the use of insulation and a vapor retarder on the outside of the insulation that is the vapor barrier prevents outdoor air from getting into the insulation and making contact with your walls.

      However this type of solution requires an excellent job be done on installing the vapor retarder, because as you know, air will find a way in. This is why by far the best solution is spray foam insulation applied directly to the back of your sheetrock. Closed cell spray foam is virtually impermeable to water, and therefore the outside humidity can never come in contact with your cool walls.
      Let me say that undoubtedly anyone who says the moisture is coming from inside your house during the summer should not be listened to. It proves that they are ignorant of the physics involved. The only possible way that interior humidity would cause a problem inside your walls during the summer is if you constantly had steam pouring into your living space!

      Now, here comes the really bad news. You really need to resolve this issue by ripping off the exterior of your home again and properly insulating and sealing the walls. Otherwise this will continue to happen again and again.

      You will be best off working with somebody familiar with building science in your area who can monitor the reconstruction process as an independent rather than general contractor who is making money off of your woes. You may wish to check Angie’s List for a spray foam insulation contractor then ask them who the best troubleshooting expert is in your area. There are a good number of people these days who specialize in moisture problems. However, do not just hire a moisture remediation or mold remediation company. I have found that many of these people are not properly trained in the science of moisture and just make money on selling band aid solutions.
      I hope I haven’t made too many typos, I’m trying to do this on this on a small screen without a proper keyboard :-) but I wanted to get back to you as soon as possible. Good luck I hope you can find a great person to help you.

  2. Hey Ted, I discovered something this past weekend in my home…
    First off, some history: we’ve lived here for 5 years. We purchased/moved in to our home in the summer of 2008. We painted EVERY room in our home and my question revolves around our bedroom. We’ve had our bedroom furniture positioned the same way since we moved in up until now, when we recently purchased a new set to replace this old one. We decide to paint our bedroom again. Being that we are now finished painting in the beginning of winter, I noticed a small interior corner of our southwest wall, where condensation was building up, causing the paint to not completely dry compared to the rest of the room.
    I had a feeling I knew what it was since our windows aren’t sealed worth a damn in this bedroom (or the rest of the house to be 100%) and I figured it may be an insulation problem. I put our space heater on it for a few hours to see what would happen. It did dry, but still remains quite tacky after 48 hours. Some moisture still condenses on it as well.
    The walls & baseboard feel much colder than my daughter’s adjacent room and even compared to the rest of our bedroom (inner attached walls). This area of moisture is the outside corner of our home, located over a crawl space addition where our furnace really has to work to heat the room to begin with.
    The front half of our home has service holes on the outside of the siding when they previous owner had blown-in insulation installed, but not on this newer addition containing our bedroom.
    I’ve never noticed it before due to our bedroom dresser sitting in this corner. covering it.
    Does this appear to sound like a simple insulation issue? The rest of our home doesn’t seem to have any issues and remains quite warmer. This moisture “patch” seems to be about 4 square inches, directly in the corner, coming up a few inches above the molding towards the carpet.
    If you have any ideas, I’d greatly appreciate them. Great, informative site also. I learned something this morning!

    1. You made an astute observation. Good catch on the condensation and your notes.

      These issues are, as you noted, typically due to insulation issues. Since you’ve determined that the corner is much cooler, and hence the reason condensation is forming there, it’s almost certainly that. It is very common for builders to miss a spot or two. Fortunately, you’re not finding lots of cold spots which could indicate a bigger problem. Probably just an oversight.
      It’s also possible that there’s no room for insulation in that area. Sometimes, there’s so many studs and other wood in the wall that it’s just a solid block of wood, which provides pretty poor insulation!

      Sometimes, the problem can be made worse if the outside of the house wasn’t air sealed well and cold air blows into the wall and under the molding. My house is terrible this way around the perimeter. But my guess based on your description is insulation.

      You could drill a small (maybe 1/4″) hole in the center of the cold spot, carefully stopping as soon as the drill penetrates the drywall. Then probe in the hole and see if you can feel insulation by poking something into the hole (like a chopstick or other non-conductive object). You should be able to feel resistance from the insulation in the wall.

      You might be tempted to get a can of spray foam and fill the wall with it but I’d advise against it. When this stuff expands, it can do so with enough force to blow out the wall.

      To fix the problem, the real solution is to carefully cut out the drywall and fill the cavity. But most people don’t want to bother with that, especially if you’ve already painted and are just finishing up.

  3. Hay i am in a ground floor apt and 2 year’s the apt over me had a very bad leak because the pipes burst. ive noticed we have a lot of mould growning in the cornor of the bedroom i got a man to come and have a look with a moisture reader and it came up flashing 70??? i asked him was that bad and the fool said he was not too sure! i think he’s trying to take me for a ride can you give me any help or what way do the water moisture readings be??

    1. Sorry to hear about that. The meters have different settings so I couldn’t tell you, but typically a number like that would be on a relative scale, of 0-100, and 70 would be a sign of high moisture. Flashing usually is a warning sign also. And of course, if you’ve got mold growing in there, that’s the sure indicator that there’s elevated moisture levels.

      However, it’s possible but not likely that a leak two years ago would be causing mold today. So they really need to find the cause of the moisture today. If they just clean the mold off the wall then repaint, the mold will just come back. Is the corner where you see mold against an outer wall of the building? If so, there’s a good chance that water is getting into the wall from above due to some construction issue on the outside of the building.

      I don’t know how it is in the UK but here in the US, there have been lawsuits relating to mold since some people have very adverse reactions to it. Because of this, most people want to deal with it properly because they fear lawsuits.

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