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.