Condensation: It’s Raining in My Attic!

December 10, 2018 | Filed under: Home Ventilation, Roof Repair, Roofing Installation, Roofing Materials, Uncategorized

As someone who helps thousands of homeowners each year with their roofing questions and concerns, I can depend on one thing like clockwork. Every spring and fall, I will field many calls and emails from desperate, scared, and frustrated homeowners who are exclaiming “I had a metal roof installed, and now it’s raining in my attic!”

Here are a couple of photos sent to me recently by one of these homeowners.

condensation 1
condensation 2

What frustrates me is that problems like this become a “black eye” for the entire residential metal roofing industry, leading folks to the fear that there is something about metal roofing that creates condensation. In reality, though, these problems are the result of contractors who did not do a good job of discussing and thinking through the project before proceeding.

A Case Study on Condensation

In the case of these photos, the home was a modular unit with a flat roof originally, and the homeowners “paid extra” to solve their ongoing flat roof problems with a metal roof, only to find themselves later in an even worse situation. Warm, moist air from inside the structure is driving into the newly-created cavity beneath the metal roof and is trapped there. When the temperatures drop at night, and the roof cools, the moisture condenses on the back side of the roofing. Of course, this was not a huge problem in the summer but quickly became a major problem with the changing of seasons.

The concept of what is happening here is very simple but usually very misunderstood. Because these problems tend to pop up worst in the Spring and Fall when evenings are cool, and also when it tends to be raining a lot, most folks assume the moisture that is condensing is from outside moisture.

What’s actually happening though is that warm, moisture-laden air that originates inside the living space is trying to drive its way out of the structure toward drier areas and, if it hits a cold surface when doing that, dewpoint is reached and the moisture condenses. This is very much like sitting that cool glass of lemonade out on the porch on a warm, humid day. No sooner do you bring the glass outside than moisture in the air is condensing on the cool sides of the glass.

This particular homeowner asked me about removing the roof panels and laminating a product called DripStop to the underside of the metal panels and then re-installing the roof. DripStop is a fibrous material designed to hold moisture and allow it to dry rather than condense. While this product works well on large open-air barns, in a tight space like the attic in these photos, it would have been overwhelmed by the moisture and quickly developed mildew and mold.

How to Avoid Condensation in Your Attic

Each year, I hear from many homeowners who either are in this situation or have heard about this situation and are frightened by it.

Avoid Problems With 3 Solutions that Work Together

There are three primary things that work together to avoid problems like these. In some cases, you might get by with just two of these things but never will you get by with one or none. To be safe, you really want to have all three. Those three things are:

  1. Insulation and Air Sealing Through insulation on top of the ceilings and making sure that there are no air leaks from the living space into the attic, you can keep the warm air from getting into the attic.
  2. A Vapor Barrier This will stop the transmission of water vapor from the living space into the attic. This vapor barrier is usually best directly on top of the ceilings and beneath the insulation. Occasionally, folks will ask about placing a vapor barrier beneath the roof deck or roof panels, but all that will do is transfer the problem from the underside of the roof to the underside of the vapor barrier.
  3. Ventilation Proper attic ventilation brings in fresh air at the bottom of the attic and uses convective airflow to exhaust it out at the ridge of the roof through exhaust vents. As the air travels through the attic, it picks up moisture and carries it out as well.

5 More Things to Think About

So, those are the things that help avoid these problems but, in assessing a project, there are other things to think about as well.

  1. Decking I never suggest a residential metal roof not installed on solid decking. The solid decking helps prevent warm, moist air from reaching the backside of the metal where it will condense. You can get by without decking on large buildings that have a lot of air exchange going on, but that is not the case with a home. Also, large buildings by nature, unless they include animal confinement, tend to have lower ambient humidity levels. (By the way – another common mistake – underlayment should always be installed on top of the roof deck before the metal roof.)
  2. Measure Levels of Air Flow and Moisture This condensation can occur on homes with decking as well. What we usually see is that, in some cases, the houses were already very close to having a condensation issue and the metal roof dropped the temperature of the roof deck enough to cause condensation where it did not occur before. In essence, the metal roof becomes the “straw that breaks the camel’s back.” If you suspect this could be the case on a home, use a smoke pen or machine to check for airflow in the attic and also check moisture levels of the insulation and lumber in the attic. No air movement and high moisture levels indicate a potential problem. The answer is often to increase ventilation in the attic.
  3. Prepare for Updates to Older Homes Older homes that are not very airtight tend to allow a lot of moisture to escape through the walls and windows. Additionally, we create a lot more moisture inside of older homes than we did when they were first built, through increased amounts of bathing, laundry, cooking, etc. If you’re working on an older home and it is perceived that at some point the owners may replace the windows, add siding and house wrap, and do other things to make the home “tighter,” then be aware that, once those things are done, more moisture will migrate into the attic. Better off to prepare for that eventuality now!
  4. Raise the Roof Deck In extreme cases, getting the metal roof up off of the roof deck can be helpful and not drop the roof deck as cool. Various metal shingle products naturally have this helpful air gap between the metal and the roof deck. Some clip-fastened standing seam products have clips that lift the metal off of the deck a bit. In other cases, some metal roofs can be installed on battens though I caution going that direction. Battens make roofs harder to walk and also less wind resistant. They also can cause gutters to need to be raised and also interfere with dormer windows and other roof protrusions.
  5. Consider a Hot Roof Another “worst case scenario” answer is to spray closed cell urethane foam to the underside of the roof deck; a process which isolates the roof deck from moisture while also having insulating properties. This sort of “hot roof” is being done with some new construction and works well. For retrofit, it is a little trickier and requires someone skilled at spraying foam insulation.

And There’s More

So, there you have it; my take on how to keep it from raining in your attic. Are there more things your contractor may not know or understand about metal roofing? You bet.

Download my list of 10 Things Your Contractor Doesn’t Understand About Metal Roofing.

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