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Community and Q&A

Moisture condensation

GBA Editor | Posted in Webinar Follow-up Q&A on

I recently installed dense-pack cellulose in a 1929 home. After the install, the amount of moisture condensing on the windows increased significantly. Why would this happen. The relative humidity in the home should be the exact same before and after. The windows are original with fully closed storms.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Although it's impossible to answer your question without visiting the site, it's important to point out that installing dense-packed cellulose in existing walls will lower the air leakage rate through the walls. Before the insulation job, the infiltration and exfiltration rates were high, helping lower the indoor humidity levels. Lower air leakage rates can result in higher levels of indoor humidity.

  2. Riversong | | #2

    As Martin suggested, your house was taking care of the excess indoor humidity as long as it leaked lots of air. Now that the air leakage is reduced, it's up to you to take care of excess indoor humidity. First try to reduce it at the sources: wet basement or crawl space, unvented dryer, kitchen range and bath showers with no exhaust fans (or fans not being used), hanging laundry indoors, lots of large potted plants...

  3. T. Inoue | | #3

    In addition to what Robert and Martin mentioned, if you use a humidifier, you may not want to use it anymore. Tight homes and humidifiers don't mix.

    Get a gauge to monitor your indoor humidity. During the winter, even 35% relative humidity will lead to condensation on many windows. You have to keep the indoor humidity in check during the winter or you can end up with mold and rot issues.

    Overall, a tight house is a good thing. It's just that you have to be aware that is has different properties and is less forgiving than your old, leaky home.

  4. Kent Mitchell | | #4

    The answers above are on track. Testing to determine ventilation values and needs are probably a good idea too. After the issues mentioned above you may find it needs some added mechanical ventilation.

  5. John Hess | | #5

    "bath showers with no exhaust fans (or fans not being used)"

    We recently moved into a poorly-insulated house with the bathroom on a corner, and with no exhaust vent in the bathroom. I used a couple of expandable shower rods and a shower curtain to seal off the top of the shower/bathtub sliding door so that the shower area is now sort of a self-contained sauna. Voila! The cool air in the shower (we keep our house at 65*) heats up much more quickly, there's no more cold air pouring over the top of the shower door, and no more condensation on mirrors and walls. Sure hope there is no radon in the water!

  6. Robert Hronek | | #6

    Given that its a 1929 home with storms tells me it has single pane windows. The glass is going to be cold. As others have pointed out the lower amount of air leaks does not purge as much moisture, The home is likely not over humidified but rather it has seen enough of an increase to allow it to condense on cold glass.

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