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Community and Q&A

High Humidity in Attic Conversion

watunicit | Posted in General Questions on

Hello, my (new to me) 1920s house has a third floor attic conversation bedroom that gets very humid–it rained last night and it was 67% this morning–so I’m looking to find ways to decrease the humidity. I’m currently using a dehumidifier and it’s not unusual for the 50 pint bucket to fill in one day. The attic above this bedroom contains the blower and ducts for the second and third floors’ a/c unit.


The roof is slate and the attic insulation, while I’m certainly no expert, doesn’t appear to be proper. There’s R-30 insulation inconsistently laid on top of the floor of the attic, along with some white fluffy material, partially inside and outside of red plastic, fitted into the floor (joists). There is no insulation on the ceiling of the attic (rafters), which I have researched a bit and can’t determine if problematic or proper due to the slate’s need to “breathe.” I believe there are no vents in the attic space.


The exterior walls of the room appear to be plaster on brick, as seen in the two places I’ve cut into the walls. We had a roof leak along the chimney for which we have had the chimney flashing repaired (along with various broken slate), though I can’t be sure the repair was proper until the walls dry out from the prior leak (which I’ve been told will take months due to the lack of breathing space given the plaster on brick), so I cut said holes in places where the plaster was “bubbling” and would need to be fixed anyway in attempt to possibly speed the drying process.


I’m curious whether the humidity issue is caused by the lack of wall insulation, the attic insulation, a combination, or something else, and how to fix the problem. I’m also curious whether I need to take any corrective steps for non-humidity reasons as well. Any guidance or suggestion would be greatly appreciated. Thanks in advance.


Photos (sort by oldest for more logical order):

Note: I’m having the roofers come back to fix the few gaps in the ridge

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  1. user-5946022 | | #1

    Humidity is caused by
    a. people being in the space
    b. activities of people in the space (showers, cooking)
    c. a leaky (ie not tightly sealed) building envelope that allows alot of air to leak between the conditioned space (bedroom) and unconditioned space (which it sounds like your attic is), when the outside air is more humid than the inside air

    For a, since the purpose of the space is for people to be in it, you can't do much about that.
    For b, you want to ensure you have exhaust fans in the baths and kitchen that exhaust to the outside.
    For c, you want to work on air sealing the barrier between the conditioned and unconditioned space. As a general rule on a historical building you often cause more problems than you solve by attempting to turn what was intended to be a vented attic into a conditioned attic, so you probably want to leave your unfinished attic as is. As a general rule, it is MUCH easier to attic air sealing before you revised insulation, and the results of air sealing are also much more immediately noticeable in terms of comfort. Without knowing more about your specific situation, I'd say start with air sealing - every little void...there are some good posts on here about using a fog machine or smoke pencil to see where the leakage points are

    ETA: after looking at the photos, you have an unconditioned attic, and unfortunately have ducts with conditioned air going through that unconditioned space. I'm going to guess that the penetrations between that unconditioned attic and your conditioned space could use significant air sealing - penetrations such as ducts, duct boots, electrical boxes in ceilings, holes where electrical wire or conduit fishes through the wall, etc. Start reading everything you can on air sealing an old house.

    ETA 2:
    1. Not sure what you mean by having roofers come by to fix the gaps in the ridge. It looks like an unconditioned attic so you should have a ridge vent, which some might interpret as a "gap"
    2. Sorting the photos oldest to newest, the second photo indicates a pieces of pink insulation stuffed in the space that usually leads to the soffit vent...if so you don't want to block that.

  2. watunicit | | #2

    Hello, thanks very much for the reply. It seems like I need to focus on finding and sealing any gaps between the third floor ceiling and the attic floor. The ceiling has 4 light fixtures, 1 fan, 3 functioning air vents, and one non-functional air vent--perhaps that last one is problematic? Would I be better off poking around the attic floor to find gaps or removing the fixtures from the ceiling?

    The gap I was referring to is the light at the top middle of the first (oldest) photo--there is a broken slate that will be replaced to close that gap. I will look to see if there is any insulation blocking a vent.

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