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Q&A Spotlight

High Humidity in the Attic

Insulating the underside of the roof deck solves one problem and creates another

Jason Advani has insulated his attic with spray polyurethane foam, and while his heating bills have dropped, high humidity remains a problem. [Image credit: Jason Advani]

Jason Advani thought he was on the right track when he insulated the bottom of the roof deck in his walk-up attic with closed-cell spray foam. As planned, the R-40 worth of insulation has drastically reduced his heating bill, but now he faces another, unexpected problem.

“Since then, the space has been plagued with very high humidity,” he writes in a Q&A post. “It’s at 60% to 70%, maybe higher. I’m forced to run a dehumidifier up there. I’m spending more money on running the dehumidifier year-round than I previously was to heat the house!”

With outside temperatures ranging between -5°F and 95°F, attic temperatures range between 48°F and 92°F. Only about 10% of the hip roof is shaded, so it gets plenty of direct sunlight.

Advani hopes to finish the attic and install a minisplit, although he’s not sure that will lower the humidity.

Most of the house is heated and cooled with a conventional gas-fired furnace and air conditioning system. But there is no effective way to circulate the air between the attic and the remainder of the house. Advani says he runs a dehumidifier in the basement during the summer and a whole-house humidifier in the winter.

“What do I do?” he asks. “Bath exhausts do go outside … all that typical stuff has been considered / examined.”

That’s where we start this Q&A Spotlight.

Sealing air leaks would help

Charlie Sullivan suggests that finding and sealing air leaks would improve conditions in the attic.

Air leaks in the lower part of the house — in the basement, for example — allow outside air with low humidity to get into the house. Advani is adding moisture to the air with a humidifier, and that hot air rises, finding ways of getting into the attic, Sullivan says. The air then leaks out of the attic to the outdoors.

“If you found and sealed those leaks, you would have much less cool air coming in, so you’d have much less need for a humidifier,” he says. “Even much further north, people who think they need humidifiers likely actually need air leaks sealed. A house that is well air sealed will tend to stabilize at a reasonable wintertime humidity level.”

In that scenario, Advani could still run the dehumidifier in the attic, but it would have to run less. In the main part of the house, humidity would probably be high enough for comfort without running the humidifier at all.

“In the summer, the problem is humid outside air leaking into the attic,” Sullivan adds. “The air sealing you do for winter will also help that problem. You probably will want dehumidification in the summer — high temperatures plus humidity are worse for mold, etc. growth than are low temperatures plus humidity.”

Calculating dew points

Sullivan had predicted that during the winter the air in the attic probably has about the same dew point as the air in the house. The relative humidity would be higher because the air in the attic is cooler. But even a relative humidity of 70% in the attic isn’t going to cause a problem in cold temperatures.

Advani seems to have confirmed Sullivan’s guess.

With a weather station in the attic to collect data on temperature and relative humidity, Advani finds the dew point is about 42°. In the main living area of the house, the temperature is higher and the relative humidity is lower — but the dew point is 43°.

“So, if I understand this correctly,” Advani says, “the attic is actually slightly dryer than the house. Right?”

Peter Yost points out that it is “notoriously difficult” to measure humidity accurately and inexpensively.

“If indeed your attic humidity is running at that high of levels, next we identify the source(s) of that moisture,” Yost writes. “I am not quite sure why you would run a humidifier in the winter at the same time you run a dehumidifier.”

Get conditioned air into the attic

In Walter Ahlgrim’s view, Advani did “half the job” of creating a conditioned attic.

“You have sealed and insulated it, now you need to condition the attic by adding supply and return registers in the attic,” he says. “When done correctly the attic will have almost the same temperature and humidity as the rest of the house.”

Right you are, replies Advani, and at some point in the future, the attic will be fully conditioned. “Having the same temperature and humidity is certainly the goal.”

Our expert weighs in

GBA Technical Director Peter Yost has already posted one comment, but he had these additional points to make about Advani’s situation:

Jason has been really responsive regarding the issues he has with his conditioned attic. Here, in order of priority and timing, is what I would recommend:

Identify moisture source(s): We are assuming that the big culprit is the forced-air system humidifier (more on this issue below). But there may be others. Take a look at this resource and make sure that you don’t have other sources of moisture.

For example, many of my building investigations are for homes with propane or natural gas cooktops/ovens. If these are used regularly, about 1 pound of water is released for each 90,000 BTU burned (and that does not include moisture coming from what is being heated). I seriously doubt that your moisture problem is being driven by bulk water leakage, but I can’t assume that and neither should you. Check penetrations and flashings to rule out this moisture source.

Test/remediate vermiculite insulation: Jason really needs a blower door test to identify air leaks in his building. He reports that he has not conducted a blower door test because of the potential asbestos content in the vermiculite insulation. So, either test the vermiculite insulation for asbestos content or jump to remediate this health hazard as part of renovating the home.

Turn off your whole-house humidifier after air sealing based on guidance from blower door work. See what happens to your house and attic relative humidity (RH).

There are two primary humidifier types — scroll wheel and steam. Jason has a scroll wheel system, as shown in the drawing below. One key to the proper operation of either type is the quality and location of the controlling humidistat. Since warm, moist air is less dense than warm dry, air, you can end up with a bubble of warm, moist air in an attic that has no air-mixing, such as Jason’s. We don’t know where Jason’s humidistat is located but I bet it is not in the attic. That disconnect (between where the moisture and sensor are) could be part of moisture issues in Jason’s attic.

Ventilate your conditioned attic: Get some air flow and movement up in the attic space. Building Science Corporation recommends 50 cfm per 100 square feet of attic floor area.


  1. Josh Durston | | #1

    It's all about the dew point. Personally I don't think there's a source of moisture. It's just that the air is close to being saturated as you cool it.
    Check out this fix the dewpoint slider at 42 and solve for %rh. Then slide the temp slider around. You can see a huge range of %rh is possible with the same amount (pounds not %rh) of moisture in the space.

    The key thing with relative humidity is that it's relative (to temperature)! High humidity doesn't necessarily mean the there is more moisture, just that the air is having a tougher time holding the moisture it already has.

    That being said condition the space and see what happens.

  2. User avater
    C&H Architects | | #2

    Great article. Thanks.

  3. Kevin Camfield | | #3


    Thanks for the link. That's a very helpful calculator.


  4. Gene DeJoannis | | #4

    This discussion should have begun with a look at a psychometric chart or calculator. Dew point temperature (horizontal lines on the psych chart) is directly related to the Humidity Ratio on the right side axis, that is the pounds of water in each pound of air. So if the dew points are about the same (42 & 43) in the lower and upper areas, that means that there is about the same amount of moisture per pound of air in both areas. That should not be surprising because unless you go to a LOT of trouble to seal all joints between the lower floor and the attic, moisture will flow between the two areas. By adding moisture to the lower floor, you increase the partial pressure of the water vapor, which pushes it to areas of lower vapor pressure, like the attic. So they equalize pretty quickly. The difference in the two spaces is the dry bulb temperature. IF you raise the attic temperature the RH in both places would be the same. The issue in the attic is that at 48 degrees F. you are approaching the 43 deg dew point, and some colder surfaces like windows you will get condensation there. That's just the properties of moist air. IF you seal up the leaks on lower floors you may do without the humidifier and stay comfortable. Use clear caulk around the window trim and along the floor-wall joint. SEal all outlets with foam gaskets

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