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Community and Q&A

High Relative Humidity in Winter Attic

brianandson | Posted in General Questions on

I live in mid Michigan.  I have a mansard style home that has a low slope hip roof above the living space.  Maybe 3/12 or 4/12 pitch.  The attic is 900sf square roof and at the peak has only 3ft of clearance.  The rest of the space has less than 3ft of clearance.  It is a tight space and tough to maneuver around in the attic and air seal and insulate really good.  I have a power attic fan that has humidistat and thermostat on it. I also have 6 static vents.  I have no soffits, but I do have vents on the lower ends of the roof.  It seems like there has good air movement.  The humidistat is set at 60%RH.  The fan is running non stop and I can not get the % RH below 69%.  The outside high temps have been low 30’s.  Do you have any guidance how to get the %RH lower?  I am concerned at 69% I will have bad mold issues.  I recently added insulation and it’s 16” deep in the center part of the attic but I am unable to get close to the ends. At all 4 eaves there is only about 6” of attic clearance space.  At 4ft up from the eaves there is only maybe 20” of attic clearance. It’s got real tight space all around the 4 sides of the square hip room.  So I don’t think I can pack insulation because it will be hitting the bottom of the roof sheathing.  It’s a real difficult roof and attic being such tight space and if I get mold it will be so difficult.  Any advice you can give me to help lower the attic RH level in the winter temps and be safe from
mold will greatly be appreciated

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  1. JC72 | | #1

    Powered attic fans will draw moist air from the living space into the attic but seeing as you have a passive vent at each end I can't tell you how much moist interior air the fan is pulling.

    The primary issue is that moist interior air is getting into the attic space. The usual suspects are air leaks around can lights, duct boots, bathroom fans, and access points into the attic. Double check the ducts for your bathroom fans as well. You don't want them exhausting into the attic. Air seal them with tape or foam.

    On an aside a lot of houseplants or water features within the living space will increase the moisture levels within the house.

    My two cents.

    1. brianandson | | #2

      Great thank you for your reply. Yeah I will add more vents for I take and hopefully that will help too. But my humidity level in the attic was at %70 before I turned on the attic power vent. I was hoping turning on the attic fan would help but it didn’t. I do also plan to remove the insulation and air seal the ceiling and then reinsulate in the spring. And helpfully this will all lower the RH levels in my attic during the cold weather months. One thing I do not understand -Right now the outside temp is 35 degrees and the outside RH is %87 according to the weather channel. If the outside RH is %87 how am I suppose to keep the attic RH in the 50’s. I monitor the inside RH level and it’s at 40%. So I don’t think the moist air is coming from the inside living space Thank you for any advice

  2. GBA Editor
    Kiley Jacques | | #3

    To reiterate John’s point: air leaks are the primary culprit when it comes to humidity in the attic. I’m quoting Martin Holladay here: High humidity in an attic is usually caused by air leaks in the ceiling of the top floor of your house. During winter months, humidity from this escaping air can condense on cold attic surfaces, leading to mold. So the first step is to seal the air leaks. For best practices, check out his article Air Sealing an Attic.

  3. igrigos | | #4

    How does the RH in the attic compare to the RH outside? If the RH is 80% outdoors, then no amount of ventilation will get you there

  4. brianandson | | #5

    Thank you Kiley for the info. Yeah that’s what’s my plan is to do. I’m going to remove all the existing attic insulation and seal the entire ceiling penetrations with foam and then reinsulate. Do you think I should put a plastic vapor barrier down on the attic floor before I add the new insulation? As an extra to the air seal.
    Thanks igrigos. That’s what I’m thinking too. Even if I have an air tight attic and proper insulation and proper ventilation, when the weather is like it is now with %87 Humidity outside and 35 degree temps I will not be able to get a lower RH in my attic. Do I have any options when the weather is cold and humid outside like it is right now? Or do I just accept that on these type of days I will have a high humidity in my attic? Can mold grow in this type of environment? I still am going to air seal and reinsulate in the spring
    Thank you

  5. Jon_R | | #6

    I live in SE Michigan and vented attic mold has not been a problem. I'd keep checking for mold and frost before assuming that you have anything to worry about.

    Maybe someone should build an attic mold prediction device. When it's humid outside, high relative humidity in the attic is normal and isn't an accurate indicator of a problem.

    While the attic floor should be sealed well enough to prevent attic mold problems, an attic fan can be the best ROI option. If so, it should be small and blow inward (which will *reduce* infiltration from the house into the attic). Preferably only running when outdoor dew point is less than the attic dew point (so it never adds moisture).

  6. Expert Member
    Peter Engle | | #7

    Mold stops growing below 50F for most varieties, so high humidity in winter isn't as much of an issue. When attics get moldy in winter, it is usually due to temperature cycling. If the sheathing gets saturated during cold snaps then warms up without drying, you get mold growth. In some attics, this is a daily cycle with solar heating providing enough warmth in the daytime and moisture accumulation during the cold/humid night.

    In all climates, air sealing and insulating the attic floor helps tremendously. In addition to the heat savings, it reduces the moisture loading from the house itself. Less moisture is always good. If the attic only has to deal with humidity from outside there is far less chance of mold. Wood acts as a sponge - it soaks up moisture depending on the %RH of the air around it. So 85% RH outside air in your attic would eventually bring the wood moisture content up to around 20%. Not quite enough for mold growth, but close. But you are still protected by the temperature. At 35F, mold's not going to grow anyhow.

    Now take that outdoor air and warm it up to 70F. Assume it's a sunny day and the attic warms up. The RH of the air coming into the attic and warming up will drop to about 30%. The wood dries out. The opposite happens at night, with radiational cooling. The roof temperature can drop below the outdoor air temperature, and condensation forms on the underside of the sheathing, soaking it. Depending on the balance of night-time wetting and daytime drying, you may or may not have mold issues during daily temperature swings.

    So the answer is to run the fan only during the day, if at all. If you have adequate passive ventilation, you don't need a powered fan. If you don't have adequate ventilation, a powered fan is going to suck some amount of (moist) air out of the house in addition to the outdoor air. I would recommend adding passive vents where you can, and doing the insulating and air sealing. Then monitor the wood moisture content, not the RH. It's WMC that you care about. Above 25% WMC and 50F, wood will rot. Lower the WMC or the temperature and you're OK. Leave the fan off and see how the attic performs. If you think you need the fan, run it only during the daytime and see how you do.

  7. brianandson | | #8

    Thank you Jon for your reply. Yeah I can see some spots of mold but it doesn’t look bad ..yet. I’m just trying to nip this in the butt before it spreads and gets real bad. The mold spots could be from long ago. I’m not sure how old it is. I’m glad to hear you say that a high RH in a cold attic isn’t the worst in the world and isn’t an indicator of definite mold growth. My fan is an power fan mounted to a hole in the roof and it sucks the air they the intake attic vents and suppose to crest air flow out of the attic. I am gonna keep checking it and try to keep RH low as possible in the attic and then in the spring air seal and re insulate. Thanks again for the info. Go green

  8. user-723121 | | #9

    I think Peter has it. We do not use powered attic fans in MN anymore or the whirlybird roof vents. They can depressurize the attic and pull warm, moist air from the house into the attic, exactly what you are trying to avoid. The attic floor should be air sealed as well as possible and passively vented.

    We get here (MN) Attic Frost buildup over the winter months in attics that have leaks to the living space below and are generally under vented. The frost accumulates on the rafters and the underside of the roof sheathing and melts when we get the first warm days in the spring. Calls are made to check the leaking roof when it is just the thawing of the accumulating frost.

  9. brianandson | | #10

    Wow 😳. Thank you Peter and Doug! Tons of great info that really relieves some of my stress about mold forming and spreading in a cold winter attic. I will follow your guidance completely and will continue to monitor the attic. Do you know of a tool that measures the WMC and where I can buy one. I want to get one right away. Thanks so much.

    1. user-723121 | | #11

      Mold anywhere in the house should be considered but mold in the attic is outside of the living space so not such a health concern. Mold in the attic does however point out a deficiency in the air tightness of the ceiling plane. Attic hatches are many times poorly sealed and a lot of moist air can pass by them.

      1. brianandson | | #12

        Perfect thank you Doug. Yup I’m going to air seal real tight and reinsulate. Do you have any guidance how to properly air seal and insulate a low pitched Square hip roof with no soffits? I have about 20” of clearance at 4ft in from the eave on all sides and I have about 6” of clearance at the eaves. It’s almost impossible for me to get in the attic and get to within 4ft of the edges of the roof. And I’m assuming that my new insulation is not supposed to touch the bottom side of the roof sheathing either. From 4ft to the eave is a real tight space and I’m not sure how to air seal or insulate that area around the roof Do you have any more expertise to offer me ?

  10. walta100 | | #13

    I am the anti spray foam guy but in a attic like yours without any possibility for soffit vents, a moisture problem and limited ridge vents.

    I would have to at least consider the possibility of conditioning the attic.


    1. brianandson | | #15

      Thank you Walter. Yeah I am like you. I’m trying to stay away from spray foam, but I would hate to try another option and have it not work and still have ceiling leaking into attic. Thanks

  11. Expert Member
    Dana Dorsett | | #14

    >"I can not get the % RH below 69%"

    69% at what temperature? The problem with RH numbers is that it's RELATIVE to the temperature, not an absolute humidity measurement.

    If it's 40F/69%RH in the attic and the outside air is 35F/85% RH the out door air is more humid than the attic air. Active ventilation under those conditions would bump the indoor RH to 70% @ 40F.

    An RH of 69% @ 40F isn't a mold hazard, but at 90F it is.

    Air sealing all penetrations at the attic floor and leaving the power vent off will probably work just fine in your climate, summer and winter.

    1. brianandson | | #16

      Thank you Dana. That is great news. I’m not a physics guy, so when I seen that RH level in my attic at 69% I freaked out because I thought that was a super high humidity level and would produce widespread mold. And mold scares me also. Yes you are correct. The temperature is cold in my attic. It is around 40 degrees with the 69% RH. And the outside air temp has been low 30’s with an RH of 80%. So the RH level in my attic is lower than the outside air. Thank you for your expertise. I will turn off my attic fan and chill out a bit. I still will be air sealing the attic floor and new insulation

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