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Indoor humidity levels: Magnitude of risk and solutions

rshuman | Posted in Energy Efficiency and Durability on

I have some questions about the risk posed by humidity levels in my house and how best to address that risk. I have read lots of stuff on GBA regarding the humidity issue. Mostly I am looking for confirmation that I am on the right track. Sorry for the length of the following in advance.

The house is a two story with a walkout basement. The basement walls were constructed using ICFs, the framed portion of the walkout basement consists of 2×8’s insulated with Roxul R-30 and equipped with poly under the moisture resistant sheet rock. The first and second stories are 2×6 construction, insulated with Roxul R-23 with a sheet of poly. The house is sheathed with Advantech and sided with cement board, there is no rain screen. The house is located on the coast in Lubec ME, climate zone 6A.

The humidity concern started with basement odors, some of which were addressed by a plumbing fix. In trying to figure out the source of the remaining odor (which comes and goes) I set out a $10 Accurite humidity/temperature monitor. Over a period of several days I conducted measurements in the basement, just outside the door of the basement (protected by a porch above) and, occasionally on the first floor of the house. The humidity readings were converted to dewpoints. I have attached a small spreadsheet that summarizes the results.

The results indicate basement temps generally fell between 61 and 64 degrees; RH’s were generally in the 70’s leading to dew points in the 50’s. The outdoor temps were generally greater than or similar to the indoor temps, the same can be said wrt the outdoor RH’s and dewpoints. First floor temps, RH’s, and dewpoints were not all that much different from the basement values.

Overall, RH’s were much greater than the ‘safe’ ranges generally cited at GBA. So, the first question becomes ‘should I be concerned?’ If the answer is yes, then a plan of attack is in order. With an eye to better sealing the house, I was thinking a sane first step would be to conduct a blower door test to isolate the biggest leaks. As far as the basement goes, and given that ICF’s were used in its construction, it seems like the first floor rim joist is likely the source of the biggest leaks. Although pieces of Roxul have been placed along the rim joist in some places, they have not in others. Where Roxul has been placed, pieces of poly were stapled in place inboard of the insulation, but it does not appear to be air-tight.

So far, I am assuming the source of the moisture is outside. In terms of indoor sources, there are a total of three house plants, the dishwasher is run 1-2 times per week, and bathroom fans are used during showers. The gas range/stove is not vented outside but cooking pressures tend to be small. Two people live in the house.

Assuming the rim joist is weak link (as far as the basement goes) I was thinking about using canned foam and/or caulking at all junctions of framing members within the bays and then insulating with R-30 Roxul. No poly would be placed in the bays.

All insights into the risk posed by the observed moisture levels, ideas about a plan of attack, and discussions of suitable fixes will be greatly appreciated.

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  1. user-2310254 | | #1


    How do you heat, cool, and ventilate the house? Does the basement have a perimeter drainage system? Is the odor problem just in the basement or also in other parts of the house?

  2. rshuman | | #2

    Hi Steve,

    The house is heated with a direct vent propane furnace, there is no air conditioning. Any ventilation is passive (through leaks) at this point in time. There is a drainage system around the outside of the foundation of the house. The slab has a couple inches of foam underneath it.

    The odor seems to be confined to the basement. As I indicated in my post, some of the odor was 'stale water' in the plumbing which has since been remedied. I can't really characterize the smell as being caused by excess humidity, but I can't rule that out either. It generally isn't an issue on most days, every now and then it is noticeable.

    I started focusing on the humidity because of the measurements I collected. I figured it made sense to address this problem regardless of whether it was at the root of the original issue (odor). So, in a sense, the odor issue that started this has been put on the back burner for now.

    The house is relatively new construction, about two years old, and we are the first to live in it full time. So some of this is a matter of learning about the house and fixing things as they are discovered.

  3. Expert Member
    Dana Dorsett | | #3

    The assumption the source of the humidity is the outdoors isn't necessarily well founded. Ihouses built wtih ICFs and polyethylene vapor barres tend to be pretty tight (but not always), and it's likely that you'll need active ventilation to keep indoor air quality high, and possibly some mechanical dehumidification (or air conditioning) to keep the indoor RH bounded.

    But the mid-50s to low 60s dew point column in your spreadsheet isn't particularly scary, and aren't necessarily much higher or lower than the outdoor dew points. The high interior RH numbers are dew to the relatively low indoor temperatures. A dew point of 60F is only 60% RH @ 70F, but your basement temps are in the low 60s, raising the RH.

    The solution is a room dehumidifer.

  4. Expert Member
    Dana Dorsett | | #4

    deleted double-post

  5. user-2310254 | | #5

    It's possible that the house needs to dry out. Framing and sheathing are exposed during construction and can accumulate a fair amount of moisture. Dana's suggestion to add a dehumidifier (Energy Star from the big box store) and a ventilation are worth considering.

  6. GBA Editor
    Martin Holladay | | #6

    I agree with Dana: The likely solutions include a ventilation system for the upper floors and a portable dehumidifier (at least temporarily) for the basement.

    One possible source of basement moisture is the slab. It isn't impossible that your contractor forgot to install polyethylene under the slab -- and if that's the case, the slab can contribute lots of moisture to the basement.

  7. rshuman | | #7


    Thanks for the input. Poly was placed under the slab so it would appear that moisture pathway isn't important. I am going to tape a piece of poly to the floor and see if I get any condensation to help confirm this.

    I am going to pursue a blower door test to see where things stand in terms of air leaks, take steps to seal any discovered leaks, and then figure out what might be reasonable in terms of active ventilation. Dehumidification is also on the table.

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