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Community and Q&A

Preventing Condensation on Sheathing

JennL | Posted in Expert Exchange Q&A on

I am extremely sensitive to mold. We are planning to build a 1765 sq ft ranch in climate zone 5 and trying determine the best time to build. Initially we were considering building in the winter due to lower relative humidity levels, but I recently read the following article and became concerned about the risk of condensation occurring due to cold wall sheathing.

Condensation on Wall Sheathing During Construction

“Damp sheathing is more common when the insulation has not yet been covered by drywall, since the interior air has better access to the cold sheathing when there is no drywall. (Without drywall, the warm, humid, indoor air has unimpeded access to the cold sheathing.)… Damp sheathing is more common when the sheathing is covered by an air-permeable insulation like fiberglass batts.” We are planning to use fiberglass batts in the stud walls.

I was hoping to do a pre-drywall blower door test, which could further delay the installation of drywall and increase the risk of this occurring if we get to this phase in winter.

According to the article, control measures include:

1. If possible, consider installing a sufficiently thick layer of continuous rigid foam or semi-rigid mineral wool on the exterior side of the wall sheathing.If possible, consider installing cellulose insulation instead of fiberglass batts between the studs. (Cellulose insulation does a better job of slowing air movement than fiberglass insulation or mineral wool insulation.)

2. Before installing wall insulation, take steps to lower the home’s indoor relative humidity. During the winter, it might be a good idea to open a few windows or operate the home’s ventilation system. It’s also a good idea to turn up the heat (as long as the heater doesn’t introduce moisture into the building).

3. During the summer, it might be a good idea to operate a dehumidifier or air conditioner.

4. if you are installing fiberglass batts in cold weather, get the drywall up fast — as fast as possible after the insulation work is done — to ensure that there is an air barrier separating the humid indoor air from the cold sheathing.

Here are a few of my concerns:

1. I am reluctant to add exterior insulation as my builder doesn’t do this often; choosing the correct thickness can be tricky; and it could affect the ability of the wall to dry to the exterior.

2. I don’t want to use cellulose insulation as this is essentially pre-digested mold food and, to my knowledge, is not recommended for those with mold sensitivity.

3. I don’t want to use the HVAC during construction as it may introduce debris which may foster mold growth and bother my asthma. We plan to purchase two powerful dehumidifiers to use in the basement and on the first floor of the ranch. We’ll purchase two window A/C units for the first floor to control humidity in the summer phase and electric heaters for use in the winter phase of building.

Here are my questions.

1. Is it better to get to the insulation drywall phase in spring or fall rather than winter (when sheathing condensation may occur) or summer (when the humidity is higher)?

2. If we want to get to the insulation/drywall phase when temps are more moderate, is it okay to pour the foundation early (prior to winter) and let it sit?  Would this help the foundation to dry and reduce humidity levels? Or would it be subject it to rain, snow, and dirt?

3. What is the best time to build, all things considered?

Thank you

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  1. plumb_bob | | #1

    The typical season for buildings getting wet to a damaging level is when the framing happens in the fall, and gets rained on before the roof goes on. But I am on the west coast, so lots of rain...

    This moisture/mould management seems to be very important to you, so I would suggest you get your foundation poured, framing completed and get the house to lock-up (roofing on, windows/doors in) and let the house sit for a while to dry out. You can then monitor all of the components and not close them in until the moisture levels are optimized. You could have fans and dehumidifiers running to help dry things out.
    If you schedule your construction this way it should not cost any more, just complete the project in phases.
    I think many houses would benefit from this approach, but the construction industry is geared to complete a building as quickly as possible.

  2. Expert Member


    I agree with plumb_bob. One thing to watch though is many construction loans have time limits for completion, and many also say that the lock-up draw will only be released when the rough in plumbing and electric are done. Having a house sit between draws can mean paying the sub-trades yourself.

  3. user-1072251 | | #3

    Completing a house in the fall means it'll be closed up for months. Completing it in the spring means you can easily let it vent. My recipe for avoiding "damp' is to properly insulate the basement, starting with good foundation drains with solid perforated pipe, draining to daylight. Next, a good coat of asphalt on the exterior. Then insulation under and around the slab (we use 6" of EPS in New Hampshire), covered with heavy poly which is taped, and aluminum faced polyiso on the interior concrete walls, which stops the inflow of moisture through the concrete. Of course you'll need a ventilation system and a good heat pump based heating system which are excellent dehumidifiers. Our basements are not damp, and the house is comfortable. We use 1500 watt electric heaters if we are building in winter; in very cold weather we might use three; typically only one is needed after insulation to keep the house near 70.

    Do not use fiberglass insulation! It holds moisture if it gets wet, while cellulose and Rockwool will dry out.

    Most of our homes are double wall with cellulose; we have used exterior foam; both work. Be sure the wall can dry either both ways or at least one direction.

    One client has no more problems with her asthma; one reports no more migraines in the over two years since moving in.

    1. JennL | | #4

      Thanks for your feedback. What model 1500 watt electric heater do you use? How many do you think would be sufficient for a 1765 sq ft ranch prior to insulation?

  4. erik_brewster | | #5

    I understand you don't want to use the house HVAC because of debris getting inside it. How about you rent a portable HVAC system (AC in summer, heater in winter, dehumidifier / humid / as needed)? Then when construction is done, the potentially contaminated HVAC system leaves, with the debris.

  5. walta100 | | #6

    It sounds easy to say “I don’t want to use the HVAC during construction” Ok how much extra money are you will to spend to make that happen?

    If you really mean it and want to add thousands for a budget line look at this Matt Risinger video with a 5-ton trailer mounted heat pump he shows being used to cool during construction.

    If I wanted to time construction to quickly dry the new home, I would try to finish early in the heating season. Open some windows when it is very cold and heat that very dry air.


    1. JennL | | #7

      I've been doing research and generating alternate solutions. Worth the effort when your health depends on it. We are planning to purchase two Sante Fe (Thermastor) 70 pint dehumidifiers (which we would have purchased anyway for later use), two electric heaters and blowers (which we also would have purchased for intermittent use in the garage - if only one is needed we can sell the other), and two window A/C units (which can be kept for backup or sold). It's a small house so I think this is a reasonable solution.

  6. user-1072251 | | #8

    Santa Fe is an excellent dehumidifier. But the real solution is to build an airtight, net zero ready super insulated house where moisture and air quality is controlled by the way you build and you don't need that stuff.

    We've used several brands of 1500w heaters; whichever is cheaper. Doesn't matter.
    Walter: electric heaters are not inexpensive to use, but they are less if you don't need much heat.
    How many would you need? Depends mostly on how tight your house is. Be sure to do a Blower Door Test before you insulate; that way you can tighten it up; much harder to do after the walls are closed up. In our houses, under 2000 SF we don't need more than two. The house with three was 3000 SF.

  7. JennL | | #9

    I'd argue that the dehumidifier might be needed during shoulder seasons when the humidity is high but the temperature is low so the AC will not be triggered. I need to keep the house <50% RH at all times. I'm going to use Lunos HRV fans for ventilation which might bring in a bit of humidity during the shoulder seasons, too.

    Yes, I am planning to do a pre-drywall blower door test. Can you give me an idea about cost? My builder is quoting $3500 but this might be a typo as I am getting quotes closer to $350. Hoping to find someone who uses infrared b/c I'm not just looking for a number - I'm looking to correct leaks. Thank you

  8. andy_ | | #10

    I think a lot of this concern about mold in the construction process is misplaced. The reality is that the finished home won't suffer mold issues if it's designed and built well to begin with.
    I'm mold sensitive too, and I built a house in the PNW in winter. It rains a lot here in winter. Like, A LOT. Due to circumstances beyond my control the unfinished framed house sat in the rain while I sat in a hospital bed and then recovered for a month. The framing got mold. A little remediation once it was weathered in and there was no sign of mold after that. You'd never know there was ever mold anywhere now.
    Job sites get messy, wood gets wet, but the finished house won't reflect that. Mold issues in homes are the result of the original design shortcomings or the occupants behavior.

    1. Expert Member
      MALCOLM TAYLOR | | #11



      1. JennL | | #13

        I DISAGREE that concern about mold during the construction process is misplaced.

        It's estimated that 50% of all buildings in the US are water damaged. Clearly we are doing something wrong. Much can be done during the time of the build to avoid future problems and ignoring that is just foolish.

        If you don't control humidity during the building process (and in the first year afterward), you may well have problems. Leaving construction materials in direct contact with the wet ground, storing them without adequate airflow, introducing moisture (and toxins) into the structure with combustion heaters, and sealing in building materials before they have dried adequately are all a set up for future problems. These are just a few examples.

        Here's another example related to dew point considerations.

        P.S. Mold can't be seen when it's hidden behind drywall. It's health effects can be insidious, difficult to diagnose, and devastating.

      2. JennL | | #14


        I was surprised by your response.

        As an expert your opinion carries considerable weight. Many members of this site are builders so your opinion has the ability to affect many people.

        Do you really believe that poor construction practices do not have the potential to contribute to mold growth in structures? Please see my comments #s 13 and 17.


        1. Expert Member
          Akos | | #15


          My current project involved removing a roof and adding on a 3rd story to an existing house. The main floor and basement were finished, as you can imagine keeping water out was a priority. Even with my best efforts, nature tends to not cooperate and storms happen, some water made it in.

          For a new construction, there is simply no way to build it without material getting wet. I guess you can erect a tent and build under it, not sure what the cost of that would be. Plus even then, when jobsite material gets delivered it might sit on the front lawn for a week or two and might get wet. It probably has already sat at the lumber yard for a week or two and got soaked. Moisture plus wood, there will always be some mold.

          About the only way to avoid this, is not to use wood for your house. Possible, but won't be cheap. You can look at ICF or CFS as options.

          I think the better option would be to accept that there will be some mold during construction. Once the place is dried in either pay a remediation company to clean it up or have somebody spray the whole place with primer to seal it in.

          1. JennL | | #16

            I respectfully disagree. We can do better. Here are some simple steps that can be taken to mitigate the risk.

            Schedule delivery of building materials close to the time of their installation.
            Once they’re on site, keep them off the ground, covered with tarps, or ideally, stored within the structure under construction.
            Ventilate during construction.
            Avoid use of portable kerosene or propane-fired heaters in enclosed spaces as they introduce large quantities of moisture into the building.
            Allow enough time to pass between various construction phases so that materials that require drying have sufficient time to do so.


            Guidelines for Storage and Handling of APA Trademarked Panels

            Tips to deal with wet framing from Matt Risinger:
            Remove standing water from subflooring asap
            Use carpet blowers and electric heaters to dry out the structure
            Use a moisture meter to determine if the wood has dried to the proper moisture content

            This is a great continuing education course if you are interested in learning about this topic.
            Mold Preventative Building Education for Homeowners and Building Pros

          2. Expert Member
            Akos | | #17

            As they say, In theory there is no difference between theory and practice, while in practice there is. So while what you say is possible, about the only way to make it happen is if you do the build yourself.

            Reality is construction is a complicated and messy process. Unless nature cooperates, things will get wet. Wet osb, there will always be a bit of mold.

  9. Deleted | | #12


  10. walta100 | | #18

    Jen have you been involved in a house build before this one?

    We are not trying to be negative but we want you to have expiations mere humans could achieve.

    It seems your expiations are so very high we fear that the reality of a job site is going to crush you.

    I found it to be complicated and stressful process. Getting what I wanted done is hard enough. Getting things done when I wanted is no small feat but if you set about micro managing every detail your builder will go insane and very well might quit on you. If you get to the point that you start telling people how to do their jobs the word will get around and the good sub will not take your calls the bad sub will bid and take your jobs but ignore your instructions and file liens if you don’t pay in full.

    When you ask people for bid and tell them you want your work to be done differently than most jobs. Both you and they will understand what you are asking will take more labor to do extra supervision and have extra risk, to make sure your odd requests happen the way you want you will be paying extra to cover the costs and everyone will be happy. If you think you will get something for nothing you had better think again.

    How will you deal when things don’t go your way?

    If it rains every day for 2 weeks and your what is to be your basement looks like a swimming pool?

    The lumber gets delivered and they set it in the mud? Did they have a choice every inch of ground within 100 feet oh the house is mud except for the strip of rock that will one day be a drive way.

    What happens when you see a spot on a 2x4 and think it is mold? Really you want the framing blasted with dry ice to remove all the dark spots?

    Will you be able to deal with the stuff is the question more than a few things will go wrong? Buying something that is a complete known building sounds a lot less risky & frustrating even if it is not your dream.


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