High Indoor Humidity and How to Control & Reduce It

High Indoor Humidity Levels and How to Control & Reduce Them During Construction

I am going to cover a topic that I have yet to find much detailed information online--a circumstance that you will likely encounter when constructing a super insulated and airtight building like a Passivhaus--high indoor humidity levels.

I believe this is a most important subject matter--for those few of us who actually build passive houses--as I virtually guarantee you will encounter high or elevated levels of relative humidity during some phases of construction of your Passivhaus.  When you do, you may be alarmed, as we were.  Don't panic, things will get better, but it does require vigilance and effective management.

It is a fact that during much of the construction period, you introduce lots of moisture into the interior spaces of your home.  It's unavoidable and there is no way around it, so it's best to be prepared for it.  Monitoring and maintaining reasonable levels of indoor humidity is extremely important as you don't want to create a mold factory during the construction period, especially behind the interior walls.

To that end, I am going to discuss, what I believe are, the key areas that contribute to high levels of relative humidity (RH) and moisture production.  The EPA recommends maintaining indoor RH levels between 30%-55%, with the sweet spot being at about 45%.  At this optimal RH level, the interior air quality tends to be the best as are the conditions that minimize the production of bacteria, viruses, fungi, and dust mites.

Lumber and Wood

We constructed a double wall system.  The exterior walls were SIPs and the interior walls were created with 2x4 wooden studs.

Lumber was everywhere and all of it was moisture rich when it was first used.  It takes a good number of years for wood to dry out.  In the meantime, its going to be a source of interior/indoor humidity and moisture for an extended amount of time.

In our particular circumstance, the extended length of time it took to construct our home only exacerbated the situation.  The duration of time that the interior of our home was exposed to the outside elements and/or unconditioned air was (much) longer than we would have preferred.  We went through full seasons of spring and summer with at least some section of the building opened to the outside.  Furthermore, we went through fall and most of the following winter without active heating from our mini-splits.

That meant, extended interior exposure time to (at times very) high levels of humidity.  Which, in turn meant that all of the wood, already moisture rich, took on even higher levels of moisture from the outside warm humid air; all of the studs, floor plywood, and OSB acted as sponges.  Keep in mind that even if this were not the case and our building would have been "buttoned-up" quickly, the wood used in new construction always has elevated levels of moisture content, initially.

To make matters even worse, since we didn't begin conditioning our home, we used propane heaters to heat the interior spaces while we worked.  And you know what that meant?  You guessed it, even more moisture and humidity were introduced inside.

Poured Concrete Slab (or Foundation Walls)

Concrete slabs are another significant contributor to elevated or high levels of indoor humidity.  In our case, we used a high PSI concrete (between 5000-6000 PSI) at a nominal thickness of five inches.  It takes a couple of years for concrete completely to dry out, especially higher density concrete.  With sub slab insulation and an airtight vapor barrier below it--just like that of exterior SIP or ICF walls--the moisture has only one pathway to go, to the inside.  Expect any basement to be a source of elevated levels of humidity for some time.

Airtightness/Vapor Barrier (with SIPs and ICFs) on Exterior Walls

Of course any properly constructed Passivhaus is going to be airtight and the best airtightness designs are those that are accomplished at the earliest possible phase of construction and at the most outer sections of the building--the exterior walls.

The significance of this, especially when coupled with SIPs or ICF construction, is that any drying out can again only occur towards the inside of the building as the moisture has only one pathway out.  That meant high levels of indoor humidity.  Be prepared for it, it's going to happen.  And since  there is only one pathway for all of the moisture to leave, it's going to take a good bit longer too.

As Lisa and I were shooting for airtightness levels exceeding 0.2 ACH50 (three times the Passivhaus standard) we made certain that we achieved the best airtight performance possible at the earliest time.  We didn't count on getting any further air tightness benefits from drywalling and insulating, as so many builders typically do.  The very first official blower door test indicated a 0.68 ACH50--already extremely airtight and that was merely after the SIPs, the windows, and airtight membranes were utilized and before we even began the iterative process of making our home especially airtight.

When a building is extremely airtight, the ACHis essentially nil and without active ventilation or dehumidification when the building is closed-up humidity has no where to go.


Insulation can be another large source of increased indoor humidity and in our case, it was.  We used wet spray blown cellulose throughout and lots of it.  Our SIPs gave us a nominal R38 R-factor.  We then sprayed another five inches of blown cellulose on the first and second floors to give us a final insulating R-value of ~R58.  In our walkout basement, we used 11 inches of blown/sprayed cellulose in addition to our superior walls to give us a similar final R-value.  11 inches of wet sprayed cellulose contains a lot of moisture.

We even added more moisture with our insulation since we insulated every interior wall and filled every floor joist cavity which had plumbing (for sound deadening).

We did let several days go by before dry walling, but even after a week in the basement, 11 inches of wet spray cellulose was still far from completely dry.  Dry walling closes any remaining moisture from insulation inside, causing it to take even longer for it to completely dry.

If you are using wet spray cellulose, keep from dry walling for as long as you can to allow the insulation to dry out as quickly and thoroughly as possible.

At this phase, relative humidity levels were in the high 60s to low 70s.  This is not a place we wanted to be for any extended period of time.  To help with reducing these extremely high levels of indoor humidity, we used several dehumidifiers running 24 hours a day and box fans to move the air around.  Be ready for a healthy electric bill during this period.  Getting the humidity down was our primary objective at this time.

Drywall & Plaster

Another source of indoor humidity and moisture was drywall and plaster.  Levels of indoor humidity were basically at their highest at this point.  We took a double hit here, because not only did the drywall and plaster significantly contribute to humidity in themselves, they slowed down the rate of drying of all of the lumber and insulation behind it.

We continued relying on our dehumidifiers and fans to assist in drying things out and they certainly did.  I can not even tell you how many times we dumped filled containers of water from these things, but it was a lot.  On average I dumped two-three containers of water each from two dehumidifiers per day for an extended period.  I couldn't get over how much water came out of the air.

Fortunately, with all of this effort, were did make progress--our indoor relative humidity levels were dropping and got into the high 50s--as long as we kept running the dehumidifiers.  If we stopped using them, the relative humidity levels increased to the low 60s again.


We used an ultra-low VOC water-based latex paint for all of our interior walls.  The effect from painting on indoor humidity was quite interesting.  Initially it added moisture, so expect humidity levels to rise after each coating.  But after each coat of the paint dried (which happened pretty quickly), humidity levels drop again and dropped even further than where they were before we began painting.

The reason?  Because once dried paint is on the walls, the rate of drying of everything behind the paint is substantially reduced.  This doesn't mean that the moisture went away, it just means that the release of moisture is substantially slowed.  Therefore the positive impact of dehumidifying is felt that much more and they really helped extracting that moisture out of the air quickly.

At this point we had made some serious progress.  We were were able to achieve and maintain RH levels in the mid (to occasionally lower) 50s.

Getting the relative humidity levels below 60% was an important milestone, as the possibility of mold formation is inhibited below this level.

Hardwood Flooring

Lisa and I chose to use a lot of hardwood flooring throughout the house.  Not only do hardwood floors look great, they also lead to a healthier indoor living spaces.  Hardwood doesn't trap odors, dust (much of it from dead skin), dirt, and dander as does carpeting.   But, unfinished hardwood floors need to acclimate to the interior conditions, meaning the wood has to dry out, which in turn means more interior moisture.  We let our hardwood acclimate for nearly five weeks.  Given the expense and the quality of the wood (Patagonia Rosewood), we wanted to be absolutely certain that the wood got adequately conditioned to the building, especially in light of the challenges we were overcoming.

To measure moisture levels of various materials, we purchased a moisture meter, available from Home Depot or Lowes.  We were able to monitor the levels of moisture content in our plywood floor, drywall, concrete foundation slab, and our unfinished hardwood.  I would certainly recommend that you purchase one to do the same.  We waited until the moisture content of the wood was within a small percentage of the moisture content of the plywood sub-floor before installing the flooring.

Mission Accomplished!

I am most pleased to report that with of this effort and running our mini-splits (when we had them installed) in dehumidification modes (during the summer) or opening the windows and doors--when outside humidity levels were very low--and heating (when needed) during this past winter, we are now at nominal indoor relative humidity levels of the low to mid 30s when empty to the optimal mid 40s when occupied--at indoor temperatures of about 70-72F and that includes the basement as well.

Using our ERVs and the pre-dehumidified air coming out of our earth air-tubes supplying the intake air to the ERVs, certainly helped in transferring moisture from the inside to the outside, as well.

Even our basement feels very comfortable and dry and nothing like how a typical basement feels.   Coming to the warmer months, we are hoping that we won't require much active air-conditioning (or any, for that matter) because our humidity levels are so optimal at this point.

I attribute our success in achieving and maintaining such healthy levels of indoor relative humidity in such a short period-- despite our early handicap--to being vigilant and because we effectively monitored, managed, and controlled our indoor humidity levels throughout the construction period.

Most conventionally designed and heated buildings can be especially dry with forced-air heating systems.  I am very pleased to see that we can maintain RH levels of 45-47% (at temperatures of 70-72F) during the winter.  Having our indoor humidity levels within this range makes for most comfortable living conditions and when inside, we are completely oblivious to the fact that we are in the winter season.  It's a wonderful feeling, indeed.