Managing Mold & Humidity (in Hawaii)

blower door hawaii

A blower door measures how tight a house is

Air sealing done as part of the install, and after hacking holes into the house (and sealing others), total leakage only increased by 65 CFM50.  Still a B-ring house.


I got a call from the east side of Hawaii, so I did what I always do.  I tested the house and collected some data.  The testing is important - no matter how well we think we know our houses, they can surprise us.  Without testing, it'd be like looking at someone and guessing if they have high blood pressure.  Sometimes it's obvious, but mostly it isn't.  I play small games with myself and try to guess how a house will behave before I test it.  I usually lose.

The blower door helps figure out why a house has a problem.  In this case we found consistently high humidity levels, coupled with a tight building envelope.  Because of the design of the house, it couldn't ventilate the moisture away at all.

HOBO data loggera Data logger takes snapshots of humidity levels

The building tightness was actually helpful; if the house were built today, it would be 25% better than Hawaii's Energy Code minimum for airtightness.  That meant that the environment could be more controlled, and a ducted dehumidifier would work.

Ultra-Aire 120H dehumidifier

Hanging the unit saved valuable floor space


The control is a "dehumidistat"

MERV 13 filter

A nice fat MERV 13 filter

This house doesn't have air conditioning, so it doesn't have a duct system either.  That can be an advantage, because a dedicated purpose-built duct system can be right-sized to match the unit.  As it's built, its joints are sealed and while you're at it, it'd be a good time to air seal the house too.  That helps keep the drier air from escaping as quickly.

ceiling registerCeiling diffusers are installed in the closets, so dry air doesn't blow directly on people. closets are also mold-prone areas.


The joints on the ductwork should be sealed, otherwise, the air you want to keep in the house is squeezed out through the joints.

air sealing

these are electrician's holes, all sealed up with blobs of expanding foam. air sealing helps to keep conditioned air inside the house.

crawlspace hawaii

the crawlspace was air sealed too - there was plenty of air leakage around plumbing drains in particular

Once the unit is installed, and everything's attached, it's tempting to think that it'll work.  Why wouldn't it?  There are lots of potential reasons:

  • faulty equipment
  • shipping damage (the unit came from Wisconsin)
  • missed ductwork connections
  • install errors
  • control wiring mistakes
  • house alterations
  • etc.

So many things can go wrong that "commissioning" a system is crucial.  As part of a startup procedure, I take measurements.  I re-do the blower door test, because in this case seven large holes were cut in the building envelope and that could make the house leakier, which isn't helpful.  It's good to make sure all the new holes are sealed as best as possible.  I also measure the pressure in the ductwork, because in the instructions for the dehumidifier there's a table that tells you how the unit's performing.

static pressure

0.11 inches water column, correlates to about 310 cubic feet of air per minute

Ultra-Aire 120H dehumidifier Hawaii

from the instructions. always read the instructions.

But if you really want to make sure it's working, and not just hope it's working, you need a good amount of data.  I left my data logger behind to see what happened, and we learned a useful thing: in 18 hours, it's possible to reduce the humidity in the house by 20 percentage points.

relative humidity graphIn about 18 hours the house went from 85% RH to about 65% RH

Did I mention that this house is off-grid?  I got a call from the homeowner that the battery bank was drained and the generator didn't kick on.  Here's the same graph as above, just for a slightly longer time period.  You can see the humidity climb (when we experienced the power interruption), how quickly it climbed, and how quickly the unit drew the humidity level back down when the power came back on.

relative humidity graph 2 with power interruption

On the right side of the graph, we can see the dehumidifier "stuttering" as power came & went. once it was lost completely, the smooth part of the graph shows the steady rise of the humidity level, and then the sharp drop-off shows rapid drying.

That was a good thing to learn - the generator was offline!  It had never been an issue before, but because an 'ohana will be added, it became clear that more solar generation will be needed.

Mold needs certain conditions to grow, and one of them is a moist environment.  You could kill the mold, but if you don't change the conditions it'll just come back.  A drier (but not too dry) home is healthier.  If you're tired of moldy closets, or your child sneezes a lot, or your furniture is moving, get in touch.  We'll figure out what will work for you.

The data will tell us.

Previous blogs:

You Need a Bigly Dehumidifier part 1

You Need a Bigly Dehumidifier part 2

Staying Dry on Waimea's Wet Side

"Before" Pictures:

mold couch

moldy furniture


mold cabinet

some building products are more moisture-sensitive, like this kitchen cabinet



I mean, you can SEE the humidity. It's RIGHT THERE!

wood movement

the humidity was so high it split the furniture apart

dpcalc results

under the house's normal conditions, mold was guaranteed.

link to blog here:

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Comment by Blake Reid on October 11, 2018 at 2:45pm

Thank you very much!

They're actually the same, though - the one with the dehu cover photo has a link that sends you to this very same blog, on my website.

Comment by John Proctor on October 11, 2018 at 1:51pm

Nice post -- much better than the one that only showed the dehumidifier.

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