Hi all,

The background info:

- We are in central North Carolina - a humid area!
- Bought the house last May
- We are having problems with mold/mildew in the kitchen cabinets. It's smelly enough that it's bothering us, and not just when the cabinets/drawers are open.
- When we first moved in there was no issue beyond maybe a mild musty smell.
- We're not just dirty people I promise you, I've never had this problem before.
- Some time in the last 5 years the HVAC was modified to zone the upstairs and downstairs separately

The recent developments:

- We had a "mold guy" come and diagnose the house. He said the foundation seemed fine, some other things seemed fine, the humidity was high (mid 70s) and he indicated that as the likely source of the problem. He tested this by cranking up the AC and we watched as the humidity levels dropped. Makes sense to me.
- His claim is that the AC is your house's dehumidifier, and for it to be effective it should be as constantly as possible, as opposed to in strong short bursts. A smoother constant cooling process will keep the dehumidification process in effect. So he thinks something is wrong with our HVAC.
- We then have HVAC guy come out and he says we're missing a "bypass damper". They install one, and the next morning the humidity is at 70%. They come back and adjust it, and things seem better although at night we're still getting that spike to 70%.
- We're hearing/reading ideally houses should be between 45%-55% humidity with a properly adjusted HVAC, and that at this humidity level mold will not grow.

So now we are a little unsure of where we stand. Is the source of the problem fixed and we can move on to cleanup?

The questions!

- Is it reasonable to expect an AC to keep the house between 45%-55% constantly? Is a spike at night when the temp drops normal?
- If such a spike is normal, is it enough to enable mold to grow?
- What about in the winter when the AC doesn't run? Does the heat have the same effect?
- What about in the fall when neither need to run (as much)? Do people just get mold then? Or does this not last long enough for mold to thrive?
- Is this a problem that is specific/inevitable with particleboard cabinets? Would a better material for cabinets not have this problem?

Thanks for info and advice, we are a bit baffled by this.

 

The A/C can only remove so much humidity. Since it's cooler at night and the A/C runs less.... the humidity will rise. Most of the humidity is caused by air infiltration from outside to inside the house. Based on the age of your house you may be lacking in the insulation area.

The basement is also another area of high humidity as it enters thru the floor and foundation.

 

You mentioned that the A/C was modified to zone the upstairs and downstairs separately. Do you mean that you now have 2 A/C systems, one for the upstairs and one for the downstairs? If so, it's possible that the downstairs system is oversized (assuming that it previously cooled the entire house). For maximum dehumidification, you want a properly sized A/C system. If it's too small capacity, it won't be able to keep the house cool on the hottest days. However, if it's too large, it won't run long enough to dehumidify the air (runs for short burst). If the downstairs system was originally installed to cool the entire house, but is now only cooling the downstairs, it will be oversized.

As Pete mentioned, basements are normally very humid in the summer. If you have a basement, you should purchase a dehumidifier and run it in the summer when it's humid. You could also purchase another dehumidifier to put in the kitchen to remove some of the humidity. Of course, you'll have to put up with the additional noise, increased electric bill, and have to empty it periodically (daily at least).

In the winter, the heating normally dries out the house, so I doubt you'll have a humidity problem once the heating season begins. I have to run a humidifier to add moisture in the winter (western New York).

 

Since the tech just added a bypass damper..... this is a multi zoned single system

 

Right, this is a multi-zoned single system. Also I have a slab, there is no basement. The first guy who came out commented that my slab was unusually high off the ground, maybe 3 feet? But he also used some instruments to measure the possibility of moisture coming up through the slab and he did not find anything.

I will try to take a look at insulation as a possible problem, although I've had it commented too that my insulation is good because the AC cools effectively - it is able to cool the house quickly and is able to keep it in the low 70's even on 90+ degree summer days.

Could an insulation problem be local to the kitchen? It's the only area we have the mold smell. It may be interesting to move the humidity gauge upstairs somewhere and see if it reads differently. Can different areas of the house have significantly different levels of humidity?

 

Are you saying you actually have mold and mildew growing on the walls and cabinets, or is it just an odor?

We frequently have people comibg here complaining about musty odors and the source is often the bacteria in the sink trap. Replacing the trap and tailpieces with clean new ones often solves that problem.

 

We have definitely seen, and cleaned mold off drawers and cabinets that are on the other side of the kitchen as the sink. We definitely notice it coming from the cabinets and not the sink specifically. The mold guy we had out did a similar check.

 

Okay. The area where the mold and mildew occurs... is it along an exterior wall? Because if it is, I would guess that possibly there is poor air barrier on that wall. When drywall is taped and finished, it forms an air barrier. If there is any drywall missing behind your cabinets, hot humid air from outside would be flooding that area... and the humidity in that part of the house would be higher.

 

70% humidity is too high, that will cause mold.

Has anyone performed a load calculation? Your system may be too large for the house.

Leaking ductwork can also cause high humidity. Has the ductwork been checked? I have found many 8 by 24 inch holes in ductwork over the years.

Many systems will have a blower capable of delivering more CFM of air than the requirement of the outdoor unit. The external static pressure should be measured and the speed tap of the motor may need to be changed to deliver the correct airflow for your outdoor unit.

 

Last night I moved our humidity gauge upstairs to the bedroom and it read almost 70% this morning. Which I guess makes sense - the temperature drops, the AC stops running, the humidity rises. As Houston says, 70% is too high, but the thing I'm not clear about is whether a nightly spike to 70% is a "problem" enough for mold to grow when during the day (and the AC is running) I'm closer to 50%. What is a normal amount of fluctuation?

In any case that experiment tells me it's not a problem super local to the kitchen, although I also can't rule out the things Sleeper mentioned because the cabinets that have the problem are the ones sharing the wall to the outside, and the wall to the garage, and in both cases at ground level (although I'm not sure the significance of that). The cabinets that just share a wall with the interior of the house have no problem.

 

Do you have any vents in your toe kick near where the mildew is present? And is this a problem on base cabinets only? Or both wall and base cabinets?

 

No vents in the toe kick. I think there may be a mild smell on the upper wall cabinets, the big offenders are the base cabinets though.

 

Are you using bath fans and kitchen exhaust wile cooking? Bath fans should run at least 20 minutes after showering. Did the mold guy test the floor for moisture?

 

We do use bath fans, but probably not for 20 mins after the shower. We have a kitchen fan that we use some times but it just goes to a filter, not an exhaust. He did test the floor, although both the floor and the back walls of the cabinets I think must need some further investigation. The problem seems local to that area. I think our next step is to have a mold guy back out to the house and re-test with our new AC config.

 

Have they sent a camera down the ductwork to look for any standing water?

There could be something to the thought that the concrete under the cabinets has high moisture. Cool concrete absorbs moisture out of the air (at your exterior walls) when it's hot and humid... not to mention it's like a wick for ground water. It could be absorbing humidity from outside and your A/C is pulling that moisture out... just like if the drywall is not air sealed, lots of humid air could be leaking into the house.

Don't fixate on the humidity levels in different parts of the house, relative humidity changes with temperature. Its only an accurate comparison if the temperature is exactly the same everyplace you take a humidity reading.

Keep cleaning the cabinets with a product like Tilex. If the cabinets are bare wood inside and absorb water you don't really want to sponge them and add MORE water, and if you do you would need to leave doors open and run a fan until they are really dry. Lemon oil might be a good way to reduce the odor you are noticing.

Air movement is one way to even out differences in temperature/humidity so turning on a box fan or two, pointing them at your problem areas would be VERY helpful.

A/C coils need to be clean in order to remove humidity efficiently, so you could also check into that... don't know what type of system you have, but maybe an a-coil is dirty, and filters need to be changed. The amount of time the A/C runs before shutting off is important, so they can diagnose that too.

 

How would Tilex be different than any other cleaner like 409 or a bleach or a vinegar cleaner? What about Lysol?

 

I said "a product like Tilex". Not that Tilex is the only cleaner you could use.

 

You have moisture issues either the HVAC is over sized or you have moisture coming through the floor.