How long for interior to dry out?
jackofalltrades777
| Posted in General Questions on
I read some articles that state it can take up to 3 years for all the “construction moisture” to finally dry out (slab, interior lumber). I am in a Zone 4B climate, ICF walls, insulated slab on grade with a vapor barrier, pretty tight house, ERV, under 1,000 sqft home.
After almost 8 months of living in it, I notice that my interior humidity this summer is always hovering around 45% inside even though it can be 25% humidity outside. I don’t cook pasta or run long showers.
Is this still some leftover moisture from the construction phase?
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Replies
Peter- for what it's worth it took my 1000 sq ft PERSIST wall house about 6-9 months to dry out with a wood stove in NH. This is probably an absolute best case scenario
Tech1234,
What were your humidity levels during that time?
Peter,
You haven't told us (a) whether you have air conditioning, or (b) what the outdoor temperatures were when you made the observation about 25% RH outdoors. These factors affect any interpretation of the conditions you are describing.
In general, it's quite possible that your house still has construction moisture. The two most common ways of reducing indoor moisture levels during the summer are (a) to use an air conditioner, or (b) to use a dehumidifier. In some cases, you can use both.
In much of the U.S., operating a ventilation system during the summer is counterproductive (if your goal is to reduce interior moisture levels). So keep ventilation at a minimum.
Without the temperature to which the humidity numbers are relative there comparing the indoor & outdoor RH is meaningless- it's RELATIVE humidity, after all, the saturation level relative to the air temperature.
When it's 45% RH @ 75F indoors as a dew point of 52F ( a measure of absolute relativity), which is teh same humidiy as 25% RH outdoors at 93F. It's normal for the dew point of the indoor air in a tight house to be higher than the outdoor dew point until/unless there is active mechanical dehumidification being used (air conditioning, etc.)
In a zone 4B climate outdoor dew points are normally in the 40s, sometimes low 50s in the summer, and ventilation with outdoor air is almost NEVER counterproductive. An indoor RH of 45% RH is still within the human-health Goldilocks zone (30-50% RH). As long as the outdoor dew point is below 55F and above 20F with an indoor temp of 75F, ventilation with outdoor air would be within that zone. Whenever the outdoor dew point is lower than the indoor dew point (which is nearly ALL the time in zone 4B) , ventilation will be moving moisture out.
In zone 4A summertime humidity is sometimes a hinderance for drying out the house with ventilation, but not so much in 4B & 4C. Even in at the humid edge of 4B near 4A say, Lubbock TX the outdoor dew points are still below 60F about half the time in mid-summer. Most of 4B is drier than that.
Martin,
A - Yes on the air conditioning (1 ton ductless minisplit - Mitsubishi) (I also have a dehumidifier that I turned on during the monsoon storms that hit)
B - It was 94F outside with 25% RH
We just had some monsoon storms a few days ago which drove the outdoor humidity into the 95% range and the dew point was in the upper 60s at that time. It's starting to dry out on the outside as the monsoon system is taking a reprieve now.
Should I vent with the ERV when it's only below 55F Dew Point outside?
I did an experiment and turned on the dehumidifier and let it run from 10pm to 6am last night. When I went to sleep at 10pm the indoor humidity was at 45% and at 6am it was at 44%. The bucket had water in it that had to be emptied in the am.
It's really weird on how the indoor humidity kind of locks in at 45%. It will be interesting to see what happens once the monsoon system passes and the outdoor dew points drop into the 40s.
I also noticed that the Mitsubishi has a "dehumidify" mode but what happens is that it will drop the interior temps from 71F to 64F and then the unit shuts down as that is the bottom temp threshold. Water does drip from the condensate line when in that mode.
I wonder what the Mitsu level is for indoor humidity? Does it dehumidify only when indoor RH is above 50% when it standard AC mode?
If your indoor % relative humidity is less than 60%, I wouldn't take any action to reduce humidity.
I think you should always vent with your ERV (to improve indoor air quality).
>B - It was 94F outside with 25% RH
The 94F/25% RH outdoors has almost exactly the same humidity levels as 75F/45% indoor air, so that sort of makes sense. Ventilating with that air won't dry the place out, but it won't add to the indoor humidity either.
>I wonder what the Mitsu level is for indoor humidity? Does it dehumidify
>only when indoor RH is above 50% when it standard AC mode?
As a general rule equipment that tests at a high SEER also has a higher Sensible Heat Ratio (SHR) will be higher (and lower latent cooling fraction) than equipment with a lower SEER. All air conditioning modes will do SOME dehumidification, but the ratio changes with how the equipment is being operated. If you want it to deliver lower SHR without using the "DEHUMIDIFY" mode, force the fan speed to it's lowest setting rather than letting it automatically shift up/down with load. With the fan at it's lowest setting maximum cooling capacity will be somewhat lower, but it also lowers the SHR, increasing the dehumidifying effect.
Which model Mitsubishi?
In a 4B climate a high SEER high SHR hardly matters though (unlike zone 4A) since most of the time indoor humidity can be controlled to reasonable human health & comfort levels via ventilation rates.
FWIW: Dew points are in the 70s in my area right now and are expected to hang there for a week or so, expected this time of year in zone 5A. In zones 3A @ 2A it's fairly common for dew points to hang in the high 70s for a week or more at a time, sometimes breaking 80F.
Jon R is right. Indoor relative humidity at 60% isn't unusual during the summer, and if the indoor RH stays at 60% or below, you're fine.
During the winter, an indoor RH of 30% is more normal. But not in the summer.
Dana,
The Mitsu model# MUZ-GL-12NA
Martin/Dana,
Do standard AC systems do a better job of removing moisture from the air? My neighbor has a standard AC system and his indoor RH is about 35% right now.
Peter,
There are a lot of variables. In general, right-sized systems (systems that have long run times) do a better job of moisture removal than oversized systems.
Lower airflow rates over the coil -- generally, 350 cfm per ton or lower -- usually result in better humidity removal than high airflow rates over the coil. Be careful with any air flow rate adjustments, however -- if your air flow rate is too low, your coil can freeze.
Again comparing your interior RH % with you're neighbor's isn't relevant without the temperature. Air at 80F / 35% RH has the same amount of humidity as 72F / 45% air.
"Standard" central air systems can have either a higher or lower SHR than a GL12NA. The SEER of the GL12NA is higher than most central air systems at 23-25, but central air drives outdoor air infiltration by creating room to room air pressure differences, which can add/subtract from the latent load, independent of it's SHR. The actual cfm through the coils will also vary by the duct design, and a lower cfm will deliver a lower SHR than a higher cfm.
With the GL12 (unlike most central air systems) you a wide set of blower speeds you can control. Setting the fan speed to "Quiet" with the remote will maximize the latent cooling (dehumidifying) in normal cooling mode while still controlling to a setpoint. If it can't keep up with the sensible load (temperature setpoint) at that speed you can bump the fan speed up to something higher, giving up some (but certainly not all) of that dehumidification.
My (very low cooling load) house hasn't seen 45% @ 75F type indoor humidity since early May. Over this weekend the sensible loads were low , outdoor dew points were in the 70s, and the indoor RH was runing 62-63% @ 73F. To deal with it I ran a half-ton window shaker (set to 60F to be sure it didn't turn off) on it's lowest fan speed for a handful of hours with all the interior doors open (except to the basement) which brought the indoor humidity into the 52-55% range @ 72F, just 1F lower than where it started. The GL12 can drop back to under a half-ton in cooling mode, and at the lowest fan speed would do even better of controlling humidity than the cheap window shaker.
With their smarter-better controls mini-splits are better protected from coil icing than window-shakers (or central air) that don't/can't vary the refrigerant volumes the way mini-splits do.
Dana,
Thanks for the information. I didn't know about turning down the fan speed increases the dehumidification. The Mitsu instructions and remote control is pretty limited in what it shows and states.
How does the fan speed control humidity?
Peter,
Adjusting air flow over the coil is common on central air conditioners connected the forced-air ductwork -- the systems found in typical U.S. homes. I didn't mean to imply that this approach works for a Japanese minisplit.
If the air moves slowly over the coil, there is more time for condensation to occur, and more time for the water drops to drip off the coil onto the pan below.
If air moves swiftly over the coil, you get more sensible cooling (lowering of the air temperature) but less latent cooling (removal of moisture).