High RH in a Green home with a “conditioned” sealed crawl space
aburickymad
| Posted in Energy Efficiency and Durability on
HELP!
I am mad as a humid hornet!. I have a sealed “conditioned” crawl space, in one-year old house in Harrisonburg, VA, that is virtually leak-proof with top of the line 2Ton HVAC by Lennox.
So why do I have high RH (60-70%) in the living space? The crawl space is even more humid 70% plus! My wife has allergies, my hardwoods are “cupping, and doors are sticking. No mold yet.
Ideas?
If I must get a dehumidifier should I:
a. dehumidify the living space and “push” dryer air into the crawl space
b or dehumidify the crawl space directly–assuming that 30-50% of crawl space air gets into the living space eventually?
Confused.
Tom
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Replies
Sealed crawl spaces are a pain in the rear; moisture management, sealing detail and proper ventilation is job #1. Installing a dehumidifier BEFORE all the following items are done, maybe a waste of money.
1. I would check on the seal job of the conditioned space. The Moisture barrier over the soil, the rim joist, exterior penetrations, and the sump may not be sealed completely.
2. Have you provided ventilation to the crawl space?
3. Is the HVAC system designed, installed AND commissioned correctly?
4. Have you provided mechanical ventilation with humidity controls for humidity?
5. How are the moisture management detailing, envelope seal and insulation jobs on the wall and roof assemblies?
6. Do you know the details of your wall and roof assemblies?
7. Do you have negative pressure in your house?
The source of humidity probably is the crawl space, and the air-sealing details are likely the problem, both between crawl and house, and between crawl and outside. Then there's the question of how well you're sealed from the ground.
I like Armando's suggestions of where to look, but I would have a pro do it, it's not really stuff that a layperson is going to correctly inspect and diagnose. The person you want is an energy auditor / building science-savvy home performance contractor.
What is the RH outdoors? I'm curious to know if the crawl and house are higher humidity than the outdoors. You have ducts in the crawl with A/C running?
Tom,
It's possible that the high moisture levels in your one-year-old house are due to construction moisture (due to drywall compound, paint, damp lumber, and rain that entered your crawl space before the roof was dried in).
A new home with a sealed crawl space sometimes needs a dehumidifier for the first 4 to 6 months to get rid of construction moisture. However, a properly detailed sealed crawl space should not need a permanent dehumidifier. If your builder never installed a dehumidifier temporarily in your crawl space after the house was dried in, I would consider installing one temporarily -- especially after confirming that your air sealing details were handled properly as Armando and David suggest.
Finally, the building code requires that sealed crawl spaces be conditioned. Was this done at your house?
The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs. Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan blowing through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)
Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)
For more information, see Building an Unvented Crawl Space.
Hi Tom,
You say that the house is virtually leak-proof. Do you mean . . .
It's well built and flashed and sealed against bulk water entry
OR
It has a low air leakage rate (determined by a blower door test)
Since this is a new house, did you ask the builder these questions? What type of commissioning and testing was performed on the house prior to occupancy? Do you have this information and data?
If not, you should probably consider an inspection / audit to determine root cause.
I'm in Augusta County --- south of Staunton. I have a sealed crawl space. If you want to talk to somebody local, I would be glad to discuss in more detail. You can reach me at (540) Four one six - 3590.