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Trapped Moisture –

Your Home’s Enemy Number One

Trapped moisture is your house’s worst enemy! We are talking about the kind of sneaky moisture that comes in on wet jackets, shoes, umbrellas – even pets. It also comes from watering house plants, cooking, showering and house leaks.


When the moisture from those sources evaporates, the water vapors travel through the house until they find a cool wall surface, toilet tank, or even a thermal pane window to collect on and turn back into a liquid state. This water vapor also finds the cool lower corners of rooms, bottom walls of closets and certainly anywhere in an unvented bathroom. It sticks and stays, creating a little environment for mold and mildew. None of this is good and can be pre-“vented” – literally.


Over the years working for NeighborWorks of Grays Harbor, we have inspected and directed the rehabilitation of hundreds of moisture-damaged homes. From the foundation to the roof, improper ventilation in one or more areas is usually the problem. The good news is that specific vents can be installed in your roof, foundation, bath and kitchen to take care of the excess moisture.



How do you know if your house is adequately ventilated? Homes in our area should have one square foot of ventilation for every 150 square feet of foundation floor area. The same formula also applies to the attic using roof vents. For example, a one-floor 1,200-square-foot house, divided by 150, would need 8 square feet of ventilation around the perimeter of the foundation and 8 square feet in the roof.


This can be achieved by adding 1-square-foot

foundation vents at every corner and evenly spaced around the foundation or skirting. For the roof, we recommend ridge and soffit venting, but there are other ways to get the job done.

Inside the house, you should also have a powered fan in the kitchen and each bathroom that is properly vented to the outside. While openable windows can help, they are not as efficient and only work if opened. By installing these vents you have taken some of the preliminary measures needed for a healthy, home and you.



This is what you want to see and smell under your house:

  • At least one good, screened or covered access point to get under the house.

  • 18 to 24 inches or more of space from the ground to the floor joists.

  • Clean, dry 6-mil black plastic on the ground, laid tightly, anchored at the edges, cut around each pier block and overlapped at the seams by 12 inches. 

  • Very few bugs, spiders or cobwebs.

  • No wood-type debris and nothing growing.

  • All areas of the foundation, especially under the kitchen and bath, should be dry. There should be lots of fresh air streaming through the vents and no odd smells, standing water or signs of animals.

  • All non-treated wood components should be dry, rot-free, away from soil and separated from any concrete using composition roofing or another waterproof barrier.



If you see lots of spider webs, that usually indicates high moisture content in the air and may indicate you need more ventilation and/or a ground cover. Installing 6-mil black plastic on the ground under the house stops this moisture source, instantly. Next, place a splash block or a pipe at each downspout to convey water away from the foundation.  Now do the math for ventilation, adding more vents if needed.

If you find a water puddle on the plastic you need to play detective. If it is gray, filmy or soapy water, it means a sink, tub or washing machine drainpipe could be leaking. Operate each fixture to find the leak point and repair it. If the air smells or looks like toilet water, call a plumber! That water is hazardous to your health! A puddle of clear water on the plastic can be from rain through a vent or a water supply line drip. First look at the water meter ‘tattle-tale’ and see if there is a leak indicated. If not, look for another source and the reason why, like rainwater.

If you have a lot of groundwater you may need a sump pump to carry it away from the house, but avoid sending groundwater into the city sewer system. That’s a big no-no!

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If you observe any signs left behind by critters—like ammonia cat pee smells, feces, hair, tracks, or even the critter!—trap or remove the animal and secure openings and under the edges of the skirt. Also, small sawdust trails and holes peppering the wood are signs of powder-post beetles or larger holes that ants or termites make. Poor ventilation and/or a steady source of moisture on wood can create a condition known to attract pests. The bottom line is you need to get rid of these pests and the moisture sources that allow them to be there! That may require the knowledge and products professionals use.


Depending on the level of damage, a professional pest spray, adding ventilation and removing all sources of moisture could cure the problem. If you wait too long, you may have to replace your foundation, sub-floor and skirting!





It’s not just vents at the roof and foundation, venting the middle of your house also makes a big difference as far as keeping trapped moisture out.


Your home’s exterior siding protects your home from the many wraths of nature’s fury, just as your clothes protect you. When properly installed and maintained, most of the siding material types are generally durable and long lasting. However, worn, cracked siding material, a bad paint job, UV light from the sun, exposed or poor caulking and rusted or poorly-installed flashing, and moisture build-up can all damage your house. All these issues can be prevented.





Here’s what you want to see as indicators that your home is well protected:

  • A great paint job on every surface

  • Good doors, windows and trim

  • Good gutters and downspouts with splash blocks, to convey the water away from the foundation

  • A sealed pipe and flapper vented to the outside for the dryer, kitchen range hood and each bathroom equipped with a fan that is vented outside

  • No woodpiles against the house giving shelter to bugs

  • As noted above – adequate venting 


If you already have bathroom and kitchen fans as well as the dryer properly vented, make sure they are not leaking moist air and that each flapper opens and closes properly when the fan is on/off. Finally, make sure the bathers and busy cooks use those fans, leaving them on for 20 minutes or more after they are done. 

Did you know that dryers put out huge volumes of sticky steam full of lint that can stick to and build up on the inside of the vent pipe and flapper to the point where it takes a long time to dry your clothes or clogs the whole system? This is also a fire hazard! An annual cleaning of the dryer vent system is easy to do and also helps prevent rats from making your dryer into their nursery. They can’t get through a flapper that actually closes when the dryer is turned off. 

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Holes in siding, windows, roofs as well as cracked putty can also be a source of moisture. So, inspect them all. If you see rusty siding nails or bleed marks in your siding, it may mean someone used the wrong siding nails and can also mean you have a moisture issue behind the siding. 


Once you have found and corrected the sources of moisture, countersink the old nails, fill the old holes and re-nail as you go with galvanized nails. Using a stain blocker over the rust marks, before painting, works well to hide and seal out moisture.  

Other common maintenance problems to look for include missing or damaged siding or trim. It’s important to repair and replace these materials to avoid moisture intrusion, causing deeper component structural problems and more unnecessary expense.  If you own vintage housing stock, pay close attention to broken or missing window parts. The sash surrounds the glass panes 

and must be kept well glazed and painted. 

The stool sill sheds the rain away from the siding and must be smooth, rot-free and well painted. The trim boards around each window make a weatherproof transition from siding to glass. They, too, must be tightly caulked and painted. When these parts fail, it can result in leaks, then wall and floor damage. When you observe, repair or replace window parts, make sure to stab test the wall sheathing just under the window sill, fixing what you find as soon as possible.

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When it comes to home maintenance to keep moisture out, your time, money and effort will be rewarded with a better looking home and fewer major home repairs in the future! 

Dave Murnen and Pat Beaty are construction specialists at NeighborWorks of Grays Harbor County, where Murnen is executive director. This is a nonprofit organization committed to creating safe and affordable housing for all residents of Grays Harbor County. For questions about home repair, renting, remodeling or buying, call 360-533-7828 or visit 710 E. Market St. in Aberdeen. Our office is fully ADA-compliant.

Do you have questions about home repair, renting, remodeling or becoming a homeowner or one of our contractors? We have rehab loan funds at tailored rates!  Call us at 533-7828, write us or visit us at 710 E. Market St. in Aberdeen. Our office is ADA compliant, complete with a designated disabled parking spot, ramp and ADA compliant restroom. 

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