Seattle Home Inspector's Blog


Are you NUTS? Why would I want to vent my crawl space in the winter?


This Seattle Home Inspector does not inhale!

I have been around and around with clients enough to know that there are many people that seem to think that venting a crawl space in the winter is a bad idea---especially in the wet NW where outdoor winter humidity can be close to 100% for much of the time.

Flooded crawl spaceWhy would anyone in their right mind want to draw all that wet air into the crawl space?  Sounds like a problem waiting to happen, doesn’t it?

To get to the answer however, we have to actually look at the science of humidity and how important to our homes (as well as to our answering the question) that we understand that humidity has a critical relationship with “temperature.”

The truth, as counterintuitive as it may seem, is that we can actually “lower” the moisture in a wet crawl space by bringing in this 100% moist air.

Some of you will be absolutely certain that I am celebrating WA State’s new Mary Jane law---but I assure you that, not only do I not inhale, but that I am not smoking anything funny at all.

Again the key is “temperature.”

Air at 40 degrees F cannot hold as much moisture as 100 degree F air can. 

Because of this simple fact, 40 degree air will reach saturation (100% humidity) much quicker.  So now let’s drag that 40 degree, 100% humidity air into the crawl space that is 50 degrees at 75% humidity.  Not only can it easily handle the moisture coming in, as the air is warmed to 50 degrees its humidity drops and results in the 75% air dropping to less than 75%.  There are a lot of factors, such as air flow, and how much the air flow will reduce crawl space temperature that will effect just how much the humidity will be lowered, but the point remains that the crawl space humidity, overall, will be lowered---not increased. 

So now let’s keep the crawl space temperature at 50 degrees and drop the humidity to a more normal 50%.  The ability of that 50 degree air to absorb moisture in the colder air improves even more.

Interestingly enough, very few crawl spaces ever drop much below 50 degrees except perhaps near the perimeter where the cold of Mother Nature occasionally “takes-out” an outside faucet---at least in the mild Northwest.  Your Mother Nature may treat you differently where you live---it is wise to pay attention to Mother Nature and understand her idiosyncrocies in your area.

Generally speaking, wood moisture content (as measured by a moisture meter) tends to be slightly higher in summer than in winter when outside air is at higher temperature and at lower humidity and then the reverse happens.  75 degree air at 50% humidity will work to raise the humidity of the air in a crawl space with 50% humidity at 60 degrees.  The cooler air in the summer cannot hold as much moisture as the warmer outdoor air and the crawl space air reaches saturation quicker resulting in higher crawl space humidity and thus raising moisture levels in the woodwork.

But before it can become a problem, the seasons change and moisture levels naturally start to go the other way.

Different climate zones are affected by these same principles in different ways---that is why they are called “different climate zones.”  It is important for the builder to understand the climate the home is built in to understand how to allow for these principles to work in ways that don’t destroy the house or provide an environment conducive to wood destroying organisms and mold.

In the South, where humidity can be high year round, and temperatures are near saturation year round, venting crawl spaces at all is especially problematic.

Of course all of this is relative only to “vented” crawl spaces.  Since we have millions of homes with crawl spaces (and more are built every day) any conversation I might have about how they should not be allowed does nothing to deal with the ones that we do have already.

It is more important to understand how to maintain good humidity levels in crawl spaces so that problems do not arise.

Me in a crawl space


Charles Buell, Real Estate Inspections in Seattle

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Comment balloon 16 commentsCharles Buell • January 19 2013 05:46AM


Thanks for the perspective on humidity in the crawl spaces.

Who was the person crazy enough to crawl under a home to photo you under the home?  :)

Posted by Tom Arstingstall, General Contractor, Dry Rot, Water Damage Sacramento, El Dorado County - (916) 765-5366, General Contractor, Dry Rot and Water Damage (Dry Rot and Water Damage Mobile - 916-765-5366) over 6 years ago

Charles, This is a very helpful post and you've explained the reason for venting a crawl space in a very easy to understand way. Hoping all your future inspections are crawl space free ;)

Posted by Jeana Cowie, Broker Associate, ABR, CRS, GRI, SRES (RE/MAX Real Estate Limited) over 6 years ago

Great information as usual....we installed a radon system and that is vented out thru the crawlspace too

Posted by Kristin Johnston - REALTOR®, Giving Back With Each Home Sold! (RE/MAX Realty Center ) over 6 years ago

I am so glad I do not have a crawl space to deal with. I am very happy with my 9ft basement and I can see everything without crawling to it!

Posted by Ellie McIntire, Luxury service in Howard County & Catonsville (Ellicott City Clarksville Howard County Maryland Real Estate) over 6 years ago

My first question, what's a crawl space? :) I'm in the no venting camp, which seems to work quite well in my climate zone. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 6 years ago

Excellent discussion on humidity and the counterintuition it requires to understand venting!  Suggested as usual.  Not as wet is drier!

Looks like you are in the crawl space with your Super Shorty (look it up) to hunt wabbits.  Where's your Elmer Fudd hat?

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Charles -- thanks for the explanation of why it is important to keep those vent open in the winter around here. As mentioned, it is a bit counter intuitive - but your explanation puts things in perspective.

Posted by Steven Cook (No Longer Processing Mortgages.) over 6 years ago

Tom the picture was taken by a student during a home inspection training course

Jeana, not much chance of that around here :)

Kristin, yes, radon is just one of the many things that have to be vented away

Ellie, I am with you on that---no fan of crawl spaces

Jim, for sure

Jay, it wouldn't fit through the hole

Steven, thanks---important to understand in the wet NW

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Charles, that picture of you actually made me inhale loud enough for Lucy to ask me what's wrong...I am very claustrophobic and I just can't imagine what it would be like doing what you do...Stay well.

Posted by Ernie Steele, Call me, let's get started!!! 717-273-3774 (Berkshire Hathaway HomeServices Homesale Realty) over 6 years ago

Ernie, none of my students wanted to come in with me either :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Charlie, We see freeze drying over here ; ) But the principle works pretty good over here when humidity levels can get down in the teens.

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago

"En cave", thats the French expression for being in the basement. It is a cave like enviroment both there and in crawlspaces. There is a reason they tend to self-regulate around 50 degrees.

56 degrees is 'cave' temperature of natural caves on unheated mines and sheltered excavations. That's the mean temperature of the earth crust without being affected by surface conditions.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 6 years ago

I think I saw one of those Buells things in my crawl space just a few weeks back.  He seemed quite at home.

Of note, you make these subjects more edible for the eyes than I ever could have imagined.

Posted by Jason Sardi, Your Agent for Life (Auto & Home & Life Insurance throughout North Carolina) over 6 years ago

Don, freeze drying does work :)

Robert, in the winter the crawl space can be a welcome warmth for the inspector in the winter

Sardi, was that infamous rodentus-bueltus---or merely a wannabe?

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Charles - another informative blog for homeowners. We don't see much crawl space in Southern California, but when we is not a comfortable place to be in.

Posted by Les & Sarah Oswald, Broker, Realtor and Investor (Realty One Group) over 6 years ago

Sarah & Les, I would be happy to share some of ours with you---we have way too many :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago