Recently, I saw a general statement telling people to block their crawl space vents permanently. I have made skeptical remarks on this, based on conditions I personally see out in the field inspecting in western Washington and based on Washington WDO laws.

While the practice of blocking vents might be a good idea in some areas, it is not a good one here. It is also mandated by WA state law, not simple building codes, that a home inspector must call out lack of proper ventilation: ineffective or blocked crawl space vents. My position is in reference to conventional crawl spaces. In this region, unless there is a basement, about 95% of crawl spaces are conventional: dirt underneath a plastic vapor barrier, posts on concrete piers, hopefully with a membrane between the post and the pier. And, lastly, vents in the foundation, hopefully not too few or too far apart. There is likely to be fiberglass batt insulation between joists.

Now, if we were talking about a conditioned crawl space -- that has some heat and a concrete floor, that is a different story and vents there should be closed or likely never put in to begin with. This is fairly new here, seldom seen, and while I do not know that they have been in my area long enough to know just how they will stand the test of time, since they are like "mini" basements they should be okay if they are designed right.

The following photos are taken from two homes that had blocked crawl space vents. What you are seeing is serious anobiid beetle damage, the primary reason, other than WA state law, that makes me resist the suggestion of blocking vents in conventional crawl spaces. These houses had the most serious WDO (wood destroying organism) issues of any houses I have ever seen. In both cases, it is my view that repair includes replacing all posts, about 80% of beams, joists, sill plate, and 30 to 50% of sub-floor. In one case it is probable that wall studs will also have to be partially replaced. While these are the two worst I have seen, over and over again, I find this type of damage is typical of the homes that have blocked, or partially blocked, crawl space vents. The WSDA also states that other wood destroying organisms, such as carpenter ants, moisture ants, termites and wood decay fungi or rot are attracted to such crawl spaces.

These houses both had adequate vapor barriers. They had gutter water that was controlled away from the foundation. They did not have plumbing leaks. They did not have standing water or creeks going through the crawl space. They did have conventional crawl spaces with blocked crawl space vents. These vents had been blocked more than seasonally -- for a decade or longer. The photos are extreme but, again they are not all that extreme, and pretty typical, of what I expect to find in older homes with blocked crawl space vents. On the opposite extreme, when I find older homes, with plenty of ventilation and also few conducive conditions, the wood is usually still sound. This is my view, and it may not apply everywhere, but if you live in a climate like western Washington, and if you have a conventional crawl space -- open the vents! The photos tell the story.

All vents blocked

Anobiid damage, exit holes sub-floor       Joist destroyed, note new exit holes in sistered repair

Post destroyed, anobiid        Anobiid, note blocked vent below

Sill plate and joist destroyed, anobiid        Post destroyed, anobiid