My crawlspace should be framed with this, right?

Crawlspaces are known for being high-moisture environments (that is a problem. See our post on crawlspace humidity). Pressure-treated lumber is for moisture and water contact, so that’s what I should use for my crawlspace, right? On the surface, that makes a lot of sense. But does it?


Ignoring the fact that your crawlspace should not be a high-humidity environment, it likely is at least more so in the interior of your home. If you have had damage to the framing in your crawlspace due to the humidity levels, it would seem to make sense that when repairing things, you would want to go back with pressure-treated lumber to help with that issue.


But you probably don’t want to do that.


Should pressure-treated lumber be used in a crawlspace?

Yes, in two specific conditions.


The first is as a sill (we’ve written about those in an earlier post), this is a board that is meant to get wet, and its job is to protect the other boards from gettin wet, so it should be made with a lumber that is intended to get wet.


The other is as the band when it is below grade. It is worth noting here that this should only be done as a repair option. By design the band should never be below grade where dirt is against the house. This creates too much moisture, a path for groundwater, and is a recipe for guaranteed disaster at some point in the future. If, however, these conditions already exist, replacing that board with treated lumber is the only viable option to give it a longer second life. In this instance, we would use a special type of treated lumber called KDAT, which stands for Kiln Dried After Treatment; we’ll explain why below.


Why would pressure-treated lumber be the wrong choice in all other conditions?


Moisture and lack of dimensional stability are the big reasons to avoid treated lumber in the crawlspace.


The chemicals used to create pressure-treated lumber are applied as a liquid. Pressure-treated boards can have moisture contents as high as 60% when on the rack at the store and will often drip when cut and squeezed. Normal healthy wood should have a moisture content of 13-19%. If you re-frame a crawlspace that already has moisture problems with a bunch of wet pressure-treated lumber you will only be adding to the moisture problem as the lumber tries to dry out.


When wood takes on water it swells, when it drys out it shrinks. If you have ever seen a new deck or new fence installed made of treated lumber when it is just finished, all the boards are nice and tight, everything is touching, and it looks great. After a few months, sometimes less, when everything has dried and shrunk, there are now lots of gaps and space between all the boards. This very same thing would happen to the framing under your house. The nice, flat and smooth floor after framing will not be in the same position after a few months of drying out.

This is why we said we would frame a new band with KDAT lumber if absolutely necessary because that is a treated product that has already shrunk, meaning the wall won’t drop 1/4 of an inch in the months following the repair.


Now, just because you are thinking about it, couldn’t you use that KDAT stuff to re-frame the whole crawlspace?


Sure, and that would avoid all the problems of adding moisture and the wood shrinking. However, KDAT happens to cost 3-5 times more than standard dimensional lumber, meaning you only want to use it where it’s really needed.