All About Pressure-Treated Lumber
Do you use pressure-treated lumber for a crawlspace?
Crawlspaces are known for being high-moisture environments (a huge problem, see our post on crawlspace humidity). Pressure-treated lumber is built to withstand moisture and water contact. So that’s what you should use for your crawlspace, right?
Ignoring the fact that your crawlspace should not be high humidity, it likely is at least more so in the interior of your home. If you have had moisture damage in your crawlspace due to humidity, it makes sense that you would want to use pressure-treated lumber.
But you might not want don’t want to do that.
When to use pressure-treated lumber in your crawlspace
There are two specific conditions.
The first condition we use pressure-treated lumber is as a sill, which is a board designed to get wet. Its purpose is to protect the other boards from moisture, so it should be made from lumber specifically intended for that.
The other condition is as the band when it is below grade. It’s important to note that this approach should only be used 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 a recipe for guaranteed disaster in the future.
If, however, these conditions already exist, using 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 Kiln Dried After Treatment (KDAT).
When not to use pressure-treated lumber
Moisture and lack of dimensional stability are the big reasons to avoid treated lumber in the crawlspace.
Manufacturers apply chemicals used to create pressure-treated lumber in liquid form. Pressure-treated boards can have moisture contents as high as 60% when in store and can drip when cut and squeezed. Normal healthy wood should have a moisture content of 13-19%. Reframing a moist crawlspace with wet pressure-treated lumber will be adding to the moisture problem as the lumber tries (and fails) to dry out.
When wood absorbs water, it swells, and when it drys out, it shrinks. If you’ve seen a new fence made of treated lumber, you’ll notice how tight and clean it looks. Every board fits snugly, everything is touching, and the whole structure looks great right after installation. 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 (yikes). The nice, flat and smooth floor will not be in the same position it was in after a few months of drying out.
This is why we would frame a new band with KDAT lumber if absolutely necessary because it 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.
Couldn’t you use that KDAT stuff to re-frame the whole crawlspace?
Sure! That would avoid all the problems of the wood swelling and shrinking. However, KDAT happens to cost 3-5 times more&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/em> than standard dimensional lumber, meaning you only want to use it where it’s really needed.</em>
We Can Help</h2>
If you’re not sure, we’re here to help. We’ll give you a free inspection and honest feedback and walk you through the options so you can make an informed decision for your home and family.</p>
