Lets stick with the humidity issue we discussed last week and start there.
If the crawlspace is not supposed to be humid, what can be done about it?
The ideal solution for a crawlspace is to take things all the way to one extreme, either be as open as possible with as much air movement as possible or be as closed off as possible and mechanically control the moisture.
Option one: wide open.
Some of the best “crawlspaces” we see are on older farm homes that have never been enclosed. The whole space is just on piers and out in the open. This option worked great when you didn’t have countless reasons to poke holes in the floor, like running electrical wires, plumbing pipes, and HVAC ducts. In the name of keeping the pests out of the house, exterior skirting started being applied to homes. Unfortunately, when you take measures to keep something out, you sometimes keep out things you wanted as well, in this case, a nice breeze. Efforts to keep things out often have the effect of keeping other things in, in this case, moisture.
The moisture that used to have lots of options for places to go, like to be blown away, is now contained in the crawlspace. The relative humidity of the air wants to be at an equilibrium and will always move from areas of high humidity to low humidity to reach that balance. In a crawlspace, that means highly humid air outside moves through the vents and pushes vapor through the bricks even into the crawlspace to reach that equilibrium. The inside of our homes, due to HVAC, tends to be dryer than the air contained in the crawlspace, which causes the humidity of the crawlspace to move into the house, meaning it congregates in the insulation and framing in the crawlspace that is along its path to the interior space.
Code does require venting of the crawlspace, but the amount and placement are based on linear ft of exterior wall, and no other factors are considered. Many homes have a garage or a porch that covers large portions of an exterior wall and eliminates many of the vents. All that to say it can be difficult to get a crawlspace to deal with moisture well if a lot of thought is not put into the design and layout to ensure lots of good air movement. The inclusion of HVAC ductwork in the crawlspace also adds to the difficulty. The ductwork can spend large portions of the summer months with a surface temp below the dew point, which leads to condensation and the pooling and collecting of water. This water tries to evaporate and creates concentrated areas of high moisture, leading to microbial growth.
So what can be done?
If getting an “open” air crawlspace with good ventilation is a real challenge, and it often is, the opposite end of the spectrum is the solution. This means treating the crawlspace like a short basement. The idea is to seal the crawlspace off from the outside and control the elements just like you would for a basement.
This process is typically known as encapsulation. It involves using a thick, durable plastic to cover the floor of the crawlspace along with any “vapor open” surface, like the masonry piers and walls. The walls and piers are sealed, but 6-8” is left visible at the top to allow termite tunnels to be seen should they show up at some point. The plastic is taped together and creates a continuous barrier over most of the crawlspace to prevent moisture from evaporating out of the dirt or moving through the walls and piers. Once the area is sealed off from outside moisture, it allows the humidity in the space to be controlled with a dehumidifier.
Some additional steps for making a higher-performance encapsulation include insulating the perimeter walls of the crawlspace and actually conditioning the air.
Having a dehumidifier, dry, filtered air in the crawlspace means you also have that same dryer air making its way into the house through any openings, however tiny, that connect the spaces. If you remember from an earlier post, air that stays under a relative humidity of 60% greatly reduces or even eliminates mold and fungal growth. This means healthier air for you to breathe and a home that lasts longer and moves less with seasonal changes. Wood does not really move too much due to temperature, but it does due to moisture; a home that stays consistent with humidity will see less moving in hardwood floors and trim throughout the home.
Next will look at some of the repairs that can be done in the crawlspace to address damage in the framing.