Why Mice Love the Fiberglass in Your Basement or Crawl Space

Many homes in the Rochester area have crawl spaces under their homes. These were done generally because of a high water table or because they are less costly than digging out a full basement. For whichever reason, there are a few traits that most crawl spaces have that are not a good idea for your home.

Let us start with the crawl space floor. You would have either bare dirt, gravel over bare dirt, gravel over plastic covering bare dirt, or concrete. Concrete is the best option but most do not have concrete because of the cost. Plastic vapor barriers are the next best option, but whether it will work depends on the plastic used. The cheaper the plastic, the more likely it will break down over time. More than likely, if you have bare dirt, you have had a mold problem at some point. Dirt is porous so any air and moisture in the dirt will make it into your crawl causing a moisture issue only feet from your feet.

Next, we will look at the walls. Most crawl spaces are cinder block although I have run into cobblestone walls in the past. Most have vents open to the outside. These were installed as a means of allowing moisture to evaporate to the outside. Unfortunately, building science shows these vents bring air in from outside thru lower pressure in the crawl space. If you have rooms above a crawl space with open vents, you have more than likely dealt with cold floors.

Lastly is the ceiling of the crawl space. Most have fiberglass batts rolled out and stapled to the rafters. There are many problems with this. First, the paper vapor barrier should face in a way where it touches the subfloor. Most face toward the crawl space because it makes installation easier. This causes a moisture problem above the insulation when warm air comes in contact with a cold paper surface. Second, fiberglass does not stop air flow, so any air movement or pressure in the crawl space will make its way into the living space of the home (welcome to the mold factory). Third, you are giving mice and other rodents a wonderful place to live. They love nesting in the fiberglass.  What’s the air quality like in your crawl space?  Remember – because of the stack effect, if it’s in your crawl-space, it’s in your house.  That’s the same air that you’re breathing upstairs.

The best way to fix a crawl space is to seal it up, and make it part of the home. If the floor is anything other than concrete, there should be plastic on the floor. The higher grade, the better (we use a reinforced 20mil poly to hold up to abuse if anyone ever needs to go into the crawl). The plastic needs to be sealed to the wall and sealed at any seams. The walls should be insulated with foam board panels and any vents should be sealed off and not used. Lastly, the rim joist and sill plate should be spray foamed to keep air from being able to blow in. This will stop heat from leaving to the outside and cold air from blowing from the outside (and also stop rodents from getting in -- yay!). You will have a much warmer living space above and fewer disgusting allergens than you have now.