Floor insulation important for comfort and energy savings
Good insulation has long been recognized as a crucial component in keeping your home warm and comfortable, and keeping your utility bills under control. Attic insulation is, of course, the first thing we think of when it comes to insulation, but you don’t want to forget about the floor either.
There used to be a common misconception that since heated air rises, floors don’t really need to be insulated. Heated air does indeed rise, but heat also moves from a warm surface to a cold surface, so a substantial amount of heated air is being lost to your cold crawlspace or basement if there’s not a layer of insulation between your toes and all that cold dirt down there. The colder the climate, the greater the potential air temperature difference between the house and the crawlspace, and therefore the greater the amount of air movement and heat loss.
Improved comfort alone is a great reason to insulate a floor. But how about those utility bills? Consider that a 1600 square foot, one-story house with 8-foot ceilings has 12,800 cubic feet of air in it (1600 x 8). If that same home has a 2-foot high crawlspace below it, there is an additional 3200 cubic feet of air down there, or 25% of what’s in the house itself. That’s a lot of air, and since the heat inside your home is constantly moving to try and equalize all the temperatures, it’s naturally trying to move into that cold crawlspace and warm it up, unless you take some steps to stop it.
What’s needed and how it’s installed
The accepted insulation method in use today is to insulate the floor itself, directly below and in contact with the subfloor. In that manner, heat loss is stopped right at the edge of the living space, known as the
the same way that insulation stops heat loss at the ceiling and the inside of your walls.
Exactly how much insulation is needed depends on what part of the country you live in and what the average cold temperatures are, as well as what’s required by the building codes. In milder climates, R-19 is usually acceptable, while colder areas typically specify R-25 to R-30, or even higher. If you’re doing this work yourself, before beginning call your local building department to verify what’s required for your area.
To be the most effective, the insulation needs to be in contact with the underside of the subfloor, as well as the sides of each of the joists. If you’re using insulation with a vapor barrier facing, the vapor barrier is installed facing up, in contact with the underside of the subfloor.
Ideally, you’d like to fill the joist cavity completely with insulation, but how to do this is dependent on a combination of how thick the insulation needs to be, and how deep the floor joists are. For example, R-25 fiberglass batts are about 8 inches thick, so they work well with a 2x8 joist. The insulation is placed between the joists and touching the underside of the subfloor, and is then held in place by wood lath, nylon twine, or other methods.
For deeper joists, you have a couple of options. For example, if you have 2x10 joists, 8-inch batts will still work. The batts are placed in the cavity and pushed up against the underside of the floor, and are then held in place with springy steel rods that are pushed between the joists, or with nylon straps stapled between the joists.
Another option would be to increase the thickness of the insulation. There is a higher initial cost for the material, but installation is simplified and, depending on where you live, the extra R-value of the thicker insulation may pay for itself during the life of the house.
Additional precautions
To prevent moisture problems and keep the efficiency of the insulation at its peak levels, you need to install a vapor barrier on the ground to prevent moisture from coming up into the crawlspace. For this application, the most common material is 6-mil black plastic sheeting, laid directly on the dirt. Overlap all the seams by at least 6 inches, and lap the plastic up the foundation walls at least 12 inches.
Don’t overlook insulation for your water pipes and heating ducts. Remember that by insulating the floor, you’ve just stopped heat from the house from reaching the crawlspace, so that area is now going to be a lot colder. Your pipes have a greater chance of freezing, as well as increasing heat loss from your ducts.
You can wrap your water pipes with strips of fiberglass insulation, or you can cover them with foam pipe insulating tubes made specifically for that purpose. Cover any exposed ductwork with fiberglass insulation to at least an R-6.
All of the materials you need for DIY floor insulation, from the batts to the plastic sheeting to the materials to hold everything in place, as well as dust masks and other safety gear, can be found at any home center or hardware store. If a weekend under the house doesn’t sound inviting, contact at least two licensed insulation contractors for estimates, and compare their bids carefully.