Q: We have a frame house with a stucco exterior. Part of it is over a crawl space about 3 feet high. The house is bolted to the foundation and the exterior stucco covers the walls down to the ground. On the inside of the crawl space, I can see the 1-by-8 redwood backing for the stucco and we have no problems with termites.

I’d like to improve the insulation and make the house more earthquake-safe. Would it be better to insulate under the floor that is accessible from the crawl space or insulate the short exposed walls in the crawl space? Is it a good idea to put plywood as shear-wall bracing over the short exposed walls inside of the crawl space, which would then cover the insulation? Must I drill any holes for ventilation in the plywood? There are vents for the crawl space that I’d leave open by not covering them.

A: Good for you. Increasing the energy efficiency and earthquake resistance of your home is time and money well spent. It won’t hurt the value either.

The short answers to your question are: Creating a sheer wall by attaching plywood panels to the short wall, known as a "cripple wall," is a great idea; insulating between the floor joists rather than the stud bays in the perimeter "cripple wall" is the way to go (it’s thermally more efficient to insulate the floors rather than insulate the perimeter walls of the crawl space); and, yes, you will need to drill holes in the plywood panels for ventilation.

This is a three-part job. Phase one is to insulate the floors. Phase two is to reinforce the cripple walls with plywood. We also recommend (phase three) that you install a plastic vapor barrier over the dirt in the crawl space. A vapor barrier impedes any dampness that might come from the soil and reduces the risk of moisture taking up residence in the insulation.

We’ll divide our answer into two parts. This week we’ll give you the "how-tos" for insulating the floor. Next week we’ll tackle reinforcing the cripple walls.

Floors over unconditioned crawl spaces are often neglected when insulating. Lack of insulation results in heat loss from the warmer confines of the house to the crawl space. We all know that heat rises. But we’re less familiar with the fact that the temperature in a conditioned space always tries to reach a state of equilibrium with unconditioned ambient air outside the conditioned space.

During the winter months heat migrates through an uninsulated floor to a relatively cooler crawl space. Installing insulation in the bays between the floor joists impedes the migration of heat from the warm house to a relatively cooler crawl space.

There are two ways to insulate a floor. The first, installing batt insulation, is definitely a do-it-yourself job. The second, spraying expanding foam insulation between the joists, is a job best left to the pros. We recommend going with the batts. It’s less costly and your 3-foot crawl space gives you enough room to do the job. The pros will tell you that foam provides better insulation. While that is true, when the added cost is weighed against the benefit the difference is insignificant in our view.

The 1-by-8-inch wall sheeting you see behind the exterior stucco tells us you have an older home. The floor joists are most likely dimensional lumber rather than engineered I-beams. Measure the width of the floor joists. Our guess is that they are 2-by-8s measuring approximately 7 1/2 inches wide. The joists should be set either 16 inches or 24 inches apart.

If this is the case, R-25 batt insulation in the right width will fit neatly and snuggly between the joists. Place the craft paper face against the bottom of the subfloor as it acts as a vapor retarder. If the joists are wider, use thicker insulation.

Do not compress the insulation. This will reduce its efficiency. The idea is to fully fill the gap between the floor and the bottom of the joists. It’s OK to use two layers of batts to fill the space. If you go this route, only the layer adjacent to the heated surface should have the vapor retarder. A vapor retarder sandwich in the middle of layered insulation is an invitation for moisture condensation and the problems it can cause.

Batts are sold in pieces or in a continuous roll. Use the rolls to minimize the number of joints to increase the efficiency of the insulation. With a sharp utility knife, cut each batt the length of a joist bay. Push one end of the batt into a bay and work the remaining insulation into the rest of the cavity. Fluff out the insulation so that it’s even with the bottom of the joists. The final step is to nail pieces of wood lath perpendicularly across the joists every 2 feet to provide support for the insulation. Without these supports, gravity will do its work and the insulation will work its way out. That’s all there is to it.

Here are three tips that will make the job go more smoothly:

  • Dress appropriately. A dust mask or respirator, a hat, eye protection (goggles that cover all of the eye), long pants and a long-sleeve shirt are all musts when installing batt insulation. There are few things that burn and itch more than these spun glass fibers.
  • Cutting batt insulation is easier if you compress the insulation by pressing down with a metal bar (a framing square works well) to provide a straight edge to guide the utility knife.
  • If there are any heating ducts, electrical wire or pipes penetrating the floor into the house, seal them with a spritz of expanding foam insulation. One brand that comes to mind is Dow’s Great Stuff.


What’s your opinion? Leave your comments below or send a letter to the editor. To contact the writer, click the byline at the top of the story.

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