Q: Our Mill Valley, Calif., home was built in 1957 and was not insulated. Every time a wall is opened up, we add insulation. Some say it would be a good idea to insulate the floor of our house. There is a crawl space under the house where we could reach the bottom of the floor. As the house is built on a slope, the crawl space varies from 2 to 8 feet tall.

Others say insulating the floor may not be a good idea. In the summer, the house gets hot, and the coolness of the crawl space will help keep the house from getting hotter.

What do you guys think?

Q: Our Mill Valley, Calif., home was built in 1957 and was not insulated. Every time a wall is opened up, we add insulation. Some say it would be a good idea to insulate the floor of our house. There is a crawl space under the house where we could reach the bottom of the floor. As the house is built on a slope, the crawl space varies from 2 to 8 feet tall.

Others say insulating the floor may not be a good idea. In the summer, the house gets hot, and the coolness of the crawl space will help keep the house from getting hotter.

What do you guys think?

A: Without a doubt you should insulate the floor that separates the living areas from the crawl space. And rest assured you’re doing absolutely the right thing by insulating the walls as you open them. You don’t mention the attic space. We assume you have some insulation there. Check to make sure it is enough.

Insulation is measured in R-value, which is a measurement used to quantify the resistance of a material to the transfer of heat. Some materials, such as most metals, are good heat conductors; others, such as fiberglass, conduct heat poorly.

Interior air is always trying to seek a balance with outside air. Left to its own devices, 70-degree indoor air wants to transform itself into 40-degree exterior winter air. Insulation inhibits that transformation. The same is true for cooler interior air in the summer. Insulation, caulking, thermal windows and other energy-saving strategies seek to maintain the temperature of conditioned interior air within the living space.

The U.S. Department of Energy provides a chart and map of recommended insulation levels for homes in various climate zones. According to this map, your Mill Valley house is in zone 3. Assuming your heating system is natural gas, the recommended insulation level for a crawl space is R-13, walls are R-13 and attic space is R-38. …CONTINUED

The Department of Energy also includes an insulation calculator on its Web site. You can plug in the first three numbers of your ZIP code and answer a few questions, and recommended R-values will be calculated for you.

In our view, these recommendations are minimums. As we mention from time to time, Kevin lives in Eagle, Idaho. That’s in climate zone 5. The recommended floor insulation level is R-25. Unfortunately, he installed only R-11 insulation in his floors. The 9 1/2-inch-deep bays between his floor joists would easily have taken R-30 fiberglass batts, which would have made his home more comfortable and saved on heating and cooling costs.

As it is, even with substandard insulation, he is able to walk barefoot comfortably on his wood floors year round.

Your 1957 home probably has 2-inch-by-8-inch floor joists that measure 7 1/2 inches deep. It will easily accommodate R-19 fiberglass batts. If the joist bays are deeper, use a thicker batt. The bottom line is to separate the crawl space from the living area with as much insulation as you can.

Remember, when installing insulation batts, the craft face or foil face vapor retarder should be installed toward the conditioned area. In this case, that’s the floor.

But remember, you’ll get the most bang for your buck by properly insulating the attic space. Attics get hot in the summer from the sun. Hot attic air will try to radiate into the living area. A proper level of insulation inhibits that.

The same is true for the crawl space air. Foundation vents are designed to allow exterior air to circulate under the house. As crawl space air reaches the ambient exterior temperature, it also radiates into the living area, making it hotter in the summer. Insulating the floor provides resistance to this heat transfer, enhancing the comfort of the interior temperature.

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