Q: My single-level, end-unit condominium, built in the early 1980s, is always cold, despite the fact that the outside walls are insulated and the windows are double-glazed.

I have a gas furnace in the attached garage. The crawl space is lined with heavy-duty vinyl, and the amount of head space is about 2 feet. There are a lot of pipes and wires. I wonder if it would help to add some kind of insulation under the flooring. How would you recommend doing it, and do you feel it would be cost-effective?

A: Floor insulation will definitely help. As part of the job, we recommend that you first conduct an informal energy audit. Check the doors and windows to see if there are any gaps that will let the great outdoors in. Next, take a look in the attic to see if the insulation up there is adequate.

The first step in conducting the audit is to check the doors and the windows. Replace any weatherstripping that may be worn. Make sure the doors and windows close tightly, forming an airtight seal.

Next, be sure there is a minimum of 12 inches of either batt or blown-in insulation in the attic and that it covers the entire area with no voids. If the insulation is short, add some. Another layer of non-faced batt insulation will do the trick. Be sure not to cover any soffit vents, as doing so will lead to condensation and potential mold growth. Because warm air rises and tends to migrate to colder air, these steps are the first line of defense.

As to whether adding insulation will be cost-effective, we believe it will be. But that’s really not the primary issue. The big issue is your comfort. When it comes to heating and cooling, cost-effectiveness is measured by the length of time it will take to recoup the dollars and cents spent on the insulation you add. Whether the job is cost-effective is subjective.

A two-year payback may be acceptable to you, but a five-year payback may not be. The bottom line is that we believe insulating the floor will make the condo more comfortable inside and thus be worth the cost, especially if you do it yourself. As with most building projects the cost of the material is minimal — it’s the labor that adds up.

There are two ways to go about insulating the 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 2-foot crawl space gives you plenty of 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.

Before you begin, be sure that the crawl space is adequately ventilated. You can tell by looking for the foundation vents on the outside of the home. Measure the size of the vents. You should have 1 square foot of vent for each 150 square feet of floor. If you are short on ventilation, add enough to meet this formula.

Measure the width of the floor’s joists. For example, if the joists are 2-by-8s, they measure 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 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 because that 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, the insulation could well work its way out. That’s all there is to it.

When you’re done checking the weatherstripping, making sure the attic has enough insulation and insulating the floor, you’ll be snug as a bug in a rug.


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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