Improve Energy Efficiency by Stopping Air Infiltration in Basement | Wisconsin Historical Society

Guide or Instruction

Improving the Energy Efficiency of Your Historic Building's Basement

Improve Energy Efficiency by Stopping Air Infiltration in Basement | Wisconsin Historical Society

The basement or crawl space of your historic house can be a huge contributor to energy inefficiency. When your house was built, energy was so cheap that builders and owners did not take measures to prevent energy loss. Fortunately, you can substantially reduce your heating and air conditioning costs by insulating and sealing certain basement or crawl space areas of your house.

Check Masonry in Your Basement

If your house is solid masonry, be sure the mortar is sound around the interior and exterior of your basement walls. You can substantially reduce air infiltration by repointing any deteriorated mortar.

Check Wood Components in Your Basement

If your house has a wood frame or a wood frame with a masonry veneer (brick, stucco or stone), it will have either a wooden sill plate or a sill beam. A sill plate is a doubled-up 2 x 4-, 6- or 8-inch plate that rests on top of the masonry foundation. A sill beam is one large beam that often measures about 9 x 9 inches. You should caulk this plate or beam to your foundation with a flexible and easily cleaned up caulk, such as acrylic latex with silicone.

The floor joists (beams that hold up the floor) rest on the sill plate or beam. The area between the joists is called the box sill. Your box sill probably allows a lot of air infiltration, so you should caulk all of the box sill edges around your entire foundation.

Many houses built before 1865 have no subfloor under the finished wood flooring. The lack of subfloor allows cold air from the crawl space or basement to be sucked up into the heated rooms above. You can prevent cold air from entering your heated upstairs area with caulk and foam board. Go to your basement or crawl space and use a strong construction light to look for light leaking through your floor from the finished space above. Caulk any joint where you can see light entering the basement or crawl space. Next, install two layers of foam board in each box sill by following the steps below:

  1. Cut two layers of 2-inch- thick, rigid foam board to friction-fit in the box sill.
  2. Install the first piece of foam board in the box sill.
  3. Apply some construction adhesive to the face of the first piece of foam board.
  4. Install the second piece of foam board.
  5. Caulk all four edges of the foam board.


Check Pipes, Wires, Vents, and Ductwork in Your Basement

Pipes and wires in your basement that penetrate the box sill area or the floor above are areas where air can infiltrate. Often these pipes and wires extend up into the interior walls as well. You should caulk all these penetrations into your basement.

Caulk around the perimeter of all forced-air heating supply vents and cold air returns where they are attached to the floor. Apply the caulk from the basement side.

If you have forced-air heating, you should insulate all basement and crawl space ductwork. If you have hot water or steam heat, you should insulate all of these pipes as well.

Weatherize Your Basement Windows and Doors

You should weatherize your basement by weatherstripping the following areas:

  • Any hinged or trap doors
  • Windows

You can also add storm windows to your basement windows to stop air infiltration.

Weatherize Your Crawl Space

You can weatherize your unheated crawl space by adding insulation between the floor joists. First, install 4-mil-thick plastic sheeting between the floor joists on the warm side of the floor, which is the room above. The plastic sheeting will act as a vapor barrier to stop warm, moist air inside the house from reaching the insulation below. When you have installed the vapor barrier, add insulation in the joist pockets.

If the floor of your crawl space is dirt, lay 4-mil plastic sheeting over the dirt to help stop ground moisture from reaching the insulation.

Always make sure your crawl space has cross ventilation. Cross ventilation is provided with vents located on opposite sides of the foundation that allow air to flow through the entire crawl space. You should close the vents in the winter and open them in the spring, summer and fall.

The information presented here is not intended to provide comprehensive technical advice or instructions on solving historic preservation issues. Any information contained or referenced is meant to provide a basic understanding of historic preservation practices. Read full disclaimer.