Insulating the Attic of Your Historic House | Wisconsin Historical Society

Guide or Instruction

Insulating the Attic of Your Historic House

Insulating the Attic of Your Historic House | Wisconsin Historical Society
EnlargeAttic insulation

Heat loss through an uninsulated roof is significant. Adding insulation to the floor of an attic helps maintain the heat within the house and also a cold roof (prevents ice damming). Source: Phil Thomason.

You can make your historic house more energy efficient by taking some cost-effective measures to seal your attic. These measures include sealing all the holes and penetrating pipes, wiring, ducts and vent pipes entering your attic space. It is important to take these measures in your attic because most of the heated air in your house moves upward like a chimney. The attic is also the best place to insulate your historic house.

Seal your Attic Entry

You should be able to access your unfinished attic from either a staircase or a scuttle hole (a square hole in a ceiling). Bring a reliable lighting source with you to find the areas where hot and cold air can escape. A construction light or a strong flashlight will help you find these areas.

If your attic staircase has a door, apply weatherstripping to the door. If you have a scuttle-hole hatch in the ceiling, use foam-based weatherstripping to seal the edges. You should also glue two layers of two-inch foam board to the attic side of the hatch.

Seal Your Chimney

Your chimney is the first place in your attic to look for areas to seal. If air is obviously moving through the gaps around the chimney, you should seal these areas. Use your hand to feel for moving air.

Fire codes require that a masonry chimney and any nearby wood or other combustible materials be separated from each other with a noncombustible material. Use aluminum flashing metal to seal the gap between the wood and the chimney. You can buy aluminum flashing in rolls from most lumber yards. Where the flashing meets the chimney, bend the edge upward and caulk it to the masonry. Also caulk and nail or staple the metal flashing to the wood framing surrounding the chimney. Be sure to caulk any joints where the flashing overlaps another piece of flashing.

Seal Pipes, Vents, and Plumbing

Seal any plumbing vent pipes or bathroom fan enclosures that enter your attic space. If the gaps are small, you can caulk them with a flexible silicone- or urethane-based caulk. If the gaps are large, you can use a non-expanding 1:1 spray foam to fill in the gaps. Spray foams that expand at a 3:1 rate add pressure to the surrounding areas, which could cause damage or even crush some vent pipes. Put petroleum jelly around the vent pipes before you spray the foam into the gaps. The grease will keep the foam in place when the pipe contracts or expands with a change in weather. You can also buy rubber or neoprene gaskets intended for this purpose at a lumber yard or plumbing supply store.

Seal Wires and Light Fixtures

Seal electrical wires that enter your attic through the framing from below. Use caulk or spray foam to seal the wires. Do not seal electrical boxes or recessed can lights anywhere other than around their edges. Heat can build up inside the electrical boxes, and sealing them completely can create a fire hazard. Seal any other pipes or penetrations with caulk or spray foam.

Insulate Your Attic

EnlargeAttic insulation

Dane County. Within this attic, batt insulation has been laid on the attic floor maintaining a cold roof. Source: WHS - State Historic Preservation Office.

Adding insulation to your attic is a big and messy job. You are probably better off hiring a professional to insulate your attic. If you decide to try this project yourself, be sure to wear a mask rated for your insulation as well as long sleeves, eye protection and gloves.

Before you add insulation to your attic, inspect the floor, floor joists and rafters for old knob and tube wiring. If you have any old knob and tube wiring, you should have an electrician verify if any of the wiring is still live. This is very important, because you should not cover live knob and tube wiring with insulation. This would create a fire hazard. If any of your knob and tube wires are still live, have the electrician disconnect them or cover them with a box to prevent contact between the insulation and the live wires.

The U.S. Department of Energy has created zones throughout the U.S. that indicate the level of attic insulation you need. The lower two-thirds of Wisconsin is located in zone 6, and the upper one-third is in zone 7. These are the two coldest zones in the country other than Alaska, so houses in these zones require more insulation than those in other zones.

Insulation is rated for its thermal resistance with an R-value. Thermal resistance means the ability of the insulation to resist or stop heat flow through the insulation. The higher the R-value, the better the insulation is rated for stopping heat. An unheated and unfinished attic space in Wisconsin needs insulation rated between R-49 and R-60. You should select a loose-fill R-49 insulation product. Insulation rated at R-60 can place too much weight on a plaster ceiling, which can cause the laths under the plaster ceiling below to loosen from the ceiling joists.

Avoid using foam insulation in your attics. Foam insulation attaches itself to everything and cannot be removed. It also contains formaldehyde that initially off-gases, which can cause illness. These foams also shrink over time.

Choose the Right Insulation

Loose-fill attic insulation is made from many types of material, including fiberglass, cellulose and rock wool. You can buy any of these insulation products at most lumber yards. Many of them will also provide a blowing machine to install the insulation.

Each insulation material is given an R-value per inch of thickness. Fiberglass has the lowest R-value at R-2.5 per inch of thickness. Cellulose is rated at about R-3.5 per inch, and rock wool is rated at about R-3.15 per inch. To achieve the R-49 rating on your attic floor, you must apply an appropriate thickness of your chosen insulation material in layers. For example, you'll need about 14 inches of cellulose insulation and nearly 20 inches of fiberglass insulation.

You should also blow insulation into the joist cavities of your attic. The dimensions of a floor joist in your attic are probably between 2 x 4 inches and 2 x 8 inches. If your joists are wide, you should choose a higher R-value of insulation to blow into the joist cavities. For example, if you blow cellulose insulation into a 2 x 8-inch joist cavity, you will get R-28 insulation. Although this is not as effective as R-49 insulation, it is better than no insulation. To blow insulation into a joist cavity, drill holes in the attic flooring between two joists.

CAUTION: Do not blow insulation directly into your eaves. Air flow from your eaves into the unheated attic space helps to prevent ice damming on your roof. When you buy your insulation, get insulation chutes to prevent the insulation from blowing into the eaves. These chutes staple to the roof sheathing between roof rafters and prevent the loose-fill insulation from getting into the eaves.

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.