How to Insulate an Older House with 6-Inch Walls

Regarding the depth of the walls, most older homes have studs that are barely four inches or less. The typical thickness of the studs is two inches. The width and depth of the new studs in the walls are 1.5 inches and 3.5 inches, respectively. These studs are perfect for R13 insulation (three and a half inches thick); however, the current US energy code mandates that all new homes have R19 insulation (six inches wide).

Future energy expenses are a significant consideration when upgrading an older property. Several items must be updated, including inefficient furnaces, drafty windows and doors, and inadequate insulation. Insulation can be installed either inside or outside of a building. The insulation value of a wall can be significantly improved by installing an inch of insulation board behind the siding while it is being replaced. For example, if the exterior is made of stone or brick, the insulation should be installed on the inside.

Six-inch insulation can be used but with much extra wood detail work. The studs can be made deeper so that six-inch insulation batts can be used. The mechanical components can be put in after the walls have been taken down to the studs. Don’t install the outlet boxes yet; leave the wire drops in place. The same goes for the rough-ins of the plumbing. If the actual depth of the wall was six inches, leave nipples at the stub outs that are at least that long.

The next task is to tear long lengths of two-by-two-and-a-half-inch wood furring strips. If the height of your studs is eight feet, then the size of the rips should also be eight feet. Create a rip count equal to or greater than the number of studs in the room, plus enough extra to circle the room twice. Before extending the top and bottom plates by 2.5 inches, you need to pack out the room by installing a ring of the shredded materials around the room’s floor and ceiling. Then, attach a strip to every stud in the room to pack them out. Furring strips should also be used to pack out the door and window jambs similarly. The completed work will result in cavity depths of 6.5 inches. Set all electrical boxes flush with the face of the new wall studs, plus the thickness of the drywall when installing.

Now we can easily install six inches of insulation and have a half-inch gap between it and the exterior wall. Lapping the insulation paper onto the stud faces achieves the finest vapor barrier. Use a small sheet of plastic to make up for the missing wall.

Upon completion of a standard drywall installation, it will become immediately apparent that the door and window jambs do not meet flush with the drywall’s outer surface, necessitating the addition of trims. Jamb fillers are used to expand the posts of a window or door to flush with the drywall surface. Cut a length to size using a portable table saw and some number two or better pine boards then set it in place. Along the back of the pine board, scribe a line flush with the drywall using a nice, sharp pencil. Jambs in older homes are typically not square and have noticeable width variations from top to bottom.

Your pine extension jamb can always be precisely flush if you scribe the line first. Buying broader widths of boards may allow you to get two extensions out of one piece of wood if you pre-measure the extensions and find they are in the 2-3 inch wide area when you buy the pine. However, it would be best to use caution because giant boards are far more expensive. The cost of a more extensive panel of the highest quality can skyrocket. If you are careful while selecting the number two boards, you might save money by buying more of them rather than better-quality wood.

If you install or replace your finish trims, the only visible difference between the old and new walls will be the depth of the window and door frames. The five-inch reduction in space will likely go unnoticed. Next winter, when your fuel bill is substantially lower, your wallet will thank you. Your home will be more “green” and energy-efficient due to the additional insulation, and you’ll save money on your heating and cooling bills.

Toby Ackerson
Wagsys.com: Your Helpful Local Building Inspector

Pete Ackerson has been a building inspector for more than 30 years and has worked in both the public and private sectors. He has experience in both the office of building design and the field of construction in the Eastern United States, having worked on a wide range of projects from schools to treatment plants, individual residences, and condo projects to major residential landscaping projects. Wagsys LLC, which he co-founded in 2006 with two other building inspectors, developed software for city departments such as building inspection, planning, and zoning.

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