Bag It and Forget It!

September 03, 2011
September/October 2011
A version of this article appears in the September/October 2011 issue of Home Energy Magazine.
Click here to read more articles about Roof & Attic

I've been an insulation installer, energy auditor, and consultant since 1980; have served as the technical coordinator for the Wisconsin Home Performance with Energy Star program since 2006; and last year started my own company. Throughout my career I have tried to find creative and effective ways to make life easier for insulation installers and air sealers like myself, while improving the health and energy efficiency of homes.

In 2010, I started looking for an inexpensive yet practical way to install a "plug" in the floor cavities between the first and second floors in balloon-framed homes, so insulation installers could dense pack the sidewalls without blowing huge quantities of insulation deep into the floor cavities. The same technique could be used in multistory platform-framed homes.

The concept of blowing up a rubber balloon came to mind, but we realized that the balloons would probably burst on nails or other sharp objects inside the floor cavities.

Early in my career, my colleagues and I made 5-inch holes in the sheathing between floors and hand-stuffed wads of fiberglass into the floor cavities, but we quickly realized that there had to be a better way. We tried to insert an empty plastic cellulose bag on the end of a dense-pack tube through a 2 1/2-inch hole in the sheathing, but the bag would usually blow out when I tried to dense pack it with cellulose (too much air pressure). We tried punching holes in the plastic bags to relieve the air pressure, but that was too labor intensive to be a practical solution. The concept of blowing up a rubber balloon came to mind, but we realized that the balloons would probably burst on nails or other sharp objects inside the floor cavities. I knew of one (local) insulation contractor" s wife who would hand-sew bags crafted from pieces of InsulWeb, but that seemed far too costly, especially for large-scale production.

A Note on Codes

I had a very candid discussion with one of the Wisconsin code officials and discovered that there are no codes in Wisconsin barring the use of plastic mesh bags in building cavities. He alerted me to the possibility that other states might bar or restrict the use of them. I suggest that you do your own research for your particular state if you plan to use plastic mesh bags in building cavities.

Then I thought of an already-manufactured produce bag that could solve the problem. I secured several onion bags and experimented. They worked very well and I became a connoisseur of French onion soup. I was then able to discover an economical and reliable source for these bags. After developing a suitable document to illustrate and describe various ways to use these bags, I called them Insul-Bags and began selling them to contractors (see "A Note on Codes").

Here are a few of the ways that these bags can be used.

Insulating the Exterior-Wall Top Plate in an Attic

Reducing heat loss at the top plates of exterior walls does much to control the formation of ice dams and icicles while saving energy and money and creating a more comfortable living environment. Many homes are not properly air-sealed or insulated at the juncture between the exterior-wall top plate and attic. This is a common bad building practice, caused by laziness on the part of some builders, the use of improper materials, the improper use of materials, ignorance of building science, physical restrictions that make sealing and insulating difficult, or the need to save money. Sometimes homeowners take matters into their own hands and make mistakes when they try to solve the problem. Oops.

Insulating an Exterior-Wall Top Plate in an Attic
Insulating an Exterior-Wall Top Plate in an AtticFigure 1. An elevation view of a knee wall side attic where there is a ventilated roof.

Insulating a Band Joist
Insulating an Exterior-Wall Top Plate in an AtticFigure 2. An elevation view of a dense-packed insulating bag installed in the floor cavity at band joist location (platform framing).

Figure 1 shows an elevation view of a dense-packed insulating bag installed at the juncture of the exterior-wall top plate and the rafter, and contacting the underside of a ventilation air chute. Where no ventilation air chutes exist, dense-packed bags will contact the underside of the roof deck. The bag is then oriented to best suit the configuration of the juncture.

Before installing the insulating bag, remove any existing insulation and debris from the exterior-wall top plate and 6-10 inches of the adjacent attic floor. Where necessary, install a ventilation air chute in the rafter bay.

Position one insulating bag filled to optimum density directly and completely over the exposed top plate. Orient the bag to best suit the configuration to provide best coverage of the top plate. The bag should to be wedged snugly between the top plate and the underside of the ventilation air chute if there is one, or the underside of the roof deck if there is no chute. Once installed, the filled bag serves as insulation, without degrading the R-value of the attic insulation. It resists windwashing and acts as a permanent insulation dam. This system significantly outperforms the folded or compressed fiberglass batt that is often used in this location.

Insulating at a Band Joist

Figure 2 illustrates a dense-packed insulating bag installed in the floor cavity at a band joist, in a platform-framed building. In this application, the insulating bag acts as a permanent plug, positioned inside the floor cavity. The bag creates a cavity to be dense packed from three possible locations: (1) through the band joist from the exterior, (2) through the ceiling from conditioned space below, or (3) through the floor from conditioned space above. Of these locations, the first is the most commonly used.

In all three cases, an empty bag is inserted into the floor cavity through a 2 1/2-inch (or larger) hole drilled in the location of choice. To accomplish this, insert the insulation fill tube into the open end of an empty bag. Grasping the fill tube, insert the bagged end through the access hole into the floor cavity. Make sure that some of the mesh bag material with tie straps remains outside the hole. Dense pack the insulating bag in place, tie the straps in knots or cinch them tight when the bag is full to create a permanent plug in the floor cavity. Leave a 6-8-inch cavity between the plug and the inside surface of the band joist. Complete the process by dense packing the cavity to effectively air seal and insulate this key juncture. Seal the access hole.

Other Uses

Insulating bags can also be used to insulate

  • an exterior-wall top plate in a knee wall attic;
  • in sill box cavities;
  • at knee walls;
  • in cantilevered floor cavities; and
  • between the floors of a balloon-framed home.

Insulating bags provide an effective, economical, durable, and easy-to-use method of creating bagged insulation (dense packed with cellulose or fiberglass) to seal and insulate key junctures in new and existing homes.

Since 1980, Kevin Hogan has worked as an insulation installer, energy auditor, and consultant. In 2006, Hogan became the technical coordinator for Wisconsin' s Home Performance with Energy Star Program and currently serves as outreach manager for the Wisconsin Focus on Energy New Homes Program and Home Performance with Energy Star Program.


learn more:

You can contact the author for diagrams illustrating the various uses of insulating bags not described in the article at

Insul-Bags are available for purchase through EFI. Order online at

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