Bag Tricks to Seal and Insulate
by: Paul Kando
Bag tricks are a quick and easy way to seal and insulate wall and ceiling cavities. Adapted to varied situations they are excellent low cost do it yourself improvements. All you need is pieces of left over insulation (fiberglass batting is fine), a plastic bag and physical access to the cavity you intend to air seal and insulate.
Let pictures speak. Figure 1. shows the top of a partition wall being blocked from the attic. On Figure 2. the upstairs floor is isolated from the unheated space behind the knee-wall. While there, you also have the chance to insulate the knee wall itself. Check also if the roof section that forms the slanted ceiling in the top floor room is insulated. If not, this is a good time to blow it full of insulation. You may even want to blow the whole space behind the knee wall full of densely packed cellulose, seeing to it that the roof remains ventilated.
Figure 2: Sealing and insulating under the knee wall using insulation stuffed in a plastic bag. Figure 2: Sealing and insulating under the knee wall using insulation stuffed in a plastic bag. Figure 3. suggests a way to block the large uninsulated space between a lower floor ceiling and the floor above. A standard 3" blow-hole is drilled in the approximate center of each bay formed by adjacent joists. An empty plastic contractor bag is inserted through the hole in such a way that its opening remains outside. The hose of the insulation blower is then inserted into the bag through the blow hole and the bag is blown full of densely packed insulation. The bag will jam against the walls of the cavity, blocking and insulating it. (Care must be taken that there be no recessed light cans near enough for the bag to come closer than 4-5" to them.) Finally the blow hole is plugged.
These bag tricks work best if the outer walls are also blown full of densely packed cellulose insulation, which tends also to retard air movement. There are also new foam products available which can be used in place of the blown cellulose. They provide a higher R value, but may also cost more.
Figure 3: A three-inch blow hole is drilled in each joist bay. A bag is inserted and blown full of insulation.Figure 3: A three-inch blow hole is drilled in each joist bay. A bag is inserted and blown full of insulation .Cautions: (1) There must be a vapor barrier on the warm side of each insulated building surface. In our Maine climate this means the indoor side. An inexpensive vapor barrier may be formed by using a shellac based primer (such as BIN) under the desired finish coat. (2) All paths for air/moisture to enter the structure (e.g. electrical outlets) must be sealed. (3) Live knob-and-tube wiring must not have insulation in contact with it; it is already overloaded by modern electricity demand and it is best to replace it with modern wiring. (4) Insulating a building from the outside requires that the amount of insulation added be sufficient to prevent condensation within the original wall cavity, or serious moisture problems will result. More on this in a future column.