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Winterizing Inside Partition Walls

Paul Kando

Inside partitions can lose a lot of winter heat if their empty interiors are open to the unheated attic. As the heated rooms on either side warms, the air inside the empty wall cavitiy through the thin skin of drywall or plaster, the air next to the drywall warms and rises into the attic. Cool attic air dropping down from the attic in the middle of the wall cavity replaces it. This cooling cycle continues as long as the heated rooms on either side are warmer than the attic temperature. Thus internal partitions often act like uninsulated outer walls.

Using the bagtrick behind an attic kneewall

The fix is to air-seal and insulate the top of the wall cavity up at the attic floor. Here is a quick and easy way to do this for both partition walls and also empty ceiling cavities. All you need is pieces of left over fiberglass insulation, large plastic bags and physical access to the cavity you are dealing with.

Fill the appropriate size plastic bag with pieces of fiberglass, stuff the bag down into the wall cavity from the top, i.e. from the attic. Repeat this procedure for every space between two studs along the length of the wall. The bagsful of insulation will compress to both air-seal and insulate the wall cavity, effectively turning it into part of your heated living space.

The same technique will work to seal and insulate the floor under that attic bedroom with the knee-walls on either side, provided you can get behind the knee-wall. In this case the insulation-filled bags are jammed into each between-joists-space along both knee-walls. (While you are behind the knee-wall, also insulate the floor there).

You can also use the “bag trick” to insulate the empty spaces between floors, using an insulation blower you can rent. From the outside, with a hole-saw, drill a 3” round hole midway between each pair of floor joists. Insert a large empty plastic contractor bag through each hole, keeping the open ends of the bags outside. Holding on to the open mouth of each bag, blow them full of cellulose insulation, so that it fills the cavity, pressing against the sides. The two joist-cavities along either of the end walls must be blown full of insulation. After you’re done, plug the blow holes in a suitable way.

Cautions: (1) Don’t insulate against recessed lights. Replace such fixtures with any of a number of available LED lights. (2) There should be a vapor barrier on the warm side of insulated building surfaces. A shellac based primer (such as BIN) under the desired finish coat will serve as a simple vapor barrier on walls and ceilings. (3) Seal all paths of air/ moisture entering the structure (like electrical outlets). (4) Don’t insulate over live knob-and-tube wiring. It is likely overheated by modern electrical loads. Replace with modern wiring. (5) To avoid moisture problems, when insulating a building from the outside, the amount of insulation added must be sufficient to prevent condensation within the original wall cavity.