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Fix the Orphan Spaces Cooling Your House

by: Paul Kando

Orphan spaces
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We expect insulation to surround us during the winter cold, rather like the sketch on the left, with the whole heated envelope, regardless of the number of rooms in it, protected from cold and wind. Sadly, a majority of our houses do not offer this protection. They are more like the sketch on the right, with every room at least partially exposed to winter's cold. In fact, in most houses, only the walls facing outside are insulated. The interior partitions are empty of insulation, let alone an air barrier. The same goes for the space between the ceiling of a lower floor and the floor above.

On windy days one may even hear the wind whizzing through these spaces; and putting a hand in front of electric outlets, even on interior walls, feel a cold draft. Recessed lights, on or off, act like chimneys protruding into the empty space between a ceiling and the floor above.

These empty spaces are thermally connected to the outdoors and there is only a half-inch thick wall - board or ¾ inch thick lath and plaster layer between them and the adjacent heated rooms. That is less than R-1 in insulating value. The warm rooms heat the layer of air next to the wallboard or plaster, creating a convective current. The warmed air and is continually replaced by cool air sinking down the middle of the cavity - a perfect cooling system. In short, each of your rooms is wasting heat in all directions.

What to do?

  1. Seal and insulate the basement or crawl space ceiling (with the air-vapor barrier facing the warm side, i.e. up toward the living space). This will prevent losing heat to the cool basement and seal bottom of interior walls from the cold below.
  2. (2) In the attic, locate the tops of partition walls (shown by arrows) and seal them. If the tops are open, you can use a "bag trick" to do this: Fill large plastic garbage bags with pieces of fiberglass insulation and jam one into the top of the wall between each of the wall studs. The insulation-filled bags will seal and insulate the gap on top of the wall. If the wall-top is not open, caulk any cracks in and around the top plate.
  3. (3) If, as part of a Passivhaus-grade energy upgrade, you wrap your house from the outside with a continuous air barrier and insulation, it will seal all air leaks in the walls and solve the problem of any between the floors spaces, by incorporating them in the heated envelope. (Sufficient insulation must be added to the exterior, without gaps, to prevent the temperature of the home's original outdoor surface from ever dropping to the dew point temperature. Failing this, moisture will condense inside the original walls, causing rot and mold damage.)
  4. Short of a Passivhaus-grade "insulation-wrap" from the outside, the space between floors (indicated by arrows on the sketch above) can be sealed and insulated using another handy "bag trick". No need to remove any exterior siding or boards, but an insulation blowing machine will be needed or an insulating contractor hired to have this job done. Here is how: (a) determine the direction and location of each floor/ceiling joist; (b) with a hole saw, cut a standard 3" blow hole in the middle of both ends of each joist-bay; (c) insert an empty, heavy gauge plastic contractor-size bag through each of the blow holes, except for those in the very end-bays that run parallel to the outdoors. (d) Holding the mouth of each bag outside; blow them full of densely packed cellulose insulation. The bags will jam against the walls of each cavity, sealing and insulating it. (e) blow the two end bays full of dense-packed cellulose. Caution: for fire safety reasons, insulation cannot be blown around recessed light cans. Any located in these two end bays will have to be removed and their holes sealed. Or they can be replaced by either a zero clearance unit or a surface mounted fixture. (f) Cover and seal the blow holes per normal practice for blown insulation.
  5. To seal the space under the finished top floor of a New England cape, get behind the knee-wall, seal and insulate it and the floor, vapor barrier facing the warm side. Then jam insulation-filled garbage bags into the gaps between each of the floor joists, except for the two end bays. These are best blown full of dense packed insulation.