Besting the balloon blues

Posted Wednesday, October 19, 2011 in Sustainable Maine

Besting the balloon blues

by Paul Kando

You have an energy audit. The blower-door test identifies leaks in your house not only in the outer walls, but in the inner partitions and the first-floor ceiling as well. You expected to be concerned with only the outer walls and topmost ceiling, like this:

Instead you are facing the unpleasant surprise depicted below. Each individual room seems to be connected to the outdoors. How could this be?     

Like a majority of Maine’s housing stock, your house is probably balloon-framed. Popular from the mid-1800s through the early 1950s, balloon-framed houses, unlike modern platform-framed buildings, have long wall studs that reach from the sill plate to the second-floor ceiling. The second floor itself rests not on top of the first-floor frame but is anchored between the long wall studs. Consequently there are pathways for air to travel between each bay formed by adjacent wall studs or ceiling joists (Figure 3.). Air can travel vertically in the outer walls from the sill to the roof and horizontally between floors, inside what you thought was part of your heated space.

Since partition walls are open to the ceiling cavity and are usually not insulated, every wall in your house is, in effect, an outer wall. It leaks air (e.g. through electrical outlets and switches) and also heat, since there is nothing but lath and plaster between you and a cavity thermally connected to the outdoors. The remedy is obvious: You must block the flow of heat, air and moisture and restore the house to what is shown in Figure 1.

There are a number of ways to do this. The most thorough method is a passive-house rehab. The whole house is essentially wrapped in a layer of air-impermeable insulation with an R value high enough to prevent the existing outer wall-surface to cool down to the dew point. The result will be a super-insulated, tight building envelope which prevents condensation from ever occurring within the outer wall structure. Thermal bridges through wooden framing members are also eliminated. Add a heat-recovery ventilating system and you are well on your way toward reducing your heating energy bill by 75 to 90 percent, eliminating the need for a conventional heating system. Unfortunately this method involves much more than sealing and insulation. You must re-side the house, do finish carpentry at every window and outer door, and perhaps tend to other expensive details. A passive-house rehab should therefore be planned in conjunction with a major remodeling.

The least expensive alternative is to seal each individual air leak with caulk, spray foam, gaskets for electrical outlets, weather-stripping and the like. This will help. However, this is treating symptoms, not the cause of the problem. Nor will all that sealing reduce copious amounts of radiative heat loss from every room in every direction. Even if perfectly air-sealed, ceilings and partition walls will remain thermally connected to the outdoors. One could blow them full of cellulose but this is not always feasible. If knob and tube wiring or recessed lighting fixtures are present, having insulation in contact with them would create a fire hazard. Also there is the mess and disruption to consider when insulation is blown into inside walls.  

The tops of some partitions can be sealed from the attic, but partitions ending at an intermediate floor are not accessible. There remain options; we may collectively call them “bag tricks.” Plastic garbage bags of the appropriate size are stuffed full of pieces of batt insulation and jammed into each opening between pairs of wall studs or ceiling joists. This method works well for sealing and insulating tops of walls in the attic, ceilings open to the unheated space behind upper-story knee-walls, and ceiling cavities between floors. In combination with the outer walls blown full of densely packed cellulose insulation, bag tricks can improve the building envelope significantly and at a reasonable cost.  

In words and pictures, we will explore bag tricks next week.  

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