The savings to be achieved in sealing wall headers are from the very large areas that will convect, where air churns within the wall, refreshed in an infiltration path, or maybe as a throttled static floor pit equalizing with attic air. I think simple math can apply on average, and on instinct choose that of a floor pit, reduced to 25% of temperature equalization with the attic. It would not be hard to confirm an assumption like this, in a serious laboratory test. Has anyone reported on this?
Further simple averaging might present a useful average interior wall area, as a multiple of a home's conditioned space area. I am interested first in one-story homes predominant in the drywall era. An example diligence report finds 2240 sf interior wall area, where home heated space is 1630 sf. Headers affect heated space, times maybe 1.4?
In this example home, I find savings of $448 per year from the sealing of wall headers. I don't know exactly how much time I spent, and cost is all labor. The sealing was done progressively as I staged work areas with stowage of tools and materials. Two man days, where staging, vacuuming and other efforts also served the sealing? Maybe four, where I reckon decking made the work possible and bearable? I needed the lighting. If I allocated $800 of job cost to header sealing, it is still as profitable as the quicker sealing of ordinary pits. Payback in under two years.
Heat carried in excess infiltration through header gaps, matters less than the wall convection. In the example home there are 140 ft of wall headers. If the average combined gap along two walls is 1/16", gap area is 105 sf. By Insulation Math, involved infiltration costs $0.555*105 = $58 per year. The corresponding reduction in a blower door test is 7.5*105 = 800 CFM50, a worthy result.
In the example home with found R18, final R50, the added insulation saves $112 per year, at cost of $1536. Simple payback awful, in fourteen years.
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I read a recent article in Fine Homebuilding about the early days of what is now called "Building Science." Some University professors constructed a test house and insulated it according to standards they developed. Then they tested the energy consumption and found it to be much higher than their theory indicated.
After stripping down the building and performing various experiments, they determined almost all the lost energy was from air currents flowing from the main house up to the attic. Small holes and gaps and points of conduction allowed a steady flow of heat to escape - completely bypassing the insulation layers.
Covering wall voids blocks a major path of this flow. Patching around lighting and other ceiling holes makes a big difference too. Ideally, the drywall used in ceiling and wall construction would be air-tight as well.
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