Sunday, February 26, 2012

How Big Is A Hole, That Matters?

I have reported that the most-reputable weatherization contractor in metro Portland claimed air sealing of 1100 CFM50 from its two hours of squirting foam, to steal an air sealing rebate. It was an audacious claim. A mere and almost-believable 300 CFM50, would have been enough. Rebate given. No questions asked. No proof required. No explanation for a learning curve in attempted justification of useless Home Performance test-in and test-out. Several blog posts, including this, ask for that justification of HPwES.

On then, with the math for air sealing. It must be math, for small changes are not measurable outside reading uncertainty in test results. Please accept my development of the math, which has been posted to the public for several years, with appeal for comment, and with never a report of error:
The math is specific to my gas-heated home in Portland, Oregon, which I think of as average for the nation.

Excerpts applicable to a single-story home:

(Portland, Oregon variables, 4400 HDD, 90% efficient gas furnace, fuel cost boosted to fairer $2 per therm)
Annual Heating Cost Due To Drafts = 0.074 * CFM50
CFM50 = 7.5 * (Path Area, sq in)
Annual Heating Cost = $.555 * (Path Area, sq in)

A required reduction of 300 CFM50 by sealing measures, corresponds to a hole of area 40 sq in, as in a circle 7" diameter.

Reduction of 1100 CFM50 corresponds to path area 1100/7.5 = 147 sq in, as in a rectangle 36" by 4", roughly the area of a ventilation chase I actually did seal. The HPwES contractor cheated, getting the reduction by closing and newly gasketing a closet door.

The reduction of annual heating cost for closure of 40 sq in, is $22.

The reduction of annual heating cost for closure of 147 sq in, is $82. But, leaving the closet as a fully-involved floor pit, walls 180 sf, has heat loss of 2.4*180/3 = $144 per year. Surely, with some throttling by insulation batts, the pit costs more than the infiltration. With the closet door commonly left open, not warned against, there was still the infiltration loss too.

I think these numbers are believable, and disgrace the notion that weatherization is related to achievable small increments of "air sealing." I submit a Diligence Report for each weatherization job, to my rebates organization, Energy Trust of Oregon. Here is an anonymous form of the report related to the 1100 CFM50 claim. I claim that savings to be achieved from sealing are more importantly of closing attic floor pits. Such savings in this job might be larger than $500 per year. They are achieved with much work. Work that is possible only with lighting and safe work surfaces. Work that can not be done by a clown dancing on joists in the dark. 

Back now to the Subject question. How big is an ordinary hole in a house, that matters, costing, say $300 per year?  300/82*147 sq in = 538 sq in. A hole 21 inches diameter. Who will not agree that a hole that size matters?

Now, what does the math say for a door left ajar all year? Say, size 36" x 84", 3024 sq in. CFM50 = 22,680. Annual Heating Cost = $1678. Isn't this math believable? At some point, it is just what your furnace costs, running full-time.

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