Wednesday, October 27, 2010

Insulating My Crawl Space

I've lived in my house in Oregon's maritime climate, for seventeen years. I have good attic insulation, and OK wall insulation. The energy usage in heating my home is below-average, but that may be from letting the house run cold. Much of the time in cold winter, I need good shoes and heavy socks to cope with floor temperature. There is only a very beginning of insulating the crawl space, but surely I will get the rest done soon, now that I have a likable plan:

Wrap joists with 16" width R11 unfaced batts. Complete filling with R30 kraft batts. 
There is absolute assurance batts can't fall, and less time thinking to pull batt edges down to cover joists.

Here is the math, for Portland, Oregon, with area 1000 sf:

Present heating cost is 1.8 * 1000/ 3 = $600 per year.

Wrap joists with unfaced R11 batts. Retain these batts with 1/2” staples both sides of joist, at spacing of about one foot. Do this wrap in parallel with R30 installation, beginning at the most-distant end of attic.
Place R30 kraft-faced batts as blocks 16” x 48”, Johns Manville Product K1242. These batts more-fully fill joists, and are readily defended as R30. The thermal shorting of joists is considered.
R = 3 + Reff, where 1/Reff = (1.5/16)/17.8 + (14.5/16)/30.
Reff = 28.2
Improved heating cost is 1.8 * 1000/ 31.2 = $58 per year.
Savings $600 - 58 = $542/ yr.

Cost of  insulation:
The R11 is calculated as length at 16” width, total 760 linear feet, where I will use Johns Manville AU397, sixteen 8-ft pieces per bag,128 linear ft per bag. Need six bags, at $266, retail. For a customer, I would charge $500 for the R11 installed.
The R30 K1242 insulation needed is 1000 sf/ 58.66 sf/ bag, seventeen bags, $666 at retail. For a customer I would charge $1300, installed.

Subtotal: Approximately $500 + $1300 = $1800.

Consider payback on this cost, with Energy Trust rebate, $.25*1000 = $250, and, a 30% Federal Tax credit on material at retail, 0.3*($266 + 666) = $280.
Payback = (1800 - 250 - 280)/$542/year.
Payback = $1270/ $542/year = 2.3 years. 

This kind of math overwhelms most customers, who have at least two just complaints. There is the fair cost of fuel, higher than their bills by about times 1.5. Like me, customers may also choose to not compensate, staying cold. I spend about $500 per year for natural gas heat, in a 986 sf home. Surely $350 of that is due to cold floors. At fair fuel cost of $2 per therm, my floors are blowing at least $500 per year, not much different from the $600 prediction. Maybe I am less thrifty with the thermostat, than I think, and less thrifty than that skeptical customer.

I have been foolish to not do the crawl space insulation sooner. I preserved overhead space for recent Pex-replacement of all water pipes, but could have managed all that crawling under the insulation I now propose. I have done an attitude check under my test bay, and don't feel more-confined. Head bangers are cushioned. On the other hand, where most installers would run crazy wires to "hold-up" the batts, I would have many snags to curse.

For further development of my crawl space as conditioned, please see this Fall-2012 post.

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