Sunday, November 14, 2010

Reason to Wrap Crawl Space Joists

I offered the joist wrapping of the previous post, as requisite for doing the awful work of insulating crawl spaces, in three bids. None were accepted. One reason could be offer of standard wired-up batts, at much lower cost, by more-willing competitors. The added cost of $500 to place R11 joist wrap was not sufficiently justified. This post presents the justification, with consistent use of my however-questionable math.

The previous post has inconsistent rigor, in considering thermal resistance of  framing over a crawl space. The annual cost of heat lost to an uninsulated crawl space with 2x8@16" oc framing is reckoned with a bit more rigor, as:

1.8*1000 (1.5/16/9.8 + 14.5/16/3) = $561 per year. Compare to computed $600 per year, where joists and all are treated as having the R3 minimum value.

The $58 per year heating cost for the proposed R11 and R30 batt placement is again considered with allowance for joist thermal resistance, as:

1.8*1000 (1.5/16/20.8 + 14.5/16/33) = $58 per year.

Without the R11 wrap: 1.8*1000 (1.5/16/9.8 + 14.5/16/33) = $67 per year. 

The puny difference is $9 per year. My customers should question the wisdom of paying $500 for such benefit, where I have confidence in achieving the $67 per year cost, if I would offer the work. 

The wrap is justified for any other installer, who, wiring up batts otherwise, fails to achieve full contact of batts, with floor sheathing. Such installers offend customers with lost savings opportunity, and offend subsequent service access with sharps and obstacles. Estimate the savings of heating cost by wrap placement, assuming no benefit, where R30 batts are out of contact, as difference from $58 per year ideal cost.

Cost = 1.8*100*(1.5/16/9.8 + 14.5/16(f/33 + (1-f)/3)), where f is fraction floor contact.

If batt contact is poor, it is very affordable to fix the problem by incrementally pulling down wires, and stapling R11 on joists. There should be no need of outside incentives, as in rebates, to fix a wrong installation. The incentive might be in denied incentive, where full contact is not achieved.

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