Prioritizing Retrofit Measures Under a Fixed Budget
A nonprofit owns a 1,600 ft² single-story house with a vented crawlspace, R-11 floor insulation that is falling down, a 20-year-old 80% AFUE furnace, single-pane windows, and a blower door result of 5,600 cfm50 (about 9 ACH50). Crawlspace RH is 78% and joists show staining; the floor above is cold in winter. The board has $12,000 and wants new windows because they look drafty. The climate is mixed-humid with 2,500 heating and 1,400 cooling degree days. The assessor must recommend a package that maximizes comfort and durability per dollar.
1. Evaluate the window replacement proposal against alternatives and justify a prioritized package.
Read the worked answer
Windows are the least cost-effective first measure here. At 9 ACH50, most of the perceived draft is air leakage through the envelope and floor, not window conductance; new windows would leave the leakage and crawlspace problems intact. A $12,000 budget spent on windows would consume most of the funds for a modest comfort gain.
Prioritized package: (1) Crawlspace moisture control, because 78% RH and stained joists indicate active durability risk; install a ground vapor retarder, correct grading and drainage, and seal foundation vents or convert to a conditioned crawlspace as appropriate. (2) Air sealing to cut the 5,600 cfm50 result, targeting the attic and floor planes and penetrations. (3) Re-support and air-seal the floor insulation and add a rigid or batt layer to reach a defensible R-value. (4) Apply remaining funds to attic insulation and duct sealing. Windows should be deferred or limited to the worst units, with storm windows as a lower-cost interim option.
2. Explain how the crawlspace conditions interact with the heating and cooling performance and what verification would confirm success.
Read the worked answer
An open, damp crawlspace at 78% RH acts as a moisture reservoir and a thermal liability. Falling floor insulation allows cold, humid air to contact the floor, producing cold floors in winter and condensation risk in summer when humid outdoor air meets cooler surfaces. In a mixed-humid climate, drawing humid outdoor air into the crawlspace raises the dew point under the floor, and any cool ductwork or joist can condense, which explains the staining.
Verification: re-run the blower door and compare to 5,600 cfm50; measure crawlspace RH over several weeks and target below 60%; use a moisture meter on joists to confirm drying; and check floor surface temperatures before and after. Also verify combustion safety if the crawlspace is closed, since the 80% furnace may draw combustion air from that space. Confirm duct leakage is reduced and that the furnace is not backdrafting after air sealing.