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CRES: Building Envelope and Insulation: Specifying Assemblies That Perform as Drawn

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Building Envelope and Insulation: Specifying Assemblies That Perform as Drawn

Building Envelope and Insulation tests whether you can specify an assembly that performs as drawn: correct material, correct installed grade, continuous over framing, and compatible with the moisture regime. Expect questions on R-per-inch, attic and rim joist details, window ratings, and the gap between nominal and effective R-value.

Materials and R per inch

  • Fiberglass batt: about 3.1–3.3 per inch, highly installation-dependent.
  • Loose-fill cellulose: about 3.5–3.8 per inch; fiberglass loose-fill similar.
  • Open-cell spray foam: about 3.5–3.8 per inch, and it air-seals.
  • Closed-cell spray foam: about 6–7 per inch, plus a vapor retarder function.
  • Rigid: XPS about 5, EPS about 4, polyiso about 5.6–6.5 with cold-temperature derating.

Grade is part of the model

RESNET grades insulation I, II, and III by coverage and compression. Grade I is near-perfect, split uniformly around framing, wiring, and obstructions. Grade III includes compressed, face-stapled batts and visible gaps; a Grade III R-13 may deliver closer to R-9. The grade must match the value entered in the model, not the value printed on the bag.

Attics and rims

A vented attic keeps insulation at the ceiling plane and requires baffles for clearance at the eaves, plus a sealed top plate. An unvented attic moves the thermal and pressure boundary to the roof deck, using closed-cell foam or air-impermeable insulation against the deck. Rim joists are both a bridging node and an air-leak node: seal first, then insulate with foam or rigid board rather than bare batts in a cold rim.

Windows

U-factor governs heat loss, SHGC governs solar gain, and both are climate-dependent: low SHGC (roughly 0.30 or below) in cooling-dominated climates, higher SHGC acceptable where heating dominates. Air leakage rating and condensation resistance complete a defensible specification.

Worked example. A 2x4 wall has R-13 cavity insulation; other layers (drywall, sheathing, siding, air films) total R-1.5. Framing fraction is 25%, and a 2x4 stud is about R-4.4. Cavity path R = 14.5, U = 0.069. Framing path R = 5.9, U = 0.170. Area-weighted U = 0.75(0.069) + 0.25(0.170) = 0.094, so effective R ≈ 10.6 - not 13. The reasoning: bridging is weighted by area, which is why continuous exterior insulation is worth more per inch than cavity insulation.

Traps that cost marks

  • Compressing a batt to fit and still claiming the nominal R.
  • Adding an interior vapor barrier behind a finished basement wall or in a cooling climate.
  • Insulating over knob-and-tube wiring or non-IC recessed fixtures.
  • Leaving a bath fan discharging into attic insulation.
  • Forgetting to count rigid sheathing in the whole-wall U-factor.