Rated versus real
Where the performance is actually going
Ranked by how much heat flow each problem adds compared with a clean install. Small areas can dominate this list — that is not a mistake, it's how heat flow works.
Every path heat can take out of your ceiling
| Path | Area | R there | Heat flow |
|---|
When a path's share of heat flow is much larger than its share of area, that path is a thermal short circuit. Those are the ones worth fixing first.
What fixing it is worth
Correcting the defects and bringing the assembly up to each target, installed properly.
| Scope | Real R after | Load removed | Est. savings |
|---|
The only way to know is to measure it.
This tool takes your description and does the building-science math on it. What it can't do is see your attic. A Diversified Energy assessment puts numbers on it — insulation depth mapped across the ceiling, air leakage measured with a blower door, duct leakage measured at the registers, and infrared imaging to find the bypasses you can't see from the hatch.
How the math works
1 · Heat flow adds up, R-values don't
You cannot average R-values across an area. Heat flow through a path is proportional to 1/R, so the correct method is to area-weight the conductances (U-factors) and then invert the result. This is the parallel-path, or isothermal-planes, method used in ASHRAE energy modeling.
This is why a bare patch matters so much. At R-38, one square foot passes heat at U = 0.026. Strip it to bare drywall at R-1.7 and the same square foot passes U = 0.59 — about twenty-two times more heat.
2 · The paths counted here
- Insulation at full depth, plus the drywall and air films underneath it
- Framing — joists and truss chords conduct around the insulation at roughly R-1.25 per inch of wood
- Compressed areas, at reduced depth with a small density credit added back
- Bare or thin areas, down to drywall and air films
- Open bypasses — gaps around can lights, chases, flue penetrations, and unsealed attic hatches
- Wind-washed edges, where insulation is thin or pulled back at the eaves
3 · Louisiana targets
All of Louisiana is Climate Zone 2A for energy purposes. The Louisiana-amended 2021 IRC prescriptive minimums are R-38 ceilings, R-13 wood-frame walls (or R-0 plus R-10 continuous), and R-13 floors, with an envelope air-leakage limit of 7 ACH50. ENERGY STAR recommends more than code for retrofits in Zone 2 — R-49 over an uninsulated attic, R-38 over an attic that already has three to four inches.
4 · What the energy numbers assume
Design-condition load is straight physics: Q = U × Area × ΔT, with a vented attic at 125°F and an indoor setpoint of 75°F. Annual cooling energy uses the degree-day method with 2,900 cooling degree days for New Orleans and a 1.5× uplift on ceilings, since an attic runs hotter than outdoor air. Equipment is assumed at SEER2 14.3 and electricity at $0.13/kWh. Heating-season savings are additional and not counted here.
Sources
- Louisiana Residential Code 2021, Chapter 11 — Energy Efficiency (Climate Zone 2A statewide; prescriptive R-values)
- Insulation Institute — Summary of Key Louisiana Residential Energy Code Requirements (R-38 ceiling, R-13 wall, 7 ACH50)
- ENERGY STAR — Recommended Home Insulation R-Values (Zone 2 retrofit levels)
- U.S. Department of Energy — Insulation and Air Sealing R-Value Requirements by Climate Zone
- 2021 IECC Chapter 4 — Residential Energy Efficiency (Table R402.1.3)
- PNNL Building America Solution Center — Climate Zone 2 roof and wall assemblies