- Document your home’s thermal envelope20 min
This step produces a room-by-room inventory of your home’s thermal envelope — the exact inputs a Manual J load calculation requires.
Contractors often estimate these values from the truck or from county tax records. Your own measured data let you verify the load calculation before you commit to equipment that will operate for 15–20 years.
Walk each conditioned room with a tape measure, a notebook, and a ruler. For every room, record:
Floor area and ceiling height: length × width of the room, plus the height of the ceiling above it.
Exterior walls: material (wood frame, masonry, ICF, SIP) and insulation R-value if known. If not known, note the build year — pre-1980 homes typically have R-11 or less.
Windows: for each wall, count the windows and note glazing layers (single, double, triple), low-e coating (visible as a faint tint), orientation (N/S/E/W), and shading (overhang, awning, mature tree).
Ceiling: attic above or no. At the attic access, measure the insulation type and depth with a ruler.
Floor: slab-on-grade, vented crawlspace, conditioned basement, or unconditioned basement.
Air leakage class: tight (0.15 ACH), average (0.35 ACH), or leaky (0.50+ ACH). Base this on your home’s age and visible drafts.
Occupants and internal heat loads: the number of people who regularly sleep or work in the home, plus refrigerator, oven, computers, pool pump, dehumidifier — anything drawing 400 W or more.
Save it and share it: keep your completed envelope inventory. You will give a copy to each bidding contractor so all proposals start from the same building data.