New construction load calculations are relatively straightforward because you have access to the plans: insulation specs, window specs, square footage, everything is documented. Retrofit jobs on existing homes are a different animal. The house was built 30 years ago, nobody knows what's in the walls, and the homeowner's description of the problem is "it just doesn't cool like it used to."
The load calc principles are the same. The data collection is different.
Gather What You Can From the House Itself
Measure the conditioned square footage yourself. Don't trust what the homeowner says. People regularly misquote square footage by 15-20%. Measure each room and add it up. While you're in the house, note ceiling heights, number and approximate size of windows, and whether you're seeing evidence of older versus newer construction practices in any exposed areas.
Check the attic. Insulation type and depth are major variables in your heat gain calculation. You can estimate R-value from what you see: pink fiberglass batts at 10 inches is roughly R-30. Blown cellulose at 12 inches is roughly R-38. If there's almost nothing there, that attic is a massive heat load in summer and cold in winter.
The Blower Door Substitute
A formal blower door test tells you actual air infiltration. Most residential contractors don't have one. You can use conservative default values for infiltration in your load calc software for older homes (pre-1980, assume high infiltration unless you see evidence of air sealing work). Newer homes (post-2005, especially in energy code states) assume moderate. Recent renovations that included air sealing get tighter values.
You're making educated estimates, not wild guesses. Document your assumptions in the calculation. If a customer or inspector ever asks, you have a record of your methodology.
Ask About Recent Changes
Has the homeowner added insulation recently? Replaced windows? Added a room addition? Changed anything about how the house is used (home office that runs equipment all day, people working from home adding internal heat gain)? These questions take two minutes and can meaningfully change your calculation.
Equipment Selection After the Calc
Retrofit jobs often reveal that the original system was oversized. Don't just replace the existing tonnage. Replace with what the load calc says. If the house needs 2.5 tons and has a 4-ton unit that's been short-cycling for 20 years, right-sizing solves the comfort problem the customer couldn't articulate.
That conversation, handled well, positions you as the contractor who actually solved their problem versus the one who just swapped equipment.
Put this into practice with WerkOps
Run accurate load calculations, build estimates, and send invoices - all in one tool.
Start Free Trial
Written by
Dina Good
Founder, WerkOps
Dina grew up in the trades. Her father was a contractor, then a building inspector, and the job site was her classroom long before any formal one was. She went on to own her own contracting company, which is where she first felt the pain of running a skilled trade business on bad software and worse spreadsheets. A love of data pulled her into programming, and the two worlds eventually collided into WerkOps - software built by someone who has actually been on the truck.