AC condenser size and foam insulation

I have a question regarding AC condenser size and foam insulation. My understanding of the “Rule of Thumb” for condenser size is 400 - 500 sq ft per ton. If we have foam insulation the sq ft coverage per ton can go up to 800 sq ft. When we put foam on the rafters and bring the attic into the envelope, we’re now including the cubic ft in the attic space to the equation.

The house is a 1 story, 3,300 sq ft, with 10’ ceilings and a massive attic space. One 4 ton AC condenser for all. I recommended that the buyer ask for the design specs or consult an HVAC contractor for further evaluation.

I’m looking for 2 items:

  1. Do you recommend even speaking to this kind of issue? Some inspectors here in TX say no. It’s not in our Standards of Practice; But, neither is using IR cameras. (We use IR cameras).

  2. I’d like some clarification on the Condenser size when foam is used at the rafters, bringing the attic into the building envelope. This is a mountain of conditioned space that comes into the equation. My HVAC guy didn’t have an answer. The only thing I can think of is that, because the foam seals air leaks in the attic, we have fewer air changes and higher efficiency.

Any help would be hugely appreciated!


That is way beyond the scope of a home inspection…Yep!

For question 1& 2 if you choose to exceed the SOP in this case be absolutely sure you can and are willing to perform the ACCA Manual J calculations.

Your kidding, right?!

“I recommended that **You **ask for the design specs or consult an HVAC contractor for further training.”

Proper design of HVAC systems for spray foam homes

HVAC sizing and analysis

Lots more. Search Google AC sizing unvented attic

I depart from every aspect of spray foam. Too many considerations and too few homes to judge. Building Science Corporation | Consulting & Architecture has many articles on the topic. Humidity control is very important.

Some new schools of thought nix spraying the rafters and instead spray an inch or two on the horizontal ceiling then adding standard insulation. I think Building Science Corporation | Consulting & Architecture has a discussion on this.

Did a 2-story 2,500sf / 130 yr old house last week with a 2.5 ton A/C and 76,000 btuh furnace. Attic was totally covered with spray on foam insulation.

Only the edges of rafters or joists were visible … Roof decking was not visible.
Could not see roof vent penetrations inside ALTHOUGH they were visible outside. It took me all of 3 minutes to decide to DISCLAIM & DEFER it out in the report.