You've got a spray-foam-insulated house and the HVAC runs for two minutes, shuts off, runs again, shuts off. Or the humidity is out of control. Most HVAC contractors don't know how to size for foam houses.
You probably came here because…
- Spray-foam-insulated home (open or closed cell) built or retrofitted in the last 15 years
- HVAC short-cycles constantly, especially in shoulder seasons
- House holds temperature but humidity is high
- Another contractor told you the system is fine, but you know it isn't
What’s actually happening
Foam-insulated houses have dramatically lower thermal loads than fiberglass-insulated houses of the same square footage. A 3,000 sqft foam house might have the cooling load of a 1,500 sqft fiberglass house. But most contractors size HVAC the same way regardless of insulation.
The result: a 3-ton or 4-ton AC installed in a house that needs a 1.5-ton. It cools the space in 3 minutes, shuts off, and never runs long enough to pull moisture out. Cool but sticky. Constant cycling. Wasted money.
The contrarian fix: right-size the system based on a real Manual J load calculation – or add a dedicated whole-house dehumidifier to compensate for a system that's already too big. We've been doing foam houses for 20 years and know both approaches.
What we do about it
Get a real Manual J
Not a rule-of-thumb sizing based on square footage. A real load calculation that accounts for your specific insulation, windows, orientation, and infiltration. Most existing systems in foam houses are 50-100% oversized.
Consider a variable-speed system
Modern variable-speed heat pumps and AC units can throttle down to 30-40% capacity, which lets them run longer, gentler cycles even in a low-load foam house. Great retrofit option for a chronically-oversized system.
Add a dedicated dehumidifier
If replacing the system isn't in the budget, a whole-house dehumidifier tied into your ductwork handles the moisture the oversized AC can't. About $2,500-3,500 installed. Immediate comfort improvement.
Why most HVAC guys refuse foam houses
Foam houses require real load calculations, tighter installations, and often smaller equipment than a tech is used to selling. Some contractors flat-out won't work on them. We've been doing foam houses since the early 2000s. Call (423) 637-1846 – we'll actually run the numbers for your house.
Ready to fix it?
Call the family that’s been doing this in the Sequatchie Valley since 1996.