Is spray foam the right move for an undersized AC in a 1920s attic?

Spray foaming the roof deck to create a conditioned attic is the right move for your HVAC performance, but you have to be meticulous about the execution on a 1920s structure. Since your ducts are currently sitting in 130-degree air, your 3-ton units are losing half their cooling capacity just fighting the attic temperature before the air even hits the registers. Moving that equipment into the thermal envelope will make those undersized units feel significantly more powerful because they won't be battling that massive heat gain through the duct walls.

Regarding the old moisture stains, you need to be 100% certain the roof is watertight before the foam goes in. Closed cell spray foam is a **vapor barrier**, meaning it won't let the wood dry to the inside if a new leak develops. If you have any doubts about the chimney flashing or the 2024 roof job, address those first. Using baffles with spray foam is usually a mistake in this specific application; it creates a tiny air channel that can acutally allow condensation to form against the cold roof deck if humid air finds its way in there. If you're going to foam, go full unvented and seal those soffits tight.

And yes, you absolutely need to plan for air management once you seal that attic. Even though it's "conditioned" space, it doesn't get the same airflow as your living rooms. You shoud either have a small supply and return register added to the attic or, better yet, install a dedicated **whole-house dehumidifier** tied into the system. In a Jersey shore enviornment, the humidity will find its way in through the stack effect and doors, and without a way to pull that moisture out of the attic air, you're risking wood rot over the long term. Don't expect the foam to fix a mechanical failure, though. If the vents are blowing lukewarm air, have that tech check the static pressure and delta-T across the coil. If the ducts are leaky or the refrigerant is low, the foam is just a expensive band-aid. But for those $800 winter bills, sealing the attic is the single most effective way to stop the heat from escaping through the top of the house like a chimney.

This sounds like such a game changer for an old house! I’m finally diving into my first big renovation and stories like this get me so pumped to fix my energy bills. Since you mentioned the Jersey shore, do you think the proximity to the salt air affects how the spray foam cures or holds up over time compared to houses further inland? I'm curious if the salt spray changes the maintanence for the exterior vents too!

I’d go with Huntsman Building Solutions Heatlok HFO for this. I used it on a rental near Asbury Park and it’s the only way to get that R-value high enought in a 1920s rafter space to acutally help undersized units. It’s got a great vapor permeance rating wich is key for those humid Jersey summers. If you're worried about the chimney, make sure they use a fire-rated barrier like a mineral wool board before spraying. The Heatlok will seal the roof deck so tight you’ll see that attic temp drop 30 degrees instantly. It’s expensive, but it’s the only brand I trust not to pull away from old framing.

Before you drop $10k on foam, have you ruled out duct leakage at the joints? If your HVAC guy is allready saying the units are too small, spray foam isn't going to magically increase thier BTU output. If those old ducts are leaking, you’re just going to be conditioning an empty attic more efficiently while the bedrooms stay warm. Have you done a simple duct blast test or looked into Aeroseal first? It seems like alot of risk for the roof deck if the real issue is just leaky, uninsulated flex duct or disconnected boots.