You will have a very airtight house with an effective vapor barrier and low heat gain. That could be a formula for high indoor humidity without the heat gain to trigger the demand for cooling.

Side question: Why are you going with open cell instead of closed cell foam? Doesn't closed cell have a much higher R value per inch of foam?

Thanks for the response. I understand the higher humidity point. I guess the need for a the two stage is to basically be running the one stage longer, thus dehumidifying, even if it isn't particularly "cooling". Is that kinda in the right direction?

I'm down in the south, and Open Cell Foam seems to be the foam of choice around here. It is less expensive, and the added cost of closed cell doesn't seam worth it.

I'm midwest... my brain is normally thinking about heating.

Anyway, I believe that's your contractor's reasoning. Two stage will allow for dehumidification during mild weather while still being able to cool the house when it's really hot outside. You run the risk of your house of being cool, but having high humidity otherwise.

Someone who has more experience with foam insulation in the south can address your question more specifically.

We are in south Louisiana and we did the whole foam envelope thing with our new construction. We have what our contractor called 2-stage lite. It's like a 2 stage but not really.....I'm not completely sure. We have 14 SEER Ruud systems. Our humidity inside, according to my Oregon Scientific thermometer runs between 30% and 40%. We also have stained concrete floors, and there's never any condensation on them, the windows, anywhere. We don't have a fresh-air intake either because our A/C guy's reasoning was, you're gonna open a door at least once during the course of the day so there's you're fresh air.

My parents-in-law did the same thing with the foam, but they have the Trane XL16i systems. They put in a fresh-air intake but wound up not connecting it to the system. No humidity problems there, either, and they are in the humidity capital of the world -- New Orleans.

Some matches with the 15 SEER single stage have better dehumidification rates than the 16i. Some 16i matches are rather poor at latent removal - that's humidity. 20i might be better.

Key is DON'T OVERSIZE! Too many do, especially with a super well built home.

Here is a link that might be useful: Sizing & cost guarantee site my foaming buddy recommends

I'm not a big fan of the XL16i

Hopefully you will have a Door blower test performed after or when they are spraying the foam since it is crucial the foam creates a sealed barrier, any tiny holes in foam in the attic can mean lots of trouble with mold.

Also remember if you spend the money on foam then why go less with an a/c system?
You want the right size system, and 2 speed systems have a greater range of humidity control than single speed. You want the best system a manufacture can offer.

spray foam doesn't give a home owner the option of spending pennies on the a/c system because they have a foamed sprayed tight home. It gives them the application that they don't need a larger A/C that cost more to run an hour and will be running longer to keep up with the infiltration heat load gain or loss.

For your clarification (OP) the slower speed still cools but it takes it longer to get to the desired temp. Running slow for a long time is how you get excellent humidity control when you don't need max cooling, like at night.

Guys thanks so much for the info!

Classic Dave, what are some other units you'd recommend then. Everyone down here is Trane crazy. You can find about any brand, but it seems like Trane has a big corner on the market in the new installs.

@obwannab which AC unit did you decide on? Thanks.

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