I will only answer a few questions. You sound like you are in the south so open cell is used. At that point you seal all penetrations bringing the attic into the conditioned space of the home which is why it’s possible to reduce the size of the system- your unit and all your ducts are running in a 90° space versus a 130° space in a sunny august day. Yes you can rely on the foam to seal the vents but I wouldn’t want to, I removed my ridge vent prior to spray foaming.

You are right, I am in Texas. thanks for responding. would the HVAC size be reduced by the same amount for open cell and closed cell?

You are right about that. That's why I choose batt insulation initially because that's more proven method and less complicated. but we are forced to change to foam. That building science seems geared towards cold climate and I am in texas, only care about hot weather.

@Charles Ross Homes You sound very knowledgeable. Thank you for your knowledge sharing. My builder mentioned about MUA, do we need either ERV or MUA or both?

With a tightly constructed home, e.g., one insulated with spray foam insulation, which has minimal air infiltration, you need fresh air ventilation to manage the concentration of indoor air pollutants. In Texas, an ERV would be the preferred option to provide fresh air ventilation. By contrast, a make-up air system would be installed to replace the volume of indoor air which is exhausted by range hoods, bath vent fans, central vacuum systems, or used by combustion devices like furnaces, water heaters, etc. that are not sealed combustion types.

I can't quote values of typical ventilation through the attic but I really doubt that foaming an attic makes your house automatically tight.

I would suspect that typical practices in Texas are not generally very tight and so your house will not generally be very tight. Do you have an independent energy rater? Are you getting a blower door test? These things are common in many areas of the country - I think GA has a blower door requirement. Infiltration is a bigger deal in the cold but is always a factor.

What tight does in warm climates is control humidity well. Including in the walls.

What you should be aware of is that foam is typically put on way too thin. Below code levels of insulation and they get away with saying it performs better which is untrue from an R value prospective. It seals better so overall it can perform better but not based on R value which is what code dictates.

Building Science applies to all climates. They know that most new construction in the US is in warm areas. But you also can't pretend that it doesn't get very cold in Texas sometimes. And tightness provides protection during power outages.

Agree with other posters about not using foam to seal vents and that attics that are sealed don't need ventilation. Closed vs open mostly having to do with thickness. Since attics typically have a lot of room for thickness, open is usually used.

" I would suspect that typical practices in Texas are not generally very tight and so your house will not generally be very tight." Depending on the age of a home and the applicable building code, installing spray foam insulation in the attic could very well result in a "tight" home. From an indoor air quality standpoint, "tight" means a natural air exchange rate of 0.35 air changes/hour or less. The natural infiltration rate is inferred from blower door test results at 50 Pa. Even with walls insulated with batt-type insulation, constructing a conditioned attic can significantly reduce natural air infiltration due to stack effect and prompt the need for a fresh air ventilation system.

"Agree with other posters about not using foam to seal vents and that attics that are sealed don't need ventilation." While a sealed, conditioned attic doesn't need "ventilation" it still needs to be conditioned in order to deal with higher relative humidity in the attic. If HVAC equipment is located in the space, it shouldn't be hard to do.

"Closed vs open mostly having to do with thickness." While the R-values per inch of insulation are indeed different, open-cell spray foam insulation is much more air (and vapor) permeable and may require some sort of vapor retarder. The OP will be well served to consult with local professionals with experience and expertise constructing conditioned attics. Doing what works in your area is better than having your home become an experiment.

Thank you all for your information, based what I learned from you, I was able to have some intelligent conversation with my builder and HVAC subcontractor. They assured me the AC unit is enough regardless it's closed cell or open cell and R value doesn't represent the effective of the seal. I would like to replace the ridge vent by cap shingles. the only remaining question is about soffit. I took a picture of it and attached here. zoom in to see the vent holes. How to close those soffit vent? Do I have to replace the soffit board? What do I use? can I cover it with some kind of plate or film then apply spray foam?

The effectiveness of the spray foam insulation as a sealant will only affect the heat loss/gain due to air infiltration in the manual J calculation. R-value still matters. A 30% reduction of the HVAC equipment size due to a conditioned vs. conventional vented attic seems like a lot to me. I'd look for a local HERS rater or third-party HVAC engineer to review the manual J/manual S calculations.

If the house hasn't been drywalled yet you can just attach cardboard over the vents from the inside. We did that to our gable vents when we replaced the blow in our house with spray foam.