Already discussed exhaustively on this and other fora (eg Green Building Advisor). Try a google search. Quick answer is that it's an ideal solution in some situations and not very good in a few. There are competing strategies that may be more cost effective, depending on the tradeoffs you want to make. No one-size-fits-all answer.

The only real choice, together with an unvented attic assembly, and exterior foam cladding too. Plus an ERV. or two.

In our current, as in a previous build, we're using 2 inches of closed cell foam on the basement walls for R24. It uses an ultra-low global warming potential blowing agent. The alternative was extruded polystyrene boards (XPS,) which are extremely labour intensive to install.

Note SPF behind steel stud wall. (BTW, great care must be used with any open flames near SPF. Toxic smoke!)

We were planning to use SPF under the flat roof, but without XPS boards topside and no attics to enable checking for leaks, we decided against it.

For authoritative information, see anything by Dr. Joe Lstiburek of Building Science Corp.

Thanks everyone. I asked about spray foam because we watched a YouTube video about it yesterday and it really enlightened us on several things. We are currently having spray foam installed in our new build. Now I’m leery. What is ERV?

What are you leery about? We removed the blown insulation in our attic and sealed vents and put in opencell spray foam. It puts your ducting inside your insulation envelope and is an airseal unlike paper, fiberglass or rockwool. IMO not worth the expense in walls that are sealed on six sides though.

In FL, when we built a custom home in 2021, our builder recommended it for our attic.

I have also seen people in the neighborhood who buy older existing homes do the attic spray foam as one of the updates to make the older houses more energy efficient.

ERV / HRV definition.

I’m leery because I’ve done some reading and it appears that a lot can go wrong if all conditions aren’t just right.

If you have or need to gain more expertise than your home designer and builder have regarding spray foam insulation, I'd say you're headed for trouble.

BTW, on-site today.

Adding another inch to the basement walls and R31 under the flat roof.

I’m not concerned about the architect or builder. I’m concerned about the spray foam product itself. Our town has very strict building codes and new homes now need to have a RES check performed. They are determining what you have to meet for R-values all through your home. Windows & insulation are key to meeting these R-value guidelines.

It's not likely that your town has unique building code requirements with respect to energy efficiency. Most residential building codes are intended to meet the requirements of the International Energy Conservation Code (IECC.) RESCheck is simple software for inferring overall heat transfer through building assemblies. It's used to demonstrate compliance with design requirements.

There is nothing inherently wrong with spray foam insulation. You do, however, need to carefully consider how and where it is installed and the implications for drying of assemblies, fresh air ventilation, etc.

Charles Ross Homes:
Thank you for your RES knowledge. Very appreciated. And we are installing a ventilation system as well.

Good for air sealing and efficiency, especially in tricky areas. But it’s expensive and hard to modify later, so great if done right, just make sure you use a reputable installer.