Spray foam has its advocates and detractors. Closed cell has the highest R-value per inch of insulation. It is also the highest cost per unit of R-value.

The advantage of closed cell spray foam is that it is a complete air and vapor barrier. If you want the most comfort possible, CC spray foam is your answer.

Open cell is also a good option, though at a lower R-value and you give up the vapor barrier qualities of the foam.

Lots of other good options. The flash-and-bat technique is something to consider. With these sorts of structures, your goal should be to (1) air seal and (2) then get as high an R-value as possible.

What is being done to heat and cool this expanded area and how large is it?

I am not sure the spray foam is going make a much of a difference in terms of your comfort. Perhaps the spray foam experts can give their opinion.

Have you asked the architect if the walls could be increased to 2X6 stud construction? That would allow a R-21 fiberglass insulation batt. The alternative would to keep the 2X4 and install an fiberglass board as part of the outside sheathing to increase the R value.

Will there be an attic above the vaulted ceiling? If not then R-38 might be marginal if the roof faces south or west. You should look into these options and compare their costs to spray foam insulation.

@mike_home our heating/cooling was upgraded in the fall and sized to accomodate our addition. Framing is already complete, so no changing to 2x6's now. Just trying to figure the best way to insulate what we have.

I know foam will enable us to achieve a higher R-value, but will the same R-value perform better with foam than batts? For example, would R-18 closed foam provide as much comfort as R-20 batts?

There is no attic, per se. The ends of the room are sloped, but it comes to flat nearer center of the room. There is some dead space in that flat area above the ceiling.

So there’s a lot of debate on the point about R-values and spray foam. My personal experience has been that my perception is that all R-values are not equal unless air sealing is properly taken into account. But the plural of “anecdote” is not “data”, and anecdote is really all you’re going to get in answer to your question.

The standard refrain is that R-value is R-value, and folks who are selling spray foam and saying that spray foam has greater comfort at lower R-values are just trying to sell you an expensive product. Maybe so.

The (reasonably coherent argument I’ve seen in favor of higher performance per R-value for spray foam is as follows:

”Comfort is a function of heat transfer in a chaotic environment. Heat may be transferred by radiation, conduction or convection. R-value testing places a heating element on one side of a material and measures how much heat makes it to the other side of a material. Airflow in this scenario is controlled and is effectively zero, so there is no convection. Thus, R-Value measures conduction and radiation, but does not address a material’s resistance to convective heat transfer.

Cellulose and fiberglass do nothing to stop airflow. So air can get through those types of insulation and get right up next to the air barrier between hot and cold, then conduction takes over. So your 8” of high R-value material is completely bypassed, with heat transfer taking place on air currents.

Foam is a full air barrier, meaning that the hot and cold air never get closer than the depth of your foam away from each other. So foam is a significantly higher performer at the same R value because it ALSO stops transfer of heat through convection.”

I don’t have the testing capability to verify this one way or the other. But my I think the point likely has merit.

Looked at another way, foam encapsulation with CC foam is basically turning the enclosed area into a giant Yeti cooler. And those are pretty temperature stable.

I know foam will enable us to achieve a higher R-value, but will the same R-value perform better with foam than batts? For example, would R-18 closed foam provide as much comfort as R-20 batts?

Open cell foam for a 2X4 wall will have an R value of about R-12 while closed cell will be about R-21. This assumes the foam is installed properly and there are no voids. For your question the room might feel a little warmer on a cold windy day in the winter if the spray is doing a better job of blocking air infiltration. But on hew construction I would expect the outside sheathing is doing the majority of the air infiltration blockage.

The other advantage of spray foam is can squeeze into tight irregular spaces. A closed cell spray foam may be worth the extra money on sloping cathedral ceiling if you can achieve a higher R value. There you may feel the difference when the sun is beating down on a hot summer day.

I have no experience with spray foam. However I have seen multiple photos on this forum of bad spray foam installations. I could be wrong but I don't think using spray foam in NJ construction is very common. If you do hire someone to do spray foam then check their references and if possible inspect a job they are currently doing.

"I've read about bonus rooms being hard to regulate temperatures and considered options to better insulate."

The most common issues with rooms above garages is heating them in the winter, the cold coming from the floor. Make sure you have access for good floor insulation and a good quality. thick foam underlayment for the carpeting.

Nothing beats closed cell spray foam in sealing and insulating garage ceilings under conditioned space.

On new home builds, I've used extruded polystyrene (XPS) boards piled and taped and fg batts over heat supplies and returns. Still frigid floors in the winter.

First time I used cc foam, tight and cozy warm.

(Theoretically, taped tongue and groove (t&g) XPS should work. In practice, standing on ladders or scaffolding reaching overhead in the gloom while trying to get all the corners is a different matter.)

For the bedroom and bonus room over our garage (garage is ICF as is rest of house to top of 1st floor so even during polar vortex when it was -30 degrees outside, it was only a little below 50 degrees in the garage; 2nd floor is 2 x 6), we did 1" closed cell spray foam for floor, ceiling, walls, and cellulose to fill out the 2 x 6s in the walls and ceiling. Don't remember what else we did in the floor. All duct work is inside the conditioned space. Very comfortable!

Anne