If you're just filling between existing studs/rafters there's no particular advantage. But more details are needed.

Mira, do you really live where it is -20 CELCIUS?

I have never experienced foam insulation personally. I watch HGTV, and the program HOLMES ON HOMES with Mike Holmes, he really believes in it. I personally would like to spray it underneath my house which has a crawl space and no insulation. But our minimum temp is about 15 FARENHEIT.

do you really live where it is -20 CELCIUS?

Hey, -20ÃÂ C. is hotter than the average temperature in Winnipeg in January.

She ain't livin' in some namby pamby American hothouse like, say, Fargo! Or Trwanana. (Native pronunciation.)

-20C is roughly four degrees below zero Fahrenheit---not unusual in many parts of the Midwestern or Northeastern U.S. in the winter months, and I'm sure not unusual for much of Canada.

Here's what we were told in a much milder climate when we had an energy audit of our house:
If the wall is open, use standard insulation. If the wall is not open, use spray.

But again, we are in a much milder climate (rarely goes below 30F or above 80F), and there may be some performance benefit to one or the other as you get into colder temperatures. Where we are, the performance of similarly rated insulation of each style is supposedly comparable (and in fact they ended up recommending that we not bother with any at all, since it is very low on the "bang for the buck" list in our climate---attic insulation, weatherproofing, etc. are far more important). I imagine this would be quite different in colder areas where the heat loss through uninsulated walls is more significant.

Hi,
By "sunroom" you mean three wall of glass? Ceiling glass, too?
With that much potential for heat loss, I don't think any amount of any kind of insulation will stave off the cold.
If you had an insulated and heated slab floor (thermal mass), along with the passive solar gain during the day, it may stay tolerably warm overnight. Has anyone done the heat-loss calculation on this room?
Casey

To get useful answers you need to better define the question.

Is the foam insulation you mention open-cell or closed-cell? Is the regular insulation fiberglass or cellulose?

Also, what is the climate and desired/required R-value? How deep are the studs? What kind of exterior cladding? Will there be an interior vapor retarder?

A bit of a tangent, but... The spray foam insulation looks like it gets really solid, so how do you deal with it down the road, when you need to repair a pipe, or remodel, or something similar?

We used 3" spray foam closed cell in our attic and gable walls. That was after putting 4" of foil faced urethane in the downstairs walls as we replace sills and rewired the house. We spray foamed along the gap we left around the foil sheets for a tight seal. The energy savings have been substantial over conventional insulation. It's not just the insulating value, but the fact that there is little if any air leakage, which accounts for 40% of the savings from what I have read. Good luck on your project.

I learned from an expert that fiberglass insulation starts to lose it's insulating properties at very cold temperatures - just when you need it to work. Cellulose and spray foam don't have this problem.