This is the difference in building for today or tomorrow. Value vs Obsolete.
Let me start by making a very bold and controversial statement. Fiberglass insulation is like having No insulation. It is typically miss installed and it allows air to move through it making it a filter not an insulator. Now with that said let me explain.
Fiberglass is woven together leaving air pockets in between the open cells. This gives a series of buffer zones if you will giving the batt an insulating quality and when tested performs as stated, but if air is introduced to the system it will pass right thru just like the filter you use in your heating and air system. This air will carry hot or cold air right thru the system and in my opinion that is just like having no insulation. I have used batt insulation in the past and I have seen install jobs that look like the picture in the next column. It very hard for batt insulators to get it right every time. Most often they end up cutting a piece too large an stuffing it in leaving virtually no insulating pockets. Another habit is to staple the batt tabs to the side of the stud leaving a full air space between the sheetrock and the sheetrock. If they had the air sealed off from outside this might work, but instead sets up a convection loop. As you can tell I am not a fan of batt or blown insulation of any kind.
After watching my crews trying to caulk every plate line and realizing that I was pushing a chain, I switched to expanding foam and have never looked back. I use batt insulation as a sound barrier in interior walls only. Foam is sprayed out of a gun in almost a paint consistency and expands like 500 times in just seconds. It is amazing to watch the foam form while filling every hole that it touches. Now that is air seal! We use open cell foam which has many air pockets in it as well. The difference is air will not pass through this system, "0". I like this analogy; when you use a paper cup and fill it with hot coffee you have to get one of those corrugated sleeves and even that gets hot to hold; however a thin foam cup will give you plenty of comfort to hold that coffee. I have a standing bet that if you built a box out of R-15 batt insulation and put a bag of ice in it out in the hot sun and bought a igloo foam cooler that has maybe an R-2 thickness with a bag of ice and put it along side the batt box. Is there anyone betting on the batt box to keep the ice fresh longer than the foam box? Anyone?