Plenty of spray foam contractors will tell you fiberglass is obsolete. We install both, which means we have no particular incentive to push you toward either one — and the truthful answer is that fiberglass is still the right product in a meaningful number of situations.
Where fiberglass genuinely wins
Open, regular, accessible framing with no moisture concerns. An unfinished attic floor with standard joist spacing. New construction walls where the framing is clean and the budget is tight. In those conditions, correctly installed fiberglass reaches the code-required R-value at a fraction of the material cost, and there is no honest argument for spending more.
The word doing the work there is 'correctly'
Most underperforming fiberglass is an installation failure, not a product failure. Batts stuffed behind wiring instead of split around it. Batts compressed to fit around an outlet box. Gaps left at the top plate, at headers, and in the corners. Any one of these can cost a third of the rated R-value, and all of them are common. When we install fiberglass we cut around every obstruction and friction-fit each bay edge to edge, because a poorly installed R-13 batt is not an R-13 wall.
Where fiberglass is a false economy
- Anywhere moisture is present — fiberglass absorbs and holds water
- Crawl spaces and basements, where it becomes a mold substrate
- Rim joists, where the goal is air sealing and batts simply cannot do it
- Cape Cod knee wall voids, where air moves freely behind and under the batt
- Cathedral and dormer assemblies with complex geometry and constant voids
- Any project where air leakage, not R-value, is the actual complaint
The air sealing gap
This is the fundamental difference and it is not a matter of degree. Air passes through fiberglass. It does not pass through cured spray foam. If your complaint is drafts, uneven temperatures, or a house that never seems to hold its heat, adding fiberglass addresses the wrong mechanism. That is not a knock on the product — it is simply not what it does.
Cost over the life of the building
Fiberglass costs less up front, and in the right application that advantage holds. But in the wrong application it fails: it gets wet, compresses, sags, gets contaminated by rodents, and eventually has to be removed and replaced — at which point you have paid for the removal, the disposal, and the second installation. Spray foam does not settle, sag or lose thickness, and in a dry assembly it lasts the life of the building.
How we make the call
We look at three things: is moisture a factor, is the framing regular and accessible, and is the complaint about R-value or about air. Moisture or air leakage points to foam. Dry, regular, accessible framing with a straightforward R-value target points to fiberglass. We will tell you which one your project falls into, and we are happy to quote the cheaper option when it is the right one.