Spray Foam vs Blown-In vs Batts: What Minnesota Homes Actually Need

Insulation salespeople tend to have one product and one answer. The honest version is that each type is correct somewhere and wrong somewhere else. Here is where each belongs in a Minnesota house.

The comparison

	Closed-cell foam	Open-cell foam	Blown cellulose	Blown fiberglass	Fiberglass batt
R per inch	6.5 – 7.0	3.6 – 3.8	3.2 – 3.8	2.2 – 2.7	3.1 – 3.4
Air barrier	Yes	Partial	Partial (dense-pack)	No	No
Vapor retarder	Yes at 2"+	No	No	No	No
Water resistant	Yes	No	No	Yes	Yes
Cost per board ft	$1.25 – $2.00	$0.45 – $0.90	$0.12 – $0.20	$0.10 – $0.18	$0.15 – $0.30
DIY friendly	No	No	Rentable	Rentable	Yes
Adds rigidity	Yes	No	No	No	No

Cost per R-value delivered

This is the number that actually decides most of these choices. Cost to reach R-49 over 1,000 square feet:

Material	Approx. installed cost
Blown fiberglass	$1,500 – $2,700
Blown cellulose	$1,700 – $3,000
Fiberglass batt	$2,200 – $3,800
Open-cell foam	$6,000 – $11,000
Closed-cell foam	$9,000 – $16,000

Foam costs roughly four to six times as much per unit of R. That is not a reason to avoid it — it is a reason to use it only where its other properties (air sealing, moisture control, high R in a thin cavity) are worth paying for.

Where each one belongs

Closed-cell spray foam — worth every dollar

• Rim joists. The single best foam application in a Minnesota house. Small area, biggest air leak, needs moisture tolerance. $1,200 to $2,800 and homeowners feel the difference.
• Cathedral and vaulted ceilings. No depth for R-49 of blown material and no room for a vent channel. Foam is the only practical answer.
• Crawlspace walls and rim. Moisture tolerance is mandatory here.
• Bonus rooms over garages. The classic freezing floor.
• Cantilevers and bay window floors.
• Below-grade and against foundation walls.

Blown cellulose or fiberglass — the workhorse

• Open attic floors. This is where the money should go. You can hit R-60 for a fraction of the cost of foam, and the attic floor is easy to air seal separately with caulk and foam cans before blowing.
• Dense-pack into existing wall cavities. Cellulose at proper density substantially slows air movement, and it goes in through small holes without gutting the house.

Fiberglass batt — fine when it fits

• New framing in open walls, installed carefully with no compression, no gaps, and split properly around wires and boxes.
• Interior sound-deadening walls.

Batts are only as good as the install. A poorly fitted batt with gaps at the edges loses a large share of its rated performance, and gaps are extremely common. If someone is charging you foam prices for batts, or batt prices for a rushed job, you get neither.

The biggest myth: foam on the attic floor

People are regularly quoted $12,000 to spray foam an open attic. It is almost always the wrong call. In an open attic you have unlimited depth available — so R-per-inch, foam's main advantage, is worth nothing. Blown cellulose gets you to R-60 for a quarter the price, and air sealing the attic floor separately gets you foam's other advantage for a few hundred dollars in materials.

Spend the difference on knee walls, the rim joist, and new windows.

The Minnesota moisture question

In our climate zone, warm moist indoor air moves toward the cold outdoors all winter. If it hits a cold surface inside an assembly, it condenses. That is what rots roof decks and rusts fasteners.

• Attic floor: insulate deeply, seal the air leaks, and ventilate above the insulation so any moisture that gets up there dries out.
• Cathedral ceiling: either a fully vented assembly with a continuous baffle, or unvented with closed-cell foam directly on the underside of the deck. A half-measure between the two is how roof decks rot.
• Basement walls: rigid or closed-cell foam against the concrete, no polyethylene sheet on the interior side trapping water in the middle.

What we would do on a typical Twin Cities house, in order

1. Rim joist: closed-cell foam — $1,200 to $2,800
2. Attic floor: air seal, then blow to R-60 — $2,500 to $6,000
3. Baffles so soffit vents stay open — $500 to $1,800
4. Knee walls and slopes if a story-and-a-half — $2,000 to $8,000
5. Walls: dense-pack if empty — $3,500 to $9,000

That sequence delivers the majority of the available comfort and savings, in cost-effectiveness order.