Attic Insulation

If you are going to spend money on one insulation job in a Duluth house, spend it on the attic. It is the largest temperature difference in the building, it is where the stack effect pushes all your heated air, and it is the only place you can fix ice dams at the cause instead of the symptom.

Call (218) 514-6594 and we will go up and look.

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Ice dams start in the attic

If you get ice at the eaves every winter, the attic is where it is coming from. Warm air leaks up through the attic floor, heats the roof deck, and melts snow from below even at 10 degrees. That meltwater runs down to the overhang past your exterior wall, where there is no heated house underneath, and refreezes into a ridge. Water then ponds behind the ridge and works back up under the shingles.

Your gutters did not cause it. Your shingles did not cause it. Heat did.

The fix is the sequence below: seal the attic floor first, then insulate to depth, then keep the ventilation path clear. Insulation alone does not do it, and more insulation over an unsealed floor is the expensive version of the same disappointment.

Full breakdown on the ice dam prevention page, including what to do about an active leak, why heat cable and roof raking disappoint, and the roofs where this is harder than usual.

What actually works

Seal every hole in the attic floor. Then insulate to depth. Then make sure cold outside air can move freely from the soffits, over the insulation, and out the ridge.

That sequence, done correctly, is the fix. It is not glamorous and it is mostly crawling around on your hands and knees with a foam gun finding leaks. That is the job.

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The two attic assemblies, and which one you want

There are exactly two correct ways to insulate an attic. Mixing them is how you get problems.

Vented attic, insulation on the floor

The attic stays cold and outside. Insulation and the air barrier live at the ceiling plane. Cold outdoor air enters at the soffits, washes the underside of the deck, and exits at the ridge.

Choose this when: the attic is unfinished storage or empty, there is no ductwork or air handler up there, and the roof has functioning soffit and ridge venting or can get it.

Why it is usually right: it is the cheapest assembly by a wide margin, it is the most forgiving of small mistakes, and if a roof ever leaks you find out about it fast.

What the job looks like:

• Air seal every penetration with spray foam and fire rated sealant at the chimney and flues
• Baffles in every rafter bay at the eave so the soffit vents stay clear
• Dam and seal the attic hatch, and insulate its back side
• Verify bath fans and dryer vents terminate outside, not into the attic, which they do more often than you would believe
• Blow cellulose or fiberglass to R-49 minimum, R-60 preferred
• Depth markers left in place so anybody can verify it later

Unvented attic, foam against the roof deck

The attic becomes part of the conditioned house. There is no ventilation at all. Insulation goes against the underside of the roof sheathing.

Choose this when: the attic is finished or will be, there is ductwork or mechanical equipment up there, the roof has dormers, valleys, and hips that make continuous venting impossible, or the ceiling below is a cathedral.

The Zone 7 requirement: at least R-30 of air impermeable insulation in direct contact with the sheathing before any other insulation goes below it. That is about five inches of closed cell. This is a condensation control requirement, not an R-value requirement, and the number scales with the climate zone. Zone 5 is R-30’s little brother at R-20, Zone 6 is R-25, and here it is R-30. Confirm it with your building official before permitting.

What the job looks like:

• Strip out any existing insulation at the ceiling plane
• Block and seal the soffit vents, since this assembly is intentionally unvented
• Five inches of closed cell against the deck, applied in two inch lifts
• Then either more closed cell or open cell below it to reach R-49 or R-60 total
• Thermal or ignition barrier as required by the AHJ

The honest downside: it costs three to four times what the vented floor job costs, and if the roof ever leaks, the water can travel a long way before you see it. Do not build this assembly under a roof that is near the end of its life. Reroof first.

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R-values for Climate Zone 7

Duluth is in IECC Climate Zone 7. Minnesota’s residential energy code puts the attic minimum at R-49. The newer model energy code raised it to R-60, and on a full attic redo we generally push for R-60 because the marginal cost of another few inches of blown insulation while the crew and the machine are already on site is small.

What R-49 and R-60 look like in real material:

Material	R per inch	Inches for R-49	Inches for R-60
Blown cellulose	about 3.5	about 14	about 17
Blown fiberglass	about 2.5	about 20	about 24
Open cell spray foam	about 3.7	about 13.5	about 16
Closed cell spray foam	about 6.5	about 7.5	about 9

How to check what you have without a tape measure. Stick your head in the attic hatch. If you can see the tops of the ceiling joists, you have about six inches, which is somewhere around R-15 to R-21 depending on what it is, or less. If the insulation is level with the top of the joists, you are around R-19 to R-22. If it is well above them and you cannot tell where the joists are, you are probably in decent shape and the question becomes air sealing rather than depth.

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What attic work costs in the Duluth area

Job	Typical range
Air sealing only, spray foam at all penetrations	$1,500 to $3,000
Air sealing plus blow to R-49 over existing insulation	{{PRICE_ATTIC_R49}}
Remove old insulation, air seal, blow to R-60	{{PRICE_ATTIC_R60}}
Unvented conditioned attic, hybrid foam assembly	$14,000 to $23,000
Unvented conditioned attic, full depth closed cell	$18,000 to $30,000

Ranges, not quotes. What moves them: attic access, headroom, how much old insulation has to come out, how many penetrations there are, whether there is vermiculite present, and roof area.

On removal. Old blown insulation does not always have to come out. If it is dry, uncontaminated, and evenly distributed, blowing over it is fine and it saves you real money. It comes out when it is wet, rodent damaged, or when the air sealing underneath cannot be done without clearing it.

On vermiculite. If your attic has loose, pebbly, gray-brown granular insulation, stop. A significant share of the vermiculite installed in North America came from a mine in Libby, Montana, and carried asbestos contamination. It gets tested and, if positive, handled by an abatement contractor, not by us. We will tell you what we are looking at and we will not disturb it.

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Things we find in Duluth attics

Not scare copy, just the actual list from this housing stock.

• Bath fans venting into the attic. Extremely common in houses from the 60s through the 90s. A bathroom fan dumping warm humid air into a cold attic is a frost machine, and it is also a direct heat source for ice dams.
• Recessed can lights. Older non-IC-rated cans are open holes into the attic with a light bulb in them. They need to be sealed, boxed, or replaced with airtight IC-rated fixtures before insulation goes over.
• Open balloon framed wall cavities. In pre-1930 Duluth homes the stud bays run uninterrupted from basement to attic. They need to be blocked and sealed at the top plate or the house will keep pumping air upward no matter what you put on the attic floor.
• Knob and tube wiring. Still live in a fair number of hillside homes. It cannot be buried. Electrician first.
• Chimney and flue chases. These need fire rated sealant and proper clearance, not spray foam directly against the flue.
• Compressed insulation at the eaves. Somebody stuffed batts to the very edge and choked off the soffit vents. Baffles fix it.
• The attic hatch. A two by three foot uninsulated hole in your ceiling, and one of the largest single leaks in most houses.

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Attic first, then the rest

If you are prioritizing, the attic is the first job in nearly every Duluth house. The exception is a crawl space that is actively wet or freezing pipes, because damage beats efficiency. See the crawl space page for that comparison, and the rim joist page for the cheapest job on the list.

Call (218) 514-6594 or send the form and we will get up there and tell you what it needs.