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I have a 1 and 1/2 story house with two bedrooms and a bath upstairs. The house was build in 1910 so there is no insulation in the walls and only R-19 in the rafters. I live in Minnesota so it gets very cold but we also have days above 90 during the summer. The rafters have drywall on them so I am looking for the best cost vs benefit of adding insulation. As of right now I am looking at pulling down all the drywall, having someone spray closed cell insulation and then installing new drywall. Our rafter bays have been furred out to 2X6 so if we got 5 inches of closed cell foam that would be about R-35. Is that benefit worth the cost? My estimate is the ceiling is about 1300 sqft and we would install the drywall ourselves. Might have a finisher come in to tape and mud. We have not had issues with ice dams that I know of but we have only been here for less than 2 years. It gets pretty warm up there when the temps rise and especially if the sun is out. I am going to convert a half bath upstairs into a full so if we decide to do this project I would like to do it before the bathroom is done.
 

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It may be more cost effective to spray a couple inches of closed cell then fill the rest with fiberglass. That will still give you the air sealing afforded by the foam and you'll end up with about R-26 assuming R-6.5 per inch for closed cell and R-13 fiberglass. You could also go R-15 fiberglass and end up with R-28.

I have open cell in my house including the roof and I love it.
 

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Since I am in such a cold region code requires something like R-49. If I decide to get this done i will likely just have them fill the bay with foam to get to the highest R value possible. About how much more would it be to spray 6 inches vs 2 inches? Is cost linear so 2 inches cost X and 6 inches cost 3 times X?
 

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Heat loss reduction is not linear to the R-value.

The air sealing and first R10 do most of it. It's more important provide a thermal break and stop air movement than to super-insulate a wall or ceiling.

It's better to do like R-25 and be able to afford to insulated walls than to have R50 ceiling and have no insulation in the walls.

If your house is framed, dense pack cellulose, foam, or air-krete can be put into the walls. Dense pack cellulose works in wall applications with no vapour barrier because it stops the air movement; air is what the carries moisture which can condense on exterior surfaces made cold by the insulation.

Minimal amount of moisture that gets into a dense packed exterior wall can get out -> there's drying potential.

With roofs, there isn't drying potential, hence needing to use foam or have a ventilated space above the insulation.
 

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Had 3 walls of my sons home done, know this up front. There is no way for them to accurately shoot an exact amount. We opted for 2 inches (i believe) on a 2 x 4 structure. The cavities ended up completely filed.

Be sure you have all wiring and other things done before spraying, no going back!

He loves it!
 

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About how much more would it be to spray 6 inches vs 2 inches? Is cost linear so 2 inches cost X and 6 inches cost 3 times X?
There is a limit to how thick polyurethane spray foam can be applied in one pass. The chemical reaction as it cures is exothermic (generates heat), and since it is a very good insulator, the heat has difficulty escaping. Excess heat can degrade the insulation, or even cause it to catch fire.
 

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Spray foam can leave a house uninhabitable if it doesn't cure right. It should be reserved for special applications like ceilings without an attic above, sill plates, around doors/windows.

Even when it cures right no one knows if it's as safe as pre-done rigid foam. Could offgas. In a factory, they have much more control over chemical ratios and conditions under which it cures.
 

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Since I am in such a cold region code requires something like R-49. If I decide to get this done i will likely just have them fill the bay with foam to get to the highest R value possible. About how much more would it be to spray 6 inches vs 2 inches? Is cost linear so 2 inches cost X and 6 inches cost 3 times X?



R-49 in new construction. You need to figure out cost vs. return. Airseal with a layer of foam, then add the fiberglass. There are cost vs. return R-value charts you can look t online...
 

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I talked to a spray foam guy today at the fair. He said the exact same thing, Stopping the airflow is the key. He will spray the rafters in my wood framed attic from soffit area to ridgeline. He puts up 3/4 squares of plywood under the attic whirligig vents and sprays them shut. I asked about the soffit vents. He said they will vent the soffit after spraying. There will be no airflow at all in the attic creating a virtually new temperate zone.

I don't know about you but this just goes against all I have been taught about attics.

Temperate to me means hot, sticky, balmy, no breeze, in other words, Yech!

Anyway, we're getting an estimate for the foam after the fair.
 

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How's the headroom up there? If you're going to be taking down the drywall you can fir out the rafters to make the cavity deeper. You can keep the roof sheathing ventilated and add a lot more R value. If you have a lot of headroom to spare, you can either add in foam strips or plywood gussets to the new ceiling joists breaking the thermal bridge that exists with solid wood rafters.
But... you still need to look at the rest of the house. If you just put a super insulated hat on but are otherwise naked, you're going to still get cold. Find and stop the drafts, get the house tighter top to bottom and more insulated over all if you want to stay warm.
 
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