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Cost diference when insulating to the same R-value; Closed Cell Polyurathane vs Fiberglass

Eric Mikkelsen | Posted in GBA Pro Help on

Closed cell Polyurethane is more expensive/R-value than Fiberglass, but as a whole when a building is insulated to the same standard, how much more expensive is the foam? I would like to build a double wall with 2x4s that is 7.5 inches wide and insulate with spray-in polyurethane foam…the r-value of the wall would be 45 or maybe a bit more. To get a similar insulated wall with the less expensive fiberglass, the wall would have to be nearly 15 inches thick. Given the extra costs of thicker walls, wider foundation, etc., what is the cost difference in this situation with a 2,500 sq’ house?

I live in cold climate, Central Mountains of Idaho @ 5,000 ft elevation where the growing season is only 69 days when the temperature is not below freezing.

Is the cost difference negligible when the cost of the entire home is considered?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Eric,
    All cost calculations are local and idiosyncratic. They depend on the cost of your materials, the cost of your labor, and the cost of your local insulation subs. So get out your calculator and pencil, and start making a few phone calls.

    In most areas of the country, a double-stud wall insulated with fiberglass or cellulose will be considerably cheaper than a wall with the same R-value insulated with closed-cell spray polyurethane foam.

  2. Brett Moyer | | #2

    "A double-stud wall insulated with fiberglass or cellulose will be considerably cheaper than a wall with the same R-value insulated with closed-cell spray polyurethane foam."

    Not to mention the environmental impacts of closed-cell spray foam.

    Eric,

    Because you have posted a question on a green building website, I'm assuming you intend on building a green home. Thus, I would avoid spraying petrochemical plastic foams whenever possible.

    If you build a tight wall assembly- sheathing joints taped, framing components sealed to one another, and airtight drywall installation- then you can use a less expensive, greener (MUCH GREENER), fibrous insulation such as cellulose.

  3. TJ Elder | | #3

    Eric, it doesn't really make sense to use double framing for a 7.5" cavity. If you're building a double frame wall, may as well add a few inches so there's a proper thermal break between the frames. Even if you plan to stagger the studs, they will inevitably line up in many locations including openings and corners.

    Also, consider instead using Thorsten Chlupp's system from the SunRise house, with sheathing over the inner frame and most insulation outside that. It makes air sealing easier, avoids having shear panels at the interior where they block access for services. Maybe most importantly, it avoids shear panels at the cold side and allows a vapor open wall toward the exterior.

  4. Eric Mikkelsen | | #4

    Thanks!

  5. John Klingel | | #5

    Eric: Too, I think when you study fiberglass (batts, anyway) insulation, you'll come to the conclusion that it really is not a great insulation, and the reported R values are misleading. I'd also vote for going a couple of inches thicker, as mentioned above. I don't think you'll ever regret the lost space. If you use TC's plywood air barrier method, you'll need to follow the "1/3 rule", I believe, so you'll need a thicker wall. Thick is good. Lock onto an on-line heat loss calculator and get a rough idea of what the numbers say about how much insulation is enough, and be sure to study what goes under/next to your slab, if you have one.

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