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Community and Q&A

Cost of spray foam under roof deck

lookloan | Posted in General Questions on

The architect used attic roof trusses on a long single floor home that create a large square attic walking space in the middle like a long bowling alley since the house is 84 feet long.  Width is 26 feet wide of the roof trusses. A perfect rectangle.  Walls are 12 feet tall. 

The rafters under the roof deck are 10 inches deep and the rafters making the roof trusses floor, or 1st floor ceiling is also 10 inches.  The center bowling alley shape is framed so I could insulate this long square walk up attic on the two sides and across the ceiling of this center area.  

When doing the math, the interior roof underdeck is 83 1/2 feet long and has 42 to 44 trusses that are 1.5 inches wide each, so about 5 feet wide of truss space to minus from the 83 1/2 feet.  I figure 5 feet x 18 x 2 x $7 a sf comes out to around +$1400 so not sure if these installers back this out.  Anyways, I’m told $1 to $1.15 is the cost or 7 times this number since the roof needs 7 inches of thickness to get R49.  When I do the divide the math on a 2 inch closed cell foam board from Lowes or Home Depot,  it comes out to about 59 cents per inch foot as the installers are technically installing $2.30 for the same 2 inch SF.  Do these foam costs seem right to some of you installing close cell spray foam?  Basically these quys want $21,000 to $22,000 to spray under the roof deck so I was wondering if I am better off with another solution but not sure what to do, as I thought this was solved until I got the quotes.  Isn’t there an option where they attach mesh or fabric of some sort and blow in a type of insulation as an option under the roof deck?  I am thinking maybe just get R49 Fiberglass installed in the 1st floor ceiling and insulate the sides and ceiling of the attic bowling alley area, but that would not allow future access to the sides of the bowling alley which may have duct work installed in it.  Thank-you everyone in advance for any ideas. 

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  1. Expert Member
    NICK KEENAN | | #1

    The price seems about right. The usual unit of measure is the "board foot" which is one square foot, one inch thick. Going rate is slightly above a dollar a board foot.

  2. Expert Member
    NICK KEENAN | | #2

    Closed cell is usually quoted at R6 per inch so you need 8 inches to get R49.

    Unless your structure can't support it usually the most cost-effective is 12 inches of blown in fluffy insulation at R4 per inch.

    If you need the air-sealing that spray foam provides do an inch of spray foam and then eleven inches of fluff.

    The closed cell foam releases potent greenhouse gases so it's recommended to limit its use.

    1. lookloan | | #3

      Is the blown in fluffy stuff mentioned cellulose insulation? On the outer portions of the center there is no sub-floor like the center of the attic bowling alley area. Can cellulose be blown in under the roof deck if a material to hold the cellulose is stapled or attached to the 10 inch rafters of the roof trusses - I was hoping to have an attic that is closed in with no air venting in the soffits.

      1. Expert Member
        BILL WICHERS | | #5

        There are other kinds of blown insulation (fiberglass, mineral wool, others), but usually on this site cellulose is what gets recommended. Yes, you can use netting to support blown insulation in an application like this.

        What you’re talking about is a “hot roof” — an unvented root assembly. Are you sure you NEED to build this way? There are more potential issues with an unvented roof than with a vented roof. If you do go with an unvented roof, you need to worry about the ratio of spray foam to fluffy stuff to avoid moisture issues, or use all spray foam.

        Unless you’re trying to finish the attic and use it as extra living space, or you have mechanicals in your attic (which should be avoided regardless), you’re better off using blown cellulose on the floor and leaving the roof vented. This is a safer, and cheaper, way to go here.


  3. Expert Member
    Akos | | #4

    For an unvented roof, your option is either SPF on the inside or rigid foam on the outside. Here the cost of either option is comparable, but if you can DIY the exterior foam option is much cheaper.

    One spot you can save on is meeting code on U factor basis. This takes into account the entire assembly so you something like an R38 roof. You can meet this with 2" to 3.5" (more in colder climates) of closed cell SPF between the rafters covered by either 5" to 6" of open cell foam or 5.5" or 7.25" mineral wool batt.

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