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Spray foam, blown in and trusses..

Atticperson | Posted in General Questions on

I have a new construction house with trusses and an unvented roof assembly. Was originally going to use only spray foam on underside of roof decking to get my r value (and a “hot roof”), but, due to LARGE cost overruns on other aspects of the project, looking for ways to cut down costs.

Since roof assembly is trusses (so only 3.5 depth – no easy way to do flash and batt etc.), unvented and hvac equipment is in attic (duct work is insulated), I am somewhat limited on my budget oriented choices now.

But I did have a thought:

What about 3 inches of spray foam on underside of roof decking (to meet 40 vapor impermeable code requirement for hot roof) and remainder of required r value using blown in on attic floor?

Walls will be fiberglass Batts.. I did air sealing using zip tape on sheathing already.

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  1. Expert Member
    BILL WICHERS | | #1

    You can't mix insulation in two locations to arrive at a code required R value. If you insulate the roof, then the attic becomes conditioned space, inside the building envelope, so insulating between the attic and the house doesn't "count". You have to either put all the insulation under the roof deck, or all of it on the attic floor, with "all of it" meaning the code required minimum R value.

    Your best option would be to insulate under the roof sheathing, which, assuming it's sealed properly, brings the attic into the conditioned space which is a big plus if you have mechanicals in the attic that you can't relocate. Insulating the attic floor only and going with a vented attic is the cheapest option, but least efficient due to those mechanicals. The happy medium might be closed cell spray foam under the roof sheathing, ideally burying the rafters a little to help with thermal bridging, then make up the remainder with rolls of fiberglass insulation running perpindicular to the rafters supported with wires. That last option is probably the cheapest way to get you where you need to be while still keeping the attic as conditioned space.


    1. Expert Member
      DCcontrarian | | #2

      Agree that the insulation needs to be on the roof and fiberglass batts are the best choice. With trusses you're not going to be able to run perpendicular, the batts have to run between the trusses. The question is how to support them and that depends on the shape of the trusses. It might work to use faced batts and staple the facing to the truss wherever it touches. If there aren't enough contact points you could nail a 1x3 to the side of the truss and staple to that.

      I would check with the insulation contractors and see how much it really saves though. The answer may surprise you.

      1. walta100 | | #3


        It seems to me this plan is risky in that the attic is now a conditioned space you are recommending vapor and air permeable insulation without any vapor or air barriers. It seems likely the warm moist attic air will find its way to the cold sheeting and become liquid water and grow mold.


        1. Expert Member
          BILL WICHERS | | #4

          The OP intends to apply at least 3" of closed cell spray foam to the underside of the sheathing, which would provide an air barrier and moisture protection. I recommended applying enough ccSPF here to bury the 2x4 upper chords of the trusses, which would be even safer from a moisture standpoint.

          If it wasn't for that ccSPF layer, you'd be correct, and the assembly would be a problem.


          1. Atticperson | | #7

            Spray foam just over chords of trusses (just over 3.5 inches) and then fiberglass insulation perpendicular to roof bays (cut and fitted in to the parts of the trusses that cross its path) seems like it may be the best bet then?

            Anyone know of any insulation support systems that may work for that (outside of using wires)? Also any sections or pictures/documentation of that method to make it easier to describe someone else?

          2. Expert Member
            Akos | | #8

            Search for insulation pins. These can be installed to the roof deck before the SPF goes on to anchor them in place. You'll have to do a bit of digging to find the extra long ones. Most I've seen are 3" to 4".

            Not a big fan of spray foam but it sounds like you designed yourself into a corner. I think you'll find that getting the rest of the insulation using open cell SPF to about the same cost.

          3. Atticperson | | #9

            Problem is no one in area does open cell - only closed cell, so can't do a hybrid approach with open/closed - so looking at other options. Thanks for the tip on insulation pins - I'll take a look.

          4. Atticperson | | #10

            Looks like I could do:
            3.5in of closed cell spray foam (r24.5)
            4in insulation pins on truss chords spaced 22 oc
            Run 24" wide r15 Batts perpendicular to trusses
            Which would give me required r value (r38+).


          5. Expert Member
            DCcontrarian | | #11

            I guess I don't get the desire to run the batts perpendicular rather than parallel to the trusses. I guess it would depend on the shape of the trusses but it seems like you would have to cut and cobble the batts all over the place to do that.

            In either case, you have the trusses. If you tack a piece of 1x3 to the truss parallel to the rafter part and out from the roof sheathing the thickness of the batts you have something firm to fasten the batts to. If you're going perpendicular the batts can just go behind it. If you're going parallel you can staple to it or use wires.

          6. Expert Member
            BILL WICHERS | | #12

            Perpindicular batts is sometimes easier, but it does depend how open the trusses are. Some trusses are so open you don't have to make all that many cuts. Either orienation works, although if the upper chords weren't buried in spray foam, perpindicular batts would be better for thermal bridging.

            For the OP: I would look at unfaced low density R19 rolls here ("rolls" are like really long batts that you cut as needed). This is often the cheapest fiberglass out there, which makes it good when you need "R on the cheap". I've used this stuff perpindicular over rafters to add insulation into areas that are too small to justify setting up to blow in loose fill insulation. When I do this, I try to use the 23" wide rolls, since they make the project go faster IF they will fit. You'll need to check your trusses to see what would fit best for you. Minimize how much cutting you have to do will make the project easier, and faster.


  2. walta100 | | #5

    The first question is the HVAC ductwork and equipment in this attic?

    The next question is the ceiling sloped or is any living space tucked into this attic? IE cathedral ceiling or half story building.

    If the answer is no to both question the very best option is to cover that attic floor with at least R38 of cheap fluffy insulation and vent the attic.

    It gets more complicated if you answered yes to one or both of my questions. But if you answered yes and like most people you are likely to move away in less than 7 years the smart financial move is to cover the attic floor with cheap fluffy insulation knowing that the high cost of moving the insulation to the roof line will not be recover in fuel savings before you sell the house and yes, your utility bills will be higher than they could be.


    1. Atticperson | | #6

      Hvac equipment is in attic, duct work is insulated but I believe at a lesser r level then it would be if in a non conditioned space.
      No living space in attic.
      Roof is already on and no ridge vent is in (since it originally was going to be a hot roof) making ventilation harder.

      That and for other reasons getting full r value from attic floor insulation isn't terribly feasible.

      I will not be moving from house for a few reasons, unless I had to.

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