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Insulation in attic ceiling and walls

Arnold Hill | Posted in Building Code Questions on

Could use some help in understanding the code requirements for different assemblies (roof and walls) in attic space.  I am helping a friend in renovating house in Maryland.  Creating a new roof profile (shed roof) that will have attic space below.  Attic space to be conditioned as it will house a new high velocity HVAC unit.  Understand that roof assembly has to be R-49.  Is that correct?  Looking to use metal roof and zip system sheathing.  After some research, we want roof that will be vented from raised heel to high ridge with use of accuvent soffit baffle.  We are considering the use R-Max thermasheath 3 and closed cell spray foam to put on underside of baffle.  R-value for R-Max 4.5″ thick is 31.  If we have 1″ of close cell spray foam (R-6.5), will be around R-37.  Will code allow for remaining R-value to be blown cellulose in floor of attic, areas that will not have plywood flooring?  Do we need to have insulation in space between ceiling and plywood flooring in attic?  Are the walls in attic to be R-19?

Thanks in advance for assistance with these questions.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    >"Understand that roof assembly has to be R-49. Is that correct? "

    Sort of, but not really. Code compliance on a U-factor basis only calls out U-0.026, which is the same as R38.5 "whole-assembly", after all thermal bridging of rafters and the R-values of all the other layers such as roof deck, roofing ceiling gypsum, even the interior & exterior air films are factored in. Installing a continuous 6" layer of R-Max Thermasheath above the roof deck in an UNvented assembly would be enough.

    With 2x6 rafters you would also be able to get there on a U-factor basis with 3" of R-max above the deck and R23 rock wool snugged up to the underside of the roof deck, no venting, without needing an interior side vapor retarder tighter than standard interior latex paint on wallboard. When splitting the a R between above & under the roof deck a minimum of 30% of the total R needs to be above the roof deck in US climate zone 4 (= most of MD), or 40% if zone 5 (=some of the NW counties.) With the 3" polyiso derated (for potentially lower performance at the winter depths) R15 polyiso and R23 rock wool you'd be at 37%, and still over 40% at the labeled R-value of the polyiso, which is fine.

    Low density polyiso is much greener than even the HFO blown closed cell foams, using only about half the polymer per R.

    >"Will code allow for remaining R-value to be blown cellulose in floor of attic, areas that will not have plywood flooring?"

    Code doesn't allow you to add the attic-floor R to the roof R- only the roof-R counts. To meet code with just cellulose on the floor takes the full R49, which takes an initial blown depth of 14.5-16" (settled depth of 13").

    >"Are the walls in attic to be R-19?"

    Current IRC code is R20, not R20. R19 batts only perform at R18 when compressed to 5.5" in a 2x6 stud bay. While you could meet code with 3" of HFO blown closed cell foam, the 3" thermal bridge through the framing severely robs the closed cell foam of it's potential performance, and it would UNDER perform R20 fiberglass or dense packed cellulose. Closed cell foam between framing is a waste- do the math:

    https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf

  2. Arnold Hill | | #2

    Thanks for points of clarity....appreciated. Will what you have proposed for roof assembly, give me the ability to have conditioned space, as we are placing HVAC and hybrid water heater in the attic, to maintain an appropriate temperature without overworking the system? Did you mean 2.5" of R-Max above as R- is 16.7?

  3. Arnold Hill | | #3

    Received a notice that you had responded, but don't see the response.

  4. David Barnes | | #4

    I’m just curious what you mean by “high velocity HVAC unit”? Some of the “high velocity, small duct systems have a lot of marketing hype, and aren’t as efficient in real world applications.

  5. Arnold Hill | | #5

    Look to have installed a Unico HVAC system.....heard of them? What is your take?

  6. David Barnes | | #6

    The Unico use a lot of blower fan energy to move air through the small, restrictive ducts. A lot of people favor the Fujitsu Ducted Minisplits RLF series if your load is small enough. They are very efficient and modulate from 3100 btu up to around 21,000 btu in heating on the RLF18 unit

  7. Arnold Hill | | #7

    Thank you for taking the time to respond and suggestion made. Load is around 72000 BTU for HVAC needs. Planning to have solar panels to help with energy usage. Right now, according to our solar contractor, we will be producing more than demand....will help with some of our high demand systems.

  8. Expert Member
    Dana Dorsett | | #8

    >" Load is around 72000 BTU for HVAC needs."

    In all seriousness, I doubt it, unless some of the windows don't have glass or it's a very large house with zero wall insulation.

    Is there enough of a heating history on this place to be able to run a fuel use based load calculation on the "before upgrades" picture of the house? For guidance on how to run those numbers, see this bit o' bloggery:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    If you're tightening up and insulating the place to current code minimums most houses will run design heating loads of 10-12 BTU/hr per square foot of conditioned space @ 70F indoors/0F outdoors (colder than 99% outside design temps in MD- see https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf ) and cooling loads of about a ton per 1500' of conditioned space. So even a 4000' house would have a design heating load of less than 50,000 BTU/hr, and a design cooling load of about 3-3.5 tons, not 6 tons (=72,000 BTU/hr)

    Lousy rules of thumb like that aren't very precise, with exceptions both on the high and low end, but even if it's a 2x4/R13 type construction your design load will be something like 15 BTU/hr per square foot of conditioned space up to maybe 20BTU/hr per foot if the foundation isn't insulated & air sealed.

    Oversizing is the enemy of comfort. ASHRAE recommends no more than 1.4x oversizing for the 99% condition load, which is enough excess for cold snaps and to be able to use overnight setback strategies, but will run at a high enough duty cycle during normal cold weather to be spending most of it's time in "warm summer breeze" mode rather than the hot flash followed by the longer chill-down.

    1. Arnold Hill | | #9

      Thanks Dana. House is vacant. No energy history. I was using a rule of thumb for calculation, which I knew was generous. We will be having an engineer design loads given insulation planned.

  9. Arnold Hill | | #10

    I have question regarding recommendation for flat roof membrane. The roof will have an area that is bout 4.5' x 22' the is for most part flat (1" rise per 12" run). Any recommendations on EPDM and insulation manufacturers? Roof R-value to meet is R-49. Would it be overkill to use Firestone products? What would you recommend for mil thickness?

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