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Zone 4A residential insulation questions

IrishGuyJohn | Posted in Energy Efficiency and Durability on

I been working with a building supply company’s designer [gray haired guy, ex-contractor] who does design and cost build-outs.  I’ve also had conversations with 3 reputable, referred, builders in the area. 

My exterior dimensions are 40 x 44, no valleys or bump-outs.  A walk out basement on the south gable and a loft on the north. 

The south ½ of the roof will be 11 7/8 engineered I-beam cathedral ceiling.  The north ½ of the roof [creating the loft] roof trusses designed to allow a ‘bonus space’. 

After talking to these guys I’ve got an idea of what I can get and cannot get built in this area. 

All 4 said they’d vent the cathedral and insulate inside 10” or so.  He also has an insulator sub who blows anything I wish, including close celled.  They will also vent the trusted section of the roof.  

Question: How much of a space is needed for a vent in the cathedral ceiling to allow adequate airflow but maximize space for insulation? 



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  1. GBA Editor
    Martin Holladay | | #1

    You have a lot of questions. You may be more successful at eliciting answers if you limit your questions to just one or two questions per post.

    I'll start by helping you with your first question.

    Q. "How much of a space is needed for a vent in the cathedral ceiling to allow adequate airflow but maximize space for insulation?"

    A. A 1-inch gap is the bare minimum allowed by code; 2 inches is better. Here are links to three articles on this topic:

    "Site-Built Ventilation Baffles for Roofs"

    "All About Attic Venting"

    "How to Build an Insulated Cathedral Ceiling"

  2. IrishGuyJohn | | #2

    Thanks Martin. I'll take your advice and split up my questions. Building on your reply, is it fair to say that 2" vent space and 10" of insulation is better than 1" space and 11" of the same insulation? Or, to put it slightly differently would you sacrifice optimal insulation for optimal venting?


  3. Expert Member
    Dana Dorsett | | #3

    Unless it's a pond loop and the house isn't very far from the pond the GSHP is going to cost more money than it's worth. Even a tight code-min house that size can be heated/cooled with a 2 or 2.5 ton modulating air source heat pump at nearly the same (or sometimes better) efficiency as GSHP. But it's silly to spec the HVAC before you've even calculated the loads, which can't be done until you've figured out the insulation.

    A 2x12 rafter is 11.25" deep. If going for a 2" gap that leaves 9.25" for insulation. An R30 rock wool batt is 7.25" thick, which leaves you 2" to play with. If the baffles are made of 2" foil faced polyiso you'd have another R12-R13, and hit R42-R43, but the exterior foil facer with the air gap gives it at least another R1. So conservatively you'd be at R13 (foam + air gap) , out of R43 total, for a ratio of 30% of the total R outside the first condensing surface (the facer in contact with the rock wool), which is an adequate for dew point control in zone 4A. (The IRC prescriptive is R15 out of R49, which is 30.6%.) If that's cutting it too close to the line for you install 3" polyiso and compress the rock wool an inch. Caulk or can-foam the polyiso to the rafters to make them air tight. Tape any seams with foil HVAC tape.

    With a simple roof design, foil-clad roof decking (so the decking facer is enhancing the foam facer), and 24" on center rafters with a very low framing fraction (minimal framing for skylights, plumbing stacks, flues, etc) that can hit IRC code performance on a U-factor basis (U0.026 = R38.5 "whole assembly").

    Staggered stud walls aren't really necessary and complicate the situation for framers. Most framers can follow an "advance framing" package with 24" o.c. studs and single top plates, minimal jack studs to get the framing fraction well under 20%. An inch of exterior foam does a lot, even without advanced framing.

  4. Expert Member
    Peter Yost | | #4

    Hi John -

    In your climate zone, 1 inch of vent depth is fine; 2 inches is considered more of a cold to very cold climate detail. I am doing some field research on this issue of how much venting is needed and as part of it I talked with a recently retired roofing guy who has done smoke-stick testing of air flow in vented roofs. He swears that 1.5 inch depth is optimal, but he also has worked mainly in the north.

    Stay tuned on this issue; more to come. But if you need to pull the trigger, go with one inch vent depth and just as important as that extra inch of insulation--if not more important--is a good air barrier in your roof system.

  5. walta100 | | #5

    Read this forum for a few weeks you will find the most common questions are how to build, fix or improve my cathedral ceiling. I say the best cathedral ceiling is the one you do not build.


  6. Expert Member
    Dana Dorsett | | #6

    Somehow when I skimmed it prior to the AM coffee I missed that it was I-beam rafters.

    With 2x flanges it would be easy to set up with a 1.5" vent channel, which for anything but a flatter pitched roof is plenty.

    That leaves 10-3/8" of depth to work with. A half-inch CDX baffle brings that down to 9-7/8. Dense packing that with 1.8lbs fiberglass would deliver about R41.5, but at the much lower framing fraction of the I-beam webbing it could come pretty close to making it to code-min on a U-factor basis.

    Otherwise, 3" of HFO blown closed cell foam (R20.5-R21) and 6-7/8" of 1.8lb fiberglass (R28-R29) would hit a hair north of R49 at center cavity with plenty of dew point margin.

    If you take Peter's advice and go with 1" vent channel ( fine for higher pitched roofs, except when a "cool roof" roofing is anticipated) there will be 10-7/8" to work with. Half-inch CDX would drop that to 10-3/8", which would hit about R43-R44 with 1.8lbs fiberglass, and should easily make it under the IRC's code-max U-0.026 (=R38.5 "whole assembly") at the lower thermal bridging of I-rafters.

    1. fourforhome | | #8

      How do you dense pack the area under the baffles and not dense pack the area of sufficient headroom?

      1. Expert Member
        Dana Dorsett | | #9

        Blowing mesh is your friend...

  7. IrishGuyJohn | | #7

    Thanks all for your replies. I'm neck deep in reading articles and following Martin's advice will be posting individual questions as separate posts so there could be some over-lap between threads. So much good info on this site, it is like drinking from a fire hose.

  8. Expert Member
    Peter Engle | | #10

    If you're after the very last bit of R-value, why not use something like Thermoply to close off the vent channel. It's only 1/8" thick, air-proof, and structural.

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