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Community and Q&A

3/12 pitch roof insulation

Robertsoave | Posted in GBA Pro Help on

Hi all.
I sent a question and got a reply on a 2/12 pitch roof.

I have an architect working on my plans now. It is a single-slope roof at the widest part (of 32 or so feet).

I originally was going with 3/12 pitch, but he thought 2/12 would be a lower roof at the tall end, so I said fine!

Now, looking at the problems with 2/12, I asked to go back to the 3/12 pitch. So: If I go 3/12, would I (or should I) use R-60 fiberglass? It will have 24 in. parallel trusses.

Would I still need a cupola? (Zone 6, lower Michigan.)

This is TMI for my small brain…


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  1. GBA Editor
    Martin Holladay | | #1

    A 3-in-12 pitch is the traditional dividing line between a low-slope (flat) roof and a pitched roof.

    With a 3-in-12 pitch, you can get away with following the conventional rules for venting roofs (without the need for a vented cupola). You'll still need all of the usual elements of a vented roof assembly: soffit vents, ventilation baffles to separate the top of the insulation from the ventilation channel, a ventilation channel, and a ridge vent.

    If there is any reason why you can't include these necessary elements, you would need to adopt an unvented solution.

    For more information on vented and unvented roof assemblies, see this article: How to Build an Insulated Cathedral Ceiling.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "If I go 3/12, would I (or should I) use R-60 fiberglass?"

    A. The type of insulation you choose depends on several factors, including the final decision on venting and the configuration of your framing members. Fiberglass batts are the least effective type of insulation available. Either blown-in cellulose or blown-in fiberglass would be preferable to fiberglass batts (assuming a vented assembly).

  3. Robertsoave | | #3

    Thank you
    I will ask my designer to change it back to the 3/12 should I run the baffling all the way up or just at the soffit
    I was thinking of asking him to use like a half of scissors truss so the ceiling would not be so high also
    And I will look into cellulose instead I guess I would just pay to have that done as I do not have the equipment to do that , well I can't do it all ! we do plan on installing the steel roofing myself
    Thank you so much

  4. Robertsoave | | #4

    Would it still be a good idea to put a foam board on top of the sheeting under the steel or does it do nothing up there
    We built our current home 30 years ago I put 36 inch fiberglass in the attic and 6 in walls with 3/4 tuff r outside and house wrap
    Our heat bills our minumum and we have not uncovered our ac unit from last winter yet ! In fact we did not have an ac unit till a few years ago we installed it then with the new furnace , to eventually sell the house
    So my aim is to beat what I have now no go the other way
    Thanks your website is super

  5. GBA Editor
    Martin Holladay | | #5

    I strongly urge you to read the article I linked to: How to Build an Insulated Cathedral Ceiling.

    If you want to install rigid foam above the roof sheathing, that means that you are choosing to build an unvented assembly. If you want to build an unvented assembly, you don't need soffit vents, a ridge vent, or ventilation baffles.

    If you want to build a vented assembly with parallel trusses, then the ventilation baffles need to extend from the soffit to the ridge.

    If you want to install scissors trusses, the ventilation baffles only have to extend from the soffit to the "attic."

  6. Jon_R | | #6

    I'd consider raised heel scissor trusses (for less ceiling height) and renting a cellulose blower (I had no problems doing mine). It's not clear to me how much gap you must have before you don't need baffles.

  7. Robertsoave | | #7

    Thank you
    This is exactly what I sent my architect last night ,to go with a 3/12 outside and a2/12 inside
    Original design was a post and beam got talked me into trusses because of the foam insilation cost now I have expensive trusses and expenaive foam

  8. Robertsoave | | #8

    OK thanks for all the information
    I think what I am going to do is keep it low put 2in of tuff r that will be r13 run purlins add 1.5 tuff r in between them that will be r23 on top
    Then have blown foam under the roof to make up the difference like 3.5 or 4in
    Does that sound like it could work?

  9. GBA Editor
    Martin Holladay | | #9

    You need to read this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    If you plan to create an unvented roof assembly in your climate zone (Zone 6) with a combination of rigid foam above the roof sheathing and blown-in insulation under the roof sheathing, then the rigid foam layer needs to have a minimum R-value of R-25. Your plan to install 3.5 inches of polyiso (which will perform at about R-19 or R-20 in cold weather, due to the performance drop for polyiso at cold temperatures) won't work.

  10. Robertsoave | | #10

    Thanks I am reading as we speak
    I was thinking high density foam under the roof up against the sheathing and I can go with 4 in on top also
    Thanks again

  11. Expert Member
    Dana Dorsett | | #11

    In zone 6 you need AT LEAST HALF the total R to be outside the first condensing surface (the bottom side of the under-deck spray foam), which gets to be an expensive proposition.

    Using 4" of recalimed roofing polyiso above the roof deck would get you the first ~R20 (after derating for stackup and climate) pretty cheaply, but if you plan to fill up the 24" truss with fluff it'll need at least 7-8" of closed cell foam (R49-R56, with an HFO blown 2lb foam) to have a reasonable dew point control on the R60-R64 of fluff filling the rest. The sprayed polyurethane foam alone would be ~$8 per square foot (!).

    Installing 6" of reclaimed polyiso above the roof deck would get you to code minimum (at it's full-R, not derated for climate) on a U-factor basis, and it would be then fairly moisture-safe to install 8" of half pound density OPEN cell foam (R30-ish) on the under side of the roof deck, which works just fine with open web trusses. That 8" of open cell foam would run $2-2.50 per square foot.

  12. Robertsoave | | #12

    Thank you never thought of used
    Fiberglass is out my my picture now if I can fiND used stuff around here I could go thicker on the top
    How to fasten steel down through 5 or 6in of foam
    So if I go with r25 on top I do r25 on the underside is the way I got a quote today of 10,000 to do r38 foam for this job it is expensive yows,
    Thanks again

  13. Expert Member
    Dana Dorsett | | #13

    Looks like there's a foam reclaimer operating out of Big Rapids MI:

    It's a little steeper than I'm used to seeing for reclaimed foam, but still a big discount from virgin stock.

    For 6" of foam it's common to put down a nailer deck through-screwed to the structural roof deck with 7.5" pancake head timber screws, and mount the roofing layup on the nailer deck as if it were any other roof deck. The details will vary depending on the type of metal roofing.

    [edited to add]

    This is more typical of the pricing I usually see, but the condition of some of the stuff in the picture is questionable:

    These folks in northern IN might be truckable-distance to you too:

    These people might have something for you too:

  14. Robertsoave | | #14

    Thank you
    I did find them ,guarantee when I need it I will not find it ,I will look into the double deck system and bolts

  15. Robertsoave | | #15

    Could I put say a 2 inch foam board up against the sheathing and then add r 38 spray foam on that just have to compare the price of a double deck roof
    It can all be done on the inside correct ?

  16. GBA Editor
    Martin Holladay | | #16

    If you want an unvented roof assembly, with all of the insulation work performed on the inside of the roof sheathing, you will need to install closed-cell spray foam, which is expensive.

    The method you are suggesting is called "cut-and-cobble." When the cut-and-cobble method is used for unvented roofs, the result is often moisture accumulation and rot -- so I don't recommend this approach. For more information, see Cut-and-Cobble Insulation.

  17. Robertsoave | | #17

    Thank s
    I did read it peanut brittle and all ,now I sure wish I would have stuck with the post and beam building I believe it would have been an easy construction method with a 2x8 deck on top with ridgid spray foam
    I think the cost of the 10,000 foam would have been offset more with ease of build , this was a over the phone quote to do the minimum r38 on about 1700 foot of roof from a local guy .
    But it is designed with trusses with 3ft overhangs attached is where I am at!
    By what Dana said now if I were to put 6in used foam board on top and a double deck with standing seam on top
    Would It be easier better and thinking about a cost difference of probably $2000 more just to bite the bullet and pay for the spray foam I just hate the thought of only r38
    So I cannot put 2 inch of foam on top of the sheeting with r38 under it with high density foam (nothing on top)
    I am glad for all the good advice from all

  18. Robertsoave | | #18

    Could purlins work instead of a sheet of osb I wonder on the second deck

  19. GBA Editor
    Martin Holladay | | #19

    Q. "Could purlins work instead of a sheet of OSB on the second deck?"

    A. Some types of roofing -- for example, most types of screw-down metal roofing -- can be installed on 1x4 or 2x4 purlins above a layer of rigid foam. If you are unsure whether your chosen roofing needs a solid layer of sheathing, ask your roofer.

  20. Robertsoave | | #20

    Dana Thanks
    I made a decision on this roof ,it will be a cold roof with 18in of cellulose if possible of fiberglass if cellulose can be done it is preferred,
    So I will have about 14 sq ft of ventilation Ridge and soffit combined then 3 sq ft of vent with cupola s
    No can lights seal it as best as I can
    I believe after all of everyones help this will work unless I am missing something?
    One more thought what about foil-faced tuff r on the ceiling just above drywall ?

  21. Expert Member
    Dana Dorsett | | #21

    Putting foil faced foam behind the gypsum board, with the fluffy insulation above the foam board and below a vented air channel works just fine.

  22. Robertsoave | | #22

    The roof design will be 24in parallel trusses so as I get to the roughing proses I will have a better understanding of my cost abd at that point I believe I can change my mind ab go with a warm roof done the way you mentioned with used foam board build up
    But from what I have read and chatting looks like it will work l just hate cupola s even if I build a modern one

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