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Insulation for 1:12 standing seam metal roof

Kevin Hoene | Posted in General Questions on

Hello all,
Newbie to GBA but we are interested in building an efficient home. We live on the edge of zones 4 and 5 in Illinois. 100% of the exterior will be EIFS. The plan is for our exterior walls to be 2×6 with cellulose in the cavity and rigid foam over the sheathing.

My question is about how to ventilate our standing seam metal roof (1:12 pitch). Our architect didn’t seem to have a good grasp on green building techniques.

My questions to architect:
Should we use deep trusses for our roof? 12 inches of insulation then have an air gap of 6 inches between the top of the insulation and the roof sheathing? Then having ventilation in the soffit or fascia at a height that would be within the 6 inch air gap (above the insulation). Do we need ventilation in the center of the roof (like a cupola)?

Her response:
What type of insulation 12″ thick are you going to get that won’t make the drywall ceiling sag? Maybe batt? If batt at 12″ you can get around R38 in the cavity. With 6″ air you will need to vent. I would only vent on ends OR top and bottom to cause cross ventilation. I would not vent in center. If there is no unconditioned space (stale air), there is no need to vent. So you could spray insulation directly to the bottom of the roof deck. 1″ rigid supposedly means R5 to R7. You will need to spray around 6″. This is hard to accurately measure. In this case, the strength of the roof members shall dictate the height for the will be greater than 6″. I think either would be fine. But I feel the spray insulation is a better insulation. On my home we have the same situation. We did the spray foam without ventilation. I think some contractors thought this was a bad idea because of lack of ventilation, but why ventilate when there is no air to ventilate. And on steel buildings we insulate directly under the roof panels and do not ventilate. There should be no difference.

Ok now this is me again. My question for all of you …. what do you feel is the best approach to insulating and ventilating our roof? Sorry, all of this is new to me and I’m trying to learn and make the best decision for our home. Thanks to all in advance!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kevin,
    You can build an unvented assembly or a vented assembly; it's your choice. Here is a link to an article that explains your options: Insulating Low-Slope Residential Roofs.

  2. GBA Editor
    Martin Holladay | | #2

    Kevin,
    A couple of other points:

    1. It's not unusual to find architects who aren't quite up-to-date on building code requirements for insulating low-slope residential roofs. These details usually aren't taught at architecture school. Many builders are also ignorant about this topic.

    2. The 2012 International Residential Code calls for roofs in your climate zone (Zone 4 or 5) to have at least R-49 of roof insulation. Because it's trickier to insulate a low-slope roof than an attic floor, many architects and builders urge homeowners to accept insulation that is less than minimum code requirements -- and some building inspectors accept this deplorable practice. I urge you to insist, at a minimum, on R-49 insulation.

  3. GBA Editor
    Martin Holladay | | #3

    Kevin,
    Three reactions to your architect's opinions:

    1. This is bad advice: "I would not vent in center." A cupola or doghouse vent is necessary for a vented low-slope roof.

    2. This statement is meaningless, wrong, and worrisome: "If there is no unconditioned space (stale air), there is no need to vent."

    3. It is a mystery to me that your architect didn't mention insulating this roof assembly with rigid foam above the roof sheathing -- the standard solution for low-slope roofs.

  4. Kevin Hoene | | #4

    Martin,
    Thanks for your feedback! That link was actually the link I had used to educate myself a little bit before asking questions of my architect. A couple follow up questions if you don't mind ....

    The link you provided recommended trusses deep enough to allow 12-16 inches of insulation and a 6 inch air gap between the insulation and roof sheathing. What type of insulation do you recommend for this 12-16 inches of insulation?

    Due to the 1:12 pitch of the roof, one side of the attic on our longest sheet of metal will have about 6' more height than the other side. Do you provide the same insulation for the entire space and have a much bigger air gap on one side?

    For the cupola or doghouse vent in the center of the roof, is snow buildup around the cupola/doghouse vent a concern? Is there a certain size/style of cupola or doghouse vent that would provide adequate ventilation while maintaining our contemporary style? I wasn't able to find a lot of info online for a doghouse vent.

    In addition to the 12-16 inches of insulation, the rigid foam should be between the metal roof and the roof sheathing? How many inches of rigid foam?

    I'm sorry if these questions are elementary. All of this stuff is new to me, but building a green home is something that has become very important to me. I have to do a lot of research and ask a lot of questions because most of this stuff is also new to the builders in my area - I originally wanted to do ICF but couldn't find a builder to do it. I will definitely insist on R-49 at a minimum for our roof! I am truly grateful for your help!

  5. GBA Editor
    Martin Holladay | | #5

    Kevin,
    Q. "The link you provided recommended trusses deep enough to allow 12-16 inches of insulation and a 6 inch air gap between the insulation and roof sheathing. What type of insulation do you recommend for this 12-16 inches of insulation?"

    A. The recommendation you describe is only for vented roof assemblies. It's also possible (and usually preferable) to design an unvented roof assembly with rigid foam insulation above the roof sheathing. Assuming that you prefer a vented roof assembly, you could install cellulose or blown-in fiberglass insulation.

    Q. "Due to the 1:12 pitch of the roof, one side of the attic on our longest sheet of metal will have about 6' more height than the other side. Do you provide the same insulation for the entire space and have a much bigger air gap on one side?"

    A. Yes. Aim for a minimum of R-49 everywhere.

    Q. "For the cupola or doghouse vent in the center of the roof, is snow buildup around the cupola/doghouse vent a concern?"

    A. Yes. If you live in a snowy climate, an unvented roof assembly (with a thick layer of rigid foam above the roof sheathing) is probably preferable to a vented roof assembly. However, if you want to build a vented roof assembly in a snowy climate, make your doghouse tall, with the ventilation openings beginning above the anticipated snow depth.

    Q. "Is there a certain size/style of cupola or doghouse vent that would provide adequate ventilation while maintaining our contemporary style?"

    A. This is an aesthetic question, so only you can provide an answer. On some low-slope roofs, a cupola in the center of the roof would not be visible from the ground. On other roofs, it would be an eyesore. Again, if you are worried about this issue, I recommend that you consider building an unvented roof assembly by installing a thick layer of rigid foam above your roof sheathing.

    Q. "In addition to the 12-16 inches of insulation, the rigid foam should be between the metal roof and the roof sheathing? How many inches of rigid foam?"

    A. You need R-49 of insulation at a minimum, and there are at least three ways to achieve this goal:

    (1) Install R-49 of rigid foam above the roof sheathing. This would be an unvented assembly.

    (2) Install some rigid foam insulation above your roof sheathing, along with a layer of air-permeable insulation under (and in direct contact with) your roof sheathing. You have to follow certain strict rules about the ratio of rigid foam to air-permeable insulation if you go this route. This would be an unvented assembly.

    (3) Install R-49 (or more) of insulation above your ceiling, along with a vented air space with a minimum height of 6 inches between the top of the insulation layer and the underside of the roof sheathing. This would be a vented assembly.

    Details describing all three approaches are in the article that I linked to in my first response (Insulating Low-Slope Residential Roofs).

    For more information on installing rigid foam above your roof sheathing, I strongly recommend that you read this article: How to Install Rigid Foam On Top of Roof Sheathing.

  6. Kevin Hoene | | #6

    Sorry Martin, we went on a short vacation so it's been a few days since I replied. After reading a couple of your articles, this is what I'm thinking ...

    - 4" of XPS above the roof sheathing (perhaps 2 layers of 2" XPS so I can stagger the seams?)

    - Enough insulation (either open-cell spray foam, fiberglass batts, mineral wool, or cellulose) directly underneath the roof sheathing to get total of R-49

    - Will make sure the roof sheathing is airtight

    - No ventilation

    Does this sound better? Any recommendations for the type of insulation used under the roof sheathing?

  7. GBA Editor
    Martin Holladay | | #7

    Kevin,
    The trick to your suggested approach is making sure that the air-permeable insulation on the underside of the roof sheathing is in direct contact with the roof sheathing.

    The best approach will depend on what type of framing members you have; whether you have good access to the attic; and what method you intend to use to confine the insulation and keep it tight to the sheathing. You don't want the insulation to sag, or there to be an air gap between the sheathing and the interior insulation.

    If you can create a tight air barrier under the framing -- for example, with OSB or gypsum wallboard -- you may be able to fill the rafter bays with dense-packed cellulose. If the cellulose is dense-packed, it won't sag.

  8. Expert Member
    Dana Dorsett | | #8

    With only 4" of XPS you have to limit the total-R to about R50 if you're in zone 5, but could go as high as R65 in zone 4. It's the ratio of R above & below the roof deck that determines it's average temp, and it's the average temp determines it's late-winter moisture content when the under-deck layers are more vapor-open to the interior conditioned space air.

    XPS isn't very enviro-friendly stuff, since it's blown with HFC134a with a global warming potential ~1400x CO2. In your climate with a 2-layer approach to the foam it's nicer to the planet to put 2" of rigid polyisocynurate on the roof deck, and 2" of EPS on top of that to hit the nominal R20. Both of those are blown with pentane, at only 7x CO2. The dual-type and the layering of the stackup is important. If the outer layer is polyiso it will underperform it's rated R in winter, but if it's EPS it will outperform it's rated R, and keep the layer of polyiso warm enough to stay in it's high performance range. The labeled-R would be R20.5 to R21.4, but it'll perform at about R20 over a wide temperature range, rising to about R22 when the average temp through the polyiso layer is 50F.

    As the HFC blowing agent leaks out of 4" of XPS over then next 50 years it's R-value drops to about R17. The EPS/polyiso stackup is pretty stable over time. The EPS is fully depeted on day-1 and performs the same forever. Foil faced polyiso eventually drops to about R11+ @ 2" so in a half-century the stack up would still be about R19.5 on it's worst day.

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