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Ventilating a Low-Slope Roof

mhunt11 | Posted in Energy Efficiency and Durability on

We have a 1:12 pitch metal roof in zone 3 (charlotte NC). The architect has cathedral ceilings with a continuous ridge but no other details on insulating or ventilating the roof.   The framing is done and the roof is on and we’re trying to come up with the most effective solution to ventilating or not ventilating and insulating it.   I’ve already read “insulating a low slope roof” and I’m left with more questions. For a vented roof:  I understand cupolas are recommended in the article but a ridge vent was already installed. I understand that is a bad idea due to wind driven rains, capillary action and possibility of sitting snow.  I’ve seen some low slope ridge vent products out there. Has anyone ever used?  Should we avoid them all together? 2. I understand 6″ of ventilation space is necessary; however, that would lower our ceiling height on the exterior parts of the ceiling to less than 8″ ( i know poor design).  Is 6″ required the entire distance up the rafter bay or could we do 4″ at the exterior and get up to 6″ as the ceiling height increases? 3. If the insulation is tucked in each rafter  (2×8) with furring strips, then won’t each bay be isolated from the next and a cupola would be ineffective anyway since it will only ventilate air from the rafter bays to which it connects?  If we absolutely have to go with cupola instead of ridge vent, then we have to figure out a way for the air to converge into a common space under the peak in order to exit the cupola, correct? We cannot have the air staying contained to each bay, correct? 4.  I’m trying to avoid any ductwork in the ceiling space to avoid air leaks- is this a good idea? 5. do we need to use a vapor barrier above sheetrock/below the insulation or can we just use a vapor barrier type paint on ceiling? 6.  How do we determine how many vents we need in soffits? The ridge vent runs the length of the roof. 7. Looking at rockwool comfortbatt 10″ insulation- thoughts on this? 8. how do you insulate under the 2×8 rafters thenselves?  Is our best alternative an unvented ceiling using closed cell spray foam? Is the air tightness of the ceiling just as important in an unvented roof assembly as a vented roof?

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  1. Jon_R | | #1

    I'd read IRC R806.5.

    I doubt you will find any hard data - so my guess is that 4" expanding to 6", similarly large vents and no cuppola would work in your case if you have excellent interior side air sealing.

    Not what you asked for, but it wouldn't take much spray foam to make a well proven un-vented design.

    1. mhunt11 | | #2

      I’d really like to keep spray foam out - the gc installed the wrong underlayment (not meant for under metal roofs) and refuses to replace it (even with lawyers involved) so it worries me that if the underlayment fails and there’s a leak in standing seam roof we would never know with the closed cell foam if we have water intrusion and mold will grow unchecked…

      But yeah I mean is this shitty vented roof just as bad?

      Code wants 2”. Is 4 doable or is 6 a must if we go with the vented roof system? Would you put a smart vapor barrier on the underside of the insulation? The metal roof and underlayment are impermeable so if we go with vented roof it has to breath / and dry from the inside. Right?

      1. Jon_R | | #5

        Everything I've seen indicates that a smart retarder helps. It reduces the moisture going in and then let's it dry quickly later. Intello is a very good one that can also be a good air barrier.

        Your mild weather makes a big difference. I think you are just a degree or so short of meeting the 45F limit where you can build an unvented roof with no foam. That suggests that very little venting (maybe 2.5"?) would suffice, despite your low slope.

        1. mhunt11 | | #13

          Jon as another poster pointed out I’m actually in zone 3! Is that better for me?

          1. Jon_R | | #14

            One option in code is to use the average temp of the three coldest months, independent of zone. What is your exact average?

        2. mhunt11 | | #15

          For some reason I can’t reply to your latest comment but temps range between 30-50 in our coldest months. I’d say average is 40’s.

    2. mhunt11 | | #35

      Jon i'm really starting to consider the un-vented design but i'm not sure how that will interplay with the front half of the house/roof which is a steap slope with ridge and soffit vents and an asphalt roof. Does the whole thing either need to be vented or unvented or can you half the metal parts unvented and the shingled parts vented...

  2. Expert Member
    Akos | | #3

    The question generally isn't how much vent space you need, but how tight you can make your ceiling. If there are no air leaks, you need very little venting, if your ceiling leaks a lot, no amount of venting will keep you out of trouble.

    So first and foremost, make sure you ceiling is tight. This means absolutely no ducts in the ceiling. Don't shutgun the ceiling with pot lights, if you must have them, keep the number to a minimum and go for slim LEDs that have a nice gasket to air seal.

    As for the vent gap, as long as most of the vent is open, you can narrow it down a bit by the lower section to increase ceiling height. 4" gap is still a decent gap (my code is only 2 1/2" for low slope).

    Your bigger issue is your ridge vent. If the vent is rated for 1:12, than by all means leave it. My guess it is not as at that slope water can still move uphill from capillary action. This means the ridge vent has to come out, install a sealed ridge cap and install a high side soffit vent as your exhaust.

    1. mhunt11 | | #6

      Can you link info to a high side soffit vent? I can’t find any info when I Google. We have exposed rafters tails so no soffits just eaves by the way. any eave at the highest point of the roof is only 4% higher than the lowest point.

    2. mhunt11 | | #7

      Do you have any tips for making sure ceiling is air tight? Would you use a smart membrane? I can eliminate hvac but there will be some lighting fixtures which we can make sure are sealed. Air still does rise through Sheetrock - so not sure how to ensure air tightness!

      1. Expert Member
        Akos | | #9

        I'm assuming the low slope is a shed roof. This means you have overhangs at both sides. High side soffit vent is the same your standard soffit/facia vent but installed on the upper section of the roof.

        Mudded and taped drywall is airtight, no air will make it through. The issue is usually holes in the ceiling or the transition from the wall to the ceiling.

        For your ceiling lights, you can get vapor tight device boxes. These have a gasketed flange that the drywall can seal against. They are pretty cheap but it might be a special order in your warmer climate.

        If you are in the land of high winds and hurricanes, I would heed the advice of the posters above and go for an unveted roof with SPF. With no venting, wind driven rain can't make it into the roof.

        1. mhunt11 | | #11

          Are you referring to these places that I have arrows to? We don’t have soffits but we have eaves/overhangs. What that be enough if every low side eave had a vent but there were only two vents in soffits up there ? That would also mean that the air coming in would have to converge and go out the high side vents and we could not keep bays isolated from one another.

          I agree unvented would prevent water intrusion but unless our builder agrees to redo the roof with the correct underlayment I’m afraid to sand which the sheathing and roof and then not be able to ever know if we have a leak and or mold growing…
          He did not use an underlayment meant for metal (high temps) or approved for a 1:12 slope. He says it’s still better than most underlayments 🥴 be used Owens Corning weatherlock g.

        2. Expert Member
          Akos | | #16

          I use soffit vents are a grab all term for any vent that is installed under the roof overhangs near the walls. With exposed rafter tails, the only difference is you put the vents into trim between the rafters instead of the soffit panels.

          What you have there is a low slope gable roof. You can try only vent it from the soffits and seal the ridge, but that is pretty long length and you'll get stagnation near the ridge.

          If you want to try and vent it, I would have the metal guys fab up a wider ridge cap, say 24" to 36" wide, this would put the intakes a bit lower down the slope and less chance of wind driven rain making it in.

          The other option is to keep existing ridge vent but install a vapor diffusion port underneath using one of the fancier house wraps. You would seal this house wrap to the existing underlayment of your roof with a quality tape (ie 3m 8067 or Henry Butyl flash), this way if any liquid water makes it in through the ridge it can't get into your roof assembly. Since the house wrap is permeable, it will allow for moisture to move out of your roof. Technically you can close the soffit vents, but I would keep them open as it won't hurt. You can read more about the diffusion ports here:

          Really the best solution at this point is a flash and batt install and going for an unvented roof. A proper snap lock or seamed metal roof is liquid tight and will not leak. By the time you have to worry about roof leaks and locating them, the metal would have mostly corroded away. Closed cell SPF is not liquid tight, if you have a roof leak, you'll see it.

          P.S You mention low slope underlayment. As far as I know, there is no such thing. Your metal roof needs to be rated for the low slope, if your metal panel is not rated for this, no underlayment will keep you out of trouble. Installing peel and stick underneath helps but not a guaranteed to last. The underlayment under a metal roof is only there to keep night time condensation out of your roof deck.

          1. mhunt11 | | #17

            Yes the metal roof is snap seam so all fasteners are hidden under the next panel. It is rated for 1:12 slope. I am not worried too much about water intrusion over the metal panels/seams but more so at the ridge vent where the panels meet the ridge (water getting between the metal and the underlayment up there or getting in the ridge vent/house itself). Also the underlayment is weatherlock g which is granulated and also not rated for high temps or 1:12 slopes (the latter being the least of my concerns with it). I’m more concerned with it melting. It’s also impermeable along with the metal so if any water gets between underlayment and metal it will be stuck. What would you do in my shoes? We already asked them to change the underlayment via a construction lawyer and they’re refusing. Do we pay for it to be redone ourselves? Sue them? Leave it? The ventilation/insulation is a whole different issue (which was my original question) but I feel like the underlayment situation affects the ventilation/insulation issue. If it was the right underlayment and the ridge was closed up completely (and done well) then I’d be ok with a ventless roof…

          2. Expert Member
            Akos | | #23

            Looks like we run out of reply, so I'll reply to here.

            I really hope your metal panels are installed over strapping. You can't put metal roof over granulated as it will destroy the paint as the panels expand and contract.

            Most metal profiles have enough gaps from striations and snap ribs that you'll get some air flow under the metal. It won't trap water.

            As long as the ridge cap is also rated for 1:12, is installed properly with the closure strip sealed to the panels and sides caulked against the ribs, not enough water will get in to matter. If it is just a standard ridge cap snapped onto perforated Z closures, you'll have issues.

            Temperature could be an issue for the underlayment but most shingle roofs also run pretty hot. The problem with regular peel and stick is if the panel is installed directly over it, it will ooze over time causing the panel to stick. This will prevent the panel from expanding in the sun which can cause it to oil can and make noise.

  3. Expert Member


    You have received good advice on how to make this work. However remember that you are creating a roof assembly that is just this side of failing. That's something that is sometimes necessary in a renovation, but not something I would ever build into a new house.

    1. mhunt11 | | #8

      This was not necessary as we added the second floor onto the first and it could have been designed differently. Unfortunately the architect didn’t do a good job planning and I didn’t know any better at the time :(

  4. Chris_in_NC | | #10

    As a brief note, you're in Climate Zone 3A, not 2A. None of NC is in CZ2.

    I'm also in Charlotte.

    1. mhunt11 | | #12

      Oh no! Chris do you do roofs or what is your specialty?

  5. Jon_R | | #18

    > not be able to ever know if we have a leak and or mold growing

    You could embed some sensors to check for excessive moisture. I don't know of any specific ones.

    I suggest that you take some good pictures of the ridge vent and post - hopefully a builder will comment on its chances of not leaking.

  6. mhunt11 | | #19

    Here’s a pic of ridge vent. I’ll get close ups tomorrow.

    1. Expert Member
      MALCOLM TAYLOR | | #20


      What under the ridge-cap? Vented closure strips? Top of the panels bent up?

      1. mhunt11 | | #21

        I'll take a pic tomorrow! That was a screen shot from a video i had but i'll get close up pics. Thanks so much for helping me...

        1. mhunt11 | | #24

          Here is a close up of ridge vent. It looks like it’s pretty much flat easy access for rain … this was what was supplied by the metal manufacturer after looking at our plans and approved by the gc

      2. mhunt11 | | #41

        Under the ridge cap is z brackets. I think we plan to seal them and go with an unvented roof system with closed cell spray foam on the interior roof sheathing. Would you agree with that plan?

  7. Expert Member
    Akos | | #22

    That doesn't look like a vented ridge cap. Most I've seen tend to be beefier units. Ie:

    There are some lower profile ones similar to shingled roof as well but still not that small.

    1. mhunt11 | | #25

      Yes I agree there are ridge vents designed for 1:12 metal roofs and what we have is not one of those. I just can’t understand why the metal manufacturer (Best Buy Metals) and GC thought this was ok.. Here’s some pics… is this a code violation? Gc is saying it’s fine…

      1. Expert Member
        Akos | | #26

        The good news is that is not a vented ridge, which means you can apply SPF to the underside without any changes.

        I would still have them caulk around the Z closure, that is asking for trouble on a low slope roof.

        Here is the detail of a proper vented low profile ridge. Notice the perf is above the ribs on the roof and the Z closure is sealed:

        P.S. Do you have an actual roof vent cut through the roof deck? Can you see the metal ridge from the inside of the house?

        1. mhunt11 | | #27

          So you would recommend going with a ventless system? They told me it was a ridge vent so now I’m confused that it isn’t 😑

  8. mhunt11 | | #28

    So no we don’t have a space cut into the framing on either side of the lvl but our gc said that was a mistake and needs to be done since a ridge vent and cap system was installed on the roof. (Or so he says?!)

    We have an attic in the front half of the house (it’s one an a half stories with living in the back and attic space in the front). That also has a ridge gap but on a steep shingled roof. we want to turn it into a conditioned space since hvac is in there. If we create an unvented roof on back half of house with the low slope metal roof, can we create a vented conditioned attic space on the front half of the second floor?? Or will this create too much air coming in which is obviously problematic with an unvented roof…

  9. mhunt11 | | #29

    Are you saying caulk here? Would there still be space for air flow if we kept this ridge vent (and cut the proper space into the roof sheathing ?

  10. Expert Member
    Akos | | #30

    Seeing the full design, low slope with lots of intersection and dormers. Add in ducts in the attic, an unvented roof with spray foam is really your best bet.

    In zone 3 you only need R5 (~1") of closed cell foam for condensation control. The rest can be fluffy insulation or open cell SPF.

    1" of closed cell foam is still somewhat vapor permeable, so there are no issues with trapping moisture in your roof deck.

    Make sure they carry spray foam down to your wall top plates for air barrier continuity.

    You want to seal the Z closure on the sides and the bottom. If it was not embeded in caulk or installed with mastic tape, than a bead along the bottom of the Z as well.

    P.S. Check out my replay at #23. If the roof is not installed on strapping, you have some real issues.

  11. mhunt11 | | #31

    Thanks so much for the help. There is not metal strapping. If gc refuses To change the underlayment would you still do closer foam spray? If we end up bringing the attic under the front more steeper pitch roof into conditioned space since it houses ducts and hvac equipment would we need to make it ventless as well or can you vent one part of a roof and not another?

    1. Expert Member
      Akos | | #32

      The underlayment and venting are not related, they are two very distinct problems.

      You can have an unvented roof using closed cell spray foam with a non permeable peel and stick above the roof deck. As long as the SPF is installed onto a dry roof deck, it won't have issues.

      If your HVAC equipment and ducts are in the attic, you want an unvented roof with conditioned attic space. You want the HVAC inside the conditioned space, this tends to reduce your HVAC operating cost by about %25 or so, so not a small cost savings. The best bet is to spray foam the steep section as well and insulate against the roof deck only.

      As for your underlayment issue. The granules of granular underlayment are essentially sand. Metal roof panels move and expand with temperature change, all those granules act as sand paper so that underlayment is slowly sanding away the paint on the bottom of your metal panels. It won't won't fail right away, but will rust through eventually. This is why you never install metal roofing over old shingles directly but always strap out the roof.

      I would talk to your metal panel manufacturer about this, I'm pretty sure they would void any warranty on their panels. Even if the underlayment is fixed now, the panels might already be damaged.

      1. mhunt11 | | #33

        Ok thanks. So if the steep part of the roof already has a ridge vent in place do we need to close that up and spray foam the underside of roof sheathing and essentially just create an entire unvented roof with 100% conditioned space under the roof?

        1. Expert Member
          Akos | | #34

          Yup, it needs to be sealed. The spray foam itself will do the job. Depending on how the vent is built, you might get some overspray outside, in which case tacking up some house wrap to seal it before hand would be good.

          The nice benefit of a conditioned attic, provided your trusses are up for it, is that you can now use it for storage.

      2. mhunt11 | | #40

        Hi Akos! I'm trying to figure out the proper underlayment to use on an unvented roof assembly. The only one that works for a metal roof on 1:12 pitch that I can find is Titanium PSU 30 by Owens Corning but it says the roof must be vented... do you think it would be fine?

  12. mhunt11 | | #36

    Akos i'm back and just want to summarize and clarify:

    1. We now plan to create an unvented roof assembly. First we'll make sure roof sheathing is dry from the inside before spraying. We'll also make sure they spray top wall plates. We have 2x8 rafters so we don't have a ton of room for both CSF and fluffy insulation so can we do all CSF? Or what combo of the two can we use to fit it within the 8" of space?
    2. We will bring the front part of the attic, which is under a steep shingled roof, into the conditioned space by also creating an unvented roof. We will need to close the ridge vents that are already in place. Do they get closed from the outside or the inside (By spraying with foam) or both?
    3. Do we need to dry wall this conditioned attic space for air tightness or can we leave the CSF exposed?
    4. Can we put duct work between the ceiling drywall and the CSF if there is room or should we still place the duct work under the drywall?
    5. Do we still need to ensure air tightness of ceiling (no leaking ducts or lighting) if we're going with an unvented assembly? I guess i'm asking, is air tightness still a priority with an unvented system or does the CSF provide the air tightness so the concern regarding ductwork and lighting air leaks is moot?
    6. We need to have the underlayment changed regardless since it's granulated without strapping installed...


    1. Expert Member
      Akos | | #37

      1. You not only need to spray the wall top plates, but the foam needs to be continuous from the wall top plate up to the roof sheathing. You want a nice foam cap over the whole house with no holes in it anywhere. You should not see any daylight inside the attic. You can use all cc SPF, a combination of about 1.5"-2" of cc SPF, the rest as fluffy insulation or open cell SPF. Since the rafters are a pretty big thermal bridge, no matter how much foam you put in there, it won't affect the assembly R value all that much, so it really comes down to installed cost. To minimize the amount of foam, my preference with 2x8 would be 2" of cc SPF + 5.5" mineral wool/hd fiberglass batts. Assuming 16 oc, that would get you about an R30 assembly with R38 of insulation.

      2.The vent should be closed to contain the SPF. This can be done from the inside with house wrap stapled across the vent gap. The nice benefit of house wrap there is that it will also act as a vapor diffusion port which would let you insulate with all open cell foam in the steep section. A combination of a layer of closed cell foam + fluffy/open cell foam for the rest would still be the more robust assembly, but in your milder climate it is not a big difference.

      3.Most codes will require an ignition barrier over the foam or to use a rated foam product. You can read more about it here:

      4. A 2x8 won't leave you all that much room for insulation if you put a duct in there. Assuming you are using something like an 8" oval duct, that would leave room for just around 3.5" of insulation. Not ideal when the roof is 180F but not terrible. The important part is that since the SPF is your main air barrier, duct leakage won't be lost to the outdoors which tends to be the bigger efficiency hit. I would try to keep the ducting inside conditioned space. Run them in the floor joists of the 2nd floor and up the interior walls, no need for any bulkheads.

      5. Since the SPF against the roof deck is your main air barrier, you don't have to worry about lights and such. A 2nd air barrier is always a good idea as a belt and suspenders approach, so at least make some air sealing effort around your light fixtures (ie get gasketed slim LED lights instead of recessed cans).

      Best of luck.

      1. mhunt11 | | #38

        Such good info thank you!

        So on the steeper sections with asphalt shingles go with open cell sf and cover the ridge vent with house wrap from the inside and then spray the open cell spf over the house wrap and obviously wall plates and roof sheathing…

        And on the low slope metal do all cc spf or some combo of cc spf and mineral wool…

      2. mhunt11 | | #39

        Hi Akos! I'm trying to figure out the proper underlayment to use on an unvented roof assembly. The only one that works for a metal roof on 1:12 pitch that I can find is Titanium PSU 30 but it says the roof must be vented... any suggestions?

      3. mhunt11 | | #42

        Would 7.5” of open cell spray foam suffice or is closed cell a must?

        1. Expert Member
          Akos | | #43

          With any type of SPF, there is no ROI for going overboard. Shoot for whatever is code min in your area. About the only extra SPF that is worth it is to flash over your rafters (or top chord of truss) with about 1" to 2" of SPF to reduce thermal briding. This increases the assembly R value by a fair bit without needing a lot of extra foam.

          Open cell is fine provided the attic space is conditioned (50 CFM/1000sqft or dehumidifier) or if you install a vapor diffusion vent. You can read more about it here:

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